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PART 3

OPERATION OF MAJOR TYPES OF RADAR

 

PART 3

OPERATION OF MAJOR TYPES OF RADAR

INTRODUCTION3-3
  
SURFACE-SEARCH RADAR3-3
Long-range search for large targets3-3
Short-range search for small targets3-4
Station keeping3-5
Navigation3-5
Auxiliary fire control3-7
Composition3-7
  
AIR-SEARCH RADAR3-7
Long-range, early warning air search3-8
Short-range search and multiple-target tracking3-8
Fighter director tracking3-9
Overland tracking and overland search3-9
Fire-control liaison3-9
Composition3-10
  
PIPOLOGY
  
INTRODUCTION3-10
  
COMPOSITION3-10
Friend or foe3-10
Estimating the size of ship targets3-11
Estimating the number of ships3-12
   Bearing and range resolution
   Effect of range on bearing resolution
   Effect of sweep length on range resolution
   Effect of receiver gain on range resolution
   One-pip areas
Estimating the number of planes3-17
General hints on composition3-18
  
FALSE CONTACTS3-18
Sea-return3-18
Minor Lobes3-19
Clouds3-19
Radar pulses3-20
Double-range echoes3-21
Second-sweep echoes3-21
Reflection echoes3-21
Wakes3-21
Miscellaneous objects on the surface3-21
  
PPI INTERPRETATION3-21
Radar shadows3-21
 
3-1

Beam-width distortion and pulse-length distortion3-21
Side-lobe ringing3-22
Low land3-22
Ships near shore3-22
  
MISCELLANEOUS CONSIDERATIONS3-24
Course changes3-24
Blind sectors3-24
  
DEFENSE AGAINST JAMMING AND DECEPTION
  
INTRODUCTION3-25
TACTICAL RADAR JAMMING3-25
ELECTRONIC JAMMING3-26
Types3-26
   CW or unmodulated jamming 
   Low-frequency modulated jamming 
   High-frequency modulated jamming 
   Random noise modulation 
   Pulse jamming 
   General 
What the operator should do3-28
  
MECHANICAL JAMMING3-30
Introduction3-30
What the operator should do3-32
  
ENEMY DECEPTION3-33
Window3-33
Discrete reflectors3-34
 
3-2

OPERATION OF MAJOR TYPES OF RADAR
INTRODUCTION

Various radars differ in physical appearance. Each has its special knobs, types of presentation, and "gadgets," depending on the primary function of the individual set. Regardless of this physical variance, there is much that can be said, in a general sense, about good operational techniques for all radar sets. It is not intended, however, that the information in this section be followed to the letter under all conditions and in all tactical situations.

In order to gain the maximum tactical advantage from radar at all times, the radar operational techniques must change as the situation changes. Methods of operation must be flexible. Commonsense, and a thorough knowledge of naval tactics must determine which of these techniques should be used in any given situation.

A brief outline of the various basic controls and indicators will form a foundation for a more detailed discussion of operational techniques.

1. Range scale. What scale should be used tinder what conditions? How often should scales be shifted?

2. Gain Control. This corresponds to the volume control of a broadcast receiver. Should it be set high, low, or medium?

3. Antenna rotation. Should the antenna be rotated continuously? How fast should it turn? Should it always search an area of 360 degrees? If stopped, for how long?

4. Range. How should ranges be read? Should the range step and associated dials be used when provided? Should estimated ranges be used with the assistance of improvised scales?

5. Bearings. What are the different ways bearings can be read? Should the bug be used, or should a cursor be used instead?

6. Scope. If the radar set is equipped with two or more types of cathode-ray indicators, which should be used, and under what conditions is one preferable to another?

The answers to these questions, for different types of radar sets operating in various tactical situations, will provide you with the foundation of operational techniques. From this foundation, each special circumstance will require variations which can only be

  determined by radar operating experience and common sense.

There are three basic types of radar:

1. Surface search
2. Air search
3. Fire control

In this section, the operation of the first two types will be discussed in general terms. However, since fire-control radars have such widely varying characteristics, recommended operational techniques will be particularized for each type, and will appear only in Part 4.

Each type of radar has been designed for one specific purpose, and there is nothing that you, as an operator, can do to modify these purposes. An air-search radar is a poor surface-search radar, and vice versa. Each of these types may serve in an emergency as a fire-control radar, but they cannot he expected to furnish ranges, bearings, and position angles with the same degree of accuracy as a fire-control radar specifically designed for that purpose. In case of failure of either the air- or surface-search radars, the fire-control gear may act as a search set.

SURFACE-SEARCH RADAR

The words surface search are misleading, since search is only one of many functions that has been delegated to this general type of radar. The six major functions are listed below, together with suggestions for optimum radar efficiency under each condition.

Long-range search for large targets.

It is essential that large surface targets be detected at the maximum possible range of the radar, so that effective attack or evasive tactics can be employed. The range scale used should be longer than the expected maximum range on ships. The gain control should be set for maximum readability of an echo at 30,000 to 60,000 yards. This setting must be previously established for each particular radar set. The antenna should be rotated at the slowest available speed; an occasional sweep should be made using the manual control, if one is provided. The "A" scope (if the radar is so equipped) will usually show the initial contact before it appears on the PPI.

 
3-3

RADAR OPERATOR'S MANUAL
If a contact is established, stop the antenna (when means are provided for stopping it on contact) only long enough to obtain an initial bearing and the "A" scope range. Attempt to classify the contact specifically by utilizing previous knowledge of the capabilities of your particular radar. Data concerning previously observed maximum ranges on different types of ships, size of pip, and composition of pips will be useful in making this decision. Two courses of action are now open to you. You may follow the procedure outlined for auxiliary fire-control, or you may resume normal search. Your decision will naturally be based on the tactical situation.

Assume for this discussion that you are not interested in firing on the new contact. However, you might desire to keep a rough track thereof in order to maneuver around it. Your procedure, then, would be to continue a long-range search, reading bearings and ranges of the contact "on the fly," without stopping the antenna. With practice, sufficiently accurate data may thus be obtained to maintain a reasonably exact track. The important consideration of the on-the-fly operating technique is that you are continuing to search for other contacts (which the Captain may later decide to attack) without sacrificing the search efficiency of the radar by stopping its antenna on a contact that admittedly is not of primary interest.

Ranges read on-the-fly will be more nearly accurate

  and easier to obtain if the "A" range scope is equipped with a scotch-tape range scale, and if the PPI is marked with thinly inked range circles. Bearings can be estimated directly from the PPI.

Short-range search for small targets.

This might be called the submarine or PT boat search, and should be conducted primarily when cruising independently. When cruising in company, the OTC will normally assign the search function of each radar in the force. The range scale used for small target search should usually be the shortest available scale, although on some sets it may be found that the mid-range scale can be used to better advantage. The receiver gain should be varied during the entire search , its setting depending on the amount of sea return present and other tactical considerations. Look for periscopes close aboard, increase the gain a little, and search near the outer limits of the sea-return area for surfaced submarines and small patrol craft. Remember that sea return is basically the same as an echo from a target, and that it must be present if a small target echo is to be detected.

Operating experience will determine the correct gain setting for different amounts of sea return. Antenna rotation should be as slow as possible; again, make occasional manual searches. New targets should appear on either the "A" scope or the PPI almost

Drawing showing friendly fire on a turning convoy.
Figure 3-1. Avoid this by using radar when changing course or leaving a formation.
 
3-4

OPERATION OF MAJOR TYPES OF RADAR
simultaneously, provided the gain is set high enough for PPI operation. These indicators should be alternately observed for equal periods of time to reduce eye strain and monotony. If a contact is made, follow the procedures listed for long-range search. If an attack is to be made on the targets, coach the fire-control radar on to the contact and resume the search immediately. There is no need to attempt to duplicate the function of the fire-control radar with the search radar, unless the search radar is required to solve a torpedo problem while the main battery fire-control radar is busy.

Station keeping.

There are some situations that will demand the exclusive use of a search radar set for station keeping. It must be understood that when this radar is used for station keeping, it is not performing its intended purpose as a search radar.

The normal requirements of station keeping are such that the antenna should be rotated continuously, using the short range scale on the indicator. Bearings and ranges on the guide, or on other suitable ships in the formation, may be estimated from the PPI scope. It has been found useful, when proper conditions prevail, to maintain a plot directly on the face of a remote PPI scope with a chinagraph pencil, or to put a spot on the master PPI representing the place on the scope where the guide should appear when you are on station. Any indication of incorrect station will become immediately evident in this system. Search should not be forgotten when keeping station, and a regular plan of shifting range scales and receiver gain should be adopted. The gain should be turned down only while obtaining necessary station keeping information.

Admittedly, there are situations that demand extremely accurate station keeping. When such is the case, auxiliary fire-control procedure should be followed, utilizing the most accurate ranges and bearings available from the radar. Search must necessarily be forgotten, or minimized, during intricate maneuvers.

Navigation.

One of the most useful functions of a surface-search radar is its contribution to navigation. However, the limitations involved when radar is put to such use must be thoroughly understood. Unless you know the contour and composition of the land that is reflecting the radar energy you are never safe in reporting a range to the "nearest point of land." For instance, if you are ranging on a sharp cliff that rises

  directly upward from the water's edge, you are safe in assuming that the range to the nearest land is positively indicated by the range obtained from the radar screen. If, however, the terrain rises gradually from a waters edge to a mountain or range some distance inland, the possibility exists that the pip on your radar screen has been produced by reflection from the mountain range, and not from the beach. It is almost impossible to determine the exact point of reflection from a sloping surface, and an error of only a few hundred yards might prove disastrous in close navigational work. Always keep a contour map of nearby land available for reference when navigational information is required. After careful practice in "radar map" comparison with contour maps of familiar land, you may become proficient in estimating reflecting surfaces on unfamiliar terrain. This discussion applies, of course, to piloting, since radar "cutons" will usually differ from visual tangents, depending again on the contour of the land.

The beam width of the antenna must also be considered when an attempt is made to obtain a radar picture of a shore line. A few illustrations will show why this is so.

The first series of illustrations in figure 3-2, show how the radar shore line changes as the ship moves from one position to another. Notice that the harbor has been completely obscured by the radar shore line in all instances, and that a ship that might be situated anywhere inside the shaded areas would not appear on account of this beam-width distortion. The explanatory remarks in the first drawing are applicable to all of the subsequent illustrations.

All of the examples have been based upon the assumption that equal reflection is obtained from all points along the shore line. While this is rarely the actual case, it is a necessary assumption for a theoretical situation. The radar shore line will differ from the actual shore line by an amount depending upon the beam width of the antenna, the contour and composition of the land in the immediate vicinity of the shore line, the bearing of the ship from the shore at any given time, and the amount of receiver gain used.

It is impossible to describe all situations that might he encountered in ranging upon a shore line with radar. Each problem has its own special features, and must be treated individually by the ship involved.

More accurate fixes can be obtained if echoes from smaller land masses are used. On the PPI shown in figure 3-3, points X and Y would provide the best navigational fix.

 
3-5

RADAR OPERATOR'S MANUAL
Six views of a ship using radar to observe different land formations.
Figure 3-2.
 
3-6

OPERATION OF MAJOR TYPES OF RADAR
It is often helpful to plot the range and hearing backward from your estimated, or DR position, and analyze the chart to determine if there is a possible reflecting surface. Good and had ranges may he identified in this manner. It should he remembered,

Drawing of PPI returns of a small land mass.
Figure 3-3. Small land masses provide accurate navigational fixes.

however, that the chart itself may he in error, so its known accuracy must be considered in this procedure. A collection of sketches of the composition of pips may be useful when you return to a particular location.

Illustration of prominent landmark pips on display.
Figure 3-4. Prominent landmark pips help in locating ship's position.

Auxiliary fire control.

You are, by definition, using your surface-search radar as a lire-control radar as soon as you start to track a contact. This is often a desirable procedure in spite of the fact that the search efficiency is decreased during such operations. If your ship has no fire-control

  radar, or if such equipment has failed, you may have to depend completely on surface-search radar for the control of gunfire.

The radar operator must furnish more accurate ranges and bearings than those provided by obtaining them "on the fly." There are two methods of developing a plot for fire-control work. These will be explained in detail in RADFIVE. Regardless of which method has been selected, you must stop the antenna to obtain accurate ranges from the "A" scope and bearings from the bearing indicator. If no "A" scope is available, the most accurate method of obtaining this data must be selected, depending upon the particular radars installed.

For radar spotting, the antenna must be fixed on the target while the shells are in flight so that splashes may he noted on the radar indicator. The torpedo-control work is usually delegated to surface-search radars and CIC, since the fire-control radar is busy furnishing necessary information for the solution of the gunfire problem. In spite of the high degree of accuracy necessary to the satisfactory solution of fire-control and torpedo-control problems, the best procedure is to make one or two complete antenna rotations every minute or so to make sure that bigger game is not approaching from a different bearing.

Composition.

Continuous practice is needed by all radar personnel before they become proficient in analyzing the pip on a radar screen. Every opportunity should be utilized when in company with friendly ships to make notes (on effects of position angle, size and type of targets, ranges, and relative bearings) on the composition of an echo.

Familiar double-peaked echoes are often noted from large ships, such as battleships and carriers, at medium or close ranges. Fluctuations of the pip are different when the reflecting object is a rolling destroyer or a more stable cargo vessel. These are among the "tricks if the trade" that must be mastered by an operator before he can he considered above average.

AIR-SEARCH RADAR

The continually changing tactics of the enemy relating to air attacks makes it difficult to outline the best operating techniques for this type of radar. Although the basic tactical situations will be discussed in this section, it would he well to remember that there are no set operating conditions that will hold true for all conditions of radar protection and offensive action.

 
3-7

RADAR OPERATOR'S MANUAL
Long-range, early warning air search.

The problem involved in this type of search is obvious. We want to make initial radar contact with the enemy attack groups at the maximum radar range. Patrol planes and snoopers most be intercepted before they can radio contact reports about our task force.

The range scale should he set to provide the longest available range in accordance with the observed maximum ranges of the particular radar, The PPI and the "A" scopes should be watched alternately, with the antenna rotating slowly. Receiver gain should he set for maximum readability of the indicator under observation. This means that the gain control will be at a different position for "A" and PPI operation.

Upon radar contact, the antenna should be stopped, and the echo scrutinized to determine the composition of the pip. The target should be challenged with the BL, and the "A" scope should show the IFF response if the target is friendly.

The slow antenna rotation should be resumed immediately, and the all-around search continued to detect other possible targets. The procedure to follow at this time will vary, depending on many factors too numerous to present in this book. The type of force, the availability of fighters, and the discovery of other bogies will influence the decision of the task force commander, but this much can he said for the general case: the discovery of a bogie demands an even more

  thorough search of the 360 degrees area around the force. Keep the antenna rotating slowly.

Bearings should be obtained from the bearing cursor on the PPI, and ranges should be estimated directly from the PPI without using the range mark. This will be facilitated by inking range circles at five-mile intervals on the glass surface of the PPI tube, eliminating the use of the unsatisfactory range scale provided with these units. When the PPI is not available, ranges and bearings must be read "on the fly" with the aid of a scotch-tape range scale on the "A" scope.

Short-range search and multiple target tracking.

This search procedure could be followed when a torpedo plane attack is imminent or probable, and when raids are approaching from different bearings. Continuous antenna rotation is a necessity. The range scale should be set at its medium position, and the gain adjusted for maximum readability of the PPI. Ranges and bearings must be obtained in the same manner as that discussed for long-range search (from the PPI).

The speed of antenna rotation should be increased as the attack closes on the force, and you must be prepared to shift to the short-range scale as soon as the targets have reached the outer range limits of this scale. You must also be prepared to change to fire-control liaison operation since it is closely allied to short-range search operation.

Drawing showing radar antenna and aircraft in the distance.
Figure 3-5. Long-range, early warning, air-search radar.
 
3-8

OPERATION OF MAJOR TYPES OF RADAR
It is particularly important to maintain a plot of all friendly planes in the air when contact with the enemy is possible. Unless this is done, a "snooper" or low-flying attack plane may appear on the screen of the radar unknown to the operator. This practice may require that the antenna be periodically stopped to check the IFF return, but once a track has been started on any particular plane, the identification problem should be simplified, and bogies detected immediately.

Fighter director tracking.

For night interceptions this type of radar operation should be carried on during night fighter work so that the fighter director officer can effectively make a PPI interception. The 360 degrees search is abandoned, and the antenna is directed over only the area in the vicinity of the attempted interception. This method, however, concentrates all your efforts on a small area, and should be utilized only if there is sufficient air-search coverage from other radars in the force.

If the operator-plotter team is unable to provide an up-to-the-minute radar picture of daylight interception, fighter director tracking must be employed. Multiple-target tracking is preferable to this method, however, since all areas are covered by the radar.

Over land tracking and over land search.

Tracking targets over land is not as difficult as it may seem at first, although it is an art which requires considerable practice. Actually it is a special type of

  fighter director tricking requiring its own special technique.

Whenever planes fly over land masses, their contacts can not be seen on the PPI. Use the "A" scope and the shortest range scale on which the plane can be seen (if you intend to track it). With the antenna in manual rotation, train slowly through the land mass, watching for a bouncing pip among the steady ones. This will indicate the presence of a plane over land, and you may then read its range almost as accurately as if land were not there. To find its bearing, adjust the antenna carefully for maximum activity of the bouncing pip.

You should practice this technique on friendly planes when in port. IFF affords an excellent method of checking from time to time to be sure that you are on the plane.

Fire-control liaison.

When attacking groups of planes have closed within 20 miles of the force, it is essential that close cooperation be maintained between the air-search radar and the fire-control radars. The guns must often be "talked on" directly from the air-search radar, or by electrical target designation systems connected to that radar.

Depending on the particular installation, the radar may be set to furnish either true or relative bearings. It is preferable to utilize true bearings, provided suitable conversions have been incorporated in the

Drawing showing a ship with planes approaching from 4 directions.
Figure 3-6. Speedy antenna rotation for short-range target tracking.
 
3-9

RADAR OPERATOR'S MANUAL
fire-control equipment. If not, relative bearings should he furnished.

The antenna should be rotated as fast as possible, and the range scales used should be the shortest available on the radar. While this measurably decreases the air-search efficiency of the ship, primary consideration should be given to gunnery when planes have closed to attacking ranges.

Side lobes are especially troublesome in this type of operation, and the operator must be quick to distinguish the extraneous echoes involved. It will help to reduce the gain as far as practicable, in order to minimize these echoes. They may often be distinguished by comparing their width (in degrees)

  with the echo received due to the main-lobe radiation.

Composition.

The "A" scope is of the greatest value when the composition of a contact is to be obtained. Constant drill by operating crews in estimating the composition of friendly planes is of inestimable value as a means of obtaining reliable data to be used on enemy raids.

Composition of raids should be checked at regular intervals, about every 20 miles of target travel, to make sure that any splitting of attack groups may be noted: the estimated size of the raid should be rechecked.

PIPOLOGY
INTRODUCTION

Pipology involves the study and interpretation of all types of contacts seen on radar indicators.

Composition is a closely related word, but not so all-inclusive, and answers the questions: what type? how many? friend or foe? Determination of composition is an art, and is perhaps the most enjoyable phase of radar operation. Given enough time, almost anyone can get the bearing and range of the target, but it takes skill, imagination, and above all, experience to determine composition. With continued experience and increased skill your predictions should be about 80% correct. Trying to identify every echo that appears will give you the practice you need, and whenever possible, get someone to find out what the target is, or was, and thus check the accuracy of your estimate.

Ability to interpret pips comes both from knowledge gained through study and from endless hours of practice on the radar. It is important not only to recognize the target, but also to recognize it in the shortest possible time. Some of the advantages of speed are:

1. It aids the plotters in assembling information.

2. It aids the ship's officers in making quick evaluations and decisions.

3. It gives the gun-director crew and computer operators much needed time in laying guns on the target.

4. It adds to the over-all efficiency of the radar watch.

Pips are of various types. Each type lends itself to interpretation. In general there are four characteristics of pips which will give you information useful in interpretation.

 
1. Size of pip.

2. Shape of pip.

3. Bobbing or fluctuating in height.

4. Movement in range or bearing.

The "A" scope is most satisfactory for observing size and fluctuation of pips, an expanded or short range "A" scope for observing shape , while movement is best seen on the PPI. The following section takes up these pip characteristics in some detail to aid you in interpreting the things you are likely to see on radar scopes.

COMPOSITION

Friend or foe.

The first thing to determine obviously is the friend or foe status of the contact. This can be done only by using your IFF interrogrator, or by securing the information from another ship in your force which has already established this status. The method of handling such a situation is a matter of doctrine. You will be informed as to whether or not you are to make the identification, generally you do.

Having established the friend or foe status of the contact, the next step is to notice the rapidity and the extent of the echo's fluctuation. Consider the height of the echo, remembering the effect of range and fades; then note the depth or thickness of the echo. If the echo is saturated, reduce the gain. Look at the top and sides of the echo for any indication of two bumps or many little humps. What is the speed at which the echo is moving? Look at everything and draw on your entire background of knowledge and experience to interpret what you see.

 
3-10

PIPOLOGY
Estimating the size of ship targets.

First of all, upon what does the size of the pip (strength of received echo) depend? The answer is, unfortunately, quite a number of things, the most important of which are:

1. Range of the target.

2. Size of the target.

3. Height of your antenna (especially when surface targets are concerned).

4. Height of the target.

5. Whether the target is bow or broadside (target angle).

6. Atmospheric conditions.

7. Material composing the target.

8. Correctness of tuning.

9. Materiel condition of the radar set.

Due to the many variables involved, it is not possible to determine the exact size of the target in every case, but you can always make a reasonably

Figure 3-7. Two medium, three small targets.
Figure 3-7. Two medium, three small targets.

accurate estimate. This much you do know: if you have a large and a small target at approximately the same range, the larger target will produce the large pip (stronger echo), other variables being equal. So, if you detect any enemy task force approaching, the picture on your PPI might appear as shown in figure 3-7. Thus, the only positive thing that size of pip

  will tell you is relative size of various targets at approximately the same range.

The best way of determining the approximate size of a target is to observe the range at which it was first detected. This method is especially good with micro-wave surface-search radars such as the SG, SF, SL, SO, etc. Radio waves from these radars travel in practically a straight line. At any given range it takes a certain size object to give back an echo that is just visible on the screen or scope with your radar tuned up as well as possible. Therefore, various types of targets or types of ships first become visible on the scope at some definite range. The echoes come from the ship's mast and upper superstructure first. The superstructure offers approximately the same size target regardless of the direction from which it is seen.

Each radar will have its own characteristic ranges for detecting the various types of targets, depending on how high the antenna is mounted, the power, and the sensitivity of the particular gear.

An estimate of the approximate size of targets at less than the maximum range can be made by considering the strength of the echo, the range, and the target angle. To facilitate this process a log should he kept for recording these data. The data can then be tabulated for quick reference, showing echo strength in E units, range, target angle, and type of -hip, as well as any special features of the pip that might be noticed.

The E system of designating echo strength is based on the ratio of the echo height to the grass height. This ratio is not affected by the setting of the gain control. See figure 1-17 in Part 1, General Radar Principles.

Target angle is an important consideration except at extreme maximum range. It is the angle measured from the bow of the target ship, clockwise (to the right) to a line drawn between your ship and the target ship. In other words target angle is the relative bearing of your ship as seen from the target ship. If you are astern of him the target angle is 1800; if you are broad on his port beam, the target angle is 270 degrees. Target angle can be found by tracking the target a few minutes (see RADFIVE, The Surface Plotting Manual ). Reference to this tabulation will

Drawing of our ship, then SS, DD and finally BB.
Figure 3-8. Relative maximum radar ranges for various types of ships.
 
3-11

RADAR OPERATOR'S MANUAL
provide one of the best clues to the approximate size of ships seen first at less than maximum range.

The bouncing motion of a pip provides another means of estimating the approximate size of a ship. A large target usually shows up as a slowly bobbing pip, varying in size from medium to large. A smaller object usually gives a more violently fluctuating pip, and, especially if the sea is choppy, may produce an echo that will flutter between a medium-sized pip and no pip at all. Of course, roughness of the sea affects the amount of fluttering of pips and this must always be taken into consideration. On a calm day echoes from stationary objects, such as a lighthouse, will produce an absolutely steady pip, hilt if your own ship is rolling, even this type of object will produce a rising and falling pip, unless the antenna is stabilized.

Another way of occasionally identifying the type of moving object is by tracking, and plotting its position over a period of time to determine its speed. Keep in mind, however, that the movement of your own ship makes the target change position on the radar screen.

Here are some examples of information which you might obtain from a radar. Try to determine from them what the target is: You detect an object at 9,000 yards. On the PPI it only shows up once every two or three revolutions. When examined on the range scope the pip is fluttering rapidly. From tracking and plotting the target, you determine its speed to he about 35 knots. Since you did not pick up the target until it was fairly close to you, this indicates that it is a small target: the rapid fluttering also indicates a small target. From the speed of 35 knots you can assume the target to be a small, fast, boat, probably a PT. The same type of target, had it been stationary, might have been a buoy, especially if you were near land where buoys might he expected.

Estimating the number of ships.

Bearing and range resolution. Targets at the same range will present separate pips only if they differ in bearing by a certain minimum angular distance. This angle is called the bearing resolution of the radar, and it varies from set to set (being proportional to beam width). On the other hand, targets on the same bearing will present separate pips only when they are separated in range by a certain minimum distance. This distance is called the range resolution of the radar, and it also varies from set to set (being proportional to pulse duration)

Figure 3-10B shows the picture appearing on the range scope with the antenna trained on a single target,

  while figure 3-10C shows the picture appearing on the PPI under the same conditions. Examine carefully the pip's size. Now carefully cheek the pip's size on figures 3-11B and 3-11C with the antenna trained on two targets within your beam, both at the same range.

On the range scope, the pip is much higher as a result of more reflected energy teaching your antenna, while on the PPI, the pip is much wider. The pip is not deeper (thicker), since the time base represents only the range of the target. Figures 3-11B and 3-11C show only one pip, since the targets were too close together for the bearing resolution of the radar used.

Figures 3-12B and 3-12C are the pictures appearing on the range and PPI scope respectively, when the targets are still at the same range, but with their bearing difference great enough to obtain bearing resolution, as indicated in figure 3-12A. Here, a new pip will appear as the antenna is trained to the bearing of each individual target; their energy will not be cumulative since difference in bearing is greater than the antenna's effective beam width.

Next, consider figures 3-13B and 3-13C. Here again you see the antenna pointing on only one target, as noted in figure 3-13A. Compare these pips carefully with those appearing in figures 3-14B and 3-14C. Notice that the pips in figures 3-14B and 3-14C are deeper as a direct result of a range difference between the two targets. Should the two targets under observation have even a greater range difference, the deep pip will appear split, as shown in figures 3-15B and 3-15C. Here, the number of individual peaks will indicate the number of targets.

Effect of range on bearing resolution. As shown in figure 3-16, the ability of a radar to separate two targets close together in bearing improves as the range decreases , because the angular difference in their bearings is increasing. Notice that the two ships are covered simultaneously by the effective part of the lobe when at a range of 18 miles. On the other hand, when the same two ships close to five miles, the effective part of the beam cannot touch them at the same time, and they can be seen as two separate contacts. The bearing resolution angle, in other words, intercepts a smaller distance at short range than it does at long range. Keep counting contacts as the range closes.

Effect of sweep length on range resolution. Due to the fact that pictures are traced on scopes by a relatively large spot of light rather than by a tiny point of light, a certain amount of definition is lost. Regardless of the range scale in use, the size of the electron beam spot remains the same; consequently, it becomes increasingly difficult for this beam to trace a clear picture

 
3-12

PIPOLOGY
of the two contacts on the same bearing as they move closer to one another on the scope. Therefore, the longer the range scale, the closer the contacts will move to one another on the range axis and the more likely they will be to blend into a single contact. This effect is more noticeable on the PPI or "B" scope than on the "A" type.

The PPI drawings in figure 3-17 illustrate the point that a four-ship contact may look like one ship when

  seen on the long-range scale, like two when seen on the medium-range scale, and like four on the short-range scale, due to improving resolution. Study composition on the shortest scale possible.

Effect of receiver gain on range resolution. The range resolution will always be best when the gain control is turned low enough to present saturation. You cannot read composition on a saturated echo (one so high on the range scope that the top is squared off),

To obtain indication of two targets, range difference must be greater than the range resolution of the radar. To obtain indication of two targets, bearing difference must be greater than the bearing resolution of the radar.
Figure 3-9. Bearing and range resolution.
 
3-13

RADAR OPERATOR'S MANUAL
Three illustrations of antenna with target in lobe, the A scope and PPI scope view.
Figure 3-12.
 
3-14

PIPOLOGY
Three illustrations of antenna with target in lobe, the A scope and PPI scope view.
Figure 3-15.
 
3-15

RADAR OPERATOR'S MANUAL
so turn the gain down momentarily when necessary. Do not make the mistake of leaving it low, since this will decrease the sensitivity of the radar, (See fig. 3-18.)

To help you get a clearer concept of resolutions, let us consider the topic from another point of view, analyzing the effect of both bearing and range resolution at the same time rather than one at a time as previously done.

  One-pip areas. The diagrams in figure 3-19 illustrate the fact that the bearing and range resolutions of the "A" scope are superior to those of a PPI on the same radar. Furthermore, they illustrate the size and shape of areas within which no resolution is possible, let us call these one-pip areas. Notice that the range resolution does not vary with range as long as the same range scale is used. Also notice that the width of the
Drawings of PPI indicator showing the appearence of contact at range of 18 miles. Appearence of contact at range of 5 miles. Then showing the 80 mile scale, 20 mile scale, 5 mile scale.
Figure 3-18.
 
3-16

PIPOLOGY
one-pip areas increases with range, the bearing resolution expressed as an angle does not vary with range, but the actual width or intercept of this angle does increase. Therefore, the one-pip areas are narrow at short ranges and wide at long ranges. For any given range, there will he a one-pip area of a certain definite size and shape, and if you detect a group of ships at that same range, they will give only one pip (no matter how many ships there are) if their disposition can be completely fitted into this area.

Now let us consider the figure 3-19. The group of ships when at long range just fits inside the one-pip area of the PPI, and as a result only one pip will be seen on that indicator (this would be true of 300 ships too, if they were disposed within the one-pip area). However, two pips will be seen on the "A" scope because the one-pip area of that scope is smaller and the disposition cannot be contained by it. In this case targets A and D will show as one pip which can be resolved in range from another pip formed by B and C. Thus by using the "A" scope you know there are at least two contacts instead of the single one shown by the PPI.

After this group of ships closes to a shorter range you will be able to tell much more about its composition. Even the PPI will then show three pips. Since B and D can be enclosed by the one-pip area they will give only one pip. When B and D are in the no-pip area, neither A nor C can fit in it; therefore, they will be resolved, and three contacts will be seen: A, B-D, and C. The "A" scope again shows its superiority in the field of composition. Notice how small its one-pip area is at this range. Only one ship at a time can be enclosed by it, with the result that four separate contacts can be recognized, In other words

  each contact can he resolved from the next in both range and bearing.

What is the significance of this discussion? For one thing, the superiority of the "A" scope for composition reading is established. Furthermore, you now realize that the smaller the area occupied by a disposition of ships, the closer you will have to approach that disposition to tell by radar how many ships are in it. Finally you realize the importance of checking composition frequently as the range closes. At any instant one pip may become several.

Incidentally, the reverse of this is true even in the case of a closing contact, if the ships comprising that contact suddenly form a smaller disposition. Radar operators have reported ships sunk, because they did not realize that there is more than one way for two pips to become one pip.

Estimating the number of planes.

One aircraft contact gives a narrow pip which bounces wildly and irregularly. A large plane echo, however, will bounce less erratically than a small one, just as a pip from a large ship will bounce less than a pip from a small ship.

Two planes will usually give a slightly wider pip (wider in range or bearing), and the pip will rise and fall more slowly and regularly. The echo of three or more planes in formation will have an uneven, jigging motion, distinctly different from two planes in that it is not regular. The echo will not decrease to or near zero, but will vary at near maximum height.

The number of aircraft can be approximated in larger formations by counting the number of individual pips and multiplying that figure by three or four (this will give only a rough approximation of

Figure showing PPI resolution and A scope resolution.
Figure 3-19.
 
3-17

RADAR OPERATOR'S MANUAL
course). The size of raids can also be estimated, using the PPI. You may become quite proficient at this if you take every opportunity to check your estimates.

An air-group contact may represent planes at some certain altitude, or it may represent a "stacked raid" (planes coming at more than one level). If the group contact divides somewhat so that you can recognize two separate groups, try to determine whether or not they fade at the same range. If they do not, they are not at the same altitude.

General hints on composition.

Inasmuch as air-search radars can detect surface targets, and surface-search radars can detect air targets, a few hints on recognition of these targets will be of value.

Land targets :

1. Not moving according to geographic plot, although the contact moves on the radar scope due to own-ships motion.

2. Pip does not bob like a moving target pip.

3. Should be at expected positions.

4. Usually cover greater area on screen than other targets.

5. Separate pips do not move relative to one another.

Ship targets :

1. Pip height bounces at fairly slow rate.

2. There are normally no fades except when range becomes too great.

  3. Speed less than 50 knots (see RADFIVE for target speed determination).

4. Narrow tent-shaped pip compared with land, although a big rock may resemble a ship in this respect.

Plane targets :

1. Speed is greater than 50 knots.

2. Rapidly bobbing pip.

3. Fades appear periodically on long-wave air-search radars. (The reason for this is explained in Part 1.)

4. One plane gives a narrow, quickly bobbing pip.

5. Two planes together give a regularly bobbing pip.

6. A mass flight may give one or several large (high and/or wide) rapidly bobbing pips. Sometimes it is possible to count individual planes by breaks in the peaks of pips.

FALSE CONTACTS

Many pips appear on radar scopes that are false in the sense that they resemble ship or plane pips but are not caused by ships or planes. Report them, but say that you think they are false, and give your reasons.

Sea return.

The pips shown in figure 3-20 are produced by the radar pulses reflecting from nearby waves. These pips are constantly shifting position, and appear as rough.

Sea return seen on A scope and PPI scope.
Figure 3-20.
 
3-18

PIPOLOGY
high grass. The rougher the sea, the stronger the reflection (called sea-return) will be. In a very rough sea, the sea-return may extend 4,000 to 5,000 yards in range from you.

Minor lobes.

The beam of radio waves sent out is not perfectly shaped like a searchlight's beam. Actually, if we could view the beam as we can a light beam, it would appear somewhat as shown in figure 3-21 (viewing it from above). We have the main lobe in the direction the

 Major, minor, and back lobes.
Figure 3-21. Major, minor, and back lobes.

antenna is pointing, and a series of smaller lobes, not wanted but unavoidable, pointing in various other directions. When these smaller lobes (called back and side lobes) illuminate a target they also produce echoes, especially if the target is large and fairly close. These minor lobes seldom reach out more than 6,000 or 7,000 yards, except when they strike high land. They produce a picture on the PPI as shown in figure 3-22.

Note that all pips are at the same range. The largest pip is the actual target; all others are minor-lobe echoes. The minor-lobe echoes may be eliminated by cutting down the gain, but that of course, may also eliminate other small targets from the screen,

  Clouds.

The radar at times acts as a weather prophet since it indicates clouds, fog, rain squalls, and regions of sharp temperature differences. Some clouds are not visible to the eye; they are called ionized clouds, although this is a misnomer. Often an echo from a cloud resembles an ordinary pip from a surface target, and at night might lead to a wild goose chase" if it were not investigated further. Course and speed of the target should he determined by tracking it. If its course and speed agree with the wind's direction and speed you might suspect it to he a cloud. Unfortunately, upper air currents sometimes differ in direction and speed from those at the surface.

Figure 3-22. Minor lobe echoes.
Figure 3-22. Minor lobe echoes.

More positive identification may he obtained by training on the target with the fire-control radar to

A and PPI scopes showing rain squall.
Figure 3-23. Rain squall.
 
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RADAR OPERATOR'S MANUAL
determine whether it is on the surface or has a position angle indicating an air target.

A rain squall or fog bank may usually be identified by the type of pip produced on the screen. It will be wide in bearing and thick in range; since neither rain nor fog forms a solid reflecting surface, the pip produced is of a fuzzy, lacy nature. A typical rain squall might appear on the range scope and PPI as shown in figure 3-23.

  The sketches in figure 3-24 indicate the type of picture which will be seen. If the interfering radar pulses do not move, they may obscure target pips. Some sets are provided with a front panel control of the repetition rate, and any change in rate will cause the interfering pulses to move and keep moving. At times the intricate patterns produced on the PPI may in themselves be interesting, but the experienced operator becomes so accustomed to such interference that he hardly notices it.
Radar pulse interference shown on A scope and PPI scope.
Figure 3-24. Radar pulse interference.
Multiple-range echoes. Three images showing the true echo, a double echo and a triple echo.
Figure 3-25. Multiple-range echoes.
Radar pulses.

Often pips which move rapidly across the screen are seen: there may be one or several. They are usually caused by another radar transmitter of the same wave length, and may have the appearance of telephone poles as viewed from the window of a moving train.

  At long ranges the radar interference will be picked up only in the direction of the interfering radar transmitter. At close ranges the interference will appear at all hearings. Radar interference will always be picked up at a range considerably greater than the range at which a returning radar echo may be detected. Hence
 
3-20

PIPOLOGY
you might pick up another ship's radar in this way long before its echo appears.

Double-range echoes.

Double-range (or double-bounce) echoes are most frequently detected when there is a large target at comparatively close range abeam. Such echoes are produced when the reflected wave is sufficiently strong to make a second or third round trip, as shown in figure 3-25. Double-range echoes are weaker than the main echo, and appear at twice the range. Triple-range echoes are so very weak that they are seldom seen at all. You recall from Part 1 that these echoes are an aid in determining zero-set errors in radars.

Second-sweep echoes.

Second-sweep echoes appear only on some radars (never on the SC, SK, SA, but sometimes on SC, Mk. 3, Mk. 4, and other sets with high repetition rate). They are caused by echoes from targets at long range; in fact, from such a tong range that the echo from pulse 1 returns after pulse 2, and the echo from pulse 2 returns after pulse 3, etc. Since they must come from contacts at a greater distance than that indicated on your scope, their pips are usually smaller than you would expect at the indicated range. Usually they will be from land targets, since that is about the only target that can be seen far enough away to appear as a second-sweep echo. Find out if there is any land in the direction of a suspected second-sweep echo.

If you vary the repetition rate of your radar, the second-sweep contact will move to a new indicated

ENERGY REFLECTING OFF MAST

Antenna bearing 180 degrees relative; target bearing 080 degrees relative.
Figure 3-26. Antenna bearing 180 degrees relative; target bearing 080 degrees relative.

  range, whereas the range indicated for a legitimate first-sweep echo will not be affected by changes in repetition rate. The repetition rate of the SC radars is variable, but do not under any circumstances try to vary the rate of the Mk.3 or Mk.4, since such action would upset the accuracy of range calibration. In any event, this false contact is so rare that you may never see it.

Reflection echoes.

Reflection echoes are sometimes seen, due to the radar wave being reflected from some surface aboard your ship. It results in a contact at the correct range but the wrong bearing. This type of echo only occurs when the antenna is on a certain relative bearing. You should know the relative bearing of your particular installation which is subject to this fault.

Wakes.

The wakes of nearby large ships will he detected by your radar from time to time, especially during turns of the target ships, and when running at full speed. They are small, ill-defined contacts on the PPI, near to but astern of the ship contact causing them.

Miscellaneous objects on the surface.

Unexplainable echoes, usually at very close range, may be from whitecaps (beyond the sea-return in the direction from which the wind is coming), from birds, from floating objects such as large metal cans or shell cases, and from seaweed.

PPI INTERPRETATION

Radar shadows.

In order to visualize land nearly as radar "sees" it, imagine yourself looking down on an area from a point high in the sky above it, at about the time of sunset. The beam of light from the low sun illuminates the parts of land that a radar on the same bearing would "see" but of course there will be shadows in the hollows and behind the mountains. These same areas will be in "radar shadows" and therefore not detected by the radar. So much for the points of similarity between these two pictures. Now let us analyze the differences.

Beam-width distortion and pulse-length distortion.

Two types of distortion are always involved in PPI presentation. One is due to the diverging beam of the radar, and can be calledbeam-width distortion.

 
3-21

RADAR OPERATOR'S MANUAL
The other is due to the fact that the pulse is not instantaneous (although very short indeed), and it can be called pulse-length distortion. Beam-width distortion results in the widening of all things detected by radar; that is, all contacts appear to spread to the left and right of their actual positions. The stronger the echo, the greater the spread. This is more noticeable on long-wave air-search sets because of their wide beam width than it is on micro-wave sets. The result of pulse-length distortion is increased depth of target pips on the range axis of the scope. For example, a small navigation buoy may give a pip 300 yards deep on the "A" scope. As you probably have noticed on the PPI, contacts spread in bearing more than they thicken in range. This becomes increasingly apparent as range increases.

Have you ever noticed that a straight shore line often looks crescent-shaped on the PPI The effect is noticeable on any radar at times, but is most pronounced on long-wave air-search sets. The slight crescent-shaped effect is due to beam-width distortion. Notice in the drawings of figure 3-28 that the coast-line distortion is negligible at points where the shore is at right angles to your line of sight, but as this angle decreases, the shore-line distortion increases is shown, reaching a maximum at Various points of tangency.

  Side-lobe ringing.

At times the crescent-shaped effect is so noticeable that according to the PPI, you seem to be in the lagoon of a coral atoll or land-locked harbor, when actually you are off a fairly straight but mountainous coast line. This complete ringing effect will be noticed only on long-wave (air-search) sets; it causes much concern among fighter director officers and others concerned with air defense. This effect is due to a combination of two things. One is the beam-width distortion already mentioned and the other is side- and back-lobe contacts.

Low land.

Radar frequently fails to detect low-lying and gradually sloping land, especially at long range. This results in another distortion of a coast line.

Ships near shore.

Ships or rocks close to the shore may blend with it and either lose their identity completely or appear as a bump on the coast line. The effect is due, of course, to the spreading of all contacts in both bearing and range. A ship may hide from radar by getting very close to shore at any point, but the best place to escape radar detection would be at a point of tangency near the shore (the higher the better with relation to the radar's position).

Radar shadow of land hides an aircraft.
Figure 3-27. Radar shadow.
 
3-22

PIPOLOGY
Two vides of a ship between to bodies of land. One showing the land features the other showing the beam distortion and radar shadows.
Figure 3-28.
 
3-23

RADAR OPERATOR'S MANUAL
Consideration of these various factors, and reference to topographical data on the land to be approached, will help you to form a mental picture of what will appear on the scope. Likewise, it will be necessary for you to refer to topographical data to interpret strange land masses. The two drawings in figure 3-28 illustrate and summarize the various distortions that have been discussed. The first shows the actual shape of the shore line and the significant topographical details. Notice also the radio tower on the low sand beach, the two ships at anchor close to shore, and the lighthouse. The second drawing shows (in heavy line) approximately how the land will look on the PPI. The dotted lines represent the actual shape and position of all targets. Notice these things in particular:

1. The low sand beach is not detected by the radar.

2. The tower on the low sand beach is detected but it looks like a ship in a cove. At closer range the low land would be detected and the cove-shaped area would fill in; then the radio tower could not be seen without reducing receiver again.

3. The diverging radar shadow behind both mountains. Distortion due to radar shadows is responsible for more confusion than any other factor. The small island does not show for this reason. Notice also that the back half of the mountains does not show.

4. The beam-width distortion (the spreading of land in bearing). Notice that it is maximum at points of tangency. Look at the upper shore of the peninsula and notice that the shore-line distortion (due to beam-width distortion) is greater at the left than at the right. This is because the angle between the radar beam and the shore line is smaller at the left than at the right.

5. Ship 1 looks like a small peninsula. Her contact has merged with land, due to beam-width distortion. If land had been a much better radar target than the ship, the contact due to the former would have completely covered that due to the ship.

6. Ship 2 also merges with the shore and forms a bump on it. In this case she has merged with land due to pulse-length distortion (range spread). Reducing receiver gain might cause her to separate from land if she was not too close to shore.

 
7. The lighthouse looks like a peninsula due to the fact that it gives a better echo than the land it is on, and consequently spreads more in bearing (due to beam-width distortion) than the echoes from land.

MISCELLANEOUS CONSIDERATIONS

Course changes.

In many cases you can tell when a target changes its course before this fact is revealed by the plot. The change is indicated by an increase or decrease in the strength of the echo, and is due to increased or decreased presentment. For example: a target may be seen end-on, giving an E-2 echo, but when the same target changes course so that you are facing its broadside, the echo suddenly increases to E-4. You will not usually be able to notice any difference in the echo strength as a result of small changes in target course; therefore any sudden, noticeable change in the echo will indicate a substantial course change. You should report this without delay, even though you cannot tell which way the target has turned. The fact that it has changed course at all will often be significant.

Blind sectors.

You have been told that radar shadow always exists behind objects that reflect radar energy. Naturally then, unless your antenna is higher than any other part of your ship, it is possible that a blind sector may exist on some relative bearing due to the effect of such radar obstacles aboard your own ship as

Graph showing blind sector.
Figure 3-29. Graph showing blind sector.

 
3-24

DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
super-structure, masts, or other antennas. If you have a blind sector you should know exactly where it is.

One way to check for blind sectors is to keep your antenna trained exactly on some steady land target while "swinging ship" through 360 degrees several times. It will be easy if your radar is a true-bearing type, since the antenna will stay on the target as the ship swings. A graph may be made using polar coordinates, showing echo height versus relative bearing of the chosen land contact. It might be useful to attach a temporary scale to the A scope to assist in determining the relative strengths of the echoes. An illustrative graph is shown in figure 3-29.

Such a graph will enable you to estimate which relative bearings are partially blind to your radar. Several graphs should be made before the final pattern is determined. If it is impossible to utilize a land echo,

  an approximation of the radiation pattern can be obtained by noting the relative strength of sea-return from different bearings. The sea should be fairly calm, since a heavy sea would give a false indication of the pattern; that is, greater reflection would occur from the wave fronts than from the troughs regardless of the actual radiation pattern.

From the foregoing discussion it can be seen that there is more to learning to be a to radar operator than just studying the information in books. It is going to take a lot of actual work on the apparatus itself, but operating time alone means nothing unless you get into the habit of thinking, observing, and remembering, making predictions and checking them, and looking for small details. Radar operating is an art.


DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
 
INTRODUCTION

The enemy has two purposes in using radar countermeasures: first, he hopes to prevent us from obtaining any accurate or useful information about his forces by the use of our radars; and second, he wishes to get information about our forces by listening to our radars. The radar countermeasures methods that may be used in accomplishing these purposes are of four types: interception, jamming, deception, and evasion.

Interception is the detection of radar signals by the use of a special receiver. By this means, the enemy learns of our presence in his vicinity, obtains an approximate bearing on our position, and he may determine some of the characteristics of our radars.

Jamming is the deliberate production by the enemy of strong signals for the purpose of hiding his movements or position from our radar by obliterating or confusing the echoes on our indicators. The jamming signals may be produced by a modulated radio transmission, which is electronic jamming, or by echoes returned from many small metal strips, termed Window.

Deception is the deliberate production by the enemy of false or misleading echoes on our radar by the radiation of spurious signals synchronized to the radar, or by the reradiation of radar pulses from extraneous reflectors. Small targets may be made to appear like large ones or echoes may be made to appear where no genuine target exists.

Evasion consists of tactics that are designed to take advantage of the limitations of our radar to prevent

  or postpone radar detection, or to avoid revealing the true position of an attacking force. If attacking enemy planes take evasive action, it may be impossible to determine the height at which they are flying, or the planes may be detected too late for an adequate defense to he made ready.

VULNERABILITY OF RADAR TO
COUNTERMEASURES

Interception.

Radar pulses become weaker as they go away from the radar. Only a small fraction of the energy of these pulses is reflected by the target. This small amount of energy becomes even weaker in returning to the radar. At ranges where the pulse is too weak to return a useable echo, the pulse may still be strong enough to be detected by a receiver. Thus, if the enemy has receivers for listening to our radars, he will be able to detect our forces at ranges greater than those at which we can detect him by radar. In addition to detecting our radar, the enemy can also determine our radar frequency, pulse repetition rate, pulse duration, and whether or not lobe switching is used, and use these data for subsequent countermeasures operations. It may also be possible for the enemy to estimate the size of the force near him by noting the number of signals intercepted, or to analyze the intercepted signals as a means of telling whether or not he has been detected by our radar.

The coverage of a radar can be charted by intelligent use of intercept receivers if the radar operator

 
3-25
CHANGE NO. 1
RADAR OPERATOR'S MANUAL
Illustration of jamming coming directly from aircraft vs. radar echo having a two way trip.
Figure 3-30. Illustration of why electronic jamming is often many times stronger than a radar echo.
is not careful of his operating procedure. For example, one of our radar reconnaissance planes charted a Jap land-based radar completely by flying toward it at various elevations and on various bearings. The Jap operator stopped his radar beam on the plane as soon as it was detected and followed its motion as long at it was in the field of view of the antenna. Thus the complete coverage of the radar was found by interpretation of the intercepted signals. Our radars are also vulnerable to such reconnaissance if the operator stops his antenna on each target as it is detected. Therefore, to avoid giving information to enemy snoopers, keep the antenna rotating.   Jamming.

Jamming signals are generated by transmitters that may be carried in aircraft, on ships, or installed at land bases. The transmitter is operated as nearly as possible on the frequency of the radar which it is desired to jam. The signal from the jamming transmitter is usually much stronger than the radar echo, since the jamming travels directly, as opposed to the round-trip path taken by emission from the radar. A strong jamming signal may overload the radar receiver, which necessarily has been designed to be a very sensitive instrument, and therefore it may be rather susceptible to overloading.

Illustration showing our ship and enemy ship bow on with our radar and enemy jamming.
Figure 3-31. Jammer located on target. Target said to be self screened.
 
3-26
CHANGE NO. 1
DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
Very often a ship may try to conceal itself from radar detection by carrying a jammer. This kind of jamming, which is illustrated in figure 3-31, is called self screening. If other ships are in company with the jamming ship, the problem of detecting them may be complicated by the fact that the jamming and echo do not come from exactly the same place. Irrespective of the position of the jamming ship relative to the target vessel, a weakness of electronic jammers is that they are not effective within a certain minimum range. When approaching the target, you will first pick up the jamming signal, owing to its high strength. At normal radar range, both jamming and echo from enemy ship will be present, although the jamming may be strong enough to obscure the echo. However, as the range closes, the strength of the

Illustration showing electronic jammers ore less effective at short ranges than at long.
Figure 3-32. Electronic jammers ore less effective at short ranges than at long.

  echo signal increases much more rapidly than the strength of the jamming signal. A point is finally reached where the target pip shows up clearly through the jamming. The range at which this occurs may be between 2 and 8 miles, depending upon the size of the enemy ship and the strength of the jammer. Figure 3-32 shows this condition as related to the range of an approaching aircraft equipped with a jamming transmitter.

When a jammer is not on every vessel to be screened, it is difficult to prevent detection of the force from every direction. For example, in figure 3-33 a very exaggerated case is illustrated. Enemy ships 3 and 5 are equipped with jammers and their mission is to conceal the force. Since enemy ships 3, 4, and 5 all bear the same from friendly ship 1, they are concealed by the jammer on 3. However, a picket ship at 2 may be able to range on some of the targets in spite of the attempt by enemy ship 5 to jam him. Even though some of the ships in a task group are hopelessly jammed, others may be relatively unaffected. Therefore, keep all the radars in operation because the situation may improve as the dispositions of the two forces change.

An aircraft carrying a jammer might pass through a fade zone while echoes from the ship he is protecting are unaffected. This would cause a sudden improvement in echo strength over the jamming, perhaps making more readable echoes that were obscured. This same effect may occur when the jammer is on the ship to be screened, since the antenna pattern of a surface search radar also is broken into many lobes and nulls by reflection of the radiation from the water. The jammer antenna must direct its radiation directly into the small area of the radar antenna, while the radar antenna needs only to cause its pulse to hit some part of the large area of the enemy ship to get an echo back. At ranges where the jammer antenna is in a null of the radar, as in figure 3-34, the jamming is ineffective, but an echo is returned from the superstructure of the enemy ship because the lobe below the null strikes the ship. Thus, it is necessary to keep the radar operating, and to maintain a close watch on the scope when jamming is encountered, because the jamming effectiveness may suddenly be reduced.

Off-target jamming is seldom produced deliberately; it usually occurs because the disposition of the jamming ships changes relative to the vessels they are attempting to screen. Jamming of this type imposes no special problem for most search radars, but if the radar is one that employs lobe switching, bearing

 
3-27
CHANGE NO. 1
RADAR OPERATOR'S MANUAL
Illustration showing two freindly ships and three enemy ships. All but one of the freindly ships are in a line. Enemy jamming is not effect on ship that is not in the line.
Figure 3-33. Jammer is on same bearing but not on board target. Radar located on friendly ship at 2 is probably free of jamming.
errors may be produced. The error arises from the fact that the jamming does not affect both lobes equally, so that the matched-pip condition may appear at the wrong bearing. In radars like the SM and SP, jamming can also produce serious errors in the height measurement. These inaccuracies are greater in some radars than in others because of the nature of the receiver circuits used. In general, however, the errors in angular measurement that off-target jamming produces in lobe-switching radars can be reduced by operating the receiver at the lowest gain setting that allows the pips to be clearly visible. When range information only is desired, better results are obtained with lobe-switching "off". No bearing inaccuracy should occur when the jammer is on the target (self-screening) or on the same bearing as the target. In order to become aware of possible bearing inaccuracy, it is necessary to determine whether the jammer is on or off the target bearing.   If the side lobes of the antenna are large in size, jamming can be received from directions other than the one in which the main lobe is pointed. In some cases, the jamming received in this way may conceal a target which is not on the same bearing as the jammer. For example, in figure 3-35 the radar on the friendly ship is jammed by the off-target jammer so that neither enemy ship can be seen on the PPI. Note that there are three distinct jammed sectors on the accompanying PPI screen, produced by reception in the three lobes as the antenna rotates. If the jamming is not too strong, the sectors of jamming caused by side-lobe reception can be reduced in intensity by reducing the receiver gain. The target in figure 3-35 might be made visible by this simple adjustment.

The relative ineffectiveness of off-target jamming, except against lobe-switching radars, suggests maneuvering until the jammer is in an unfavorable position.

Illustration of radar lobes and nulls.
Figure 3-34. Jamming ineffective because the jammer antenna is in radar null.
 
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Jamming reception in side lobes from off-target jammer.
Figure 3-35. Jamming reception in side lobes from off-target jammer.
The enemy will monitor both his own jamming and the signal that he is attempting to jam so that he will know if either changes frequency enough to make the jamming ineffective. He may be expected, then, to train his jamming antenna for maximum jamming, and it will be very difficult for a single ship to maneuver in such a way that the jamming effectiveness will be decreased. However, it is always well to search the areas around the jammed sector in the hope that either the enemy operator is not alert or that some of the ships that he is trying to conceal will have strayed outside the zone of effective concealment. The possibility that the jamming is being used for deceptive purposes must be considered. Therefore, a thorough search must be maintained throughout the full 360 degrees because the jamming may be sent out to attract our attention to a sector away from the direction from which the enemy plans to attack.

Microwave radars are less vulnerable to electronic jamming than long wave types. The narrow beam width allows targets to be seen close to the jammer bearing, and the concentration of high power in a single direction makes for high signal strength relative to the jamming. Adequate jamming is hard to produce against microwave radars because it is difficult to develop high power at these frequencies.

A radar operating at peak efficiency is far less susceptible to jamming than one which is out of adjustment.

  This is especially true with regard to the transmitter-weak or erratic emission seriously reduces the chances of the echo signal strength being strong enough to override the jam. Report any falling off in equipment performance to the technician.

The A scope is less vulnerable to jamming than the PPI or B types, and therefore should be used when jamming is encountered. However, in the case of noise jamming, the A scope is little better than any other type of indicator, but it will be found more useful against most other types of jamming. PPI or 13 presentations are preferred only when it is desired to find the bearing of the jammer. It is not desirable to keep the antenna stopped for long intervals while trying to read through jamming on the A scope. All-around search must always be maintained.

Jamming can also be accomplished by the dispersal of many strips of reflecting material, called Window. Since Window jamming consists of a cloud of particles that occupy a definite place in space, the vulnerability of radar to this type of jamming is different from the vulnerability to electronic jamming. Unlike cases in which electronic jamming is employed, the location of Window relative to any targets which it is supposed to screen is continually changing. Window moves with the wind at a speed approximately 2/3 that of the wind, while the speed of the enemy ship may be greater or less than the speed of the wind. If the Window area is not large, the enemy

 
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will have difficulty in staying in the Window-infested area because the Window is hard to see in the air. Watch for stragglers outside the infected area. Often they will appear on the windward side of the Window blob.

Window produces pips that are quite similar to those from real targets, whereas electronic emissions fill the scope screen with patterns totally unlike those normally encountered. Also, the reflected signals from Window will occupy only a portion of the trace, while electronic emissions cover the entire sweep.

The first indication of an impending raid may be Window pips on long range search radars, so that the jamming may work against the group responsible for it. With fire control or height-finding radar, on the other hand, properly distributed Window can ruin the accuracy of determination of bearing and height. Because Window can ruin the accuracy of AA fire control radar, the principal use of Window has been against this type of radar as a means of escaping from AA fire after an attack.

If the enemy intends to infect a large area with Window to prevent search or fire control by radar, he will drop packages of it while flying a course that will provide good coverage. A single plane may fly a flat spiral or a figure-of-eight course; when several planes are working together, they may fly straight parallel courses, dropping packages of Window at periodic intervals. Surface craft may infect smaller areas by firing Window-filled shells or rockets. However, to be effective in concealing targets within the Window cloud, the packages must be dropped at close enough intervals that each bundle will not exist as a separate cloud, but that all the Window bundles will blend into one large cloud. The better the range and bearing resolution of the radar, the closer the Window must be sown to conceal the targets within the cloud.

Deception.

Although it is possible to deceive radars by the use of electronic devices, the necessary equipment is difficult to design and operate. Test operations using electronic deception have indicated that the results seldom are good enough to warrant the trouble involved. Since the enemy faces a great problem in this field because we have more radar on more frequencies, it is unlikely that electronic deception will be encountered to any great extent.

However, the use of mechanical devices for deception is entirely feasible, and both the Germans and

  the Japanese are familiar with these deception techniques. For example, the Japs have equipped sampans with reflectors, so that they appear to our radars like large craft. The sampans are sent out in advance of a convoy, on courses calculated to lead our ships well out of the way by the time the real targets arrive. Other types of reflectors may be floated or suspended from balloons, and designed to give false echoes like those from submarine periscopes, surface vessels, or aircraft. Many of these devices produce echoes that seem very similar to genuine echoes in their behavior on the scope. Often the only way of revealing the false nature of the deception echo is to plot its track, since most airborne mechanical devices drift down wind at a speed somewhat less than wind speed. Thus, radars are very vulnerable to attack by deception for at least a short period of time. Often this short time is long enough to permit enemy planes to get out of gun range.

Evasion.

Low-frequency radars, such as the SK and other air search sets, can not detect low-flying targets at long range, because the antenna pattern is such that the beam does not provide good low cover. The enemy is quite aware of this shortcoming and his air strikes frequently approach "on the deck". Air-search radars are not able to detect changes of altitude nor are they able to detect aircraft flying over land with any certainty. The Jap knows these limitations too, and makes full use of them to avoid detection by radar. These failings are serious, but they will soon be remedied by new equipment that is being produced.

DISTINGUISHING JAMMING FROM INTERFERENCE

Interference is caused by the reception of confusing signals accidentally produced by the effects of either friendly or enemy electrical apparatus and machinery, or by atmospheric phenomena. Interference should not be confused with enemy countermeasures.

Accidental interference from many types of electronic gear and electrical machinery has been noted on radars. The signals may enter the radar receiver by shock excitation of radio antennas or guy wires, through the power line, by way of inter-connecting cables between various units, or because of inadequate shielding of the radar equipment. It is difficult to predict the effects these signals will have on radar operation. In some cases accidental interference may

 
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DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
be confused with deliberate jamming attempts. Methods of distinguishing between the two are outlined in the following.

Internal equipment faults. Cluttered scope patterns caused by internal faults in the equipment may be distinguished from jamming or external interference because the scope pattern remains the same regardless of the direction in which the antenna is trained. If trouble persists, call the technician. However, with strong jamming the same effect may be observed if the receiver gain is not reduced.

Interference from other gear aboard own ship. It usually is possible to take a bearing on interference of this type. The relative bearing will always remain the same, but the true bearing will change with changes in the ship's course. If both bearings change, the trouble is not on board.

Pulse interference from other radar. Pulse interference causes light, tall pips on A-scope screens. These pips move back and forth along the trace in a random manner, giving the pattern the name "running rabbits". The spacing between pulses is usually much larger than the width of the pulses.

It is possible to take bearings on pulse interference from other radars in the same frequency band and having approximately the same pulse repetition rate. At short ranges, the indications appear over a large arc of antenna train. The usual method is to consider the center of the arc as the correct bearing, instead of training for maximum strength or distinctness of the signal. This system has been used to home on ships in convoy or on shore installations.

Pulse interference on the PPI results in a series of broken spirals. Under normal conditions,

  neither this pattern nor the one on the A scope causes serious difficulty because the pips are considerably more distinct than the interference, and because of its prevalence, operators soon become familiar with it. However, if the effect is found annoying, it may be minimized by changing the pulse repetition rate of own radar until the most easily read pattern is obtained.

Interference from radio transmitters, beacons, etc. Keyed CW may be read as dots and dashes. Rotating the radar antenna may or may not produce any difference, depending on how the interference is getting into your radar.

Radiotelephone transmitters on board the ship sometimes produces interference, but they may be distinguished by one or more of the methods discussed in the preceding paragraphs. This same type of interference coming from nearby ships may sometimes be very confusing to the operator. Therefore, every effort should be made by the operator to learn to identify it so that he will not confuse communications interference with enemy jamming.

Spark interference caused by commutator or ignition sparking appears on the A scope as a series of narrow, regularly spaced pulses. Interference from spark transmitters, diathermy apparatus, etc., will produce wider pulses, more closely spaced. Generally the train of interfering pulses will move across the screen. Spark interference may blank out the screen of PPI scopes in one or several sectors, depending on the signal strength.

Atmospherics. Returns from rain clouds or other atmospheric condition are not likely to be confused with transmission jamming. They are, however,

A scope and PPI scope with 'running rabbits'.
Figure 3-36. Interference from other radars- "running rabbits".
 
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somewhat similar to Window jamming. The pips produced by storms are often lacy in character on A scopes and may occupy quite a large portion of the

Photo of interference produced by keying of radio transmitter on
board ship.
Figure 3-37. Interference produced by keying of radio transmitter on board ship. Note appearance of dots and dashes and non-directional effect produced. In this case the interference is not entering by means of the radar antenna, but directly into the radar receiver itself.

  screen. They show but a slow change in range. The appearance of both cloud and Window echoes differs from that of genuine echoes in the amount of beating that becomes apparent when the receiver gain is reduced, and the motion of the echoes always agrees with the prevailing wind direction. On the PPI, a storm will be indicated by a filled-in area having less definition than a genuine target. Some thunderstorms (the isolated convection type that occur in summer) give solid echoes surrounded by a cloudy haze. These echoes can be wrongly interpreted as real targets.

Lightning has been observed to produce large pulses on P band radar. St. Elmo's fire has also caused severe interference with P band radar on occasion.

CLASSIFICATION OF TYPES OF JAMMING

The two general classes of jamming are (1) the electronic type, in which use is made of a radio transmitter, and (2) mechanical or Window jamming.

Electronic jamming.

Types of electronic jamming may be classified according to the nature of the emission employed by

Photo of atmospheric interference on PPI.
Figure 3-38. Atmospheric Interference.
 
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DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
the enemy. Any kind of transmissions can he used, although with varying degrees of effectiveness. The list includes unmodulated continuous wave (CW), frequency or amplitude modulated CW, pulsed signals, and mixtures. The modulation frequencies may be high or low, and either synchronized or not to the pulse repetition rate of our radars. We may expect a wide variety of different patterns to appear on our radar scope screens, depending on the particular type of signal the enemy is sending out. Special equipment is needed to identify the exact nature of these emissions. However, the operator can often obtain enough information from a particular scope pattern to make a good guess as to what AJ measures to apply without delay. Also, it is of obvious benefit to the Navy to receive prompt reports on the kinds of jamming being employed by the enemy.

The first general characteristic the operator may easily note about modulated transmission jamming is whether it is synchronous or nonsynchronous.

Synchronous jamming refers to signals which are modulated at an exact multiple of the pulse repetition rate of the radar against which they are being used. Thus, if your equipment is operating on a PRR of 60 cycles and the jammer is modulated at four times that,

  or 240 cycles, the jamming is synchronous. This produces a stationary pattern on radar scopes. Jammers not modulated at an exact multiple of the radar PRR are called non-synchronous, and produce patterns which traverse the screen, causing blurring. An intermediate condition, known as semi-synchronism, results in an erratic stop-go motion. Noise is the only type of modulated jamming which cannot be synchronized.

A second consideration when attempting to distinguish between types of jamming is the approximate determination of the modulation frequency of the jammer. Jamming modulation frequencies may generally be classified as low, medium, or high.

On short range scales, or when using an expanded sweep, the electron beam which traces the time base moves much faster than on long range scales. This means that for a given type of jamming signal, the pattern will be less complex when the indicator uses a short range scale. Changing the range scale has the same effect as changing the frequency of the jamming modulation, so that what appears to be high-frequency jamming on a long range scale may look like low-frequency on the short scale. It is necessary, therefore, to specify the particular range scale in use when describing a jamming signal.

Synchronous and non-synchronous jamming signal modulation.
Figure 3-39. Synchronous and non-synchronous jamming. In (A), the same portions of the jamming signal waveform appear during each sweep an the radar scope causing a stationary pattern. In (B) the jamming is an different port at cycle during successive sweeps. This produces blurring an the scope screen.
 
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 Effect of same jamming with different sweep times illustrated.
Figure 3-40. Effect of same jamming with different sweep times. The pattern in the tong range will appear to be more complex, because a greater portion at the modulation cycle will appear.
An estimate of the effect of the jamming modulating frequency may be learned by studying the following sketches and their captions. A range scale of approximately 40 miles has been assumed, except where otherwise noted.

Four views of the PPI in the presence at jamming. A. Reverse presentation due to receiver overloading. B. Bright sector caused by non-synchronous jamming. C. Striations or bands due to low-frequency, semi-synchronous jamming. D. Dotted bands produced by synchronous pulse jamming.
Figure 3-41. The PPI in the presence at jamming.

  The text which follows is concerned almost entirely with type A scopes, because PPI and B presentations are less useful for identifying jamming as they react in nearly the same manner to all types. However, a few points on the behavior of PPI scopes may be noted. They respond to non-synchronous jamming by showing one or more bright pie-shaped sectors. If more than one sector appears, the brightest is due to the major antenna lobe and the others result from minor lobes. An overloaded radar receiver is indicated by a reverse or "negative" presentation, with the jammed sectors dark on a light, speckled background. If the jamming signals are synchronous or semi-synchronous, striations (lines) show up on the screen within the jammed sectors.

Unmodulated CW jamming. Weak unmodulated CW jamming (or strong jamming at low gain control settings) causes an increase in both signal and noise level on the A scope. The trace may become distorted, (figure 3-42B), as the radar antenna first comes into and as it leaves the jammed sector. If the antenna were stopped in the jammed sector, the trace would assume a position along the normal base line. It will be noted that the pip is double sided and is more or less "filled-in" depending on how close the jammer frequency is to our radar frequency. As the jamming becomes stronger, both the grass and a single-sided pip may appear below the baseline (inverted). Finally, as shown in C, complete overload occurs, wiping the trace clean of both pip and grass.

 
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DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
A. Normal presentations. B. Moderate CW jamming. C. Strong CW jamming overload occurring. Dotted lines indicate saturation level on scope.
Figure 3-42. Unmodulated CW Jamming.
Unmodulated CW jamming is seldom used, although similar effects may appear due to accidental interference from radio equipment. Often when a small percentage of modulation is used on the jamming carrier wave many of the effects of unmodulated CW will be apparent on the indicator.

Low-frequency amplitude-modulated CW jamming. Somewhere within this region, depending upon the sweep time, the effect known as "tramlines" will occur. Tramlines appear as a number of adjacent, sometimes crossing, horizontal baselines on the A scope. The reason for the multiple pattern is that successive traces are deflected vertically different amounts by the jamming modulations. The target echo will appear on each trace, and it may be double-sided or inverted, as in CW jamming. If unsynchronized, the tramlines move up and down, so as to seem to breathe.

  Medium-frequency amplitude-modulated CW jamming. The A-scope presentation produced by this type of jamming is predominantly vertical, even on short range scales. As the frequency is increased, the horizontal or crossing-line characteristic of low-frequency modulation gives way to more evenly spaced, upright indications, usually of constant amplitude. When the pattern is synchronous or semi-synchronous, it is sometimes called "basketweave". The non-synchronous condition is evidenced by blurring, with lines or shading at regular intervals. These do not block out the target indications which run up through the jamming as a vertical line or series of pips which are atop each other, and have greater definition than the rest of the pattern. The echo will also appear on top of the jamming pattern if the gain control is adjusted so that saturation is not occurring. Care should be taken that the earn is not reduced so much
A. Normal presentation B. Appearance of tramlines.
Figure 3-43. Low-frequency jamming.
 
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A. Weak jamming. B. Strong jamming, higher frequency.
Figure 3-44. Medium-frequency modulated jamming ("basket weave").
that signals will not appear if for any reason the effectiveness of the jamming is suddenly reduced. In figure 3-44 (A) the pip is double-sided, the grass is riding the modulation, and the gain has been adjusted so that saturation is not occurring. B shows a modulation frequency about twice that of A. Saturation is occurring so that the pip does not appear above the jamming pattern, and the normal -receiver grass is not riding on the pattern. This is not dependent upon the modulation frequency, but rather upon the relative strength of the jamming with respect to the echo. The grass could ride on either modulation pattern. If the jamming were not   synchronized, it would probably be difficult to see much difference between these two modulating frequencies.

High-frequency amplitude-modulated CW. The pattern obtained from this type of jamming is of the vertical type described under medium frequency and cannot readily be distinguished from it when the jamming is non-synchronous. The echo will have more of the appearance of riding on top if saturation is not occurring, and the lines of shading will be closer together.

Frequency-modulated CW jamming. Frequency-modulated jamming is not distinguishable from amplitude-modulated types except in the special cases of

A. Normal presentation. B. FM 'bells'.
Figure 3-45. Low-frequency, wide-band FM jamming.
 
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A. Normal presentation. B. Appearence of jamming pulses or 'railings'.
Figure 3-46. Pulse jamming.
low-frequency, wide-band, FM. This produces "cobs" or "bells", on an A-scope screen. These can be fairly confusing if they are moving across the screen at a slow rate. However, an echo pip remains visible as a break in the trace, or by riding the bells.

Pulse jamming. Pulsed jamming transmissions are usually sent out at many times the pulse repetition rate of our radars, so as to cause a number of high vertical pips on the screen. These pips are usually wider than echoes, move together, and are evenly spaced. They may look like a picket fence, or a fine tooth comb, the indications becoming closer together as the jamming pulse rate is increased. The name "railings" has been given to this type of jamming.

It is possible to see echoes through railings that

  are moving across the screen rapidly, as they do when not synchronized to the repetition rate of the radar. The effect is similar to looking through a picket fence white traveling past it. If the jamming is partially or entirely synchronized, the pattern shows little or no motion, and echoes are harder to, identify but can be found on the baseline and possibly on top of the jamming signal. Railings should not seriously interfere with radar operation, unless they overload the receiver completely.

Mixed jamming. Two or more different kinds of jamming may be sent out simultaneously. The A scope will then exhibit the characteristics of each of the particular types. For example, a variety of "German mixture" is made up of a combination of

A. Normal presentation. B. Pip lost in jamming.
Figure 3-47. Random-noise modulated jamming.
 
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low- and high-frequency amplitude-modulated CW. The high-frequency modulation gives a vertical, closely-spaced, and usually blurred presentation higher up on the screen.

Noise jamming. Noise jamming produces abnormally high grass on A scopes without increasing the level of the desired signal. If the height of the grass is several times that of the echo, it is very hard to work through: With the regular patterns produced by other types of electronic jamming, it is usually possible to adjust the radar receiver controls so as to make the target pip produce some irregularity in the picture, which can then be ranged on, but the random nature of noise does not permit this. It is regarded as the most effective type of jamming, and, if sufficiently strong, very little can be done to combat it. However, effective noise modulation is the hardest to obtain technically, and unless it is good it may be as easy to work through as high-frequency modulated signals.

Window jamming.

Mechanical jamming by means of Window is now being employed by the enemy to a much greater extent than any kind of electronic jamming. However, this situation is changeable, making it important to learn anti-jamming measures against both general types.

The name Window probably results from the fact that the material originally consisted of squares or oblong pieces of aluminum foil. When this foil was dropped by an airplane, the light reflections looked like those from many windows. It was found later that more efficient use could be made of a given amount of foil by cutting it up into narrow thin strips. The Window now used consists of such strips which are packed together in bundles when carried in an aircraft and which disperse when dropped. The

  strips are cut approximately one-half wavelength long with respect to the frequency of the radar they are to be used against. Mixed Window cut to two or more different lengths corresponding to the frequencies of several radar types is sometimes used. The strips may or may not be paper-backed.

The first operational use of Window was in July 1943, when 700 British bombers dropped 2300 tons of bombs and 30 tons of Window in a raid on Hamburg. A lane or corridor 40 miles wide and 80 miles long leading into the target was infected with the material. As a result, bomber losses were much less than experienced on a previous, similar raid where no Window was used. Since then most combatants have used Window in attacks on land and sea forces.

Window dropped by aircraft first appears as a series of pips trailing out from behind the sowing plane. The indications closely resemble those from aircraft, except that Window gives a very rapid beating effect in contrast to the steady rhythmic beat of real aircraft targets. This is because the strips flutter as they drift downward. When a very large amount of Window is released, the signals saturate a sizeable portion of the range scale. (See between 1 and 2 in figure 3-48 A). The characteristic beating causes minute oscillations or "ripples" in the trace where it is not saturating. Later, individual pips are seen, as in B. These become broader, more ragged, and less like actual targets as time passes. They occupy a greater portion of the trace when the material disperses, but show only a very slow change in range. Window drifts downwind at approximately two thirds of the wind velocity. It falls at about two to three hundred feet a minute. Therefore, an important thing to remember about Window is that i t remains relatively motionless compared to an airborne target. On PPI scopes, Window appears as an island or cloud-like mass which gradually spreads out, and

Freshly sown window pips hidden; Window breaking up pips visible; Window breaking up pip visible in gap pip visible to windward.
Figure 3-48. Typical Appearance of Window jamming.
 
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changes range in the direction of the wind. (See figure 3-48 C.)

The length of time Window will remain on our scopes depends on the altitude from which it is released. When dropped from 10,000 feet, it may be troublesome for as long as thirty minutes on air search radars. If dispersed by projectiles at low altitudes to screen shipping, the time is much less.

Other Mechanical Jamming. On frequencies below 200 megacycles and in the microwave region, long streamers of reflecting material are used in preference to short strips. These streamers are usually parachute supported, and are released in the same manner as packages of Window. This material is called Rope and produces a large echo having a lower rate of flutter than Window.

 
CLASSIFICATION OF DECEPTIVE
COUNTERMEASURES

Window as a deceptive device.

One of the most important uses of Window is to decoy or otherwise deceive the opposing force. It will serve this purpose in a number of different ways. One deceptive use of Window is to create false echoes for the purpose of weakening our defense. In this case a few enemy aircraft make a low-altitude approach to escape detection by long range search radar. When within radar range, they climb to higher altitude, drop a considerable quantity of window, and then retreat in the same manner in which they approached. If our operators do not know how to distinguish Window, they will report a large force approaching. Fighters sent out to intercept the

Window being used to divert attention from attacking aircraft coming in on a different bearing.
Figure 3-49. Window being used to divert attention from attacking aircraft coming in on a different bearing.
 
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"large bogey" would find nothing. The enemy, on the other hand, having drawn out our fighter protection may be able to send in bombers from another direction while we are looking for him in the Window-infected area. This illustrates the necessity for maintaining a search over 360 degrees.

Another deceptive use of Window is to hide the strength of an attacking air group. During a lengthy operation the enemy may feint a mass attack with only a few aircraft and then suddenly attack with a considerable number of aircraft. In both cases the pip produced may look the same because of the use of Window.

A variation of the above is to use Window to hide changes in course. The Japs have used the material in this way. The aircraft turn while under cover of Window and approach their target from a different bearing. Or, they may appear to be making for one target, but while protected by Window, turn to strike at another.

Window fired by special projectiles may also be used to hide the number and location of surface craft. When sown in this manner, as a countermeasure against surface search or fire control radar, the Window cannot be dispersed from too great an altitude, or it will be above the radar beam. It thus falls in a relatively short time.

  Corner reflectors.

Corner reflectors, constructed of three planes at 90 degrees angles to each other, have been used as decoys. These will efficiently reflect a radar pulse back along the same direction from which it came. They may be supported by a balloon or a parachute so as to give an echo like an aircraft, and employed tactically in much the same manner as Window. Other uses include floating them on the surface of the ocean, so as to simulate a surface vessel, or installing them on a small craft to make it appear like a large vessel. Chicken wire may also be spread over a small vessel for this same purpose.

Corner reflectors have been developed for use in life rafts to facilitate finding their location by search planes. The size o these corners is such that they produce good echoes on S-band radars.

Other decoys.

Another type of decoy consists of balloon supported metal strips or wires, which are secured to floats by anchor lines. The streamers are designed to return sizeable echoes to radars of widely different frequencies. The track of the balloon is downwind, at a speed slightly less than wind velocity. The pip indications show less flutter than those from Window, but do have a characteristic beat.

Use of Window to hide true strength of on attacking air
group.
Figure 3-50. Use of Window to hide true strength of on attacking air group.
 
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DEFENSES AGAINST ENEMY RADAR COUNTERMEASURES
Balloon and cork-floated decoys have been released by submarines. They might be used to attract attention of attacking aircraft or vessels to allow a submarine time to escape; to divert attention of escorts while a submarine makes an attack on a convoy; to cause an attack to be directed at the decoy, which may conceal a mine; to invite attempts at recovery of the device, which may contain a booby trap; or to cause vessels to open fire, thus disclosing their position to enemy submarines or surface vessels.

Illustration of balloon-supported decoy streamers.
Figure 3-51. Balloon-supported decoy streamers.

Parachute or balloon borne reflectors have been reported in use by the Japanese. Little is yet known of them as none have been captured. It is reported that they return an echo which is considerably steadier than those from aircraft-the opposite case to that of Window. They first appear as if a real target had suddenly divided and become two; but the false signal will stay still, while the aircraft which has dropped it keeps on moving. The balloon type stays on the screen longer than window-one hour or more, according to accounts-and gives a strong pip over a relatively wide band of frequencies. The reflectors have not as yet been released in sufficient quantity to clutter an area completely but they do cause some confusion. The name " Kite " is applied to this type of reflector.

The deceptive devices mentioned here are only a few of those possible. It must be realized that the enemy has radar of his own on which to experiment and he is very skilled in devising various sorts of deception. Deception is especially troublesome when used with jamming, and it must be anticipated that more effective countermeasures will be developed by the enemy.

ANTI-INTERCEPTION MEASURES

The measures that an be taken to prevent enemy interception of our radar generally result in interference with the normal operation of the set, and so

  they reduce the amount of useful information that can be gained from the use of radar. The type of anti-interception measures taken, then, must be decided by the OTC after he has weighed the relative advantages of obtaining all the information of which the radars are capable against the advantages that the enemy might derive from intercepting our signals.

However, there are a few measures than can be employed which will not reduce the effectiveness of our radars too greatly. It is very desirable to keep the antenna in constant rotation partly to make it more difficult for an enemy at any one place to hear the signals for a long enough time to make full use of what he may detect, and partly to deprive him of the knowledge that we have found him by our radars. In some cases, a great deal of useful information can be gained from the radars even though they are operated intermittently, and the long times between the periods of operation may defer the time when the enemy will intercept enough of our signals to use them effectively against us. Some of the new radars are being fitted with circuits that will assist in intermittent operation, particularly for submarines, and several existing radars are equipped with radiation switches that have "momentary" positions to be used for transmitting intermittently. Radar silence of course deprives the enemy of the chance to intercept the shut-off radars, but it also deprives us of the information that those radars could obtain. Conditions of radar silence will be prescribed by the OTC.

GENERAL ANTI-JAMMING MEASURES

Anti-jamming, often abbreviated AJ, is the art of avoiding enemy jamming or of reducing its effectiveness. The purpose of such measures must be to prevent jamming signals from getting into the radar receiver if that is possible. If this cannot be done, then AJ measures and devices should attempt to prevent the jamming signals from appearing in the output of the receiver, so that the jamming will not be apparent on the indicator screen. In many eases, even this will not be possible, so that AJ techniques must be directed toward creating some sort of discontinuity in the pattern produced by the jamming in order that at least the range of the echo can be determined through the jamming.

Taking direct action.

Bearing information is needed so that the source of jamming may be located and steps taken to destroy

 
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the jammer. It is easy to find where jamming is coming from by the use of a PPI or B scope. The correct bearing is given by the center of the brightest jammed sector. Turning down the gain helps to distinguish this sector from other areas of the screen which may be illuminated.

If a PPI or B scope is not available, then train the antenna for the highest jamming on the A scope. The gain control should be reduced if the receiver is saturated since variations in the strength of the jamming signal are not apparent in this condition. The provision made in some fire control radars for determining precise bearings enables these sets to indicate very accurately the direction from which jamming comes. For example, tests indicate that the Mark 12 can D/F on jamming with an accuracy of -/+ 5 minutes of arc.

Bisect brightest jammed sector to find jammer bearing.
Figure 3-52. Taking bearing on Jammer on a PPI. Only the brightest sector is shown. Jamming will also appear at other bearings due to pick up n side lobes and because of reflection from ports of the ship.

Usually two or more radars at different locations are assigned to take bearings on a jammer. This permits obtaining a "fix" from which range as well as bearing information can be secured. By taking several fixes at different times, it becomes possible to tell in which direction and at what speed the jammer is moving.

A single aircraft can locate a surface vessel jammer by triangulation because the speed at which the plane travels is so much greater than that of the ship that the latter may be considered to be stationary. The enemy location is determined by turning say, 300, and then flying a straight course at a constant speed, calling out to the navigator when the jammer relative bearing is exactly 60 degrees and 90 degrees. When there is relative motion between your ship and the jammer, a similar process can be used. For instance a land-based jammer could be located by a single ship.

When Window is being sown by aircraft, the sowing aircraft must be at times ahead of the infected

  area. Since this plane is causing the trouble, it may be ranged on and action taken to destroy it. However, the presence of Window close to the dropping plane may cause angle errors in lobe switching radars that will handicap anti-aircraft fire control in shooting down this plane.

Employment of radars at different frequencies within one band.

Emission from electronic jammers is confined to a relatively narrow band of frequencies. Jammers cannot effectively blanket more than a few megacycles when tuned to a given frequency, and even then their output falls off sharply on either side of center frequency. Thus, if more than one radar of the same type is to be used, (i.e., in a squadron of ships) it is wise to pretune them to different frequencies within the band, so that some are almost certain to remain effective in the presence of jamming. This measure also has the desirable effect of reducing accidental pulse interference between radars when several vessels are in company. No improvement is obtained against mechanical type jamming such as window, which is broadly resonant within the band it is cut to cover.

Employment of radars on different frequency bands.

An electronic jammer designed for operations against long-wave radars will not interfere with microwave equipments, and vice versa. This indicates the importance of using radars of widely different frequencies at the same time.

Probably the most desirable situation is one in which several radars are used on many different frequency bands, with the frequency of all sets that

Figure showing spectrum with different radars represented.
Figure 3-53. Shows the desirability of operating radars widely separated in frequency in each bond, and of using several types to cover different frequency bonds. Block areas indicate frequency coverage of individual jammers.

 
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operate on a particular band spread as widely as possible within that band. Figure 3-53 shows something of what the enemy is up against under this condition. Note that in. the top picture the enemy needs only two jammers to jam all our radars, and that even with these two, the radars in the middle are jammed by both jammers t once. In the lower picture, however, with three types of radar and as wide a frequency spread as possible between equipments in each type, the minimum number of jammers needed is one per radar, and, very likely, three different types of jammers. With this prospect confronting him, the enemy might decide that the effort necessary would not justify the results. In any event, he could not economically jam as intensely in the bottom case as in the top one. Moreover, with several types of radar you sharply decrease your chances of being jammed, for each type is vulnerable in different degrees to each kind of jamming.

OPERATIONAL ANTI-JAMMING TECHNIQUES

Expect jamming.

Be prepared! Tests indicate that an experienced operator, after applying elementary AJ techniques can detect targets through several times the transmitted power required to jam a novice. The enemy has achieved complete success if the radar operator thinks his equipment is at fault when jamming is received and shuts down.

Recognize jamming.

Certain types of interference cause patterns to appear on radar scopes which are very similar to those caused by deliberate jamming. It is very important that the operator be able to recognize interference when it appears, in order to avoid giving false information about what he may otherwise believe to be a jamming attack.

Continue to operate.

Keep operating your radar equipment even if the jamming signals are extremely effective. The effectiveness of jamming will vary as the disposition of the force changes, and if you are persistent enough, some information may be obtained. For example, the jammer antenna may move for a short time into a null in a radar antenna pattern, perhaps allowing the targets to be seen clearly for long enough for you to determine sufficient data to help in their destruction. Remember, too, that there is a minimum range of self

  screening, so that as the targets approach, you will at some time be able to see them through the jamming, but only if the radar is in operation. However, even if you can do nothing with the jammed indicator, at least you are immobilizing the jammer, and perhaps keeping him from jamming another radar on a slightly different frequency. Continuing operation may indicate to the enemy that his jamming is ineffective, which may discourage him from further attempts.

Do not forget to search continuously through 360 degrees unless this duty has been assigned to other radars and you have been specifically instructed to confine your search to a designated sector.

Report jamming.

As complete information as possible on jamming signals should always be reported immediately to CIC. Report presence, bearing, and nature of the jamming , and state whether it is possible to read through it or not. The tact that jamming is being employed may indicate that important enemy action is under way. Reporting the bearing of jamming permits direct action to be taken to destroy the jammer-the best AJ measure of all. Lastly, if the nature of the interfering signals is made known, more effective AJ devices can be perfected.

Photo showing radar operating reporting.
Figure 3-54. Reporting Jamming. "Bearing zero seven nine-range two-O double-O. Many fuzzy targets-looks like Window. Targets are stationary."

Keep radar operating at peak efficiency.

Unless the radar is carefully maintained, its overall performance will decrease over a period of time. If the output power falls off, the set will be easier to jam since there will be less echo power to compete

 
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with the jamming signal at the receiver input. Therefore, check the level of performance of the set often, and call the technician whenever a decrease in the efficiency is noticed.

AJ TECHNIQUES FOR USE AGAINST ELECTRONIC JAMMING

Training the antenna.

This is a good AJ measure with search equipments when the target and the jammer are on slightly different bearings. If the antenna is trained across the target bearing, it should be possible to reach a point where the edge of the major antenna lobe receives the desired signal, while only low intensity signals are received from the jammer. Attempting to operate in this way must never be allowed to interfere with all-around search, for the enemy may be attempting to jam the radar simply to attract attention in a direction away from the direction from which he plans to attack.

Drawing showing own ship with two ships, a target and jammer a bit off axis.
Figure 3-55. Training the antenna so as to pick up the target but not the jammer.

Use of indicator controls.

Examine the A-scope pattern carefully while adjusting the various controls. Remember that electronic jamming signals ordinarily move across the screen, whereas the echo pip remains relatively

  stationary. This allows the echo to build up in intensity relative to the jamming on successive sweeps, and it provides the best chance you have of finding the target. Therefore, change the pulse repetition rate, if possible, to get the jamming pattern in motion. Look for small breaks in the baseline, bright streaks in the background of the jamming pattern, and bright pips at saturation level of the indicator. Vary the range scale to change the appearance of the pattern produced by the jamming. Sometimes a particular range scale will produce a pattern that is easy to read through. Often an expanded, or type R, scope simplifies the jamming pattern greatly. If the pattern seems regular, observe any discontinuities or breaks in its makeup; signals can be detected by watching for these indications even though the screen may appear hopelessly jammed at first glance.

Use of receiver controls.

There are some controls on all receivers that are useful in combating jamming. Some receivers have certain anti-jamming features incorporated in their circuits so that they have additional controls not found on more conventional receivers. If any of the controls are displaced from their normal settings in the process of trying to read through jamming, be sure to note the correct settings, since the jamming may suddenly cease, or the antenna may have to be turned away from the jammed sector in order to search over the rest of the area. If the correct settings are noted, the set can be restored to normal operation in a minimum of time. Expect interaction between the controls. Adjustment of one often makes readjustment of the others necessary.

Although not all radars have all the controls listed below, they are grouped together as a means of presenting them simply. Remember that even though your radar may not have some of the controls mentioned, you can still do much against most forms of jamming with the adjustments normally found on all receivers.

CONTROLCOMMENT
GainProbably the most useful. Adjust slowly, trying both reduced and increased settings. There is an optimum setting for each target pip.
Local Oscillator (L.O.) (Receiver Tuning)Try swinging tuning very slowly in both directions. Pip may decrease in size or become distorted but this does not matter if readability is improved. Unmodulated jamming is easy to tune away from. In addition, there may be holes in the frequency coverage of the jammer you are working against. Care must be taken to note the original and correct setting of the control so that normal operation can be resumed immediately on cessation of jamming.
 
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CONTROLCOMMENT
AFC Switch (Automatic Frequency Control)Try both "on" and "off" position. You cannot vary the L.O. tuning with AFC on.
AVC Switch (Automatic Volume Control)Try both "in" and "out" positions. On receiver used with SC/SK series radars, where a three-position switch is provided, try all three positions starting with position #3.
Rejection SlotsBest against low modulation frequencies, ineffective against noise. If two controls are provided, vary first one and then the other very slowly to improve readability. If no success, return to "out" position.
Video FiltersIf several types are available, try each in turn in conjunction with L.O. tuning. These filters introduce a constant range error which should be accounted for on fire control equipments. Also they distort the pip, so do not try to center in notch. Instead, align leading (left) edge of pip with leading edge of notch.
Video GainTry different settings while also adjusting the main gain control.
Pulse-Length Selector SwitchTry various positions. Longer pulse lengths are generally better against electronic jamming.
Balanced VideoThis control is usually of no use for improving A-scope presentation. It is only advantageous on the PPI, particularly against pulse or railing type jamming. Best operation is secured when the pulses are reasonably square.
AJ TECHNIQUES FOR USE AGAINST WINDOW JAMMING

Observe windward side of Window area.

Since the material may blow away from the targets, the windward side of the jammed area should be watched with particular care. Watch for pips beyond the Window cloud-targets on the same bearing but outside the infected area will be detected as readily

  as those on totally different bearings from the Window. Search on all bearings as the purpose of the jamming may be to divert attention from an attack coming from another bearing.

Look for holes in the Window cloud.

The enemy plane may have had to take evasive action with the result that the Window is not properly sown, and holes may exist through which targets

Drawing showing flight of planes with first sowing window and others following with the impact on radar.
Figure 3-56. Relation between Window dispersal, jammed area and attacking aircraft.
 
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can be spotted. Also, if your radar has a short pulse length and a narrow antenna beam, the Window may not have been sown closely enough to interfere with detection of targets because of the superior range and bearing resolution available in your set.

Continue to try to track targets through the Window.

After some experience it may be possible to track aircraft targets through Window by noting the difference between the violent beating of the Window echoes and that of the target. If it has been possible to track the target for a time before the Window is sown, some estimate of the targets speed may have been obtained. By using this information, it may be possible to pick up the target as it comes out of the Window area by moving the range step through the jammed area at the estimated rate of speed of the target. An expanded presentation, such as the R scope, is especially helpful in assisting the tracking of targets through Window because it permits full

  use to be made of the resolution inherent in the radar. Any holes in the Window cloud will be much more noticeable on the expanded scale than on any other range scale.

Window jamming can be worked through more easily at short ranges than at long. In figure 3-56 long range Window contacts jam a larger area in the range scope than Window at short range because of the lower angle penetration of the radar emissions into the Window. For example, if the radar antenna is elevated to a higher position angle, the radar will be jammed between ranges OC and OD instead of between OA and OB. Planes flying at a constant altitude may emerge from the jammed zone as the range closes.

Use of receiver controls.

Many of the controls useful against electronic jamming will be entirely ineffective against mechanical jamming. Only these controls listed below have some possibilities.

CONTROLCOMMENT
GainThis is one of the most effective controls. Try changing the position both up and down to determine the optimum setting and to prevent saturation.
Pulse-Length Selector SwitchUse the shortest possible pulse length. This will improve the range resolution of the radar.
IAVC and FTC (Instantaneous Automatic Volume Control and Fast Time Constant Coupling)Both of these special devices, incorporated on the more modem radars, increase the ease of working through Window.
ANTI-DECEPTION MEASURES

If deception is carefully carried out, it will be impossible to reveal the echoes as false within a short period of time. If only a few minutes pass before the deception is detected, it may have served its purpose. Therefore, operators must learn to observe and remember all the characteristics of true echoes in order that they will be able to detect quickly even small variations from normal that may be apparent with some types of deception.

Deceptive devices usually can be revealed as false if a plot is made of their course. If the device is hung from a balloon or a parachute, or is floating free in the air, its motion will always be in the direction of the wind aloft, and the speed will

  compare reasonably closely with the wind speed, but never exceed it. It is well to realize, however, that the wind speed and direction at 2000 feet may be different from that at the surface.

Since it is difficult to make a deceptive device that can affect all radar frequencies equally, deception may sometimes be revealed by comparison of the echoes on several radars that operate on different frequencies. This may be done quickly on a repeater PPI by simply turning the selector switch.

ANTI-EVASION MEASURES

The enemy can resort to evasive tactics to prevent or postpone radar detection only because he

 
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is aware of the limitations of our radars. Since it is quite apparent that the Jap knows how to take advantage of these shortcomings, there is little that an individual operator can do to combat evasion, except to practice faithfully in order to become so expert that it will be very difficult for the enemy plane to get out of the radar beam. If the limitations of our present radars, such as the poor coverage against low-flying planes or the inability to indicate altitude accurately and continuously, are overcome in new radars, it will then be nearly impossible for enemy planes to avoid detection or to confuse the operators by radical maneuvering. However, about the only means we have of combating evasion at present is to attempt to extend the coverage of our radars by deploying picket ships as far as 50 miles away from the main force, and establishing extensive coordination among all the CICs in the force.  
PHOTOGRAPHS OF JAMMING ON RADAR INDICATORS

This section illustrates scope presentations in the presence of various types of jamming that have been employed. Electronic jamming on type A scopes is shown in Figures 3-57 to 3-72, on type PPI scopes in Figures 3-73 to 3-81, and Window jamming on both type A and PPI scopes in Figures 3-82 to 3-85. The captions of each figure give appropriate AJ measures for the type of jamming illustrated, in order of preference. Unless otherwise mentioned, the sweep for A-scope presentations corresponds to a 40-mile range scale.

It should be realized that, while the photographs represent as closely as possible commonly encountered conditions, the actual pattern is usually in motion. For this reason, the camera cannot duplicate what the eye sees.

A. Air search radar A scope. The trace is wiped clean and is distorted by the overloading of the receiver.   B. ASB type L scope. Note characteristics gap between traces and baseline.
AJ-Reduce gain slowly to find optimum setting. Detune L.O. Rejection slots effective.
Figure 3-57. Unmodulated CW Jamming.
 
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Unmodulated CW Jamming (Mark 4 radar), Range scope on left, Train scope on left.
Strong jamming is overloading receiver and almost wiping trace clean. Echo is strong enough to produce inverted pips which are visible on Train and Elevation scopes due to fast sweep.

AJ-Adjust gain control to optimum setting. Try video filters. Try L.O. detuning. Turn off lobe switching if range and only approximate bearing ore desired.

Figure 3-58. Unmodulated CW Jamming (Mark 4 radar).

 

Unmodulated CW Jamming (Mark 4 radar), Range scope on left, Train scope on left.
Moderate jamming with gain control at optimum setting. Echoes are double-sided, filled-in, and fuzzy Bearing accuracy is probably impaired.

Figure 3-59. Unmodulated CW Jamming (Mark 4 radar).

 
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Low Frequency AM Jamming
This type of jamming is known as "tramlines." Less than one cycle of the jamming modulation appears on each trace. Echo present on each trace.

AJ-Adjust gain to optimum setting. Try detuning L.O. Try rejection slots. Use video filters (FTC is a form of video fitter). Try various selections of AVC time constant, or use IAVC.

Figure 3-60. Low Frequency AM Jamming.

 

Low Frequency AM Jamming.
An example of "Basket weave" Jamming. The jamming is synchronous, and has a modulating frequency higher than that in Figure 3-60. Approximately two cycles of the modulation are appearing on each trace.

AJ-Adjust gain to optimum setting. Try detuning L.O. Try rejection slots. Use video filters or FTC. Use IAVC or try various settings of AVC time constant.

Figure 3-61. Low Frequency AM Jamming.

 
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Low Frequency AM Jamming
A. A non-synchronous, "basketweave" type of jamming. Pip shows as break in each trace.

AJ-Adjust gain control to optimum setting. Try detuning L.O. Try rejection slots. Use video filters or FTC. Try various selections of AVC time constant, or use IAVC.

 

Low Frequency AM Jamming.
B. Same condition as in A, except with rejection slot set to carrier frequency of jammer. Jamming modulation has been practically removed, but still shows up as a thickening of the baseline.

Figure 3-62. Low Frequency AM Jamming.

 
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Figure 3-63. Medium-Frequency AM Jamming (ASB radar).

An example of semi-synchronous jamming modulation. Because the baseline on the ASB is vertical (type L presentation), low-frequency tramlines appear vertical and medium- or high-frequency modulations give a horizontal pattern.

AJ-Adjust gain control to optimum setting. Try detuning L.O. Try rejection slots.

  Medium-Frequency AM Jamming
 
Medium-Frequency AM Jamming   Figure 3-64. Medium-Frequency AM Jamming (Army radar).

Jamming modulation is non-synchronous, Modulation on frequency is difficult to determine. Note vertical striations in body of pattern. No AJ devices are being used and gain is set at normal setting. Pip causes break at baseline and runs through jamming pattern, but is difficult to find.

AJ-Adjust gain control to optimum setting. Try detuning L.O. Try rejection slots.

 
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Medium or High-Frequency AM Jamming
A. Jamming modulation is non-synchronous. Striations in body of pattern are less visible than in Figure 3-64, Gain setting is normal. Pip causes break in baseline, and runs through the jamming pattern, but is almost invisible.

 

Medium or High-Frequency AM Jamming.
B. Same condition as in A, except gain control has been set to optimum setting. Pip now produces marked irregularity in pattern. Rejection slot also being used, but it is not completely effective because of presence of small amount of frequency modulation.

Figure 3-65. Medium or High-Frequency AM Jamming.

 
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High-Frequency AM Jamming (Mark 4 radar).
A. Moderate jamming. Modulating frequency of jammer is about 200 kilocycles per second. Possible to obtain range, but train and elevation pips are too fuzzy to be used.

AJ-Adjust gain control to optimum setting. Try video filters, starting at lowest number. Try detuning L.O.

 

High-Frequency AM Jamming (Mark 4 radar).
B. Same condition as in A, except that AJ measures have been applied. Only gain control adjustment was necessary in this case. When weak jamming is encountered. video filters should be used with caution. Angular errors result unless jammer is definitely known to be on target. Do not forget to "spot" the range when filters are used. On Train and Elevation scopes, do not match total heights, but rather the height of the pip above jamming pattern.

Figure 3-66. High-Frequency AM Jamming (Mark 4 radar).

 
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Pulse Jamming.
A. Synchronized jamming pattern, sometimes called "railings".

 

Pulse Jamming.
B. Similar to A, but on Army radar. Jamming has higher repetition rate or longer range scale is in use. Non-synchronous pulse jamming of high PRR is hard to distinguish from high-frequency AM jamming.

AJ-Adjust gain control for optimum setting. Change PRR if possible, to make jamming move rapidly across scope. Try balanced video to improve PPI'S readability. Try detuning L.O.

Figure 3-67. Pulse Jamming.

 
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Figure 3-68. Low-frequency FM Jamming.

Synchronized presentation commonly called "bells" or "cobs" Pip rides on top of pattern. Other types of FM jamming produce patterns similar to AM, and are no harder to work through.

AJ-Change PRR to obtain rapid movement of bells. Reduce gain if pattern is saturating.

  Low-frequency FM Jamming.

 

Mixed AM Jamming.   A. A combination of 240 cycle and 10 kilocycle jamming modulations. The lower frequency causes the tramlines, and the higher frequency the vertical indications.

AJ-Gain control. L.O. detuning. Rejection slots. Video filters or FTC. Short time constant on AVC or IAVC.

Mixed AM Jamming.
B. Jamming pattern called "German Mixture" on Mark 4. Made up of 50 cycle and 100 kilocycle jamming modulations. In presence of jamming, Mark 4 often gives appearance of mixed jamming when lobing is used, because pattern is "chopped up" at lobing rate.

AJ-Adjust receiver gain. Try video filters. Try detuning L.O.

Figure 3-69. Mixed AM Jamming.

 
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Random Noise Jamming.
Moderate jamming-jam-to-signal ratio slightly less than effective value. Gain control was adjusted for optimum pattern. On train and Elevation scopes, do not match total heights, but rather height of pip above jamming pattern.

Figure 3-70. Random Noise Jamming (Mark 4 radar).

 

Random Noise Jamming
A. Weak pip lost in grass produced by jamming. A difficult type of jamming to work through, but keep trying. Effectiveness of noise jamming depends on the strength of the jamming relative to the echo strength.

AJ-Adjust gain control to optimum position. Vary L.O. tuning.

Random Noise Jamming.
B. Jam-to-signal ratio is below effective value. Either echo is stronger or jamming is weaker. Gain control has been adjusted to optimum value.

Figure 3-71. Random Noise Jamming.

 
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Random-Noise Modulated Jamming
Strong jamming of almost saturation level. While this jamming may he practically impossible to work through, change of position with respect to the jammer may reduce the jamming strength to a level where targets could he detected.

AJ-Adjust gain control to optimum setting. Try detuning L.O. Try video filters.

Figure 3-72. Random-Noise Modulated Jamming (ASB Radar).

  Moderate strength jamming.
A. Moderate strength jamming. Striations appear at outer edges of jammed sector because of continuous rotation of sweep.

Same as in A after applying AJ measures.
B. Same as in A after applying AJ measures. Gain was reduced (too much in this case) ant the FTC cut in. Slightly higher IF or video gain would restore more of the targets to their normal brilliance.

AJ-Adjust gain to optimum setting. Try rejection slots. Try detuning L.O. Try video filter or FTC.

Figure 3-73. Unmodulated CW Jamming.

 
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On left, A. Jammed Scope; On right, B. IAVC alone.

On left, C. FTC alone; On right D. IAVC and FTC combined.

Strong jamming. Very few echoes show in A because the receiver is saturated by the jamming received not only in the main lobe but also in the minor and back lobes. Note the successive improvement as a result of using IAVC, FTC, and in D, a combination of both.

Figure 3-74. Unmodulated CW Jamming.

 
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Low-Frequency AM Jamming.
Jamming is synchronous. Low and medium frequency modulations are characterized by radial bands when synchronous or semi-synchronous condition is present. Setting of gain control has been reduced so that side lobes are not visible.

AJ-Try video fitter or FTC. Adjust gain control to optimum setting. Try detuning local oscillator. Try rejection slots.

Figure 3-75. Low-Frequency AM Jamming.

 

Low-Frequency Sine Wave AM Jamming
A. Jamming appears over a wide sector because it is quite strong. Note radial bands characteristic of synchronous low-frequency modulated jamming.
  Low-Frequency Sine Wave AM Jamming
B. Same as A after applying AJ measures. FTC was cut in and gain reduced Note improvement in definition in ground clutter area at short range due to use of FTC.
 
AJ-Try video filters or FTC. Adjust gain to optimum setting. Try detuning L.O.

Figure 3-76. Low-Frequency Sine Wave AM Jamming.

 
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Low-Frequency Sine-Wave AM Jamming
A. Twelve-mile sweep of airborne radar being used. Target is lost in jamming at about 7 miles. Note radial-hand characteristic of jamming.

AJ-Try video Alters or FTC. Adjust gain to optimum setting. Try rejection slots. Try detuning L.O.

 

Low-Frequency Sine-Wave AM Jamming
B. Same as A after applying Al measures. FTC was cut in and gain control was adjusted. FTC has eliminated jamming and considerable clutter. Note ship echo and echo from dredge anchored near shore which were both previously lost.

Figure 3-77. Low-Frequency Sine-Wave AM Jamming.

 
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Medium-Frequency Sine-Wave AM Jamming.
Note that radial hands are closer together than in Figure 3-77 because of higher frequency of modulation. Jamming is stronger and is entering side lobes of radar antenna. Spiral lines are caused by interference from another radar.

AJ-Try video filler or FTC. Adjust gain to optimum setting. Try detuning L.O.

Figure 3-78. Medium-Frequency Sine-Wave AM Jamming.

 
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High-Frequency AM Jamming
A. Airborne radar, 5-mile sweep. Strong jamming entering side lobes. Echo not visible. Note striations characteristic of semi-synchronous jamming. Dark sector caused by photography.
 

High-Frequency AM Jamming
B. Same as A after applying AJ measures. Gain control adjusted and L. O. detuned. Modulating frequency too high for effective use of FTC. Target now visible at 12 o'clock.

AJ-Adjust gain to optimum setting. Try detuning L.O.

Figure 3-79. High-Frequency AM Jamming.

 

Taking a Bearing on Jamming
A. High-frequency sine-wave jamming entering multiple side lobes.
 

Taking a Bearing on Jamming
B. Same as A. but jammed sector can now be accurately bisected after reducing gain to eliminate side lobe indications.

AJ-Adjust gain to optimum setting. Try detuning L.O.

Figure 3-80. Taking a Bearing on Jamming.

 
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Random-Noise AM Jamming. Arrows point out the Echo and Jammed Sector
A. Moderate jamming with gain control setting reduced. Jammed sectors produced by random noise, unmodulated CW, and non-synchronous high-frequency modulated jamming are somewhat similar in appearance. Random-noise jamming produces ragged edged sectors. The inner part of the jammed sector looks like excessive snow at reduced gain control settings.

 

Random-Noise AM Jamming. Arrows point out Jammed Sectors.
B. Same as A after jamming strength has been increased. Jamming is entering side lobes of radar antenna and echo is obscured. The spiral lines are pulse interference from another radar.

AJ-Adjust gain for optimum setting. Try detuning L.O. Try video filter or FTC.

Figure 3-81. Random-Noise AM Jamming.

 
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Window Jamming
A. Plane which is fourth target from the left, has just sown Window. Window cause the multiple ragged indications immediately beyond this pip. A target appears at a greater range than the Window.

 

Window Jamming.
B. Same condition is in A, except at a later time. Plane, which is second pip from the left, is now nearby and has sown enough Window to give saturation returns. No AJ measures have been applied, and the target is lost.

AJ-Turn down gain to prevent saturation. Look at edges of Window area. Cut in FTC. Use shorter pulse length, if available. Use most expanded sweep possible. Look carefully for the relatively fast beating of Window echoes on A scope.

Figure 3-82. Window Jamming.

 
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Window Jamming on SG Radar
A. Aircraft at about half range, bearing 100 degrees true is about to sow Window to screen a destroyer, visible just beyond the plane.
 

Window Jamming on SG Radar
B. Result of Window jamming shortly after photograph in A was taken. Aircraft flew a spiral course while sowing. Destroyer is completely screened.

 

Window Jamming on SG Radar

C. Fifteen minutes after photograph in A was taken. Window mass has drifted with wind. Destroyer is again visible on windward side of Window area.

Figure 3-83. Window Jamming on SG Radar.

 
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Window Jamming. Area infected by window laid by one airplane.
Another example of Window dropped by aircraft. The aircraft probably flew a zig-zag course over the target to be screened.

Figure 3-84. Window Jamming.

 

Window Echoes on Different Radars
A. Window echoes on Mark 4 range scope. Material cut for Mark 4 radar frequency.
 

Window Echoes on Different Radars
B. Echoes from same Window as in A. Little effect because not cut for SG frequency,

Figure 3-85. Window Echoes on Different Radars.
 
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TECHNICAL ASPECTS OF ANTI-JAMMING TECHNIQUES

The nontechnical radar operator must apply AJ techniques by trial and error. However, much greater proficiency can be attained through a partial technical knowledge of the problems involved. The radio technician must also have some knowledge of AJ methods because the application of some techniques require his presence at the main frame of the radar transmitter.

Generally speaking, the successful application of AJ techniques may be accomplished through an understanding of:

1. The nature of echo and jamming (electronic) signals.

2. Radar receiver operation in the presence of jamming.

3. The principles of AJ techniques and devices.

The Nature of Echo and Jamming Signals.

The echo signal. Pulsing, as employed in radar transmitters, is a form of modulation. The transmitted pulse and the corresponding echo pulse may be considered to be a complex modulated signal made up of a carrier and many side bands. If the pulse is square in shape, the frequency spectrum and the relative amplitude of the various components could be illustrated by Figure 3-86.

The spectrum is made up of individual components, which are separated in frequency by an amount numerically equal to the PRR. The width of the spectrum (in megacycles) from the carrier frequency out to the first zero amplitude point is equal to 1/lambda, where lambda is the pulse duration in microseconds. To receive this spectrum, the IF band width of radar receivers may vary from 0.8 to 2.0 times 1/lambda depending on the design of the IF circuits and the extent

  to which it is desired to maintain the pulse shape. Absence of higher frequency side-band components causes the pulse shape to become rounded off or distorted, whereas excessive band width reduces the signal-to-noise ratio because more of the receiver noise components are passed.

CW jamming would theoretically consist of an emission on a single frequency. However, due to instability in the jammer, a small amount of frequency modulation is usually present unintentionally, causing the signal to cover a narrow band of frequencies.

Modulated jamming may either be of the amplitude-modulated or frequency-modulated type. Both types produce similar patterns on the radar scope-AM being more commonly used. The frequency spectrum for an AM signal will consist of a strong carrier and side bands spaced symmetrically above and below the carrier frequency. If a carrier is modulated by a single frequency, the spectrum will consist of the carrier and only one pair of side hands. Modulating frequencies are referred to as low, medium, and high, and have the following approximate limits:

Low-up to 10 kilocycles
Medium-up to 100 kilocycles
High-100 to 1000 kilocycles.

If the jamming modulation pattern can be made to stand still on the scope (i.e. synchronized) by varying the PRR control on the radar, an estimate of the modulating frequency can be made by counting the number of modulation cycles which occur during a given sweep. For instance, if 4 cycles of the modulation appear on the 40 mile range sweep, the modulation frequency may be calculated to be about 8 kilocycles.

Mixed modulation , consisting of both low and high frequencies used simultaneously, has been employed by the Germans.

Wave diagram showing the carrier.
Figure 3-86. The Radio-Frequency spectrum of on echo pulse.
 
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On the left RF Signal in 'IF' Amplifier on the right After Detection. Pairs are given for:
Echo Pulse-Normal Pip
Echo and CW Jamming in Phase-PIP Above Baseline
Echo and CW Jamming 180 degrees out of Phase-PIP Below Baseline
Phase Between Echo and CW Jamming Changing Periodically-combined waveform.
Figure 3-87. Echo and CW jamming changing phase.
 
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Pulse jamming is a form of AM having a frequency spectrum similar to a radar echo signal. It consists of pulses like those produced by a radar transmitter. The jamming pulses are of the same order of width as some radar pulses except that they usually have a higher PRR and the time between pulses approaches the duration of the pulse. Pulse jamming may also be obtained by 100% modulation of a CW carrier with a square wave.

Random noise modulation. This will consist of a large number of components randomly varying in frequency, phase and amplitude. The total spread of the relatively strong components of the spectrum will usually not exceed 6 to 8 megacycles. In some cases the carrier is suppressed and only one side band is transmitted.

Radar Receiver Operation in the Presence of Jamming.

The IF Channel. When both echo and jamming signals are present in the receiver simultaneously, the echo signal adds with or subtracts from the jamming, depending on the phase relation between the jamming and echo carriers.

If the jammer is on the frequency of the radar, the phasing of the two signals will change from echo pulse to echo pulse within the limits illustrated in figure 3-87 B and C. These figures show that the pip may appear either above or below the normal baseline on the scope. After a number of sweeps the

Example of Echo and Low Frequency MCW and of Echo and Low Frequency MCW Modulation 100% Echo in Trough
Figure 3-88. Echo in presence of Amplitude-modulated jamming

  persistence of vision and the persistence of the cathode-ray tube screen will cause a series of pips such as illustrated in figure 3-87 D to appear. If the jammer is not exactly on the radar frequency, the phase relation will also change during the echo pulse at a rate determined by the frequency difference. This condition is shown in E. The pip patterns will not be as distinct as those shown, but will have a blurred, "filled-in" appearance caused by the constant movement of the trace in the directions indicated by the arrows. This condition is illustrated in the photographs shown in figures 3-59 and 3-60. The jammer will usually be "off frequency" because of frequency drift inherent in both the radar and jamming transmitters, and because of the difficulties encountered in constantly monitoring the jamming transmission.

When the jammer is amplitude modulated, the pattern corresponding to the modulation frequency appears during the entire time of the scope sweep, in addition to the effects noted above.

If saturation is not occurring, the pip is superimposed upon the modulation pattern (figure 3-88A). A special case occurs when the per cent modulation of the jammer is 100 percent or over. When the echo occurs at the same time as a trough in the modulation, the pip will be normal (i.e. not double-sided), provided the recovery time of the receiver is short enough, because the amplitude of the jamming is small. Unless the modulation is synchronous, the same portion of the modulation cycle will not occur at the same time on successive sweeps. The modulation will therefore appear to move across the scope and the pip will jump up and down, being sometimes normal and sometimes double-sided (figure 3-88B).

Overload and Instability in the Radar Receiver

How overload occurs. Many stages of amplification must be used to obtain the high gain required to amplify a small echo signal to a usable level. If a strong jamming signal which is many times the amplitude of the usual echo is amplified by a number of RF and IF stages, the jamming may reach a level sufficient to overload the receiver. This overloading may take the form of plate current saturation or grid limiting, depending upon the components and operating bias of the stage concerned. A receiver stage thus affected will not be able to amplify small changes in amplitude, such as the echo modulation of the jamming. The echo signal is said to be "wiped off."

 
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Shows First I.F., Last I.F., Detector (Diode), Video
Figure 3-89. Plate-Current saturation on Positive Peaks Occurring in Last IF Stage.
Prevention of overload. For purposes of illustration, let us assume that overload is occurring due to plate-current saturation on the positive peaks in the last or next to the last IF stage (figure 3-89). It will be noted in figure 3-90 that the echo may be restored in the output of the last IF amplifier by reducing the setting of the IF gain control, but that it is double sided.

In this case gain control is accompanied by changing the amplification of the first IF stages-one of the methods frequently used. No change is made in the operating point of the last IF stages by manipulating this control. The echo signal is reduced in amplitude by the same ratio as the jamming but the echo is now amplified by the last IF stage instead of being wiped off". Weak echoes may not be

  visible after the IF gain control has been reduced because of the loss in amplifier gain.

Another method of seeing a pip in the presence of jamming, under certain conditions, is to turn the IF gain up instead of down and to observe the pip as a black opening at the base of the jamming pattern. This can be done only with jamming having a relatively high percentage of modulation, and is dependent upon the recovery time of the receiver. Overload will occur in certain portions of the jamming modulation cycle but not necessarily in the troughs. The amplitude of the jamming in the troughs may be so small that the echo may appear as a normal pip at the baseline which makes a gap in the jamming pattern.

Overload is made less likely in radar receivers of

Shows First I.F., Last I.F., Detector (Diode), Video
Figure 3-90. Receiver Gain Reduced-No Overload Occurring.
 
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present design by the use of back bias" circuits. By means of these circuits the signal level is made to control automatically the operating point of various stages in the IF amplifier. The presence of the strong jamming then causes the grid bias of an IF stage to change from its usual value to a much higher negative value. As a result, the input signal may still be relatively large without wiping the pip off on one half of the input cycle. In effect, the tube circuit is now acting as a Class "C" amplifier, and is actually discriminating against the jamming in that the average amplification of the stage for the jamming signal has been reduced relative to that for the echo.

In order to make full use of this principle, the operating point of each IF stage must be suitably adjusted. "Unamplified back-bias" is used in the earlier stages; in the later stages, where a greater voltage swing is required, "amplified back-bias" (AVC or IAVC) is used. IAVC (Instantaneous Automatic Volume Control) is similar to AVC except that very short time constant circuits are used. AVC circuits tend to remove jamming modulations

  because of their degenerative characteristics. Time constants determine just how high a modulation frequency is removed. The recovery time constant also determines the effect on pulse jamming, Window, and other clutter.

Receivers are in production with the following back bias schemes:

(a) Unamplified back-bias for all IF stages-time constants shorter or equal to radar pulse duration.

(b) Unamplified back-bias for first IF stages, back-bias (AVC) for last IF stages.

(c) Short time constant amplified back-bias (IAVC) for last stages.

The modulating frequency appears in greater amplitude at the output of the detector as the modulation percentage increases. Under these conditions, it will sometimes be noted that a jammed echo may be visible on the A scope but only a blanked out sector with no echoes visible appears on the PPI. This blanking may be caused by grid limiting in the video amplifier. However, when only a cathode follower

Use of AVC (Back Bias) in IF stage. Shows change in bias produced by AVC in the I.F. Stage.
Figure 3-91. Use of AVC (Back Bias) in IF stage.
 
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is used after the detector, the difficulty may usually be traced to the limiter used to prevent the PPI screen from overloading. Some of our present radars are now provided with a video gain control. Suitable adjustment of this control will at least make the jamming visible as a bright sector on the PPI. However, since the PPI is an "intensity modulated" device, small changes in intensity, such as an echo might produce in the presence of strong jamming, are not as easily detectable as on the A scope.

In the presence of jamming, another effect comes into play-the large jamming signal may cause the receiver to become unstable. This may be the result of "ringing" or oscillation in some of the IF tuned circuits. Such a condition may sometimes be remedied by a careful setting of the gain control or by changing the response time of the AVC circuit.

The principles of AJ techniques

The enemy will attempt to tune his jammer so that the center of the jammer frequency spectrum corresponds to the center frequency of our radar echo spectrum. This is a difficult thing to do, and keeping the jammer exactly on the radar frequency over a long period of time is even more difficult because the frequency of both the jammer and the radar transmitters vary independently. Therefore the enemy

  must constantly monitor the radar transmissions by means of an intercept receiver capable of looking through" the jamming he sends out. However, even with constant monitoring it is almost impossible to keep the jammer exactly on frequency. Some of the following techniques may be useful in combating this jamming, after the radar gain control is adjusted to prevent overloading the receiver.

Take advantage of frequency difference. Usually the jammer will be slightly off the radar frequency. This allows the beat between the carrier of the echo and the carrier of the jammer, and other strong components around the two carriers, to be used as an echo indication. The strong complex beat may produce an easily identified discontinuity in the jamming pattern to he observed on the scope.

Figure 3-92 shows a medium-frequency amplitude. modulated jammer having side hands marked (1) and (3). In (A), the response of the IF amplifier, as indicated by the curve showing the "acceptance hand" is very nearly but not actually zero at the frequency of component (3). Therefore, if component (3) is strong enough, it will pass through the IF amplifier and beats will he produced after detection with components of the echo.

Figure 3-92(B) shows the spectrum present at the input of the video amplifier. The portion under

I.F. Spectrum with the Jammer Carrier is shown on left A, Video Sprectrum with Beats are shown on right B.
Figure 3-92. Utilizing Beat between Echo and Jamming as Echo Indication.
 
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the dotted line shows the spectrum that would be present if no jamming were occurring. The beating that would result is more complicated than indicated, and the amplitudes of the frequencies shown are probably not entirely correct. The beat between (1) and (2), which produces the confusing pattern on the scope, occurs continuously while all other beats occur only during the time that the echo is present. Previously the video amplifiers in our radar equipments have not had sufficient frequency range to pass the beat between (3) and the echo. The present tendency is to improve the frequency response of video amplifiers so that the frequency range is about equal to the width of the IF acceptance band. This suggests the use of the beat between (3) and echo as an echo indication and removing all other frequencies below this band of frequencies by means of suitable video filters.

The AJ technique used in this particular case is to shift the radar frequency a small amount so that the jammer carrier is attenuated by placing it at the outer edge of the IF acceptance band. This tends to reduce the amplitude of the objectionable beat between components (1) and (2).

It is desirable for the operator to have direct control over small frequency variation so that the most

Normal L.O. Setting shown above Result of Readjustment of L.O.
Figure 3-93. Effect of Adjusting Local Oscillator Tuning Control.

  readable echo-jamming pattern may be obtained. On equipments having a transmitter power control at the receiver-indicator, slight frequency changes may be made by varying the transmitter plate voltage. Multimoding may occur with transmitters employing magnetrons if the plate voltage is changed too much from its normal value.

Local oscillator tuning. The local oscillator tuning provides another means of getting "out from under the jamming" which is directly under the control of the operator. Changing this control causes the spectrum of both the jamming and echo signals to appear about a different center frequency in the IF amplifier, but the acceptance band of the IF remains the same irrespective of the setting of the local oscillator. However, the IF amplifier output will drop off when the center frequency of the signal no longer corresponds with the center frequency of the IF acceptance band, because some of the components will be more attenuated than in normal operation. For this reason changing the L. O. tuning will also act like a gain control and will tend to prevent receiver overload.

When both jamming and echo signals are present in the IF the effect may be used to discriminate against the jamming when the jamming and echo carriers are not on the same frequency. A slight frequency variation may add materially to the effect obtained. Figure 3-94 illustrates what can be done with L.O. tuning. Here we are attempting to shift the greater part of the jamming spectrum outside of the receiver acceptance band, without losing too many of the echo components. The carrier of the jammer is attenuated considerably, but continuous beating between the side-band components of the jammer will still be visible on the scope. Thus, the result of detuning is not to remove all or the jamming signal, but only to cause the echo signal to become strong enough relative to the jamming to permit a definite discontinuity to be seen. In many cases, though, the jamming may be almost entirely removed. In a complicated case of jamming the best method of observing the echo may be to advance the gain control so as to obtain limiting of the jamming pattern while simultaneously varying the L.O. tuning. The echo would probably appear, if at all, as a dent in the baseline. When the local oscillator is detuned, however, extreme care should be taken to note the correct setting of the control, so that the radar can be restored to normal operation as soon as the jamming stops.

IF filters. The IF rejection slot is so called because it puts a "slot" in the IF acceptance band of the receiver.

 
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The small band of frequencies that it removes from the acceptance band will not affect the shape of the echo appreciably. The characteristics of such a filter suggest its use against CW jamming and amplitude modulated jamming. The principle used is that of removing the carrier of the jamming interference so that beating will not occur between side bands and the carrier of the jamming. Beating will still occur   between the two jamming sidebands but at higher frequency and with smaller amplitude.

If another slot is available, it could be adjusted to eliminate one of the jammer sidebands. As the slot is moved across the acceptance band of the receiver (as indicated by the arrows) several minimums in the jamming pattern will be noticed as the sidebands are passed. Some slight improvement may be

Using L.O. tuning to Discriminate Against Jamming. Normal L.O. Setting on top, Detuning After Readjustment of L.O.
Figure 3-94. Using L.O. tuning to Discriminate Against Jamming.
 
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Slot shown above, Medium Frequency Jamming below.
Figure 3-95. Setting IF Rejection Slot to eliminate carrier frequency of jammer.

expected in removing the carrier of amplitude modulated noise jamming, but since there are so many other components present, the improvement may not be very noticeable.

Video filters. The types of filters now used as AJ devices may be classified as fast time constant, high-pass, and band-pass. The purpose of using them is to remove objectionable modulation frequencies or to improve the visibility of the echo. The elimination of strong modulation components also prevents overload in the video amplifier. Filtering may be accomplished by relatively simple R-C circuits or with more complicated L-C circuits employing many sections.

When the modulation frequency spectrum of the jammer consists only of low or medium frequencies (i.e. less than 100 kc), a simple R-C filter is satisfactory for removing the jamming modulation. This is accomplished by introducing fast time constant coupling (FTC), preferably between the detector and the first video stage. The time constant of the coupling condenser and the grid resistor of the video stage is made to equal one to five times the radar pulse length. In general, this type of filter is very effective in removing low frequencies, but it becomes less effective as the modulating frequencies increase due to its poor cut-off characteristics. The effect of such a filter is to allow the echo frequencies to pass without much distortion, while preventing the

  passage of the jamming frequencies, but when the jamming includes high frequencies, the filter is ineffective. If the jamming modulation consists of a number of components scattered throughout the echo spectrum, removal of the jamming modulation becomes more difficult. Any simple filter which would attenuate or remove these widely scattered modulation frequencies would also attenuate the echo in the same ratio. However, a sharp cut-off high-pass filter may be used to advantage when the jammer and echo carrier frequencies are slightly different. For example, if the jammer frequency differs from the radar frequency by 0.5 megacycle, a 500 kilocycle beat exists for the duration of the radar pulse. If a 500 kilocycle high-pass video filter is used it will pass the 500 kilocycle beat and attenuate all lower frequencies. As a result, the jamming modulation is removed and a "beat frequency echo" appears in place of the usual echo. This echo may be filled-in or have a more fuzzy appearance than the usual echo, but nevertheless it is usable. Extra video gain must be used, for otherwise only strong jamming will produce a beat large enough to be observed.

Short time constant AVC and IAVC circuits have been designed which will remove frequencies up to 10,000 cycles per second. However, it is sometimes impossible to use these quick-acting circuits in the presence of jamming because of the tendency of circuits in the receiver to break into oscillation.

The problem of removing jamming modulation is somewhat complicated when barrage jamming (several jammers having their carrier frequencies staggered over a given band) is encountered, because of the beat frequencies set up between jammers. A band-pass filter may be used in this case. This filter will tend to remove the modulation frequencies in the low side of its characteristic and the jammer beat frequencies on the high side.

Video filters must be used cautiously with fire control radar equipments employing pip-matching. High-pass and band-pass filters remove nearly all of the frequency components of the echo, which necessitates observation of the beat frequency echo both for ranging and pip matching. When the jamming is strong enough, which means that it exceeds a certain jamming-to-signal ratio, depending upon the equipment in question, the amplitude of the beat will be proportional to the amplitude of the echo. If the jamming is weak, the amplitude of the beat varies with the amplitude of the jamming-an undesirable condition. Therefore, serious angle errors will result when high-pass or band-pass filters are used in the

 
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presence of weak off-target jamming. They should not be used against weak jamming when angle information is desired, unless the jamming source is definitely known to be located on the target.

The insertion of filters in the video amplifier, with the exception of the simple R-C filter, delays signals passing through the amplifier. Such delay results in

  range errors if not compensated for, and different filters introduce different delays. When a selection of several types of filters is available, time delay compensation is applied to make the total delay (filter delay plus compensation) the same for all filter positions. It is then possible to "spot" the range a given amount no matter which filter is used.
 

* U. S. GOVERNMENT PRINTING OFFICE: 1945-6388S9.

 
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