GENERAL RADAR PRINCIPLES
GENERAL RADAR PRINCIPLES
|Visual detection methods and limitations ||1-4
|Radar overcomes visual limitations ||1-5
|Derivation of word "radar" ||1-5
|Information given by radar ||1-5
|Importance of radar || 1-6
| Battle of Britain ||
| Battle of Midway ||
|Navy types of radar ||1-6
| Search ||
| Fire-control ||
| Special ||
|Navy letter system ||1-8
|Radars on board ship ||1-8
|Importance of radarman ||1-8
|BASIC PRINCIPLES OF RADAR ||1-9
|Frequency code ||1-11
|Equivalent measurements ||1-11
|Principle of pulse reflection ||1-11
|Bearing determination ||1-12
|Range determination by sound ||1-12
|Range determination by radar ||1-13
|MAIN PARTS OF A RADAR SYSTEM ||1-13
|The transmitter ||1-14
| Wave length or frequency ||
| Duration and power of pulse ||
| Pulse rate ||
|The antenna system ||1-16
| Transmission line ||
| Antenna: an emitter of radio waves ||
| Non-directional antenna ||
| Directional antenna ||
|How does radar determine bearing? ||1-20
| Concept of lobe ||
| "E" units ||
| Maximum echo method ||
| Accuracy consideration ||
| Minimum echo method ||
| Lobe switching ||
| Minor lobes ||
|The receiver ||1-28
| Signal amplification ||
|The indicator ||1-28
| Basic electron theory ||
| Structure of electrostatic cathode-ray tube ||
| Concept of sweep and time base ||
| Electromagnetic cathode-ray tube ||
| Echo indication by deflection and by intensity method ||
| Standard C.R.T. controls ||
| Development of trace by the electron beam ||
| Continuous and discontinuous sweeps ||
| Internal calibration ||
| External calibration ||
|Types of scopes ||1-38
| The "A" scope ||
| The "J" scope ||
| The "R" scope ||
| The PPI scope ||
| The "B" scope ||
| The "H" scope ||
| Summary ||
|The modulation generator ||1-42
|The duplexer ||1-44
|Summary of one cycle of radar operation ||1-44
|GENERAL RADAR CHARACTERISTICS ||1-47
|Air-search radar ||1-47
|Surface-search radar ||1-47
|FACTORS AFFECTING RADAR RANGE ||1-47
|Maximum range factors ||1-47
|Minimum range factors ||1-47
|HOW DOES RADAR DETERMINE ALTITUDE ||1-47
|Position angle and range method ||1-47
|Air-search radar for altitude determination ||1-48
|Addition and cancellation of radio waves ||1-49
|Fade chart ||1-50
|SPECIAL USES OF RADAR ||1-54
|Direction finding ||1-55
|Fire control ||1-55
|Fighter direction ||1-55
|FUTURE OF RADAR ||1-55
The primary purpose of the Navy is to "destroy the enemy," and all the
Navy's activities exist
only for this purpose. Only those pieces of equipment which will enable
it to do this effectively
are considered as being of any value.
Closely related to this primary purpose of the Navy is the secondary
purpose: self preservation.
The ship that fights the most effectively has the best chance of coming
safely into home port
again. A poorly fought ship may never have another chance to fire its
guns. Sunken ships do not shoot.
You, as a radar operator, may be wondering how radar helps the ship fire
the guns and come
safely home again. The purpose of this hook is to clear up that question.
Radar performs the old
task of finding the enemy but uses new methods. If you understand how
this task was performed
before the development of radar, it will help you to realize the
superiority of radar over any
device formerly employed.
Visual detection methods and limitations.
Ships have used visual lookouts since the early days of sail. A lookout,
however, cannot see
or smoke or darkness for any great distance, and even on a clear day
cannot see far beyond the
horizon. Such limitations of visual lookouts have hampered ships for
centuries-and still do,
Navy men of the past realized that when an enemy vessel appeared over the
horizon or out of a
fog bank too little time remained to prepare for battle. So when in
search of the enemy, they
sent out pickets, a line of the faster, smaller ships in the direction
from which the enemy was
expected. Then, when the ship farthest ahead saw the enemy rise above his
horizon, it signaled
the next vessel, and thus the word was passed.
But the picket system did not help much in bad weather or at night. The
black of night or a
curtain of fog still could hide an enemy's approach. After radio came
into general use, pickets or
patrol vessels could relay the information beyond the horizon at night or
through fog. But even
so, the visual lookout of the patrol vessel was handicapped by his
limited field of view.
Even if the presence of an enemy were known, battles could not be waged
very successfully on
Figure 1-1. Ship to ship communication before radar.
GENERAL RADAR PRINCIPLES
nights because of the difficulty of locating the target. Firing guns
blindly in the dark is not
effective. Besides being a waste of ammunition it can do harm by betraying
your position to the enemy.
Figure 1-2. Indiscriminate firing betrays your position.
Radar overcomes visual limitations.
Radar, generally speaking, can reach out beyond
the visual horizon. It can detect through darkness,
fog, and smoke as well as through sunshine. You no longer have to wait
until the enemy appears
over the horizon before you know several facts about him-his presence,
number, size, course,
and speed. You can be preparing a plan of action before the enemy even
knows where you are.
Thus, radar enables your ship to "shoot, the guns," even in the dark, and
to make hits with the first or second salvo.
Remember that for all practical purposes radar is not affected by visual
limitations. It can
detect equally well through darkness and smoke, and almost as well through
fog. True, it does
have a maximum range, a radar horizon, but this is usually well beyond the
visual horizon. The
wider radar horizon gives you earlier warning of the approach of the enemy,
precious minutes in which to prepare for battle. Moreover, radar is more
than a walking stick
for groping in the dark. It actually gives valuable information as to
identity, size, and location of
objects, which, without it, would be undetected because of distance or poor
Derivation of word "RADAR".
Let us digress for a minute to study the derivation of the word radar. We
know that radar uses
radio techniques, hence the first two letters RA. We know that it is used
in detection and this
gives us the letter D. In addition to detection, radar is useful in giving
the range of an object.
This then gives the last two letters AR, A for and, and R for ranging. If
we combine them we have:
|RA ||Radio equipment for
Information given by radar.
Up to this point, we have discussed the function of radar without
mentioning just how it
operates, other than remarking that it uses radio methods and techniques.
Now let us consider
what information it furnishes, and how it functions.
Radar gives the following information:
1. Presence of an object|
4. Position angle (angle of elevation) or altitude.
Radar operation consists of sending out a series of radio frequency (R.F.)
pulses from a high
power ultra-high-frequency radio transmitter. These pulses are directed
into a beam by a
directional antenna. When this beam strikes an object in its path, most the
R.F. energy will go
around the obstruction, and a small amount, depending on the size of the object, will be reflected
toward the sending antenna, the transmitter position there is a highly
sensitive receiver which will receive or detect the small amount returning R.F. energy. From the receiver the
|RADAR OPERATOR'S MANUAL|
returning pulse goes to the indicator where it can be observed.
In radar the reflected R.F. energy is called an echo. The presence of an
object is indicated by the
echo appearing on the indicator.
Since the echo returns to the antenna when it is pointed at the object,
it can be said that the
object must have the same direction, or bearing, as the antenna.
The range to an object can be determined by measuring the time it takes
for the pulse to go out
from the transmitter to the object and return. To avoid confusion, enough
time is provided
between pulses to allow an echo to return from the greatest distance at
which radar can be
expected to function.
Position angle is the angle above the horizontal at which a plane may be
seen. Since protection
against enemy aircraft is of great importance, some radars have been
adapted to give this
position angle as well as range and bearing. This has been accomplished
in several Navy sets
with sufficient dependability to permit full radar control of AA
As an operator you can learn to get the information previously mentioned
with relatively few
hours practice. A superior operator, however, can gain far more
information than this. He can
determine the composition of the target, including the number and type of
units involved. With
experience and a reasonable amount of practice you will soon be able to
recognize the difference
in appearance of the blip (radar indication) representing a surface
vessel and that representing
an aircraft. Presently, you will be noting differences in blips caused by
large ships as compared
with those caused by small ones, and can estimate the size of the ship
from the size of the blip
and the way it behaves. You will also be able to estimate the number of
planes or ships
producing blips on your screen.
Remember that you will learn how to do these things only by keeping your
eyes open and
actually trying to learn. The extra information gained can be of vital
importance to your ship.
You should not be disappointed because you are unable to establish such
data the first time that
you stand a radar watch, since this ability comes only from skill and
familiarity with your set.
However, you cannot use inexperience as an excuse for laxity. Excuses
will not save your ship;
your ability and experience can.
Importance of Radar.
It is imperative that you as a U. S. Navy radar
operator realize the importance of this super-weapon which you are about
to master. The
importance of the role which radar is playing in the present war can best
be set forth by
relating actual instances in which it proved beyond a doubt its superior
Battle of Britain. The Nazi Luftwaffe, intent on bombing England, was
itself defeated in part
through the use of radar. With it, the British beamed directly on Germany
and occupied Europe
and saw the enemy planes shortly after they rose from the ground. As the
approached, the RAF, at that time vastly outnumbered, was always at the
right place at the right
time to intercept them.
On a Sunday evening, January 17, 1943, the then mighty Luftwaffe appeared
in force over
London in reprisal for RAE raids on Berlin. There was a bright moon and
everything seemed to
be in their favor. Much to their surprise, however, the searchlights,
which previously on
similar occasions had scanned the skies in futile search action, now
followed the planes with
unerring precision. Radar was not only directing the few planes of the
RAF to the right spot at
the right time, but aiming the searchlights and guns as well.
Without radar, the Air Ministry has said, the Battle of Britain, one of
the greatest decisive
battles of all history, would have been lost. Radar helped turn the tide
of the war. In a sense, it
probably saved our entire civilization.
Battle of Midway. Another illustration of radar's effectiveness
occurred in the Battle of
Midway. A large Japanese force was approaching the island, presumably to
At the island there were several squadrons of land-based bombers. Near by
was the carrier
Yorktown. Without radar a continuous patrol depending on visible
detection would have been
necessary. With radar, the enemy was detected while still about a hundred
miles distant, and his
course plotted. The Japanese were first allowed to close in (thus saving
fuel); then the carrier
planes and the land-based bombers were sent to the attack. Directed by
the large radar stations
on the carrier and on Midway, our pilots went straight for their targets.
Thus our force, though
considerably outnumbered, was able to disperse and defeat the larger
Navy types of radar.
The fundamental principles of all radar sets are alike. However, radar
lends itself to many
different uses. Each use requires a different application of these
principles. In this section, the
different types of
|GENERAL RADAR PRINCIPLES|
Navy radar will be discussed. Every radar operator is to some extent a
specialist on certain
types of equipment. It is important that he should know his own apparatus
Search. Due to the great speeds with which enemy aircraft make their
approach, need for
detecting them in ample time has become only too evident. A large portion
of shipboard radar has
been designed for just this purpose. It picks up targets far at sea,
giving our own ships
sufficient time to prepare for immediate action. Reports from this radar
are given at regular
intervals to the combat information center (covered later in this book).
This makes possible the
rapid and accurate plotting of the enemy's course and speed. It is a most
valuable aid in sending
our own planes aloft to intercept the enemy.
There is also a need for detecting surface targets and securing knowledge
of their movements.
For this purpose every ship in the U. S. Navy has some type of surface
search radar aboard. This
equipment is not only invaluable for the location of the enemy task
force, but is equally
important in the location of surfaced submarines (it can detect even
their periscopes), or for
obtaining positions of ships in convoy.
Both surface and aircraft search radar are fundamentally alike. However,
since each has a
different task to perform, special consideration was given to the design
of their respective
antennas. In the days following the birth of this revolutionary weapon, a
group of technical
experts devoted a considerable amount of time to the development of the
antenna, realizing that
therein lay the means of obtaining more accurate target information. For
instance, it was found
that surface search radar, in order to do its job efficiently and
effectively, requires an antenna
beam width comparatively narrow in a horizontal plane, yet large enough
in the vertical plane
to compensate for the roll of the ship. On the other hand, long wave
aircraft search radar must
have a beam that is wide in the vertical plane because the aircraft
targets may be at any angle
with respect to the horizon. The horizontal beam width is also large
because of a necessary
compromise between the need for a narrow beam width, and at the same time
a reasonably small
Engineers and designers were also agreed that aircraft search equipment
should have a greater
range capability. This was necessary because of the rapid approach of the
Fire control. During the first year of the war,
radar was widely employed as a searching device. Even at that time there
were those who
thought it could be used equally well in controlling the fire of the
ship's guns. Before long, a fire
control radar was produced: radar that not only gave the direction and
distance of the enemy, but
also aimed the guns. This particular type of radar has been used
extensively and effectively in
the vast Pacific in night operations against the Japanese fleet, and it
did an equally fine job in
silencing the shore batteries at Casablanca.
Shipborne fire control radar, like search gear, is divided into two
general classes, namely, anti-ship and anti-aircraft. Here again, the main difference lies in the
antenna and beam it emits. An
anti-ship fire control antenna must provide a beam which is extremely
narrow in the
horizontal plane in order to obtain sharp bearing accuracy. This beam is
made somewhat wider
in the vertical to allow for the roll and pitch of own ship. Otherwise,
the beam might go over or
fall short of the target. An anti-aircraft
Figure 1-4. Disadvantage of a narrow beam.
|RADAR OPERATOR'S MANUAL|
fire control antenna must provide a beam that is very narrow in both
vertical and horizontal
planes; it must also be made so that it will elevate from horizon to
zenith (0 degrees to 90 degrees) as well as
in any direction on the horizon.
Special. As you progress in your study of radar, you will encounter
equipment that is entirely
portable and can easily be moved about from one location to another. This
especially valuable when a beachhead has been established, and it is used
to warn against both
aircraft and surface targets.
IFF (Identification, Friend or Foe) equipment is
a part of the radar in use today. This equipment, rather than being an
actual radar, is an aid to
radar. It has its own transmitter and receiver and answers the all
important question as to
whether the target is enemy or friendly.
Navy letter system.
In conclusion, it is important that you learn a little about your Navy's
way of naming the gear
with which you will soon work. There are many radars in the Fleet, each
doing its own
particular job, each having its own particular name.
First, there are two large divisions of radar, those used for searching
action, and those used for
fire control. To distinguish them, their first letter is always "S"
if the instrument is for search, and "F" if for
The second letter in the designation of search radars is usually an
indication of the model of one
particular type. For instance, a model SC radar is a search radar and is
older than an SK. An SO
model, on the other hand, is more recent than an SK, all, however, are
used for search. When a
modification is developed, the Navy uses numbers to designate the new
model; i.e., an SC-2 is a
modification of an SC.
All models and types of lire-control radar are now named by the Mark
system. This system is
employed by the Bureau of Ordnance and is used in connection with all
gunnery equipment. Some
of the earlier models of fire-control equipment were also known by the
letter system used on
search gear. For example, the Mark-3 radar was also referred to as FC;
the Mark-4 as the FD;
the Mark-8 as the FH, etc. Later fire-control models are known only by
their Mark number
such as the Mark-9, Mark-12, Mark-19, etc.
The identification of airborne radar follows the same general rule; i.e.,
in the case of ASE, "A"
is for aircraft, "S" for search, and "E" for the model of the equipment.
Other examples of
identification letters are the ASB, ASC, ASD, ASG, etc.
Usually, all types of recognition (IFF radar) equipment used with radar,
both airborne and
shipborne, have the letter B in their designation. Examples of these are
the BL or BK models.
Following this same system, the airborne model becomes the ABK. The
combinations of IFF
units are designated Mark IFF radars; i.e., Mark 3 IFF, Mark 4 IFF, etc.
Radars on board ship.
It should be mentioned that there are certain natural combinations of
search gear from the
standpoint of the functions the sets serve and the ships which carry
them. The SG (surface
search) and SC or 5K (air search) always go together. These sets are
designed for combatant
ships of DD size or larger. The SL (surface search) and SA (air search)
also go together. These
sets are designed for ships of the DE class. Another group of sets
consists of the SO and SF (the
SL can also be included). These sets are all for surface search and one
is used on small ships and
auxiliaries, such as PT's, PC's, SC's, AK's. and AT's, which do not carry
an air search radar. You
will be expected to become an expert operator of one of these
Importance of the radarman.
Since radar does let you "look" through fog and
smoke and darkness, you find that it is a great help not only for
detecting the enemy, but also for
locating your own vessels and for warning you of nearby rocks, islands,
icebergs, and similar
objects. It is most important for the safety of your ship that you have
this information. Of
course, if your radar is operated carelessly, you cannot expect it to
give good results. If you fail
to do your task well, fail to notice instantly the indication of an enemy
ship or of a rock, you
may well be responsible for the sinking of your own ship. Radar is
capable of performing its
task well, but only if there is an efficient, alert operator at the
controls. Yours is indeed a big
responsibility, and the safety of your ship and shipmates depends on how
well you do your job.
Keep this fact before you every minute you are on duty.
You as a radar operator, are the first aboard to know of the enemy's
presence, his strength, and
|GENERAL RADAR PRINCIPLES|
location. Before your Captain can begin to maneuver the ship, before the
gunnery officer can
give the command to fire, you must pass on your radar information to them.
The information you receive is of no use whatsoever if you fail to relay to the proper place the
information your radar gives you. This equipment is not installed merely
for the purpose of
satisfying your curiosity. Remember that information, in order to be used
effectively, must be
received by those with authority to act on it. Your duty is to see that
those in command get the
information they need. They are depending on you. Remember this: what you
tell them or fail to
tell them may determine the fate of your ship.
The more familiar you become with your equipment, the more you will
realize the importance
of keeping what you know to yourself. You are being entrusted with a
vital military secret when
you learn about radar. It is imperative that you keep it a secret. Stop
and consider what radar
does for you in guarding your ship and in helping to protect your life
and the lives of your
shipmates. This miracle weapon is largely on our side. That is where it
will remain if security functions. Keep what you learn to yourself!
BASIC PRINCIPLES OF RADAR
Before beginning with the basic principles of radar there are a few
terms, symbols, and
abbreviations that you should learn in order to help you understand the
material that follows.
The word cycle is familiar to most of us, occurring in such expressions as
"vicious cycle," "cycle of prosperity," "cycle of life," etc.
The cycle is one complete series of events at the end of which conditions
are back at the starting
point. Beginning with any particular phase or condition, one cycle is
completed as soon as it
starts repeating itself.
|RADAR OPERATOR'S MANUAL|
Frequency means only how often something is done,
as, for example, the frequency of eating, which should be three times a
day. The frequency of
your heartbeat is about 72 beats per minute. In general, frequency
indicates the number of
times something occurs in a certain period of time. Frequency in radio is
the number of cycles
per second, or the rate at which the cycles occur.
Since these cycles occur regularly, there must be a definite time
required for each cycle. This
time is known as the period. Remember that it is a measure of time required for
one cycle to occur.
Hence it is reasonable to expect the time for each cycle to decrease when
the number of cycles
per second increases.
If one second is divided into one hundred equal parts, each part will be
one one-hundredth of a
second long. The period is 1/100-second when the frequency is 100 c.p.s.
(cycles per second).
Now, note this carefully: The period is the same as one divided by the
Another term that will help to make our discussion
simpler is wave length. This is the actual distance
traveled by the energy while it is completing one cycle.
You are familiar with the fact that as long as you travel at a definite
speed, you can cover a
distance proportional to the time. In other words, if you travel at a
speed of 30 miles an hour
for one hour you go 30 miles; if you travel for one-half hour, you go
only half as far; if you
travel fur two hours, you go twice as far, etc. The distance increased
with the time or,
as expressed before, distance is proportional to the time. The distance
you can go in a certain
time is equal to the speed, or velocity, multiplied by the time. Radio
energy too, can travel a
certain distance in a given period (i.e., in a definite amount of rime).
This distance is called the
wave length. It is a measure of length, just as feet and inches and yards
and miles are measures
of length. Using the rule: distance equals velocity multiplied by time,
you find that
wave length (distance) = velocity (speed) x period (time).
Usually, you will not know the period, but this information is not
necessary for finding the
frequency or wave length. Therefore, a formula which gives the wave
length when the period is
not known will be better for our purpose. Remember that the period equals
1 divided by the
frequency: P = 1/f. Substituting 1/f for its equal P the new formula will
wave length = velocity x 1/frequency, or WL = V/F
this is the usual formula used in calculating the wave length.
Radar energy travels at a velocity (or speed), of 162,000 nautical miles
a second. If frequency
is 2,000 cycles per second, what is the period, and what, then, is the
Frequency (f) = 2,000
Therefore the period 1/f = 1/2,000 second = 1,000,000/2,000 microseconds = 500 microseconds.
|Abbreviation or symbol ||Meaning ||Example or equivalent
|Kilo ||Prefix indicating 1,000 ||10 Kilocycles = 10,000 cycles
|Mega ||Prefix indicating 1,000,000 ||10 Megacycles = 10,000,000 cycles
|Micro (u) ||Prefix indicating one millionth ||10u sec = 10 microseconds 10/1,000,000 sec.
|Milli ||Prefix indicating one thousandth ||1 ma = 1 millampere = 1/1,000 ampere
|V,S ||Velocity, Speed ||
|V,S,C ||Speed of light or radio waves ||300,000,000 meters per sec.
162,000 nautical miles per sec.
|˜ ||Cycle per second ||100˜ = 100 cycles per sec.
|o ||Degree ||360o = 360 degrees
|F,f ||Frequency ||
|λ (lambda) ||Wave length ||
|GENERAL RADAR PRINCIPLES|
Note: 1 second = 1,000,000 microseconds.
The wave length = velocity/frequency.
162,000/2,000 = 81 miles.
Thus you see that the energy travels 81 miles while it goes through one
You have learned that radar may be used to determine the range and
bearing of a target. Range is
the distance of the target from you. Bearing is the direction of the
The table at the bottom of page 1-10 gives the most common symbols and
you may encounter in reading the various radar publications,
Radar equipment operates on many frequencies. some of which are just
being explored today. To
safeguard this system, operating frequencies or wavelengths are
classified and described only
by the following code:
|Frequency in megacycles ||Code
|0 to 300 ||P
|300 to 1,500 ||L
|1,500 to 5,000 ||S
|5,000 to 10,000 ||X
|10,000 and above ||K
|1 inch ||= 2.54 centimeters
|1 yard ||= 36 inches
|1 meter ||= 100 centimeters
|1 meter ||= 39.37 inches
|1 meter ||= 1.09 yards
|1 nautical mile ||= 2,000 yards (approx.)
|1 statute mile||= 1,760 yards
Principle of pulse reflection.
Now that you know something of the importance of radar, its various types
and their uses, and
have built up a vocabulary of terms frequently used, let us consider next
just how this
equipment functions in securing information.
If you understand the principle of sound echoes you have mastered one of
understandings of radar. Suppose that you are in a canyon and that some
distance from you is one
of the walls of this canyon. You shout loudly-then wait. What happens?
The shout comes back in
the form of an echo. Why? It is simply that the sound wave from your
voice travel through the
air, hit the wall of the canyon, and bounce back. You hear the reflected
sound wave. If you want
to hear it distinctly you do not shout continuously, but utter one brief
sound and then maintain
silence until the echo returns.
By shouting for a short interval and then waiting, it is possible for you
to shout loudly again the
next time. In other words, you are sending out pulses of energy of short
time duration, thereby
making it possible for you to shout at maximum strength without straining
This is the basic principle of echo ranging: sending out brief pulses of
energy and measuring the
it takes them to return.
SENDING ENERGY ....... TIME TO RETURN
Figure 1-7 Timing echoes.
|RADAR OPERATOR'S MANUAL|
Radar also works on the principle of pulse reflection. A strong pulse of radio energy is sent out into space from the
radar. If there is a target
such as a ship or aircraft in the path of this radio energy, some of the
radio waves upon striking
the target rebound just as sound waves do, and produce an echo. This echo
which is not heard,
but seen on a special device called the cathode-ray tube, is called a
blip or a pip.
In radar we deal not with sound waves, but with radio waves. In
shouting, the sound waves are
produced by your vocal cords. In radar the radio waves are produced by a
unit called the
transmitter. This radar transmitter is turned on for a very short period,
so short that the time
is measured in microseconds (millionths of a second). This short time
during which the radar is
sending out radio energy is called the pulse width, or pulse duration.
During this short time the
transmitter produces a maximum amount of energy.
After the transmitter is turned off, there is a definite amount of time
microseconds) before it is turned on again. This is called the rest
period. It is during this rest
period that the echo returns, if there is a target in the path of the
radio waves. This rest period
must be long enough to allow time for an echo to return before another
pulse of energy is sent
out. Accordingly, we have the pulse width during which the strong radio
signal is sent out into
space, and the rest period during which time the echo returns. The
transmitter is also turned on
a certain number of times a second, thus setting up a pulse repetition
Suppose that there are several large cliffs or walls in different
directions from you in a canyon
in which you are shouting, and that you desire to receive an echo from
one certain cliff. By
shouting and sending the sound waves in all directions, you have no way
of telling which wall is
sending the echo back to you; perhaps they are all sending back a small
amount of the energy. By
placing a megaphone to your mouth, or by cupping your hands around your
mouth, it is possible
to direct most of the sound waves in any desired direction. If you desire
to receive an echo from
one certain wall or cliff, you point the megaphone in the direction of
that cliff and shout into it.
The sound waves are concentrated into a narrow beam, go out, hit the
cliff, and are reflected
back from the same direction. Since you know the direction
in which you have sent the wave, you know the direction from which
the reflected waves come.
In radar the radio waves are concentrated into a narrow beam by a special
antenna which will be
described later. This narrow beam is much the same as the concentrated
beam of light a
searchlight sends out. Just as you can point a megaphone or searchlight,
you are also able to
point the antenna and direct the narrow beam of radio waves in any
desired direction. If now you
receive an echo while the antenna is pointing in a certain direction, you
know the target is in
that direction. Radar works the same at night or during bad weather as it
does in daytime or
during good weather. Accordingly, it is possible to detect a target with
radar when it is
impossible to see it with optical equipment. By turning the antenna
through a complete circle or
360 degrees around you, it is possible to detect any target. You will know the
direction or bearing of
this target by knowing the direction the antenna is facing.
Range determination by sound.
Detecting the target and knowing its bearing are not enough. You must
also know how far it is
from you, or the range of the target. By knowing both the bearing and
range of a target, you
locate the target exactly.
The following analogy will help to make the concept of range clear. If a
stone is dropped into a
pool, a small wave will start out from where the stone hit. This wave
spreads out in a circle in
all directions. If there is a pole or piling in the pool a short distance
from where the stone is
dropped, the wave going out will hit the piling and a reflected wave will
start back. Assume for
the sake of explanation that the wave is traveling at a speed of one foot
per second through the
water. If you start a stop watch when the stone hits the water and note
how many seconds elapse
before the reflected wave returns to the starting point, you can easily
tell how far away the
piling is. For example, if it takes eight seconds for the wave to go out
and return, then the
distance traveled is eight feet. The range, which is the distance out to
the piling, will be one-half of the total distance, or four feet.
A special stop watch could he devised with the face marked off in feet
instead of seconds. The dial
would read distance instead of time. For example, in place of one, two,
three seconds, and so on,
the face would read one, two, three feet, etc. Better yet, since we are
interested only in the
distance to the target, the face
|GENERAL RADAR PRINCIPLES|
of the watch could read one-half foot in place of one
second, one foot in place of two seconds, etc. Thus, at the instant the
wave returned to the
starting place, you could either note where the hand of the stop watch
was, or you could stop the
watch and read the exact range out to the piling.
What you actually do is measure the elapsed time from the instant the
stone hits the water until
the reflected wave returns to the starting place. Knowing the speed of
the moving wave, you
multiply the time by the speed. in order to compute the distance. As
range is only half the
distance traveled, you divide the distance by two and obtain the range.
Put in the form of a formula, R = (s x t)/2 where R is range, s is
speed, and t is time.
Let us go back to the example of sound echoing in a canyon. Suppose you
want to know how far
away that cliff (i.e., its range) which sent back the echo is. You need
to know that the speed of
sound is approximately 1,100 feet per second, and you also need a watch,
or better still, a
stopwatch. It is now an easy matter to obtain the range. If, for
instance, four seconds pass from
the time you shout in this canyon until you receive the echo, you know
that the distance traveled
by the sound waves out to the cliff and back is 4 x 1,100 feet or 4,400
feet. You know that
range is one-half the distance out and back: therefore
the range is (4 x 1,000)/2 or 2,200 feet. Since you
divided by two, why not divide by two at the start, and make a statement
that for sound, range
equals 550 feet multiplied by time? Thus, it would he 550 x
or 2,200 feet.
On your stop watch, you could make a special face to measure range in
terms of echoes. Since for
sound one second is equal to 550 feet range, in place of one second on
the watch have 550 feet, in
place of two seconds have 1,100 feet, etc. Thus, whenever the echo
returns, it is possible to
note where the second hand is at that instant on the face of the watch
and to read directly the
range of the target.
Range determination by radar.
How does all this fit in with radar? Let us next see how see are able to
tell how far away the
target is, or how to obtain the range with radar.
Light travels so fast that it is almost instantaneous. Radio waves,
electricity, and light, all
travel at about the same speed, which is 186,000 land miles per second,
or almost seven and
one-half times around the earth in one second. Expressing this in
(a nautical mile is approximately 2,000 yards): radio waves travel
162,000 nautical miles in
one second, or 300,000,000 meters in one second. Knowing the speed of
radio waves, it is
possible to obtain the range of a target with radar by the same method as
that used in the case of sound.
For example, suppose that the transmitter sent out a short pulse of radio
energy and that the
reflected wave was received after 1,000 microseconds. The distance
traveled out to the target
and back is 1,000/*1,000,000 x 162,000 = 162 nautical miles. Referring to the sound analogy, remember
that in order to
compute the range, the total distance out and back must he divided by
two. So also with radar;
the total distance must be divided by two. In this case range is 162/2 or 81 nautical miles.
However, the speed of radio waves is much greater than that of sound.
Therefore, an ordinary
stop watch cannot be used. A special timing device is needed to measure
such small time
intervals as microseconds. This special timing device is called the
cathode-ray tube, and on it
there is a special time base which takes the place of a second hand on
the stop watch. Instead of
being marked off in divisions of one, two, three microseconds, etc., this
time base can be
marked off directly in miles or yards of range.
By sending out a very short pulse of energy from a high powered
transmitter, and receiving the
echo which is called the pip, you have detected a target. By knowing the
direction the antenna is
facing, you know the direction or bearing of the target. By measuring the
time it takes the wave
to go out to a target and return, you have a means of obtaining the range
of the target. If you know
the bearing and range of a target, you then know the exact position of
this target at any instant.
In good weather or bad, in daytime or at night, whether surface craft or
aircraft, you are able to
establish the desired data. But these are only some of the many things
you are able to tell about
the target. In the pages which follow you will learn about other
information which a good
operator can get through the medium of radar.
MAIN PARTS OF A RADAR SYSTEM
In the preceding section you have learned about the principles of radar.
This section will deal with
* The 1,000 microseconds must be divided by 1,000,000 to give seconds.
|RADAR OPERATOR'S MANUAL|
the main parts of a radar set. If you are familiar with the several units
that go to make up the
radar set you operate, the chances are that you will be able to operate
it more skillfully and
intelligently. In studying the derivation of the word radar you found
that the first two letters ra
came from the word radio. You know that in radio there must be a
transmitter and antenna
system to send our the program; to reproduce the program there must be an
receiver, and loudspeaker. Likewise in radar the main parts are the
system, receiver, and indicator which functions in radar in the same
manner that the
loudspeaker functions in radio.
Wave length or frequency. To make radio detection and ranging
possible, it is necessary at the
outset to send out a pulse of radio waves. It is the function of the
transmitter to generate the
pulse. Since the transmitter creates the pulse, it is the source of the
radio energy. There are
well defined differences between the radar transmitter and the radio
transmitter that operates
in the radio station. One of the main differences is in wave bands on
which the two types of
transmitters operate. Radio stations operate on either the broadcast or
the short wave bands,
but radar uses the ultra (very) short waves that were previously used
only for experimental
purposes. Another difference is that radio broadcast transmission is
continuous while radar
transmission is intermittent.
Duration and power of pulse. An important point to remember about the
transmitter is its ability to send out pulses of radio energy as powerful
as, and in many cases
more powerful, than the transmissions from the biggest broadcast
transmitters. High power is
necessary in radio transmissions to carry the broadcasts to the listeners
at distant points. In
radar very strong pulses must be emitted in order to get back even a
small echo or reflection
from the waves striking the target while the rest of the waves continue
into space until they die
out. Unfortunately, only a minute amount of the energy of the pulse sent
out is bounced back. In
generating the strong pulses that are needed, high voltages, dangerous to
life, are required.
Everything possible, however, has been done to make the equipment safe
for the operator, so
long as certain safety precautions are observed.
In the section on the principles of radar, the point was stressed that
short duration pulses
rather than continuous transmission were used in order to provide time
for the echo to return.
Continuous operation would drown out all reflections or echoes. The
actual time the transmitter
is sending out radio waves is measured in very small units of time, micro
When you read the expression, pulse width, banish any and all thought of
expressing this value
in feet, inches, or meters, because the pulse width is a measurement of
the time the transmitter
is working. At first, it may be bewildering to consider time reckoned in
denominations, but later on it will serve as a constant reminder of the
importance of speed in
all phases of radar.
One advantage that results from the transmitter's working only in brief
periods is the long rest
or idle period during which sufficient electrical energy can be stored up
to provide the
extremely high power necessary for the next pulse. Thus the overall or
average power output of
the transmitter is low, and within the transmitter's capacity. During the
brief moment of
transmission, considerable heat is generated by the tubes. During the
rest period the tubes cool.
Should the transmitter be allowed to operate continuously at such high
power the unit would be
badly damaged or even destroyed by the intense heat. Motor-driven fans or
cool air inside the cabinet of the radar set to aid in keeping the
temperatures at safe operating
levels. The length of the rest period (expressed in microseconds) is
dependent upon the pulse
interval (the time between the beginning of one pulse and the start of
the next one) and the
pulse width (the working time).
Pulse rate. The keyer unit performs the task of pulsing the transmitter.
The keyer, which will
be described later in this section, does exactly what its name implies,
it keys the transmitter,
turning it on for an instantaneous surge of radio energy for a few
microseconds (or even a
fraction of a microsecond); then, after the pulse, it turns it off for a
comparatively long time
until the next pulse. It is during this period while the transmitter is
the echoes return from any object that was in the path of the outgoing
pulse. The rate at which
the keyer pulses the transmitter is the pulse rate or pulse repetition
rate, which simply means
the number of times the transmitter is sending out a pulse of radio waves
each second. The keyer
operates at a constant rate, spacing the pulses so that the interval
between any two is always the
same. The length of time of
|GENERAL RADAR PRINCIPLES|
Figure 1-8. Block diagram of a typical radar system.
|RADAR OPERATOR'S MANUAL|
the pulse interval is a direct result of the pulse rate. For example, if
a transmitter sends 50
pulses at regularly spaced intervals in one second's time, the repetition
rate of the unit would
be termed 50 cycles per second (a cycle being one complete operation). To
calculate the interval
between each pulse, divide one second (one million microseconds) by the
1,000,000/50 = 20,000 microseconds-the pulse interval. (Assume a pulse
width of 10
microseconds.) 20,000 microseconds - 10 microseconds = 19,900 microseconds, the rest period. Note the extremely long rest period!
The antenna system.
The antenna system is one of the most important parts of the radar
equipment since it radiates
the radio frequency energy into space and receives the reflected energy
or echo. The antenna
system is made up of two parts: (1) the transmission line, and (2) the
Transmission line. The purpose of the transmission line is to carry the
high frequency energy
from the transmitter to the antenna, and to carry the reflected energy
from the antenna to the
receiver. This transfer of energy must he done with a minimum of loss.
Two types of transmission lines are in general use
in radar equipment: the concentric or coaxial line,
and the hollow wave guide. The coaxial line consist of one conductor
inside another. Both
conductors may be tubular, or the outside one may be a hollow tube and
the inside a solid
conductor. Since the inner conductor must be exactly in the center of the
outer conductor, a
great number of insulating disks are required. Moisture on these
insulators may cause a flash-over or breakdown between the two conductors so it is necessary to keep
the line filled with
nitrogen or dry air at a pressure of about five pounds per square inch.
The wave guide is a
hollow metal pipe, the cross section may be rectangular or circular. The
rectangular wave guide
is most commonly used today.
Now the question arises, "Why have two types of
transmission lines?" The reason for this is that coaxial lines are more
suitable for radars
operating below 3,000,000,000 cycles while wave guides are better for
radars at and above
When comparing the coaxial line with the wave guide, the following
advantages seem to favor the
wave guide: (1) construction is simpler. (2) losses are lower, and (3)
power capacity is
The wave guide's principal disadvantage is that unless the frequency is
very high, the size of the
pipe must be unreasonably large.
Antenna: an emitter of radio waves. In order to utilize the radio energy
created by the
transmitter pulse, it must be converted into radio waves, which can be
shot out to strike any
object in the outgoing path. The antenna functions as the converter of
the radio energy into radio
waves which can be radiated in any desired direction into the atmosphere.
To better illustrate
this fact, consider the situation resulting when a flashlight is supplied
with new batteries but
lacks a bulb. Even though the necessary power is available, the means of
using it are lacking. As
soon as you insert a bulb in the socket and close the switch, the battery
energy is put to work
and its conversion to light waves results in a beam of light. The
transmitter can be likened to
the battery-filled flashlight, for it, too, is a source of stored
energy-radio energy. The
transmitter is inoperative without an antenna for the same reason that
the flashlight is
inoperative without a bulb. The antenna converts the pulse of radio
energy from the transmitter
into radio waves, just as the bulb converts the battery energy to light
The fundamental element from which most antennas are built is the
half-wave antenna, or
dipole. This is a metal rod or wire one-half wave length long. Since the
velocity of radio waves
is constant, 300,000,000 meters per second, the length of a
Figure 1-9 Coaxial. Wave-guide.
|GENERAL RADAR PRINCIPLES|
|RADAR OPERATOR'S MANUAL|
dipole gets shorter as the frequency increases. This is another
interpretation for the formula;
wave length equals the velocity of a radio wave divided by the frequency
of a radio wave.
Therefore, a given antenna works best at only one frequency.
Non-directional antenna. A single dipole will send its energy out in all
directions around itself.
The greatest amount of power goes directly outward at right angles to the
length of the rod with
decreasing amounts of energy out in other directions except in the
direction of the rod, where no
energy is sent out. Consequently, the side view of the lobe looks like a
pair of circles touching
the dipole, as shown in figure 1-11.
A target at right angles to this dipole would give a strong echo, while
one in the actual direction
of the dipole would give a very weak echo (theoretically no echo). One of
these dipoles mounted
vertically would send a strong signal toward anything around it and on
its level. It would give a
weaker signal to anything above it. Some of our sets use antennas like
this so that any target
around them, regardless of its bearing, can be detected. Because the
radio waves are going out in
all directions, bearing cannot be indicated and only range can be
Directional antenna. A reflector can be used with a radar antenna to make
it unidirectional, that
is, to cause all the waves to leave the antenna in one direction rather
than in two directions.
This reflector serves the same purpose, and functions in the same manner
as the reflector used
in a flashlight. The four most popular types of reflectors are: (1) the
flat or "bedspring" type,
(2) paraboloidal or "dish pan", (3) parabolic or "barrel stave", and (4)
The type of radar antenna using a number of dipoles, called a curtain
array, or bedspring array,
has a metal screen reflector. The dipoles are a definite distance forward
of the screen. The more
dipoles used, the sharper the beam. However, the energy is strongest in a
approximately at right angles to the screen. The metal reflector is
perforated to reduce
Figure 1-11. Dipole and radiation pattern.
weight and wind resistance, or it may be merely a wire mesh.
If the antenna is wide, you can expect a narrow horizontal lobe because
there will be several
dipoles horizontally. If there are many dipoles vertically, the
Figure 1-12. Wave lengths.
vertical lobe will be sharp. Of course, the physical size (in feet and
inches) of any particular
lobe width will depend on the wave length. When comparing antenna sizes,
be sure to measure
them in square wave lengths.
As an example, if you have an antenna like the one shown in figure 1-12
you can measure the
area easily by noting that each rod is one-half wave length long, and
that each row of rods is one-half wave length
Figure 1-13. Wave lengths.
away from the next. This antenna, then, is three wave lengths broad, one
wave length high, and
has an area of three square wave lengths.
The antenna in figure 1-13, though physically smaller, is four wave
lengths wide, two wave
lengths high, and has an area of eight square wave lengths-almost three
times as great as the
As a general rule, a sharp vertical lobe is not desirable for search sets
because the lobe might
shoot over the target and miss it as the ship rolls. If this happened, an
echo would he reflected
from the target only occasionally, instead of almost every time the
|GENERAL RADAR PRINCIPLES|
antenna completes a rotation. To avoid this, only a few dipoles are
stacked vertically. Remember
that the greater the horizontal dimension of the antenna (when measured
in wave lengths), the
sharper the horizontal lobe; the greater the vertical dimension of the
antenna, the sharper the
Fire-control sets must give accurate bearings, and (if used to control
accurate position angles. By reducing the wave length (increasing the
frequency), the antenna
necessary for this bearing accuracy can be reduced until its size is
physically practical. If,
also, the reflector is bent into a semi-parabola, the sharpness of the
vertical lobe can he
increased. From the side, this antenna has the appearance of a "V" with a
rounded bottom, the "V"
being on its side with the wide opening toward the target.
This antenna is not entirely satisfactory for anti-aircraft fire control.
Being a wide antenna (in
wave lengths), it gives satisfactory bearing accuracy, but lacks
sufficient position angle
accuracy for AA gun laying. To increase this accuracy, a modification has
been made. In effect,
two of these semi-parabolic antennas have been fastened together, one
atop the other. In this
way, the sharpness of the vertical lobe
has been increased. The antenna shape is shown in figure 1-14
As you go to higher frequencies, dipoles and curtains are of less
concern. Reflectors of the type
used with searchlights can be employed. This is due to the fact that
radio waves at the higher
radar frequencies behave much like light.
Paraboloidal reflectors, bowl-shaped, are usually called "dishpan"
reflectors, or "spinners".
They are used for surface-search and some air-search and fire-control
sets. Their hearing
accuracy depends on the diameter of the spinner measured in wave lengths
(the unit of measure
used for other types of antennas).
Since this type of antenna produces a "pencil" beam, the beam is as
narrow in the vertical plane
as it is in the horizontal, which for some types of radar necessitates a
helical search (a spiral
search, changing the position angle as well as the bearing angle).
On some sets, a sharp, vertical lobe is a definite disadvantage. If a
large spinner is used (for
good bearing accuracy), the vertical lobe may be too narrow. To increase
the vertical beam
width, the rep and the bottom parts of the spinner are cut off, which
reduces the vertical size of
the spinner without reducing the
Figure 1-14. Mark 4 antenna.
|RADAR OPERATOR'S MANUAL|
width, thus increasing the width of the vertical lobe without affecting
the horizontal beam
width. This type is called the parabolic reflector and, since it
resembles a barrel stave, is also
known as a barrel slate reflector. Many of our surface search radar sets
use this antenna.
Another type of antenna involves an entirely different principle. Tapered
plastic rods about
three feet long are used, and the energy comes out along the full length
of a given rod. A rod by
itself will produce a beam about 30 degrees wide. By placing 14 of them side by
side, the beam is
narrowed to 2 degrees. Three vertical rows are used to narrow the beam to 6 degrees in
the vertical plane.
When these rods are energized at different times, the lobe goes out to
one side-toward the side
which received the energy last. Constantly changing the amount of delay
causes the lobe to move
steadily from 15 degrees to the left of the center line to 15 degrees to the right in 1/10 of a second. The great
advantage of this system is that 30 degrees can be seen at once, with the
accuracy of a 2 degrees beam
(similar to television scanning). This permits accurate spotting in both
range and deflection.
Such an antenna, however, is extremely heavy and complicated.
Figure 1-15. Parabolic or barrel stave antenna.
The foregoing discussion of antennas has avoided technical treatment of
the subject because for
purposes of this handbook only general ideas are needed. It will he
helpful to keep the following
1. The antenna size, in wave lengths, determines the size and shape of
2. A wide antenna gives a narrow, sharp horizontal lobe, while a narrow antenna produces a broad horizontal lobe and poor bearing accuracy.
3. Use of high frequencies permits use of parabolic
reflectors, which give narrow beam widths, presenting better
4. The wave length is the unit used in rating antenna size.
How does radar determine bearing?
You have already learned how radar detects the presence of a target and
determines range. In
this section you will discover just how it establishes the direction, or
bearing and position angle.
When you shout in the direction of a cliff or big building you hear an
echo, but when there is no
cliff or building to reflect the energy there is no echo. If the radar
energy, like sound, is sent
out in one general direction, you can tell approximately the direction of
an object by simply
observing the direction from which the echo returns. By knowing this
direction, you know the
target's direction since it is the same.
When you shout, the sound as you know, does not go only straight out, but
can also he heard to
either side of the direction you are facing. The degree to which the
energy is scattered will
determine how accurately you can judge from the echo whether or not you
are facing the cliff. If
the sound energy is scattered over a wide angle, perhaps you can receive
an echo when you are
facing, and shouting, in a direction far off to one side of the target. A
small amount of the energy
goes off in the direction of the target, and will he reflected as an
echo, but it will be a weak echo.
You are probably wondering now how you can tell whether you are getting
the echo back from
the direction you are facing, rather than from a direction off to one
side? The answer to this is
that you get the largest echo when you are facing the target directly.
Concept of lobe. Your radar set sends out its energy like sound in a
general direction, but with
some scattering. You receive radar echoes from a target even if you are
not pointing the antenna
in the exact direction thereof. But, as in the case of sound, you get the
strongest echo when you
are directly on the target.
Since you get the strongest echo from an object when the antenna is
pointed directly at it, you
can reasonably expect to locate a small target in that direction more
easily than in any other
direction. Likewise, you can detect an echo from a ship at a greater
range in the direction the
antenna is pointing than in any other direction. It is logical to expect
that a ship will reflect a
stronger echo when it is only slightly off the antenna hearing than when
it is considerably
|GENERAL RADAR PRINCIPLES|
off the antenna hearing for if a ship lies to one side of the antenna
bearing it cannot give as
strong an echo as one directly on the antenna bearing, unless it happens
to be nearer or larger.
In fact, if a ship sails around your antenna hearing in such a way as to
always produce the same
strength echo, it will follow the path shown in figure 1-16. This shape
is called a lobe, and is
actually a representation of the range at which you can get an echo of
any particular size in any
direction from the antenna. It represents approximately the amount of
power sent out in any
direction. Figure 1-19 shows how great a change in echo height results
simply from turning
the antenna around, sweeping the lobe past the target.
"E" units. Referring to the actual height of the echo in inches is of
little value because you can
increase or decrease the height of the echo at will merely by varying the
gain control. Even a
weak signal can easily be "blown up" in this manner until it approaches
saturation, but this
increase may not help because of the corresponding increase in grass
Grass is a disturbance on the cathode, ray tube (C.R.T.) screen, caused
by noise in the tubes,
static, etc. It shows up as a fuzzy, jumpy fur along the time base on "A"
and "R" scopes, and as
many small, bright spots (sometimes called snow) on the B and PPI scopes.
It is always present
to some degree, and pips smaller than the grass are very difficult to
find. The grass seems to
wave just as actual grass does, and may cover the pip. The height of this
grass can be controlled
by the receiver sensitivity (or gain) control low sensitivity means low
grass height and high
sensitivity means a large amount of grass. You know that the pip height
can he made larger by
increasing the gain, and smaller by reducing the gain. Since both the pip
height and the grass
height vary together,
the pip size can he compared to the height of the grass for this
discussion. The comparison is
made in units.
The "E" unit system of discussing echo strength is standard in the Navy.
It is a convenient
system to use; by providing common terms in which to discuss the strength
of echoes confusion
and misunderstanding are reduced. A small pip about the same height as
the grass is an E-1 echo.
It will always he difficult to detect, and only a wide-awake operator
will notice it. An echo of
this strength will probably he overlooked on a PPI scope by even the very
best operator. That is
the main reason that an operator should not devote all his attention to
the PPI scope.
The complete E system includes five values: E-1, E-2, E-3, E-4, and E-5.
E-1, as is indicated
above, includes those signals whose ratio of signal to noise, or of pip
height to grass height is
one to one (i.e., the pip and grass are the same height). An E-2 echo is
an echo whose pip is
twice as high as the grass. An E-3 echo is one producing a pip four times
as high as the grass. If
the pip is eight times as tall as the grass, we say we have an E-4 echo.
An exceptionally great
echo reaching saturation, or 16 times, or more the height of the grass,
is spoken of as an E-5
An E-1 echo is very weak, an E-2 echo is weak, and F-3 echo is good, and
E-4 echo is strong,
and an E-5 echo is very strong. The F-5 echoes are the echoes we get from
large, nearby targets.
The E-number system enables you to report echo strength in a definite
way. You can also use the
E-number system for labeling lobes which, as you know, are
representations of the range at
which you can get an echo of any particular size in any direction from
the antenna. If lobes were
drawn to represent all echoes for a particular size target from E-1 (very
weak-) to E-5 (very
strong) they would appear as
|RADAR OPERATOR'S MANUAL|
shown in figure 18. This picture is only for one particular size of
target. A larger target would
have lobes that extend out farther, while a smaller target would not have
any lobes as far out as
the E-1 lobe in our illustration. What may he an E-1 lobe for a destroyer
might be an E-3 lobe
for a battleship. Therefore, you could draw a series of the same picture
for different types of
The closer the ship is to you, the larger the blip produced, provided
that it stays in any one
direction from the lobe center. Also, for any range, the size of the
returned echo (and thus the
blip) is smaller as the direction of the target gets farther and farther
the direction of the lobe. As the target ship steams toward the DL.
(direction of the lobe) the
opposite will happen. The blip will get bigger and bigger, reaching its
greatest size when the
antenna is pointing the lobe directly at the ship.
Maximum echo method. Knowing the D.L., you have a way of telling just
where the ship is.
Simply turning the antenna until you get the largest blip or the
brightest spot, will give you the
direction of the target.
Now, let us repeat the procedure in a brief, summarized form. As the
target comes steaming in,
keeping always at a certain angle with the D.L., the signal
Figure 1-17. "E" unit system for measuring echo.
|GENERAL RADAR PRINCIPLES|
gets bigger and bigger. (The item of fades which might enter here will
he explained later.) The
target is changing range without changing its direction from the D.L.
This is exactly what is to
he expected, because, going hack to the analogy of sound echoes, you know
that the nearer you
are to the cliff, the louder the echo you will hear. Similarly, the
nearer a radar target, the
greater the echo received by the radar set.
You cannot affect the range of the target, and hence are unable to do
much to increase the height
of the blip in this way, while keeping the target on a certain angle with
the DL., but if you swing
the antenna around a larger echo might come hack to you. lithe target
steams along, keeping at
the same range, but moving nearer and nearer the D.L., you know that an
increasing amount of
the energy is hitting it, resulting in a bigger and bigger echo. Conversely, as soon as
it begins to move away from the D.L., the echo begins, to grow weaker. As
the target crosses, so
that its direction coincides with the D.L., the biggest echo possible from
that target and at that
particular range is received. This is very convenient, because, as we
have said before, you can
determine the direction of the target by just turning your antenna around
until you get the
biggest blip or brightest spot. You know where you have "aimed" the lobe,
so if you set the
antenna for the maximum echo, you know the direction of the target.
This method of setting the antenna (which determines the direction of the
lobe), to find the
bearing of a target is called the maximum echo method, since you read
the direction from which
you get the maximum echo. It is the easiest, and consequently the most
frequently used method.
Figure 1-18. Lobe and corresponding echo height.
|RADAR OPERATOR'S MANUAL|
Figure 1-19. Determining correct bearing by echo height.
Accuracy consideration. Let us look at the picture again, noticing
especially the nose of the
lobe, which is rather flat. That flatness is significant to a radar
operator, because it causes
greater difficulty in getting accurate bearings. Perhaps you are wondering why this should be the case. To begin with, in
this case accuracy depends on effecting a big change
in what you are looking at, through just a small change in what you are
adjusting. If the blip
height is what you are looking at (or what you are using to determine
when you are on the
target), and you are training the lobe in the direction of the target,
you can get the bearing of
the target accurately if just
a small change in setting the antenna gives you a big change in blip
height. When you are on the
target, you will know it is time to read your bearing indicator. You will
then know that the
target is on this bearing.
Minimum echo method. Look at the lobe again. Where can you get a bigger
change in blip height
than near the D.L.? The biggest change you can get will be right at the
edge of the lobe. For this
sketch, a ship at "A" would be on the barely pick-up, or the minimum echo
line of the lobe. It
would give you an E-1 echo. If you swing the lobe around toward the
direction of the target (the
|GENERAL RADAR PRINCIPLES|
echo will get larger, naturally, but it will increase in size quite
rapidly! In fact, for the sketch
shown, the echo strength would change perhaps five times as much in the
first degree of lobe
trace here near the edge as it would for an equal swing in lobe direction
near the center of the
lobe. It changes in strength more per degree of lobe train right at the
edge than it does anywhere
else in the lobe. You can tell when a target is in the edge of the lobe
five times as easily (from
this sketch) as you can tell when it is right at the center of the lobe.
That means that you can set
your lobe so that the target is in the edge much more accurately than you
can for the maximum echo.
However, you find one difficulty: you can read the hearing of the lobe
(or the antenna), but not
the bearing of the target. Since you want the direction of the target,
you are interested in the
antenna direction only if it can give you this information. In your
present situation, it is
obvious that you do not know the difference in direction of the antenna
and the target.
Consequently when the target is in one edge of the lobe, you cannot find
the direction of the
target simply by knowing the direction of the antenna. You need to know
If you can find the angle between the antenna direction and the direction
of the target when it is
in the edge, you can either add or subtract this angle from the bearing
of the antenna and arrive
at the desired answer.
You can set your antenna so that the target is accurately in one edge of
the lobe and then in the
other edge, getting two bearings: one larger than the bearing of the
target by a certain
(unknown) amount, and the other smaller than the bearing of
the target by the same amount. You now know that the bearing of the
antenna is halfway between
these minimum echo bearing readings. Average these two bearing readings
and you have the
accurate bearing of the target. This is accurate because you determined
the two minimum echo
bearings several times as accurately as you could have found the bearing
corresponding to the
Lobe switching. So far, you have found two ways of setting your antenna
(to direct the lobe), to
find the bearing of the target: the maximum echo setting and the minimum
echo settings. The
minimum echo setting gives you the bigger change in echo size for each
degree change in antenna
train; hence it is the more precise. However, there is an even more
accurate way of setting your
antenna, and for two reasons: first, because the side of the lobe is used
instead of the blunt end,
consequently the size of the echo is extremely sensitive to any small
change in antenna train,
and second, because an improved method of indication is used, based on
the comparison of two
pips heights (it is easier to judge when two pips are the same size than
to judge when a single
pip is at maximum size). This is the procedure called lobe switching.
If you have an object that increases in height while another object
decreases, the difference in
their comparative heights will change twice as fast as the height of
either one. If two echoes
work together in this way to show when you are on the target (one going
up and the other down
when you get off the target), you can get the antenna set in the target
direction just twice as
easily as you could by looking at the change in only one echo.
Figure 1-21. Angle between antenna and target direction.
|RADAR OPERATOR'S MANUAL|
You could create such a situation in this way:
1. Direct the energy out to the port side of the antenna direction and
get an echo hack. The size of
this echo will depend, of course, on the part of the lobe in which the
target appears (see fig. 1-22). With this sketch, the echo would be about an E-2 echo.
Figure 1-22. Lobe to port.
2. Then direct the energy out to the starboard side of the antenna
direction to see how big that
echo is. Of course, the echo size depends on its position in this lobe
(see fig. 1-23).
Figure 1-23. Lobe to starboard.
In comparing the size of these echoes you found that they were not the
same. Why? Simply
because the target is nearer to the center of one lobe than it is to the
other. The target is nearer
the edge of
the port lobe (in our example), so you get an E-2 echo back from the port
lobe and an E-3 echo
from the starboard lobe. Now, if you turn the antenna toward the
direction of the target, you
will be increasing the echo size of the smaller echo, and decreasing the
size of the larger echo,
with the difference in height changing to ice as much as either echo
height. When you have the
target exactly halfway between the two lobes the echoes are matched in
height: about three-quarters of an inch high in our example. Each echo has changed perhaps
one-quarter of an inch,
but the difference changed one-half inch, or twice as much.
Several things must be kept in mind in lobe switching. One point is that
you send the energy out
on only one direction at one time. You send a few pulses out to one side
of the antenna direction,
and then an equal number of pulses out to the other side of the antenna
bearing. You are sending
the energy out one way or the other; not both at the same time. Another
is that you need to
separate the echoes so that you can compare them easily. You can make the
blips from the
starboard lobe show up a little to one side of the blips from the port
side (fig. 1-24), so that
the blips will appear side by side. It is vitally important that all the
echoes returning from
pulses sent out to port appear at the same definite position, and that
all those returning from
starboard pulses appear at another precise position.
Notice that you do not change the direction of the antenna between these
pulses: you simply
switch the lobes from one side to the other. When you change the antenna
hearing, you change
the direction of the lobes as well, maintaining their position a certain
number of degrees from
the antenna bearing. When you get the blips matched you know that the
hearing of the antenna is
very close to the desired bearing of the target, but you realize that the
target is not in the center
of either lobe. Since this is true the echo height is smaller than it
would be if the target
happened to be centered in either lobe:
The lobe switching method is used in some of our fire-control sets, where
bearing accuracy is
absolutely vital. When bearing accuracy is as important as in the case of
fire-control gear, you
usually have a separate scope on which to match pip heights. You can find
details of the actual
method employed in Part 4 of this book, and in the instruction books
furnished by the
manufacturers of the various sets. On radars such as the SJ or SA, in
which cases the precise
bearing is not as important as it is in a fire-control set, we use
|GENERAL RADAR PRINCIPLES|
the conventional range scope ("A" scope) to show the rips to be matched.
You have found that you cannot read the bearing of the target directly,
but only the bearing of
the antenna. But if you know when your antenna is bearing directly on the
target, you can read
the bearing of the antenna as the bearing of the target. If the antenna
is off the target slightly
when you read, you naturally get a bearing that is incorrect. But, if you
can tell when you are
off the target, you can tell when you are on the target. The simpler the
method for finding when
you are off, the mote accurately you can read the bearing of the target.
The bigger the change
produced in whatever you are looking at (the blip height, for instance),
with a small change in
antenna bearing, the greater the accuracy will be. The maximum echo
method is the least
accurate of the three methods you have studied here because the change in
echo strength is small
per degree of antenna train. The minimum echo method is more accurate
than the former, but it
takes more time, and so it is not used very often. The lobe switching
method is the most accurate
method in general use. It gives a big change in height difference for a
small change in antenna
Minor lobes. So far, in our discussion of lobes, antennas and bearings,
we have assumed that
most of the
energy goes in one direction and that the amount drops off rapidly as we
move away from the
direction of the lobe. Unfortunately, this is not always true. Sometimes
there will be a
considerable amount of energy which has a definite direction different
from the direction of the
When enough energy goes our to form a distinct lobe like this, we say
that we have minor lobes,
or side lobes (see fig. 1-25).
What does this mean to you as an operator? The primary thing is that you
can pick up a target in
a side lobe and report it as being in the main (or major) direction of
the antenna assuming that
the antenna is trained in the direction of the target. If you have a
target in a side lobe, however,
and are unaware that it is there, you might think that your antenna is
bearing directly on the
target, when such is not the case at all, and any bearing reading that
you may take will be
incorrect. If you have side lobes, the assumption that you can pick up
targets only when your
antenna bears on them is wrong.
For example, let us suppose that you have side lobes 60 degrees to each side of
the main lobe. If your
target actually bears 135 degrees, you might read a bearing of 195 degrees or of 075 degrees
if you have it in one of
the side lobes. That is evident, for your antenna is actually directed
60 degrees to one side or the other
of the target.
|RADAR OPERATOR'S MANUAL|
You should remember that it is the bearing of the main lobe you read on
the bearing indicators,
and not the bearing of side lobes.
These side lobes occur to some extent with all types of antennas. They
are generally most
noticeable and troublesome with a curtain array. They occur because the
dimensions of our
antennas are only a small number of wave lengths. We cannot escape them
entirely with our
present radar techniques; we must recognize their presence, and be
careful to avoid errors
resulting from them.
Although these minor lobes may cause errors in establishing bearing, they
will not result in
incorrect range readings. How to recognize pips from side lobes is
discussed in Part 3 under
To all intents and purposes the radar receiver bears a close resemblance
to an ordinary radio
receiver. Of course, the two sets differ in frequencies of operation, for
the receiver must be
tuned to the same spot in the wave band as its associated transmitter,
and as earlier emphasized,
the bands used by radio and radar are widely separated.
Signal amplification. The receiver used in radar must be very sensitive
so as to operate on weak
echoes. The power represented in the echo would be of little value if it
were not built up in some
way. This reinforcing or strengthening action takes place in the
receiver, and is called
amplification. It is the amplifier that builds up the weak radio signals
into energy that is finally
converted into sound issuing from the loud speaker of a radio set.
In the radar receiver the echo is amplified or increased by similar
amplifiers. A manually
operated gain control enables you to control the amount that the
echo is built up. A relatively strong echo would require less gain or
increase, while a weaker
signal would require more gain. Actually, the gain control performs the
same task as the volume
control on the home set. The reinforced or amplified echo is converted
into signal energy, but
this energy is not fed to a loud speaker or head phones, since you do nor
wish to hear a radar
echo. It is your desire to see the signal and to derive the information
The new signal, having been made much stronger and lowered in frequency,
is now one that you
can use. So far, however, you have nothing to indicate your receiver
output. For this purpose,
you must connect the receiver to a cathode-ray tube, or INDICATOR, which
will indicate the
return of the echo. The speaker on your home radio set corresponds to the
In radar, extremely small divisions of time are measured. The unit is the
millionth of a second. Obviously, no ordinary time-measuring device will
serve this purpose.
However, it has been found that a device used for many years by
television people fits radar's
demands well. This is called a cathode-ray tube, or the scope. Learning
about some construction
features of the C.R.T. (cathode-ray tube) will help you to understand how
it is used to measure time.
Basic electron theory. Before discussing the functions of the various
parts of the C.R.T. you
should know some fundamentals of electron theory with particular
reference to how an electron
beam is used to measure time.
Scientists tell us that everything is made up of atoms. which are
extremely small particles of
Figure 1-25. Main, minor, and back lobes.
|GENERAL RADAR PRINCIPLES|
of there atoms has from one to 92 electrons circulating around the
center in much the same
way as the planets rotate around the sun, except that in the case of
atoms the units are billions
and billions of times smaller. These electrons continue circulating about
the center of the atom
until shaken loose by some great impact. When two atoms collide with
sufficient force some electrons are shaken loose.
Figure 1-26. Structure of on atom showing electrons in their orbits.
Scientists discovered that by heating material such as metal it is
possible to make the atoms
within the metal collide with sufficient force and such rapidity that the
metal will emit or give
off electrons in large numbers. They also discovered that some metals
give off more electrons
than others, indicating that some materials release their electrons more
easily than others.
How well these electrons flow in materials is quite important when
selecting materials for
conductors and insulators. Conductors are materials used for electrical
wires because the
electrons are held together loosely and can move through the material
with relative freedom.
Insulators, on the other hand, are materials which hold their electrons
very tightly around the
center of the atom. Electrons have great difficulty in moving through
Electrons have a small negative charge. Usually, this negative charge is
cancelled by the
positive charge of the center part of our atom. This is true, of course,
only if the atom has
exactly the right number of electrons. If an atom has too many electrons,
a negative charge
exists, if it has lost some electrons, it is said to have a positive
In conducting materials a large number of extra
electrons may he stored up, resulting in many negative charges. Such
matter is said to be at a
negative potential. Of course, electrons do not do this of their own
accord. If free to do so, they
will scatter so as to get as far apart as possible (until there is a
uniform distribution of the
charge. To make them congregate, you must do something to overcome their
You can collect them by stroking cat fur, or by rubbing a glass rod with
silk, or by connecting
up a battery or generator.
If it is possible for them to do so, the electrons will return to atoms
which do not have enough
electrons. As soon as the electrons are paired off with atoms lacking
electrons, everything is
back to normal, or a state known as zero potential.
Whenever you collect electrons in one place to produce a negative
charge, you must get these
electrons front somewhere. When electrons are taken from atoms, those
atoms do not have
enough electrons to be neutral. Since there is a lack of electrons the
atoms in question have a
There are certain facts about negative charges and positive charges that
may be stated in the
following general law: electrical charges of like kind repel each other,
and charges of unlike
kind attract each other. In terms of negative charge and positive charge
the law is: a negative
charge will repel a negative charge; a positive charge will repel a
positive charge; a positive
charge will attract a negative charge. This is exactly like the principle
of magnetism: like poles
repel and unlike poles attract each other.
The following is a summary of the main ideas we have discussed on the
fundamentals of electron
1. Electrons are small particles of negative electricity.
2. Each atom has a certain typical number of electrons. So long as it has
this number of
electrons, it has no net charge, and thus has zero potential.
3. Conducting materials have some electrons attached very loosely. The
atoms can gain or lose
4. Non-conducting materials have the electrons firmly bound to the atoms.
Any free electron
finds great resistance to its movement.
5. When electrons are caused to assemble on, or in, something, that
object is said to have a negative
6. When electrons are taken away (so that there is an insufficient number
to satisfy all the
atoms), the object has a positive charge.
7. Electrons will go from a negative charge to a
|RADAR OPERATOR'S MANUAL|
positive charge unless special measures are
taken to prevent their doing so. This movement is called a current.
8. Like charges repel and unlike charges attract each other.
In order to measure the time it takes for the energy to go out, and echo
hack, we must have some
instrument to indicate its return. Mechanical means, such as an ordinary
pencil with gear and
lever arrangements Could not operate rapidly enough for this: their
weight prevents them from
being moved quickly enough to give an indication of the presence of the
reflecting object. Since
electrons are so extremely light, a beam of electrons may be made to move
when the echo
returns, and move in just a fraction of a microsecond. This electron beam
is used as a
convenient pencil to draw the picture of what is happening.
Structure of the electrostatic cathode-ray tube. In a cathode-ray tube
the hot metal from which
the electrons are boiled is called a cathode. This is usually only a
small piece of fine wire
(something like the filament of an ordinary light bulb), which is heated
by an electric current.
In order to make the electrons boil off more easily, this wire is usually
given a chalky covering
of a special material, which is merely a substance from which electrons
can easily be boiled.
The cathode, then, just furnishes the electrons.
You could control the number of electrons by varying the cathode
temperature, but this is a
very slow process. Some other method of controlling the number of
electrons must be used. A
man by the name of De Forest found that a negatively charged piece of
metal in the path of these
electrons could stop them altogether, while a wire with a smaller
negative charge would permit
some of them to go by, the number depending on how negative this wire
was. He also found that
by weaving wire into a grid he could do this more easily than by using
just a single wire. We
still call this controlling part of the C.R.T. the grid, but its actual
physical shape is more like a
miniature tomato can fitted around (but not touching the cathode). There
is a small hole in one
end to let the electrons out in a stream, or beam. The potential of
this grid, being usually negative, repels most of the electrons, but a
definite number get
through for any particular grid voltage. The less the negative potential
(or voltage), the more
the electrons that slip by. Figure 1-27 shows the grid (with cathode
The electrons coming out of the grid are moving relatively slowly, and
more or less at random.
If they are to he used, they must be speeded up (accelerated) greatly so
that they will get from
the cathode to the screen in a short time. To do this two more parts are
placed in the C.R.T.: the
first anode and the second anode. Remembering the properties of
electrons, you know that a
positive voltage attracts the electrons and causes them to rush toward
it. This force of attraction
depends on the magnitude of the voltage, so to get a strong force, the
anodes are put at a high
positive voltage. This causes the electrons to move very rapidly toward
the anodes, and to shoot
through them toward the center of the screen.
The anodes are cylindrical (see fig. 1-28) and are mounted so that the
electrons may shoot
through the hole in the grid, the hole in the first anode, and the hole
in the second anode. Since
emerge from the second anode at extremely high speed, the complete
arrangement of the cathode,
grid, first anode, and second anode is sometimes called the electron gun.
|GENERAL RADAR PRINCIPLES|
These electrons continue on and in a very short time strike the glass
front of the tube. They are
much too small to be seen even with the most powerful microscope, but
they can produce effects
which are visible. Certain materials will glow and give off light when
these electron bullets
strike them. If the inside of the tube could be painted with some of this
material, you could tell
just where they hit by looking for this glow. That is precisely what is
done. Since the glass
front, painted on the inside with fluorescent paint (a substance that
glows), is what the picture
appears on, we call it the screen. Its purpose corresponds exactly to the
screen in a motion-picture theatre. Without it, you could not see the pip.
You know that you can attract the electron with a positive charge and
repel it with a negative
charge, and that the attraction or repulsion is proportional to the
charge (or potential). You can
move the beam upward by placing a positive charge near the top, or a
negative charge near the
bottom of the tube, or by both. Likewise you can move the beam to the
side by putting positive or
negative charges to the side. It is convenient to put these charges in
place by using two pairs of
metal plates, one pair horizontal and the other pair vertical. These are
known as the deflection
plates, since they deflect the electron beam.
One of the vertical plates is above and the other below the beam. If
there is a positive charge on
the upper one and a negative charge on the lower, the beam will move
upward, since the positive
charge attracts the electrons and the negative charge repels them. These
charges can be placed
on the plates this way by connecting the + terminal of a battery to the
upper plate and the - terminal of the same battery to the lower plate. Since these plates move
the beam up or down,
they are called the vertical deflecting plates (V.D.P.). Remember that
they lie horizontally in
the tube, but the direction in which they move the beam is important.
They are called the
vertical deflecting plates because they can cause the beam to move only
up or down, never
horizontally. The vertical position (up or down) depends, of course, on
the amount of the charge
on these plates, or the potential difference between them. A large
difference will cause the beam
to be either far above or far below the center position, while a smaller
voltage difference will
cause the beam to appear nearer the center. The beam (or the dot on the
screen) may be moved
to the right or the left while it is going up or down, but its distance
above or below center is
always determined by the voltage on V.D.P. Its vertical height, in radar,
is independent of the
sideways position. For a given voltage on these plates, the dot will
appear at a certain height
above or below center. For every voltage on these plates, there is a
certain definite vertical
position of the dot, regardless of its horizontal position.
There is another pair of plates which are exactly like the vertical
plates, except that they are
turned half-way around so that they are at right angles to the vertical
plates. They are able to
move the beam to the right or the left, depending on the charges on these
plates. Since they
control the back-and-forth
position of the dot, they are called the horizontal deflection plates.
It is obvious that by merely varying the vertical position and the
horizontal position of the dot,
you can make the dot take any position you want, anywhere on the screen.
adjusting the voltages on the two sets of plates properly, you can make
the dot appear at any
place on the screen. You can make it move in any definite manner by
changing the voltage on one
or both sets of the plates properly.
Concept of sweep and time base. If the dot moves from left to right at a
certain speed, you can use
this spot movement as a yardstick with which to measure time. Suppose the
distance the dot
moves is three inches, and that it covers this distance in exactly 1,200
Figure 1-29. Movement of electron beam with change in voltage on vertical
|RADAR OPERATOR'S MANUAL|
Figure 1-30. Movement of electron beam with change in voltage on horizontal deflection plates.
microseconds. If its speed is constant, it must have moved one inch from
the starting point in
one-third of the time, or in 400 microseconds. If you drive a car 3 miles
in 12 minutes, you
could go one mile in one-third of 12 minutes or in 4 minutes. If you set
your mileage at zero
when you started, and kept the same speed continuously, you could measure
the time by reading
the distance you had traveled. You know that it would take one minute to
travel one quarter-mile, and therefore for every quarter-mile you had traveled, you would
have been traveling for
one minute. If you had gone two and one-quarter miles, you would have
traveled nine quarters of
a mile, and since it took one minute to travel one-quarter mile, it would
he nine minutes from
the time you started. Since you move the same distance every minute, the
time that has passed
since you started is exactly in proportion to the distance you have gone.
The spot on the screen of the radar cathode-ray tube moves across the
face of the tube in the
same way. It starts at one place and moves toward some
other place on the tube face. You can control the voltage variations,
causing the dot to move in
such a manner that it travels from its starting point to the finish point
in a certain length of
time. It moves across at an unchanging speed (approximately) and in a
definite length of time.
This is important the whole ranging procedure used in radar depends on
it. Movement of this dot
is termed the sweep.
RADAR OPERATOR'S MANUAL
It takes about 12 microseconds for radar energy to
travel to and return from a target a nautical mile away. You can,
therefore, determine the range
(in nautical miles) corresponding to any point on the sweep by dividing
the time represented by
that point by 12 microseconds. Suppose the dot traveled a distance of
three inches in 1,200
microseconds. Each inch represents 400 microseconds as before. Any
distance along this sweep
represents. a certain definite time, and therefore a certain definite
range. Since each inch
represents a time of 400 microseconds, each inch represents 400 / 12 =
33 1/3 nautical
miles (approximately). If an echo returns when the dot
Figure 1-31. Electrostatic cathode-ray tube.
|GENERAL RADAR PRINCIPLES|
is one and one-half inches from the start, it would indicate a round-trip
time of 600
microseconds, or a range of 50 miles. These actual numbers, of course,
can be used only for a
sweep that moves three inches in 1,200 microseconds. A sweep of any
other speed will have
different numbers to tell you what range an inch distance on the scope
Since the time and the range to a target always have a fixed
relationship, let distance on the
scope be marked in range instead of time. Do not forget, though, that you
are actually measuring
time. In all radar sets, the path of the sweep is marked in units of
range such as miles or yards
(and not time) for convenience in reading. This sweep-line is sometimes
called the time basis.
So far, you have studied the electrostatic cathode-ray tube. You have
found that you can deflect the
beam by changing the charges (or voltages) on the deflection plates. You
have learned that the
sweep is produced by varying the voltage on these plates in a very
Electromagnetic cathode-ray tube. However, getting the sweep on a PPI
scope is more difficult
because the sweep must change direction but not speed. It is troublesome
to get the voltages to
cooperate and vary properly. Fortunately, another type of cathode-ray tube
is available; using
this tube you can get the sweep to move in the desired way with little
difficulty. This type is known as the magnetic cathode-ray tube.
In order to understand how this tube works. you need to know something
about the principles
involved. You may have wondered how an electric motor could pull as
strongly as it does. The
explanation is as follows. The motor is an arrangement in which large
wires carrying heavy
currents pass through a strong magnetic field. When these currents flow
magnetism, a strong force tends to push the current out of the magnetism.
This force is always
al right angles to both the magnetism and the direction of the current.
Consequently, the force is
in such a direction as to turn the motor. No wire is needed to carry the
current because the
streaming of these electrons from the cathode to the screen in the
cathode-ray tube makes up a
current. What happens, then, when you hold a magnet across the neck of
the tube? You have
placed some magnetism across the beam, and the beam is a current, so the
beam tends to move
out of the magnetism. It is neither repelled nor attracted by the
magnets, but is bent sideways in
tending to get out of the magnetism!
The stronger the magnetism, the harder it tends to get out, and
consequently the farther from
the center of the screen it will appear. If you vary the magnetism
between a pair of poles
mounted vertically across the neck of the tube, the spot on the tube face
will move sideways,
farther from the center if you increase the magnetism, and closer to the
Figure 1-32. Magnetic cathode-ray tube.
|RADAR OPERATOR'S MANUAL|
you decrease it. Then you can produce a sweep, by varying the magnetism.
If you have ever experimented with electromagnets you know that it is
possible to vary the
amount of magnetism simply by changing the current through the coils
around them. That is
just what is done in radar. To get the spot to move and form the sweep,
you increase the current
through the magnetic coils. The magnet (or held) current determines the
position of the spot on
the screen. The current in the control coils above and below the neck,
controls the horizontal
position of the spot, and that in the coils to the right and the left
controls the vertical position.
By varying these currents (and thus the magnetism) you can change the
spot position in exactly
the same way as was done by changing the charges on the deflection
plates. By varying the
magnetism properly, you can make the spot draw any desired picture,
duplicating any picture
that could he drawn on the screen of an electrostatic tube. However, it
is easier to produce
certain voltage changes than it is to change the currents
correspondingly. Likewise, current
may sometimes be changed more easily than voltage. Making the spot on the
outward first in one direction and then in another (to form the PPI
sweep) without changing the
sweep speed is relatively easy in the magnetic tube. This action is
difficult to achieve in an
electrostatic tube. You know that the spot must be made to move in this
manner, if you are to
have a true picture of the PPI (Plan Position Indicator) scope.
The deflection coils are mounted around the neck of the tube within easy
reach. Since the sweep
is perpendicular to the direction of the magnetism, you can turn the
coils around the neck of the
tube to change the direction of the sweep. To produce a PPI scope, then,
you merely need to
rotate the coils as the antenna turns, change their magnetism properly,
and intensify the beam
when the echo comes back. It would be troublesome trying to use rotating
deflection plates on the
outside of the neck of the tube, otherwise we could use an electrostatic
C.R.T. as well as we can
use the magnetic C.R.T. Even if the coils are not rotated, it is easy to
make the sweep behave
properly for PPI purposes by employing simple electrical devices which
keep the currents
Echo indication by deflection and by intensity
method. You were told that the reason for using an electron beam is to
have a "pencil" that can be
moved very rapidly. Why is that necessary? Before you
can measure the time required for the radio waves to strike an object and
be reflected you must
have some indication of the precise moment that the echo comes back. You
can connect the
receiver (which makes echo voltage larger), to either the upper or lower
plates of the C.R.T. When an echo is received, the receiver puts a
voltage on one or the other of
these deflecting plates, but only for a short time. What would happen if
it took six microseconds
for the pencil to be moved? The range would always measure about a
thousand yards too large.
Six microseconds is a very short time in which to move anything
By connecting the receiver so that either a negative voltage is applied
to the lower vertical plate
or a positive voltage to the upper vertical plate of the C.R.T., it is
possible to move the electron
beam up and then down again in a very short time. It may jump up and back
down again within a
fraction of a microsecond producing the pip or blip. What happens
vertically does not affect
appreciably the horizontal movement, so time can be measured in a
regardless of how much the beam jumps up and down tracing pips. It jumps
up almost instantly
when the echo comes bade, giving an accurate indication of the time that
the echo comes back.
Therefore, you can measure how long it has been gone with very little
error. The electron beam,
our "pencil," can be moved with amazing speed.
By making an echo move the electron beam, you get a pip as an indication
of the echo's return.
This is called the deflection method of showing the echo's presence,
since the beam is deflected.
There is another method by which the echo can be detected, and that is by
the beam causing a
bright spot to show along the sweep at the instant the echo returns. This
method is called the
intensity method, since the echo causes the screen to be momentarily
illuminated by the greater
intensity of the beam.
How would you make the sweep brighten up at a particular spot to tell you
when the echo
returned? Since the grid voltage controls the intensity (or brightness)
of any C.R.T.,
disconnecting the receiver from the vertical plates and reconnecting it
to the grid causes the
positive output voltage of the receiver (when an echo returns) to
intensify the beam and make a
To make it easier for a radar operator to see this bright flash, an
average voltage is maintained
on the grid just sufficiently negative to keep the sweep from showing up
except when the echo
returns. Then it
|GENERAL RADAR PRINCIPLES|
is bright enough to be easily seen. Thus the target indication is a
bright spot or bright smear.
Standard C.R.T. controls. The cathode-ray tube is one of the most important
parts of a radar set.
It may be thought of as the information center of the equipment.
All cathode-ray tubes have certain characteristics in common. For one
thing, all use an electron
beam which produces a spot on the screen. There must be some way to
control the brilliance and
position of this spot. There must also be some adjustment to bring the
spot into sharp, clear
focus. Controls to make these adjustments are found on the cathode-ray
tubes. Let us see just
what is the function of each of these controls.
The intensity or brilliance control enables you to adjust the grid
voltage (with respect to the
cathode) and thus control the number of electrons striking the screen. It
should always be
adjusted for minimum intensity allowable.
The horizontal centering control of an electrostatic (I.R.T. controls the
direct current voltage on
the horizontal deflection plates, permitting you to move the complete
trace to the right or left.
The vertical centering control of an electrostatic C.R.T. permits
adjustment of the voltage on the
vertical deflection plates so that the complete picture may be moved up
On a PP! scope, horizontal and vertical positioning is effected by
actually moving the focus coil
up and down or sideways. These are semi-permanent adjustments made by the
The centering control of a PPI scope permits making the sweep start in
the center of the screen.
This is done by controlling the direct current flowing in the deflection
coils of the magnetic
A focus control is used on every cathode-ray tube to permit bringing the
dot to a sharp focus.
This is done by varying the voltage between the first and second anodes
of an electrostatic C.R.T.,
and by varying the current in the focussing coil of a magnetic C.R.T.
The astigmatism control is a secondary focus control. By varying the
direct current voltage on
both vertical (or both horizontal) deflection plates, any part of the
sweep may be brought into
sharper focus than is possible by using the regular focus control.
Development of trace by the electron beam. In order to see more clearly
the action of the C.R.T.,
let us follow a pulse from the transmitter and see what happens on the
scope screen (see fig. 1-33).
At the same instant that a pulse is sent out the dot starts at the
left-hand side of the tube screen
and forms a peak. This peak is traced, because as the transmitter sends
out a pulse, the radio
receiver detects some of the energy and applies it as a voltage to the
vertical deflection plates in
such a way that the dot is pulled upward following the shape of the
transmitted pulse. As the
radio wave continues toward the target, the dot also moves at a constant
rate of speed from left to
right, leaving a trace behind it. In other words, we can look at the dot
and see what the wave is
Now, as the radio wave strikes the target and returns as an echo, the dot
still keeps moving.
Then at the same instant that the echo reaches our position, another peak
is formed by the dot
because the radio receiver detects and amplifies the echo energy and
again applies a voltage to
the vertical deflecting plates. This peak, caused by the echo, is called
After the pip is traced, the dot still continues its travel to the right
on the scope until it
completes the time base. Notice that before the next pulse is sent out
there is a definite lapse of
time after the echo is received. The most distant echo should have time
to return before the next
pulse goes out. This interval allows us to identify each echo and the
exact pulse that caused it.
For the purpose of explanation, we have slowed down the action of a
single pulse, its returning
echo, and corresponding pulse trace and pip on the scope. The actual
speed and flow of radio
energy is that of
|RADAR OPERATOR'S MANUAL|
the speed of light, so that the time base with the pulse trace and echo
pip on the scope will
appear to he stationary or fixed. Keep in mind that there is a definite
relation between the rapid
dot travel on the scope and the radio wave travel between our position
and the target.
The distance between the pulse trace and the pip on the time base is
actually the "yardstick" for
measuring the distance between own ship and the target. The C.R.T.
translates the smallest
fraction of time into exact physical distance of yards or miles. It can
be seen that the scale most
be calibrated so that its indications actually divide by two the total
distance traveled by the
Continuous and discontinuous sweeps. You know that the electron beam
travels across the screen
in a definite manner and in a definite time. You also realize that there
is one sweep for every
pulse sent out by the transmitter; consequently, the time between pulses
puts a limit on the
time length of the sweep Sweeps that use up all of this time differ
slightly from those which do
not, so there are different names for them.
A continuous sweep is a sweep in which the dot needs the complete time
pulses to get across the scope. It travels continuously, because it jumps
back and starts over
immediately after it completes one trip. A discontinuous sweep is one in
which the dot completes
its crosswise trip considerably ahead of the time it is to start across
again, so it "rests" awhile.
It shows no echoes during the rest period, as it does not sweep across
Suppose that you have a radar set pulsing 833 times every second. This
means that the time
between starts of sweeps is 1/833 of a second, or about 1,200
microseconds. If you are
interested only in targets within about 10 miles, you need consider only
echoes coming back
within about 10 X 12, or 120 microseconds. Since the dot can be set to
travel at any speed you
choose, you can make it go across the screen in 120
microseconds. When it reaches the end, it has to wait 1,080 microseconds
before it can start
over. You would refer to this as a discontinuous sweep because of the
rest period, during which
the scope is blanked out (turned off).
If you decided to look for targets as far as 25 miles, you would have to
slow the dot down until it
took 25 X 12, or 300 microseconds to cross the screen. Its rest period
now will be 900
microseconds (with 833 pulses per second). This would he called a nominal
range of 25 miles,
because that is the biggest range that can he read directly on the scope
with this sweep speed.
You can increase nominal range to 50 miles by further slowing the dot
until it takes 600
microseconds to complete the trace. Then an echo from a 50-mile target
would get back just in
time to show up on the scope. If an echo returned from a target beyond 50
miles, it would arrive
during the 600-microsecond rest period, the time when the scope is
What would happen if the dot were slowed until it took the full 1,200
microseconds to go
across? For one thing, the nominal range would be 100 miles. Furthermore,
there would be no
(appreciable) rest period. Up to this point slowing the dot increased the
nominal range. It is not
possible to continue to increase the nominal range by slowing the dot
still more since you are
allowing only 1,200 microseconds between starts of sweeps. Hence, any
further reduction in
speed using this pulse repetition rate will only shorten the length of
the trace. Further, slowing
the dot cannot increase the pulse interval -the maximum time of travel.
It will only shorten the
distance the dot travels, which usually is not desirable.
You have found that so long as you have a discontinuous sweep, the
nominal range can be changed
by changing the speed of the sweep. The only way you can change the
nominal range of a
continuous sweep is by changing the pulse repetition rate. The nominal
range is determined by
the time the dot takes to cross the screen, and that, for a continuous
Figure 1-34. Continuous sweep.-Discontinuous sweep.
|GENERAL RADAR PRINCIPLES|
is the same as the time between starts of pulses. Increasing the number of pulses decreases the time between them, and so reduces the nominal range.
The following is the equation for calculating the nominal range of a
N.R. = 1,000,000/12 X P.R.R. (approx.).|
N.R. is the nominal range in nautical miles.
P.R.R. is the number of pulses the transmitter sends out every second.
There are advantages associated with both the continuous and the
discontinuous sweep. As long as
a discontinuous sweep is used you can switch scales simply by turning a
knob. Since the only
thing that needs to be done to change the nominal range of a
discontinuous sweep is to change the
sweep speed, you can put in a rather simple control to make a quick, easy
Changing the sweep speed is easy. The pulsing rate, on the other hand,
cannot, in some cases, be
changed much, so sets with continuous sweeps usually have only one scale.
The sweep speed of a continuous sweep can be changed on some radars so
that the sweep starts
slowly, then speeds up, and finally slows down again without altering the
time of the start and
ending of each sweep. The total range indicated by the sweep is unchanged
since the total time of
the sweep remains the same. However, both ends of the sweep register a
greater proportion of
the total range, for they represent a greater part of the total time due
to the reduced speed of the
spot. The center portion that has been speeded up now represents less of
the total range since its
time is reduced, but it represents an increased part of the physical
length of the sweep. This
results in expanding the picture of any objects appearing in this center
section of the sweep
because it now covers a greater part of the sweep length. This has
certain technical advantages
which makes accurate ranging relatively easy.
Calibration of continuous and discontinuous sweeps is discussed in the
following section on
"Calibration." There you will find that the calibration of a continuous
sweep is different from
calibration of a discontinuous sweep.
Radar sets, like every other precision instrument, must be calibrated
before they can give
correct information. Calibration is the process of making the radar read
the correct range,
bearing, and position angle. It is a common error to think that the term
tuning the receiver, adjusting the dial lights, throwing switches, and
everything else necessary
echoes. Calibration simply makes the set indicate proper range,
bearing, and position angle.
Range calibration is necessary in the ease of every
radar set. Every operator must check his range calibration every time he takes over a watch. There are two things that must
be done to make the
scope read correct range, internal calibration and external calibration.
Internal calibration. The purpose of internal calibration is to make the
divisions on the scales
the correct length. It is the same problem as making the inch-marks on a
ruler exactly one inch
apart. You could not measure distances accurately with a ruler if the
regular inch-marks were
really three-quarters of an inch apart, for you would be reading each
distance too long. An
actual three inches would only occupy four of these three-quarter inch
spaces, with the result
that you would think the object had a length of four inches.
Radar internal calibration deals with the same sort of proposition. If
the spot moves across the
screen in, say, 900 microseconds when the scale is marked off for a
50-mile nominal range,
you would read a range of 50 miles for a target that actually was at 75
Internal calibration is the process of making the time of the sweep match
the scale: making the
dot move from the start to the end in the correct amount of time. For a
continuous sweep, this
time is the time interval between pulses, so you must get the correct
pulse repetition rate for a
continuous sweep. If you have a discontinuous sweep, this time depends
only on the speed of the
sweep. Consequently, internal calibration requires determining either the
repetition rate or correct sweep speed.
External calibration. External calibration is the process of making the
zero setting correct, to
avoid any constant range error. Unless you make this adjustment, you may
read a range too
large or too small by a definite amount.
Imagine a yardstick with the first five inches cut off (so that the
five-inch mark is right at the
start). Unless you made proper allowances you would read a length five
inches too great for
anything measured with this yard stick. The 25-inch mark would be at the
edge of a 20-inch
box, and the length of the box would appear to be 25 inches. If you moved
the yardstick over five
inches, placing the five-inch mark five inches from the left edge, you
could read the correct
length of the box. Its edge would line up with the 20-inch mark.
You can do exactly the same thing with your radar set. You cannot
actually read a zero range
(because of the transmitter pulses), but you can detect nearby
|RADAR OPERATOR'S MANUAL|
targets on your radar and make their pips appear at the correct places on
the scope. Double
range (or multiple range) echoes are useful in doing this (see the
section on Multiple Range
Echoes in Part 3).
All radars should have the zero setting checked s often as possible,
either by comparison with a
Figure 1-35. Measuring a box with two different scales.
known to be accurate or by the double range echo method, which is the
most accurate and
convenient method to use, especially when at sea.
Double range echoes occur at close ranges and result from the reflected
energy striking your
ship, returning to the target, and being reflected a second time.
Therefore, you should see a blip
created by the reflected energy returning on the first trip; then a
second blip, which will be
smaller, should appear at exactly twice the actual range if the zero
setting is correct. With this
information, the exact range to the target can be determined, for it is
the difference between the
second trip echo and the first trip echo (6,500-3,500). Next, you will
learn how to find the
zero setting so that the actual range will be correct.
If you subtract the range you actually read for the double range echo
from double the range read
for the target, you will get the correction you should make in the zero
setting, and reduce the
zero reading by this amount. In the example, double the range read for
the target was 7,000
yards. The range reading for the double range echo was 6,500 yards.
Subtracting 6,500 yards
from 7,000 yards leaves 500 yards, the number of yards too many your set
If you move the zero setting back 500 yards, the normal echo will appear
at 3,000 yards and
the double range echo at 6,000 yards. Twice 3,000 is 6,000, so you know
your zero set is
This method is useful for calibrating fire-control radars. It can be used
to a lesser degree with
search type radars, if there is another ship parallel to yours at a short
Range calibration consists of two steps. If you
have a discontinuous sweep, you must get the sweep
speed (internal calibration) and zero setting (external calibration)
correct. If you have a
continuous sweep, you must get the pulse repetition rate (internal
calibration) and zero setting
(external calibration) correct.
Bearing and position angle calibration should be checked against the
optical methods. Some of the
search equipment in use does not require frequent calibration for bearing
or position angle.
This is not true of the fire-control radar, where extremely accurate
hearing or position-angle
readings are needed, and such a radar should be checked as often as
Types of scopes.
Through the use of radar it is possible to get information when all other
methods fail, but this
information is absolutely useless unless it can be put in understandable
form. For certain uses,
this information is more understandable when presented in a particular
way, while for another
job, some other manner of presenting the data may be more desirable.
There are several ways
in which the same information
Figure 1-36. Zero set is off 500 yards.
|GENERAL RADAR PRINCIPLES|
can be shown, and the particular method used will depend on the specific
The information you are interested in primarily is detecting the presence
and finding the
location (range and bearing) of objects. Just how does the radar set show
this? If there were a
few million vacuum tubes, coils, condensers, and resistors in the set,
you could make the beam
perform somewhat as shown in figure 1-37.
Unfortunately there is no room for the many complicated circuits required
for this outfit, so
instead, simpler, easier methods may be used-many of them. Six of these
methods and their uses
will he discussed here: the "A" scope, "B" scope, "PPI" scope, "H"
scope, "R" scope, and "J" scope.
The "A" scope. The fundamental type is known as the "A" scope. It is the
type most frequently
referred to during previous explanations. You remember that the spot
moves from the left to the
right side of the screen at some approximately constant speed, enabling
you to measure time
from the start to any point along the time base. The spot jumps up
whenever a reflected echo
returns simply because the receiver output is connected to one of the
vertical deflection plates.
The echo comes out of the receiver and regardless of where the dot
happens to be on the
time base, it gives the dot a kick upward, forming a pip or blip which
indicates a target.
We may determine the range, then, by measuring the time from the start of
the sweep to the
spot at which the pip appears, translating the time into range. Since the
speed of radio waves
does not change, the indicator is usually marked off directly in yards or
miles. Hence, the "A"
scope indicates range horizontally and presence vertically. It tells
nothing about the bearing of
The "A" scope has advantages which often outweigh its failure to tell the
bearing of a target. One
important characteristic is its ability to tell you what you are
"looking" at-just what the blip
indicates (see section on Composition in Part 3).
The "A" scope is useful when trying to detect the
presence of objects at long ranges. Fairly weak echoes may sometimes be
detected on the "A"
scope before they can be seen on another type of scope (especially when
the pulsing frequency is
Ranges can readily be determined with accuracy on an "A" scope. Some
scopes provide a range
step in the sweep, and control its position electrically to facilitate
the task of range
measurement, The step appears because a voltage is suddenly applied to a
plate, which causes the remainder of the sweep to he shoved down.
Dials or scales which will read the range of our step directly and
accurately are provided. When
you move the step under the blip of the target, the range of the target
appears on these dials.
This greatly simplifies the task of accurately determining range (this
step is usually not so
sharp as that shown in the
Figure 1-38. Two groups.
|RADAR OPERATOR'S MANUAL|
sketch, and it may be altogether different in shape, but the basic idea
is the same).
Before leaving the "A" scope, let us review briefly the facts known about
it. First of all, when an
object is reflecting the transmitted energy the spot jumps up, tracing a
(this might well be the reason it is called an "A" scope since the blip
resembles the letter "A"
without the cross bar). The presence of the target, then, is shown
vertically. The farther an
object is from you, the more time the energy will require to go out and
return, so the spot
tracing the time base will get farther across the screen before the echo
returns. Range can be
found by seeing how far the blip appears from the start of the sweep.
Range may he determined
directly with ease and accuracy; presence may he detected on the "A"
scope although the echo is
weak. Variations in appearance of the blip can tell you much about what
is on the reflecting end
of the radar beam.
The "A" scope itself tells nothing at all about the bearing of the
target. However, you can tell
when the antenna is pointed directly at the object, thus the antenna
bearing will be the same as
that of the target. Thus by reading the bearing of the antenna you find
the bearing of the target
relatively easily, but there must be a bearing indicator to show the
antenna bearing. This is
usually made up of two circles marked off in degrees; one reads the
relative hearing, the other
the true hearing. A pointer (called the bug)
which rotates in synchronism with the antenna points to the proper
reading on the dials. This
type bearing indicator is found on many radar sets in use at the present
The "J" scope. A new model radar recently introduced to the Fleet has a
circular sweep. The dot,
instead of traveling from one side of the screen to the other as on the
"A" scope, goes around and
around (somewhat like the sweep on some sound ranging gear). When an echo
returns, the dot
suddenly jumps outward to form a blip. This blip is straight out
("radially," we say), and time
is measured by its distance from the start of the sweep around the
screen. On this scope, range
is shown circumferentially (around the circle) and presence radially.
For any particular size of cathode-ray tube, the time base is about three
times as long as it
would be on a corresponding "A" scope. So, by wrapping the "A" scope
around in this way, it has
been possible to increase the range accuracy about three times. The "J"
scope presentation is
the name given to this novel indication.
The "R" scope. A modification of the "A" scope, of great value in
determining the composition of
targets (see section on Composition), is now found on some sets. It is
called the "R" scope, and is
a magnified (expanded) portion of the "A" scope with a control which
enables you to choose the
part to be magnified.
Figure 1-39. Magnifying a portion of the "A" scope with an expanded sweep.
|GENERAL RADAR PRINCIPLES|
To magnify the pip, you cause the spot to move across the scope in a
short time. It may cover the
five inches (approximately) of the time base in just 25 or 30
microseconds. This, of course,
will make the pip appear very wide. For instance, a pip five microseconds
wide might cover a
whole inch on the time base. Two targets separated by only a half mile
will appear almost an
inch apart. Consequently, the operator is not likely to read the
indication as being a pip from a
By increasing the width of the pip in this way, it is possible for the
operator to recognize
definite features thereof. You can count the separate peaks running up or
down the sides of the
pip, and separate targets close together which might otherwise be
mistaken for a single target.
The job of estimating size, number, etc., is greatly simplified by use of
the "R" scope.
The "A" scope, the "J" scope, and the "R" scope all operate by deflecting
the electron beam when
an echo returns. Hence we say that they indicate presence by deflection.
However, it would be
possible to have a bright spot appear when the echo returns instead of
deflecting the spot. This
brightening of the spot is called the intensity method of showing
presence, and it is used in both
the PPI scope and "B" scope presentations,
The PPI scope. The PPI scope gives us a top view of the vicinity, with
our own position in the
center. Draftsmen call a top view a plan view; since the scope indicates
a top (or plan) view of
the position of everything around you it is called a plan position
indicator, or as abbreviated, a
The pictures of the surroundings appear by this simple process: the sweep
begins in the center
and goes outward (at its constant speed), toward the edge in the
direction the antenna is
pointing, and a bright spot appears at a distance proportional to the
range of the target. Those
who have done any plotting will recognize this method: placing a mark in
the correct direction
and at the correct range on a polar chart. That is all that is needed to
draw a true map of
everything in the vicinity. This scope shows everything that can reflect
the radar energy, and
shows it in the proper place on the PPI map. You will see islands, your
own ships and planes, as
well as the enemy's craft, and anything else that happens to reflect the
energy; all objects
appearing in their actual positions. You can readily see how helpful this
PPI scope is in task-force operations, in convoy duty, in working navigation problems, or in
any one of various technical uses.
Remember that the sweep moves outward in a
direction representing the antenna beating. Since the antenna can pick up
an echo from a target
only while it is pointing at that target, and since the rotating antenna
points in any target's
direction for but a short time, you can expect the bright spot to appear
for only a moment. As
the radar beam swings away from a target, you fail to receive any echo
from it, and
consequently, the beam does not make a bright spot any longer. (You
recall that the beam can
intensify only contacts on one bearing at a time.) This means that in
order to continue to see that
particular target, the screen must continue to glow after the sweep
leaves it. In making the
cathode-ray tube, the screen is painted with a chemical coating that
glows longer than the "A"
scope screen coating, and a tube with a longer persistence screen
results. A screen is termed
persistent because it persists in glowing after the sweep has left it and
has moved on to another
spot. Tubes which employ the intensity method are used in PPI and "B"
The "B" scope. The "B" scope resembles both the "A" scope and the PPI
scope, but has some
characteristics of its own. If you turn an "A" scope on its side, the
range will be indicated
upward, or vertically, and the blip horizontally. With the receiver
disconnected from the
horizontal deflection plates, it will not be possible to get a blip
sideways. By connecting the
receiver to the grid instead, a bright spot will result which will
indicate a target just as a blip
does. Seeing a bright spot is the indication of the presence of an
object, and the spot's position
tells you the range of the object causing it.
The horizontal deflection plates have been disconnected, and are not in
use. What use could be
made of them? Range is determined vertically and presence by intensity,
so the one item of
major information lacking is the bearing. The horizontal plates, then,
can be used to determine
the bearing of the object in the following manner.
If the whole time base could be moved, bright spot and all, to one side
for a distance proportional
to the movement of the antenna, it would provide a means of indicating
the bearing of anything
giving an echo, because the scope would show how far the sweep was from
its usual position. For
example, suppose the time base moved one inch to the right when the
antenna was trained five
degrees to starboard. If a bright spot showed on the time base one and
one-half inches to the
right of the center, the antenna must have been pointing seven degrees
and thirty minutes to
starboard when the echo was received. Consequently, the target was at an
angle of seven degrees
|RADAR OPERATOR'S MANUAL|
and thirty minutes to starboard. In some sets (such as the Mk. 8), the
scope tells the angle of a
target to the right or the left of the line along which the director
Usually, the "B" scope is used with sets which do not show targets for
the full 360 degrees about the
ship. Most of them show targets in only a limited area, such as a sector
from 285 degrees forward, to
075 degrees relative, Other sets search a sector only 30 degrees wide, 15 degrees to each
side. How you set the
"blinders" depends on what you are looking for.
What, then, does the "B" scope show? Since it is similar to an "A" scope
that does not stay right
side up, it shows range vertically. The electron beam brightens up the
screen when an echo
returns, and consequently shows the presence by intensity. Finally, the
horizontal plates are
connected so that it shows the bearing horizontally. It gives the same
information as the PPI
scope, but in a different manner.
Since the sweep always starts at the bottom and goes upward-always in the
can measure range accurately. A movable pointer is sometimes used, or
moving the sweep past a
stationary pointer can serve to measure the range with reasonable
accuracy. There is no
problem here of measuring different directions, as in the case of the PPI
scope. When no
critical range accuracy is required, horizontal lines are placed on the
screen to be used in
estimating ranges, and vertical lines are used in estimating the bearing
of a target.
The "H" scope. An "H" scope is a modified "B" scope. Azimuth is given
horizontally, and range
vertically. The signal appears as two bright spots, displaced laterally
with reference to each
other. The slope of the line that can be imagined as joining the dots
gives an indication of target
elevation. The "H" scope is often designated by the term double dot scope.
Summary. The following summary gives in brief form the function of each
of the scopes, and
provides a means of comparing features of the various types.
|Bearing: ||None indicated
|Advantages: ||Ease in detection, ranging, and determination of target composition.
|Bearing: ||No bearing indication
|Advantage: ||Great ease in determining composition.
|Presence: ||Double dot
|Bearing: ||Indication horizontally
|Advantages: ||Provides bearing and range plus data for altitude determination,
|Range: ||Around circumference
|Advantage: ||Increased range accuracy.
|Range: ||Distance out from center of scope (radially)
|Bearing: ||Direction of sweep
|Advantages: ||Complete picture in a few seconds. Shows all objects in true relative positions.
|Advantages: ||Good bearings at any range; good bearing resolution or target separation at short ranges as contrasted with PPI; shows all targets at the same time. Similarity to cross-hairs makes it especially good for gunnery.
The modulation generator.
So far, you have discovered that the radar set has
these parts: the transmitter, the antenna, the receiver,
and the indicator. There is more to the set than this, however. You know
that an operator is
needed to key the communication code set, for someone must turn it on and
off to form the dots
and dashes. Similarly, radar energy must he sent out in pulses (or
extremely short dots) if you
are to get the accurate range of an object. These pulses must all he of
the same length, and must
he spaced evenly over a period of time. This means sending a pulse only
four or five
microseconds long, and 1,640 of these must he transmitted in one second.
Some radar sets
require a device to do just that, so an electrical means of
|GENERAL RADAR PRINCIPLES|
Figure 1-40. Scope presentations.
|RADAR OPERATOR'S MANUAL|
keying the transmitter, called, naturally enough, the keyer, is used. The
keyer is also known as
the modulation generator.
The modulation generator keys the transmitter, forming pulses of definite
duration and at
regular intervals. The definite duration is called the pulse width or pulse
duration, and the number of
pulses every second is called the pulse repetition frequency or the pulse
The modulation generator also controls the start of the sweep. Why is
You remember that range is determined by measuring the time taken by the
radio waves to leave
you, travel out, reflect from an object, and return. In other words, you
measure the total time
between those occurrences.
You recall that the keyer turns the transmitter on and off to form the
pulses. If it can also he
used to start the sweep every time at the same instant the transmitter
pulses or at the same
time after the transmitted pulse, each blip will appear at an identical
spot on the time base
every time. Fortunately, it is not difficult to utilize the keyer to do
this. It does this second job
by sending a synchronizing pulse to activate the indicator. Synchronizing
means that the pulse
makes the sweep start always at the same time. This is also called the
synch pulse. Thus, in
addition to its other functions, the keyer furnishes the synchronizing
pulse, which starts the
sweep at the same definite time with respect to the transmitted pulse.
The sweep may not start at the same time as the transmitted pulse. By
knowing how long the
start of the sweep is delayed you can figure in that time in calculating
the range. In some sets,
range is determined by changing this time delay until the blip is moved
to the center of the
sweep. You can then measure the amount the sweep was delayed and know the
You will find the same basic units in practically every radar set. Often
several, and maybe all of
them, will he in the same box or cabinet, but they are always represented
in some form.
Most radar sets have large, bulky antennas. This is necessary in order to
obtain good bearing
accuracy. The transmitter, you remember, must he "coupled" to the
atmosphere if it is to
transmit its energy. The receiver, too, must he "coupled" to the
atmosphere if it is to receive
any of the reflected energy.
You recall that you transmit in pulses, and receive the echo while the
transmitter is not
Consequently, while the receiver is receiving the transmitter is not
transmitting, and vice versa.
The transmitter uses the antenna only while it is transmitting (during
this time no echo can be
received), so it is possible to let the receiver use the same antenna
while the transmitter is off.
In that way, the weight of antennas needed, and the difficulties in
tuning them are reduced. In
addition, we have assurance that while receiving the antenna is pointing
in the same direction
that it was pointed when transmitting. Certain advantages, therefore, are
gained by using the
same antenna for both purposes.
However, this arrangement presents one difficulty. If the receiver and
transmitter are both
connected to the same antenna at the same time, the receiver will be
unable to carry the load;
hence means have been provided to disconnect the receiver from the
antenna while the
transmitter is sending out the pulse, and reconnect it when the
transmitter shuts off. This
switch is called the reprod (receiver protective device), or the TR box
or the duplexer.
If you are interested in nearby targets, this switch must operate very
rapidly. A delay of only
one forty-thousandth of a second in re-connecting the receiver would
cause you to miss any
targets within about two nautical miles. There are no mechanical switches
which will work fast
enough for this, so electrical switches are used, switches which have no
moving parts except the
tiny electrons in a tube. To work properly the duplexers must he tuned
technician should be called upon to do this.
Let us review the action again, and follow the course of a single pulse.
As the wave travels out
from your ship (see fig. 41), the action will be stopped from time to
time so that you can see
how far the dot has traveled on the scope.
As the pulse leaves the transmitter, (1), the dot starts at the left side
of the tube and traces out
the shape of the pulse. When the pulse has traveled half the distance to
the target, (2), the dot
has completed one-fourth of its travel. As the pulse strikes the target,
(3), the dot has traveled
but half the distance to the place on the scope where the pip will appear.
As the echo, (4), moves toward your position you can see that the dot
must travel the remaining
distance an the time base before the pip is formed. When the echo, (5),
reaches the antenna the
radio receiver detects and amplifies the energy, which, when applied to
the vertical deflection
plates, causes the pip
|GENERAL RADAR PRINCIPLES|
to be traced. Therefore, the time base actually measures the total time
for the pulse wave to go
out and return as an echo to your position. Because the speed of wave
travel in each direction is
the same, the scale on the scope can be calibrated to give the true range
directly in yards or
Broadly speaking, there are five parts involved in radar apparatus: the
antenna, duplexer, and transmission lines; the keyer; the receiver; and
the indicator or
indicating devices. Of course, you must also have the required power
supplies and controls in
addition to the five basic units.
The transmitter is used to generate very short pulses of electrical
energy which are radiated
out into space.
From the transmitter the energy flows through the transmission lines to
the antenna which may
he highly directional and concentrates the radio energy into a narrow-
beam. As the wave
travels out into space, reaches the target and returns as an echo, the
scope traces a line along
the screen forming the time-base. The echo strikes the antenna and is
detected and amplified by
the radio receiver and applied to the indicator scope in such a way that
it causes the pip to
appear on the time base.
Since the same antenna is used for both sending and receiving, you must
have a receiver
protective device to guard the receiver from the powerful outgoing
pulses. By using the so called
|RADAR OPERATOR'S MANUAL|
Figure 1-42. Block diagram of a typical radar system.
|GENERAL RADAR PRINCIPLES|
(which might he compared to a simple valve), you can keep the heavy flow
of outgoing power
away from the receiver. When the echo is received, the duplexing
equipment works in reverse
and forces the greater portion of the received signal into the
receiver-indicator channel. The
block diagram should assist you in fixing these component units of the
equipment in your mind.
GENERAL RADAR CHARACTERISTICS
There are some common characteristics of air-search, surface-search, and
than can be summed up as follows:
|Long-wave radar ||"P" frequency band
|Antenna ||Bedspring (curtain or flat)
|Maximum range ||Average 100 miles
|Minimum range ||Relatively long
|Bearing resolution ||Poor, due to wide beam
|Range resolution ||Poor, due to wide pulse
|Pulse duration or width ||Long
|Fade zones ||Observable
|Accuracy ||Expected range measurement error on 75-mile scale of PPI and
|"A" scope using scotch tape scale is +/- 1 mile.
Note: SM radar is an exception to the above.
|Micro-wave radar ||"S" or "X" frequency band
|Antenna ||Dishpan or barrel stave
|Maximum range ||Approximately the line of sight.
|Minimum range ||Relatively short
|Bearing resolution ||Relatively good
|Range resolution ||Relatively good
|Pulse duration or width ||Short
|Fade zones ||None
|Accuracy ||In general better than that of air-search radar. For specific sets see Part 4.
Fire-control radars vary so much that it is difficult to generalize about
characteristics of individual sets the reader is referred to Part 4.
However, bearing and range accuracies are comparable for all fire-control
radars. The average
expected error in range measurement is from +/- 15 yards plus 0.1% range
to +/- 40 yards. The
average expected error in bearing measurement is from +/- 2 to +/- 4 mils (+/- 1/10 degree to +/- 1/5 degree).
FACTORS AFFECTING RADAR RANGE
Maximum range factors.
In order to give you some reason for the variation in range performance
of radar sets, we shall
list the factors affecting the maximum range of any radar:
1. Wave length.|
a. Long wave length radar is best suited for air search.
b. Micro wave length radar is best suited for surface search.
2. Size of target.
3. Height of target.
a. Height of mast for surface target.
b. Height of plane for air target.
4. Target presentation (target angle).
5. Material of target.
6. Height of antenna.
7. Output power radar.
8. Sensitivity of receiver.
9. Atmospheric condition.
10. Type of indicator ("A" scope most sensitive).
11. Pulse repetition rate (determines maximum range scale that can be used).
12. Beam concentration.
13. Condition of radar equipment.
14. Operator's technique and skill.
Minimum range factors.
There are also factors affecting the minimum range. They are:
1. Pulse width,
2. Receiver recovery time.
3. Height of antenna.
4. Receiver gain setting.
HOW DOES RADAR DETERMINE ALTITUDE?
When enemy planes appear it is necessary to know their elevation before
anti-aircraft guns into action against them. How can this be done?
Position angle and range method.
At short ranges (up to several miles), the most accurate way to find the
altitude is by
|RADAR OPERATOR'S MANUAL|
using the angle of elevation (or position angle) and the range to the
target. The anti-aircraft fire-control set is equipped to do this.
How does this information-the range and position angle-give you the
elevation? For every size
position angle, there is a definite ratio of the vertical side opposite
this angle to the slant range.
This ratio is called the sine of that angle. Tables which list values for
all angles are available. As
soon as you find the position angle and the range, look up the sine of
the position angle and
multiply this ratio (the sine) by the range (the slant distance to the
plane). In some of the later
radars you can even make the set do this calculating for you. The
elevation equals range times
sine of the position angle.
The foregoing method works well for short ranges, but is not satisfactory
for longer ranges. At a
few miles range, you can get the elevation accurate within a very few
feet, perhaps within 25
or 30 feet, or even less. However, several factors cause the accuracy to
drop off at greater
ranges. One of these factors is that you cannot measure small position
angles (angles of
elevation) accurately. When you use vertical lobe switching, for
accuracy), the lower lobe is distorted by the ocean when you try to
measure small angles.
Naturally, this reduces the accuracy. With some fire-control sets, you
cannot measure position
angles of less than ten degrees. Of course, the higher the frequency of
the set the smaller the
angles you can measure, but you must train the lobe a few degrees above
the surface. A very
distant plane will be only a few degrees above the horizon; hence, the
difficulty of measuring a
small angle arises.
Another factor reducing accuracy is the curvature of the earth. Close
targets can be considered
to be above the same flat surface you are on, even though this is not
actually the case. The
resulting errors are small for targets within range of anti-aircraft guns.
The fighter director officer is also interested in elevation. He must get
the elevation of planes at
long ranges. Once an enemy plane is within anti-aircraft range the main
effectiveness of the
fighter director is gone. His business is to direct his fighter planes on
a course which will bring
them within visual range of the enemy planes and effect an interception
before the enemy gets
within antiaircraft range.
It was found that phenomena known as fades could
be used in doing the job. The use of charts indicating areas of these
fades (called fade charts) is
the long-distance method of finding elevation. It is not very accurate;
the closest you can expect
to come to the proper elevation is about 500 feet, but you stand a chance
of making greater
errors unless you are exceedingly careful. Errors of 15,000-20,000 feet
may he made, and
errors of 2,000-3,000 feet are common enough. Still, it is the best
method available for
getting elevations at great distances.
Air-search radar for altitude determination.
Air-search radar operators in times past often believed their sets were
not operating properly,
for the echoes from a plane coming in seemed to fade out more or less
regularly, until the pip
was no longer visible. Of course the pip became visible again before
very long, but the fading
out was disturbing.
This situation continued to puzzle the radar experts for some time. The
strangest part of it was
that not all planes faded at the same range. Then someone made the
discovery that all planes
flying at the same elevation (or altitude) faded regularly at the same
ranges. Planes flying at a
different altitude faded at different ranges, but any plane flying at a
specific altitude was
consistent: at any specific altitude all faded at the same range. A plane
at 5,000-feet elevation,
for instance, might fade at 72 miles, again at 50 miles, at 35 miles, and
at 21 miles. All planes
at this elevation faded at these same ranges. Planes at other elevations
faded at other ranges.
This was an important discovery. The fighter directors had long been
seeking a reliable way to
find the elevation of planes coming in. Here at last was a means by
which that information could
The fade points were different for different radar sets, depending upon
the frequency of the set and
|GENERAL RADAR PRINCIPLES|
height of the antenna, but once these fade points
were found for a given set, they were constant. By sending our planes out
at definite altitudes,
the location of the fades zones for each elevation could be found and
plotted on a chart.
Addition and cancellation of radio waves.
Before learning any more about fades, you should know why an echo from
any type target should
fluctuate in size, going from a maximum echo to a minimum or no echo,
thence to maximum, and
repeating this cycle as the target closes or opens. (A target is closing
when coming toward you
and is opening when going away.)
Since the radar waves leave the antenna at such a number of different
angles from the
horizontal, some of the energy (or radar waves) will hit the earth or
water, and bounce off
(like rubber balls) at the same angle as they hit (fig. 1-44). Certain of
these waves will strike
the water fairly close to the ship and may reflect right back into the
antenna as sea-return,
while others, due to their decreased angle from the horizontal, will
strike the water farther
out, be reflected, and continue on. Others with a smaller angle, will
strike the water still
farther out, until finally they will begin to miss the water on the
horizon altogether, and so
must be traveling nearly parallel to the waters surface.
When this occurs you must consider what is known as the phase
relationship between the two
waves shown going out from the antenna in figure 1-44.
One cycle contains 360 degrees. When a wave travels a distance of one wave
length, it goes through one
cycle, or 360 degrees. If the two paths of the waves are of different lengths,
the two waves will go
through different numbers of degrees in traveling from the antenna to the
If the two waves arrive as illustrated in figure 1-45, both starting a
cycle at the same time,
they are said to be in phase.
Figure 1-45. Two waves in phase.
When the two waves are in phase, they are always acting in the same
direction. They may be
compared to two forces pushing on an object in the same direction;
together they produce an
effect as great as that of one force equal to their sum. The two waves in
phase produce the same
effect as one wave whose strength is the sum of the two, as shown by the
dotted line in figure 1-45.
Figure 1-44. Determining elevation.
|RADAR OPERATOR'S MANUAL|
If the two waves do not arrive in phase, they are said to be out of
phase. When this occurs, you
must specify the amount they are out of phase. Thus, if one wave starts
its cycle 60 degrees after the
other wave, the two are 60 degrees out of phase. One wave may start a cycle
anywhere from 0 degrees to
360 degrees after the other. Of course, if the two waves are 0 degrees or 360 degrees out of
phase they are in
reality in phase. Figure 1-46 shows two waves 60 degrees out of phase. It is
seen that during some
parts of the cycle the two waves are opposing, and during the rest of the
supplementing each other. The resultant will therefore he smaller than it
would be if the two
waves were in phase.
Figure 1-46. Two waves 60 degrees out of phase
If the waves arrive as shown in figure 1-47 they are 180 degrees out of phase.
In this case the two
waves are always opposing, and if equal in strength they will cancel out
and the result will be
zero. Figure 1-48 shows two waves 270 degrees out of phase.
Figure 1-47. Two waves 180 degrees out of phase.
Now, let us redraw figure 1-44, representing the radio waves traveling
along both the direct
paths by wave forms similar to those used in the discussion on phase, for radar or radio waves will combine and behave in the same manner.
Figure 1-48. Two waves 270 degrees out of phase.
As these waves continue to leave the antenna, traveling paths of unequal
length, there are places
in space where the direct and reflected waves will be in phase. Thus,
their forces add, as in the
case of two forces pushing on an object in the same direction. In figure
1-49, plane B is in
space where the direct and reflected waves reinforce each other.
Therefore, the two waves
striking it will add and return to its a maximum reliable echo. In radar
these areas are called
At other places in space (see fig. 1-50, plane A) these radio waves
traveling paths of unequal
length will arrive at the target when they are 180 degrees out of phase. Thus,
their forces will cancel,
as in the case of two forces pushing on an object in opposite directions.
Therefore, if a plane is
flying through such an area, the two waves striking it will cancel each
other and a weak echo, or
probably none at all, will be returned. In radar, these areas are called
The result of ground reflection is to break the single free-space lobe
into a number of smaller
lobes with gaps between them. Figure 1-51 illustrates this effect.
The above theory is applied to the construction of a fade chart showing
the places in which you
get cancellation (fade area), or reenforcement (maximum area) for a
specific antenna. By use
of this chart, it is possible to determine the altitude of planes quickly
and without the use of
mathematics. Fade charts vary in appearance, because each one has to be
made to fit its
particular antenna installation.
Figure 1-52 shows a typical fade chart. The line on the left side of the
chart is marked off in
|GENERAL RADAR PRINCIPLES|
altitude. The bottom of the chart is marked off in nautical miles of
distance, or range. The cross
hatched lines on the chart represent the fade areas, or areas where the
waves cancel, while the
clear spaces represent the maxima areas, or areas where the waves add.
The precise boundaries
between the fade areas and lobe areas must be determined during
The chart as reproduced here shows the theoretical maxima and minima
areas, and is for
practice in determining the altitude of a plane. It is not intended that
this chart be used for
actual altitude determination. Whenever theoretical charts of this nature
are employed they
should be checked when tracking planes of known altitude. Thus, the
accuracy of the fade chart can be tested and any necessary changes made.
Let us take a hypothetical case in which the planes are assumed to be in
horizontal flight and see
just how their altitude is determined.
At 1000, radar reports large bogey (unidentified planes), zero-nine-zero
true, range 90 miles. After several reports, the operator reports that the echo is
fading, and finally at 1011 it
disappears at zero-eight-six, range 56 miles. This indicates that the
planes have entered a fade
area, but you do not know which one, since it could be any one of three
fade areas shown on the
chart at that range. Therefore, it is necessary to mark each line on the
lower side of each fade
area at that range, since the planes are closing (points A, B, and C).
(If planes are opening, the
mark will be placed on top of fade area.) At 1014, radar reports
|RADAR OPERATOR'S MANUAL|
echo reappears, zero-eight-one, range 48 miles. This indicates that the
planes have emerged
from the fade area, so we place a mark on the top of each fade area at a
range of 48 miles (points
D, E and F). This is enough information for an estimate of the planes
altitude. To find the
estimated altitude, select the pair of points on the fade area which line
up horizontally. The
estimated altitude of the planes, indicated on the altitude scale, is
Later, radar reports that the planes have again entered a fade area at a
range of 31 miles.
Operator picked up target echo again at a range of 28 miles. This
indicates that the planes have
gone through a second fade area. By plotting the points as before on the
fade area, you again find
that the altitude is 10,000 feet. This verifies the original altitude
The use of a calibrated fade chart has just been demonstrated. These
charts are calibrated to
show not only the fade and lobe centers, but the exact size and shape of
the area in spaces where
planes will fade. The fade and lobe centers can be calculated
mathematically, given the antenna
height and wave length (see the engineering manual for air-search radar).
However, the fade
areas and lobe areas, i.e., the exact areas of radar visibility and
radar invisibility, must be
determined by observation using planes flying at various known altitudes.
This is why fade
chart calibration exercises are held from time to time. It is essential
that your set be in
excellent condition during the calibration exercises, because any change
in the power radiated
from the antenna will result in a change in fade zone size and
re-calibration will he necessary.
Conversely, the fade chart, when calibrated at a time when the radar is
in good operating
condition, serves as a first-rate device for checking performance. If the
fade areas increase in
size, the materiel condition of the radar has slipped below par.
One other thing which affects the size of the fade areas is the size of
the target. Fade areas drawn
for one small plane will be larger than those drawn for a large plane or
many planes. In other
words, the bigger the target the smaller the fade areas. Some fade charts
show fade zones not for
just one size target but for several.
Completely un-calibrated fade charts (showing only
the positions of fade centers and lobe centers-points
of minimum echo and maximum echo respectively) can be used fairly well
even though the
limits of the fade areas are not indicated, provided they have been drawn
for the correct antenna
height and wave length. If you have been unable to calibrate your fade
chart, or if the materiel
condition has changed considerably since the last calibration, you can
still get a fairly good
solution by estimating the range at
|GENERAL RADAR PRINCIPLES|
which a plane passes through a lobe center (and gives its strongest
echo) and the range at
which it is in a fade center. This use of the chart does not provide as
rapid a solution of altitude
in some cases as would be obtained with a calibrated chart, since it is
necessary to wait until the
plane has flown through a maximum and a minimum point. However, the
condition of the radar
and the size of the target need not be taken into account when using the
chart in this way.
You have seen that in using the fade chart, the solution is reached in two
steps. As the plane
crosses a boundary between zones of visibility and invisibility (in other
words, when it enters
a lobe or fade) you get a number of possible solutions (one for each lobe )
because you do not
know which lobe the target
is entering or leaving. When it crosses a second boundary, however, all
solutions but one are
ruled out on the assumption that the plane is in level flight. Under
certain conditions it is
possible to tell which lobe a plane is in at the instant contact is made.
You can do this when the
plane is picked up at very long range. Anything beyond 120 miles would be
in the lower (first)
lobe or else its altitude would be over 40,000 feet, which is unlikely.
Look at your fade chart
and see what the maximum expected range is for contacts in the second lobe.
Instantaneous estimates are also possible when the plane is detected at
fairly short range: For
example, suppose you have been on the air-search radar for twenty minutes
and the screen has
Figure 1-52. Fade chart.
|RADAR OPERATOR'S MANUAL|
Then if a contact arrears at a range of 20 miles you
can reason that it is most likely at an altitude of 300
feet and entering the first lobe. The plane is not apt to be entering the
second lobe at an altitude
of 3,500 feet, because if it had made a horizontal approach at that
altitude you should have
detected it entering the first lobe at a range of about 65 miles. In
fact, if the plane had been at
3,500 feet, you should have detected it at all times between the ranges
of 65 miles and 24
miles. Similar reasoning rules out the possibility that the plane could
be entering the third or
fourth lobes, so the altitude must be about 300 feet and the plane must
be entering the first
lobe. This example illustrates not only one technique of altitude
determination, but also the
necessity for alertness on the part of an operator. If a plane is closing
at three miles per minute
and it flies in (low) under the first lobe, it may easily come to a range
of 20 miles before an
alert operator detects it. You can give the ship a maximum of seven
minutes warning if you are
on your toes, or less if you are not.
So far, only planes in horizontal flight have been considered. It is
possible for a plane to fly
down a lobe so that it does not fade at all. Likewise it is possible for
one to fly down a fade zone
and escape detection for long periods of time. This accounts for an
occasional plane getting in to
less than the usual range of detection. It does not happen often and it
is not done deliberately.
Before leaving the subject of fade charts, let us summarize briefly: To
use the charts you must
assume horizontal flight. Uncalibrated fade charts show the positions in
space of lobe centers
and fade centers. They can be used as they are if drawn for your antenna
height and wave length,
but it is best to calibrate them (by working with planes at known
altitude) to show areas in
which a plane will be seen and areas in which the plane will be in a
fade. The size and shape of
these fade areas will depend upon two things: the materiel condition of
the radar and the size of
the target. The better the materiel condition the smaller the fade zones
will be for all types of
air targets. The smaller the air target, the larger will be the
corresponding fade zones. The fade
chart can show fade zones for several sizes of air target. Calibration
exercises should be
conducted with the radar in the best possible materiel condition. The
fade chart affords one of
the most effective checks on the condition of the radar once it has been
calibrated. If a plane is
detected at extremely long range, or if it is detected at short range for
the first time, it must be
in the first
lobe, so immediate altitude estimate is possible. Constant use of fade
charts will familiarize you
with the certain capabilities and limitations of your radar.
SPECIAL USES OF RADAR
Actual operation in the Fleet has proved that radar may frequently be
called upon for special
jobs that are ordinarily not in the daily routine of the radar operator.
It is well, therefore, for
the operator to have a general knowledge of these special applications of
radar so that he may
carry out intelligently any special assignment when called upon. The
recommended in each case is discussed in some detail in Part 3.
Because fairly accurate bearings and ranges can be obtained on landscapes
with radar, it is
frequently used as an aid to navigation. The PPI is particularly useful
in this respect, because it
gives a fairly accurate picture of the surroundings. It is obvious that
the PPI can be of great
value in navigation when visibility is poor. Buoys can be seen-on the
PPI, as well as islands,
jetties and shore lines.
With sufficient experience the radar operator may
aid navigation by taking tangents on landscapes. This calls for
considerable practice, but
the method is approximately as follows: The operator swings the beam
toward the island or land
until pips just begin appearing on the scope. He then assumes that the
effective edge of the beam
is just striking the land, and swings the antenna on it about half the
effective beam width. The
center of the beam should then be on the edge of the land and the bearing
can be taken. A range is
then taken either on the edge of the land or the nearest point of land on
that bearing, and a fix
may be obtained. Tangents on low-lying landscapes are to be avoided as a
rule, because the
operator can never be sure that the tangent point is actually being
detected. Practice in taking
tangents will reveal other equally effective methods of obtaining fixes.
By training the antenna in the direction of the target, it is possible to
watch the shell pip move
along the time base. A shell in flight will give the appearance of a
mouse running under a sheer,
as seen on the "A" scope. When the shell strikes the sea, the resulting
geyser of water gives an
even better echo for several seconds. Do not try to range on the splash,
|GENERAL RADAR PRINCIPLES|
to estimate the range difference between the splash and the target.
In estimating this range difference, it is helpful to know the range
width of an expanded "A"
scope, the notch width, the width of a typical echo (expressed in yards)
as seen on the "A" scope,
and the range dimensions of anything else that can be used for
comparison. If there is a scotch
tape range scale on the "A" scope, it too can be used to estimate the
The radar receiver and antenna may be used as a direction finder to
obtain the bearing of
another radar or a jamming transmitter. It may be impossible to obtain
the range of a radar or
jamming transmitter, but bearing fixes from two receivers at separated
positions may provide
a fix. The receiver must be tuned to the same frequency as the radar or
jamming transmitter in
order to get an indication on the screen. The operator will see moving
pulses, humps, or other
indications on the screen, and the antenna should be trained until this
indication is at a
maximum. There are several reports of ships making a rendezvous with
other ships by this
method when visibility was poor, or radio silence maintained.
The fire-control radar may he put out of order during an engagement,
whereupon the search
radars will be called upon to give bearings and ranges for fire control.
The operator should be
prepared to handle this problem by knowing the range and bearing errors
of the equipment.
Constant check with fire-control radars will give the operator this
When shifting targets during an engagement some fire-control radars must
be coached on the
new target by the search-radar operator. The latter, having a complete
picture of the situation,
can easily and quickly coach on the fire-control radar with bearing and
The search operator never knows when he will be called upon to aid in
Although he may be on a destroyer or small craft, there is always the
possibility that someone
aboard his ship may be acting as a fighter director officer in an
emergency. The operator should
practice giving bearings and ranges rapidly and accurately on a large
number of targets so that
he will be able to handle this strenuous job if called upon.
FUTURE OF RADAR
Radar will play an increasingly important part in our lives during the
period following World
War II. It will be on the job then as now, protecting our lives, and
making this a safer world in
which to live. Read what David Sarnoff, president of RCA, has to say
about the postwar prospects
"Television and radar add new dimensions to radio. Wireless telegraphy
was its first dimension,
and broadcasting its second. Application of these new developments of
radio to peace creates new
fields of activity on land, at sea, and in the air.
"Radio instruments will emerge from the war almost human in their
capabilities. They will
possess not only a sense of direction, but a sense of detection that will
open new avenues of
service. The radio direction-finder, which heretofore had only an ear,
now also has an eye. The
safety of aviation will be greatly enhanced, for the aviator will be able
to see the ground through
clouds or darkness. By the scientific application of the radio echo, the
radio "eye" will avert
collisions, while the radio altimeter will measure the altitude and warn
of mountains ahead or
There is no doubt about it-radar is a coming field. Learn all you can
about your equipment, its
maintenance, and care. What you learn will be of use to you in the future.