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CHAPTER 16

SOFAR, HARBOR DEFENSE, AND OTHER SONAR SYSTEMS

Sofar
 

PRINCIPLES

Sofar (Sound Fixing and Ranging) is a long-range position-fixing system that uses explosive sounds in the permanent sound channel of the ocean. A fix is determined from the differences in arrival times, at known geographic positions, of a signal that is sent from any given point. The useful range from the signal source to the monitor stations can exceed 3,000 miles.

Sound Channel

A sound channel is formed by a layer of water that has a negative velocity gradient overlying an adjacent layer that has a positive velocity gradient. Under these conditions any sound signal that originates at a depth above and below which there is a higher velocity, is refracted back and forth so as to become horizontally channeled. Sound rays originating with an initial upward inclination are refracted downward, and those originating with an initial downward inclination are refracted upward. A central bundle of rays is channeled so that the rays never reach either the surface or the bottom. These rays, plus the low frequency of the sound source, are responsible for the long ranges obtained.

As a result of channeling, the signal intensity is not subject to attenuation resulting from (1) normal spreading because the spread is confined primarily in the horizontal plane, or (2) surface and bottom scattering, because the sound is always confined at middepths. If explosive charges, which produce low frequencies (30 to 150 cycles per second), are used, losses caused by absorption and volume scattering are kept at a minimum-about 0.002 db per 1,000 yards in the permanent sound channel of the ocean.

The vertical velocity distribution that produces the sound channel is caused by changes in

  temperature with depth. Above the axis of the sound channel there is a pronounced negative thermal gradient with a corresponding marked negative velocity gradient. This negative thermal gradient is due to the thermocline-the water layer in which most of the change in temperature between the surface and the bottom takes place. Below the axis of the sound channel the thermal gradient is only slightly negative or is negligible. The positive velocity gradient that results from the pressure effect of the water column exceeds the negative velocity gradient that is due to the slight decrease in temperature. Consequently, the velocity increases.

The depths at which the signal originates and is received govern the strength of signal and hence the range of operations. The optimum depth for maximum signal intensity is that of minimum velocity (axis of the sound channel). The velocity of sound in the sound channel affects the distance determinations and hence the accuracy of the method.

Factors Affecting the Use of Sofar

In addition to the velocity of sound and the depth of the signal, the following other factors affect the use of sofar:

1. Number of stations receiving the signal;
2. Accuracy of the geographic coordinates of base stations;
3. Geodetic data used at base stations;
4. Local departures of the earth's shape from that assumed;
5. Depth at which the receiving hydrophone is placed;
6. Depth at which the signal is fired; 7. Mean velocity of transmission;
8. Number of sound rays in the sound channel;
9. Shadows cast by obstacles (sea mounts, islands, and coast lines);

 
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10. Arc of reception at the hydrophone site;
11. Local topography at the hydrophone site;
12. Length of cable from the hydrophone to a shore station;
13. Background noise level at the hydrophone site;
14. Reverberation from the bottom adjacent to the hydrophone site;
15. Intensity of the signal.

Position Determination

The accuracy of any position determination using sofar depends to a great extent on a knowledge of the velocity of transmission. Of equal importance is the number of sofar stations receiving a signal. When three or more stations receive a signal, the location where it originated can be determined fairly accurately (within less than 10 miles). As the number of stations receiving a signal increases, the accuracy of the fix increases.

For ease in illustrating the principles involved in making a sofar fix, a plane surface will be assumed. In actual systems, compensations are made for the curvature and the ellipticity of the earth.

Example-Three stations on the same shore-A, B, and C-receive a signal. Determine the position of shot P when the arrival times are 11:00:00 at A, 11:02:30 at B, and 11:05:00 at C. In figure 16-1 the arrival at B was B - A (2 min 30 sec) later than at A, and the arrival at C was C - A (5 min 00 sec) later than at A. With the velocity known, circle B can be constructed with radius b equaling the velocity times the time lag at B. Similarly, circle C can be constructed with radius c equaling the velocity times the time lag at C. Then the center of the circle that is tangent to circles B and C and passes through A must define the location P from which the signal originated. Point P is at a distance x from A, x+b from B, and x+c from C.

If the problem involved stations on opposite coasts, it could be solved in the same manner. In actual practice one of the stations would act as central plot. The other stations would send their arrival times by radio to central plot. There, by use of special charts, the position would be determined within a few minutes after all arrival times were received. Because of the relatively slow travel time of sound in water (less than

  5,000 ft/sec), it may be as much as 1 hour between the time the signal is thrown overboard and a fix is obtained at central plot. (In sofar terminology the signal is the sofar bomb.)

Determining the Range From the Signal Duration

The distance that a signal has traveled can be estimated from the duration of the signal. The greater the range of transmission, the longer is the signal. This phenomenon is related to the ray paths followed in the sound channel. The first sound to arrive is weak and comes over the longest path, but it arrives first because of the higher velocities encountered along this path. The last sound to arrive comes over the shortest path-along the axis of the sound channel and the path of minimum velocity. Thus, the longer the distance, the greater the time differential between the first and last arrivals. Signal duration can be affected also by changes in the vertical velocity distribution and the depth at which the signal originates.

Because the time interval between successive arrivals decreases as the path along the axis is approached, and because the number of axial crossings and overlappings of arrivals increases as the path along the axis is approached, the intensity of a signal increases with duration of

Making a sofar fix.
Figure 16-1. -Making a sofar fix.

 
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signal until the last sound arrives over the axial path. The end of the signal is abrupt. The effect is similar to the interruption of a kettledrum crescendo, the sound becoming louder and louder and terminating abruptly. This effect makes those signals that originate at or near the axis of the sound channel easy to recognize either aurally or visually.

At shore stations where the hydrophone is on the bottom at a depth equal to that of the axis of the sound channel, the signal is followed by reverberation, which results from the backward scattering of sound off the bottom slope behind the hydrophone. Although the intensity of this reverberation is relatively high, it is not likely to mask the distinguishing pattern of the sound-channel arrival. A visual representation of a typical signal arrival as recorded by a power-level recorder is shown in figure 16-2. This figure shows that the signal

Figure 16-2. -Typical sofar signal.
Figure 16-2. -Typical sofar signal.

duration, X, is a function of both range and noise level. Because the noise level is variable the signal duration can be used to give only an approximation of the minimum range.

Determining the Mean Velocity of Transmission

All fixes in sofar depend on resolving into distances, the differences in arrival times of a signal at different monitoring stations. This computation can be made only if the velocity of sound transmission is known. The velocity can be determined either by (1) computing the sound transmission velocity at the axis of the sound channel from temperature, salinity, and depth observations or (2) making empirical determinations from shot travel-time data taken over known distances. Because of the scarcity of travel-time observations, velocity determinations are customarily established on the basis of the hydrographic data.

The velocity of sound transmission for a given temperature, depth, and salinity can be determined

  from graphs, tables, circular refraction slide rules, or hydrographic slide rules. If the circular slide rule or the British Admiralty Tables are used, the velocity is based on a salinity of 35 0/00 (35 parts per 1,000). If the observed salinity is greater than this value, the velocity must be increased at the rate of 4.3 feet per second per part salinity. If the salinity is lower, the correction must be subtracted. The effect of depth is to raise the velocity 1.82 feet per second per 100 feet of depth.

Because of changes in the depth of the axis of the sound channel with latitude, and because of other changes in the velocity structure with latitude, it may be necessary to have a series of zones with different mean velocities for varying distances from each station. Such a system is more applicable when monitoring in a north-south direction than in an east-west direction. For example, from Hawaii to California tile axis of the permanent sound channel varies not more than about 400 feet in depth and the velocity is constant within 8 feet per second. From the Aleutians to Hawaii the axis depth changes about 2,000 feet and the velocity about 36 feet per second.

How well can mean velocities based on hydrographic data serve for operations? This question cannot be answered satisfactorily until a pattern of shots fired at known positions is triangulated at a network of shore-based stations. Geodetic errors of position for the monitoring stations may be so great that empirically derived apparent velocities have to be used to obtain an accurate fix.

Effect of Depth at Which Signal Is Fired

The depth at which the signal should be fired is the depth of the axis of the permanent sound channel. This depth changes with oceans and with latitude. The axis of the permanent sound channel in temperate latitudes is about 350 fathoms in the Pacific Ocean and about 680 fathoms in the Atlantic Ocean. However, in Arctic and Antarctic areas the axis may be at depths of less than 100 fathoms.

In general, the firing off the axis of the sound channel distorts the signal. The distortion results from changes in both the geometry of the ray paths and the velocity of transmission. Shooting well off the axis of the channel changes the travel-time relation between the axial ray and the bounding ray of the sound channel. The bounding ray is

 
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that ray which just grazes the surface or bottom without being reflected. As a result of the changes in paths, the sound along the path of the bounding ray may or may not arrive first. At very shallow depths the sound along the axial path may arrive first; if so, the peak intensity comes at the

Signal (underwater sound) Mk 22.
Figure 16-3. -Signal (underwater sound) Mk 22.

  beginning of the signal rather than at the end. Records of signals from depth charges that were fired near 50 fathoms when the sound-channel axis was at 700 fathoms, show the peak intensity at the beginning of the signal. Upon the arrival of the signal, there is an abrupt rise from noise level to a high intensity, after which there is a gradual decay to noise level. This picture is the exact reverse of that obtained for a signal originating at the depth of the axis of the sound channel. As ray paths are reversible, a signal with a peak near the beginning also can be expected if the shot is fired at the depth of the axis of the sound channel but is received near the surface.

SOFAR BOMBS

Requirements

The problem of creating a bomb and detonator for use with sofar is particularly difficult because of the wide range in firing depths required. In addition, there are other problems, such as weight, cost, sinking time, and reliability of detonation.

The general requirements are as follows:

1. A sofar bomb must fire at the pressure at the axis of the sound channel, which varies from 500 to 2,000 pounds per square inch.

2. The detonator must fire either by pressure or, preferably, by a reversal of velocity gradient when at the depth of the axis of the sound channel.

3. The bomb must contain a sufficient explosive charge to be heard over the area of operations; must have a sinking time of not more than 5 minutes; and must be reliable, light, compact, and relatively inexpensive.

4. The bomb and detonator must be safe against accidental detonation.

Various types of bombs have been developed and tested for use with sofar. The only one to reach full production is the Mk 22.

MK 22 MODS 0 AND 1 SOFAR SIGNALS

General

Signals (underwater sound) Mk 22 Mods 0 and 1 were developed as explosive sound sources for use with the sofar system. The two modifications of the signal are fundamentally the same; each contains 4 pounds of TNT and a pre-settable hydrostatic fuse for exploding the TNT at any one of six depths between 1,500 and 4,000 feet. Specially

 
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prepared charts, which show the sound channel depth in various areas of the oceans, are supplied with the signal. The Mod 0 signal, shown in figure 16-3, has a tail vane and is intended primarily to be dropped from an aircraft in distress. The Mod 1, not illustrated, has no tail vane, and is for dropping over the side from a disabled ship or life boat.

Description

Each signal consists of the body part or signal tube, containing 4 pounds of TNT, and fuze Mk 175 Mod 0, which is screwed onto the nose end of the tube. Addition of the tail vane to the other end of the body identifies the Mod 0 signal. Before the signal is launched or dropped over-board, the cotter pin that locks the arming plunger

  must be removed. As long as the arming plunger protrudes through the diaphragm retainer the fuze is unarmed and safe. If the signal is to explode at a depth of 1,500, 2,000, 2,500, 3,000, or 3,500 feet, the bottle cap that covers the appropriate depth-setting port is taken off by means of the attached bottle opener. Removing the bottle cap exposes a rupture disk that closes the inner end of the port. Depth settings are indicated on the nose plate. If the signal is to detonate at 4,000 feet, no cap is removed, because at this depth, sea water operates the fuze through an open port directly below the nose plate. When the signal reaches the desired depth, the exposed disk is ruptured and sea water fills the fuze-head cavity, and exerts pressure on the firing diaphragm. The firing diaphragm is snapped forward and causes the shear wire, which
Fuze used with sofar bomb.
Figure 16-4. -Fuze used with sofar bomb.

239276°-53-19

 
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Map of northeast Pacific sofar network.
Figure 16-5.-Map of northeast Pacific sofar network.
holds the stab-type firing pin in place to give way. The firing pin then strikes the detonator.

The detonator remains in the safe position, even after removal of the safety cotter pin until, at a depth of between 750 and 1,200 feet, the pressure of sea water acting on the arming plunger and on the stiff copper arming diaphragm, moves the detonator permanently into the armed position. (See figure 16-4.) The arming plunger seats so that the detonator is aligned between the firing pin and the lead-in to the booster. The explosive train follows the following path: detonator, leadin, booster, and main charge.

Another type of bomb that has been developed for use in signaling by means of sofar, is the UNUSL multiple-shot bomb. This bomb is composed of four explosive sections, which fire at predetermined intervals, with a 90-second period after the detonation of the first section. The timing of the shots is controlled by a fifth section of the bomb. By means of this device coded messages

  may be sent from the ships to the sofar monitoring stations.

THE SOFAR NETWORK

The northeast Pacific sofar network (figure 16-5) consists of three monitoring stations located at (1) the U. S. Coast Guard Light Station, Point Sur, Calif., (2) the U. S. Coast Guard Lifeboat Station, Point Arena, Calif., and (3) the Naval Air Station, Kaneohe Bay, Oahu, T. H. Each station consists, essentially, of hydrophones planted offshore on the ocean bottom and connected by underwater cable to amplifying and recording equipment on the beach.

All three sofar monitoring stations are equipped with two channels, each consisting of (1) a beach amplifier located in a beach hut, (2) land lines connecting the beach amplifier and main monitoring equipment, and (3) the main monitoring equipment.

 
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Block diagram of one channel of a sofar monitoring station.
Figure 16-6. -Block diagram of one channel of a sofar monitoring station.
The main monitoring equipment is composed, essentially, of the following units:

1. Western Electric Co. 121A amplifier for each channel;
2. Power-level recorder for each channel;
3. Automatic switching unit;
4. Monitor-speaker amplifier and speaker, which may be switched to either channel;
5. Chronometer and related time-tick circuits for both channels;
6. Signal generator and calibration set for putting known calibration signals into the beach amplifier.

A simplified block diagram of one channel of a sofar monitoring station is shown in figure 16-6. At the left are the sea cables terminating at the lower panel of the beach-amplifier rack. The hydrophone numbers correspond to the numbers appearing next to each jack on this panel. No other numbers are used to designate hydrophones.

 
APPLICATIONS

The most publicized application of sofar has been for position location in air-sea rescue work. The outstanding advantage that sofar has over other air-sea rescue systems of signaling is that it operates automatically. The only action required of the operator is removal of the cotter pin. Then, regardless of whether the signal bomb is thrown overboard or sinks with the wrecked craft, the bomb fires and sends the signal because it is armed and detonated by pressure.

Another application of sofar is for long-range submarine signaling. By means of the multiple-shot bomb, signals can be sent at any fixed time interval. In this way coded messages can be sent great distances from any craft at sea. Special equipment is not needed, and with a network of monitoring stations messages are sure to be received.

 
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Harbor Defense
 
HARBOR UNDERWATER DETECTION

Harbor underwater detection, a little-publicized field of naval defensive warfare, is a part of the answer to enemy submarine threats to ships in allied harbors the world over.

To ensure detection of any vessel regardless of size or type, a number of different devices are used in the detection line. Most vessels are built of steel and have magnetic properties; consequently, one device is used which detects a ship's magnetic field. Propeller and engine noises are transmitted to the water and provide another means of detection by listening devices. That part of a vessel below the water line provides a surface from which underwater sounds of short duration may be reflected, thereby providing the requisites for echo-ranging devices.

Harbor echo-ranging and listening devices are used to provide precise tracking information to the patrol vessels and are placed adjacent to the patrol area inboard of the listening and magnetic detection lines. Magnetic indicator loops are placed to seaward, because experience has shown that they usually are more reliable in detection ability than the other systems. Because the magnetic loop is less dependent on the human element for its warning efficiency, it is very useful for the first warning. The radio sonobuoys or cable-connected hydrophone listening devices are placed just inboard of the loops where they serve to indicate what segment of the loop has been crossed and to provide additional information as to the direction, speed and type of vessel.

Detection Tactics

The purpose of fixed underwater detection is to eliminate the element of surprise from an enemy attack and allow necessary defensive action to be taken by patrol craft and harbor-defense batteries. The system, then, must provide a harbor-detection line which cannot be evaded and which is as firm as topographical factors and technical limitations of the equipment permit.

The magnetic indicator loop, which is laid on the ocean's bottom, records any distortions of the earth's magnetic field caused by the presence of an iron body over it. The magnetic field of a vessel

  passing over the loop is recorded on chart paper and the recorder mechanism in the station sounds an alarm.

The cable-connected hydrophones detect underwater sounds generated by a vessel's propulsion machinery and transmit the resultant electric impulses to a shore station by means of a submarine cable. These hydrophones are placed behind the magnetic indicator loop for the second line of detection. Radio sonobuoys perform the same function as cable-connected hydrophones but send the underwater sound ashore by means of radio instead of through a cable. They are used in place of hydrophones when water depths are excessive or when time does not permit the laying of submarine cable required for the installation of hydrophones.

Tests have shown that a submerged submarine running at "silent speed" usually cannot be heard when it is more than 500 yards from a listening device, so hydrophones and sonobuoys are installed less than 1,000 yards apart to force any vessel entering the protected area to pass within range of one of them. By noting the unit from which the signal is loudest, the operator can estimate the position of the target, and an experienced operator can usually determine the type of ship by the noises it emits.

A third type of harbor-detection equipment is the herald. The word "herald" has been derived from the first letters of the words "Harbor Echo-Ranging And Listening Device." From a tactical viewpoint the herald is the most precise of the underwater sound detection devices, in that with it the operator is able (1) to listen, (2) to obtain the bearing on a source of sound by virtue of the supersonic qualities of the system, and (3) to range on that source of sound by transmitting a signal and by listening to the returning echo and measuring the elapsed time required for the signal to go to and return from that object.

Because of the ability of the herald to obtain ranges and bearings, the target position can be pinpointed and harbor patrol craft can be directed to the exact location of the enemy. The harbor detection system, then, is usually composed of three lines of defense:

1. Magnetic loops which are the most dependable and require the least attention of the operator.

 
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When a passing vessel sets up currents in the loops, the equipment automatically records these currents and sounds an alarm, notifying the personnel that a vessel has begun to penetrate the defended area.

2. Cable-connected hydrophones or radio sonobuoys-listening equipments with which the operators can verify the contact and establish an approximate position for it.

3. Heralds, which give the bearing and range of the target and allow precise positions to be given to the friendly attacking vessels.

MAGNETIC-LOOP STATIONS

The loop is a very sensitive detection device when properly laid and operated. The distortion of the earth's magnetic field by a metal object crossing the cable causes magnetic unbalance between the two areas enclosed by the cable, generating minute currents which are indicated by a sensitive recording fluxmeter galvanometer in the shore station.

The loop itself consists of cables laid along the ocean bottom in the form of a figure "8". The average length of the loops is between 2 and 3 miles, but may be as short as 1 mile, or as long as 6 miles. In general the lengths should be kept as short as possible in keeping with the number of fluxmeters available. A shorter cable allows greater accuracy in localizing the target, and a reduction of ambient noise permits the equipment to be operated at higher sensitivities.

The spacing of the cables is usually 200 yards, which is the average length of most craft that will be passing over it. When the loop is designed for the detection of small craft and midget submarines the spacing may be made less.

When the cable is laid, great care must be taken to provide the proper tension on the cable as it is paid out. If too much tension is kept on the cable, lengths of cable will be suspended between high spots of the ocean's bottom. These suspended portions of cable will move with the movements of the water and limit the usable sensitivity of the system. If the cable is laid as slack as possible it will conform closely to the contour of the bottom, and movement will be materially reduced. However, some tension will exist as the cable leaves the ship because of the weight of the cable hanging in the water. Figure 16-7 shows the effects of the

  Correct and incorrect methods of laying
submarine cable.
Figure 16-7. -Correct and incorrect methods of laying submarine cable.

correct and incorrect methods of laying submarine cables.

Discriminator

In most magnetic-loop systems the maximum sensitivity of the fluxmeter cannot be realized because of interference caused by cable movement. For example, if a 10-foot length of cable moves 1/6 inch, the recorder pen is deflected far enough from the movement of the cable to be confused with a target ship. Because the frequency of interfering signals caused by cable movement is much higher than the frequency of signals caused by a ship passing over the loop, it is possible to construct a filter that removes the unwanted signals caused by loop movement, yet does not interfere with those from a ship. The discriminator has this function.

The discriminator consists essentially of two circuits, or channels, as follows:

1. A filter circuit, which passes frequencies of from 0 cycles per second (d-c) to 0.03 cycles per second. This circuit is made up of a filter network and a two-stage amplifier with a gain of slightly more than one. A limiter and an output stage are also included by which the output of the discriminator can be controlled so that the recorder pen does not exceed the limits of the recorder tape.

2. A limiter circuit which operates the recentering relay of the fluxmeter recorder to recenter the galvanometer. This circuit is required because the output of the filter circuit is so delayed that the galvanometer coil would be out of control if the usual centering action operated by the pen controls were in effect. This circuit can be adjusted to operate the recentering relay of the recorder at any desired deflection of the fluxmeter galvanometer, applying a return voltage to the galvanometer coil.

 
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Discriminator OS fluxmeter, recorder, and junction box.
Figure 16-8.-Discriminator OS fluxmeter, recorder, and junction box.
 
Figure 16-8 shows the external appearance of a discriminator with the OS fluxmeter and recorder.

The fluxmeter and recorder are devices to convert the minute changes in loop current to deflections of the pen in the recorder. The fluxmeter is mounted on a concrete block so that vibrations are not transmitted to the very sensitive galvanometer movement.

Multiturn Loops

From time to time the use of multiturn magnetic detection loops has been suggested for obtaining greater sensitivity. The suggestion is based on the fact that the size of a signature (the trace left by a passing ship on the recorder) increases in proportion to the number of turns used in the loop. However, there are relatively few locations where any real gain in sensitivity can be obtained by this means. In many locations the fluxmeter, on even a single turn loop, cannot be operated at its maximum sensitivity because of the perturbations due to cable movement or other causes. With multiturn loops these cable movements are increased in the same ratio as the ship signatures so that the signal-to-noise ratio, which is the usual limiting factor, is not changed.

 
CABLE-CONNECTED HYDROPHONES

The cable-connected hydrophone system is designed to pick up underwater sound noises from ships and to convert them into electric impulses. These impulses are transmitted by cables to shore equipments where they are amplified and monitored by an operator.

Tests have shown that hydrophones should be spaced no farther than 1,000 yards apart in a line across the channel to be protected. With this spacing, at least one of the hydrophones can be depended upon to pick up the sounds produced by a slowly moving submarine even if it is in the presence of a noisy surface vessel. The hydrophones are usually not placed at depths of more than 400 feet, but they will withstand pressures at depths up to 925 feet.

Hydrophone Assemblies

The tripods are approximately 8 feet on a side and 8 feet high and are constructed of extra-heavy 1 ½-inch iron pipe. They are designed to hold the hydrophone in a vertical position with the bottom of the hydrophone approximately 1 ½ feet above the base of the tripod. Each foot of the tripod

 
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Assembled cable-connected hydrophone.
Figure 16-9.-Assembled cable-connected hydrophone.

consists of a 1-foot cube of concrete weighing approximately 200 pounds. A hydrophone is shown in figure 16-9.

The hydrophone itself is approximately 55 inches long and 2 ½ inches in diameter. It comprises a long skeleton-like steel cylinder within which are supported eight crystal assemblies at 6-inch intervals. One end of the cylinder is closed by a watertight barrier, through which extend two insulated leads. The remainder of the cylinder is enclosed in a rubber jacket, and the entire unit is filled with castor oil.

Several hydrophones are placed in a line with relays at each hydrophone to connect it to the transmission cable when the system is used for listening to each one individually. A maximum of 20 hydrophones has been established for any one line, even though not more than seven on one line are recommended because the listening cycle would be too long.

An automatic-manual switching unit was designed to permit the automatic scanning of a line of 20 hydrophones, each one in succession. In

  this type of equipment automatic operation may be cut out at any time by manually operating one of the keys provided for that purpose. There is one 3-position key for each two hydrophones with the center position as normal. With all keys in the normal position, the switching is performed automatically.

If it is desired to listen to one particular hydrophone for a longer interval of time than permitted by automatic operation or not to wait until its regular turn in the automatic scanning, the key with the number of that hydrophone above or below it may be operated in the direction of the number desired. Odd numbers with indicator lights for each appear above the keys. Even numbers with indicator lights are below the keys. For example, if it is desired to stop automatic switching and connect No. 4 hydrophone through to the amplifier, the second key from the left should be moved to the down position. Other equipments may have rotary-type selector switches.

The hydrophone listening equipment, therefore, can be used in such a manner that the operator can listen to each of the hydrophones in succession automatically, or he may select any particular unit, to localize or confirm a contact.

RADIO SONOBUOYS

The radio sonobuoy is used for the same purpose as the cable-connected hydrophone. Fundamentally, the radio sonobuoy comprises a buoy barrel containing a medium-powered f-m transmitter, an antenna for transmission of the radio wave, a suspended crystal hydrophone, and a separate battery float and anchor.

In practice, the buoys are generally immediately behind the loops for the purpose of localizing the point at which one of the loops has been crossed. The buoys should not be spaced farther than 1,000 yards apart, as in the case of cable-connected hydrophones. Radio sonobuoys generally are used when there is not sufficient time for the installation of the necessary cables for the cable-connected systems, or when the water is too deep to allow placing of the cable-type hydrophones.

Model JM-4 Radio Sonobuoy

Function.-The purpose of the model JM-4 radio sonobuoy is (1) to detect underwater sounds

 
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produced by moving power-driven watercraft and (2) to transmit these sounds to a shore-listening station as a warning that a craft is moving in the waters within range of the buoy. Under normal conditions, a radio sonobuoy can detect a vessel underwater at ranges of about 1,500 to 2,000 yards. The buoys are spaced 1,000 yards or less apart to assure satisfactory coverage under unfavorable water conditions.

A receiver on shore-up to 19 miles from the most distant buoy-picks up the signal from the buoy. The receiver is continually tuned either manually or automatically, so that each buoy is listened to at least once each minute. When a ship's sound is picked up by a particular buoy this buoy is selected for continued listening to verify the presence of the vessel. If the ship's sound is heard on more than one buoy, the loudest buoy is assumed to be the closest to the vessel. A report is then made to the patrol activity that a vessel is present.

Description.-The model JM-4 radio sonobuoy equipment is shown in figure 16-10. It consists of an f-m transmitter contained in a buoy, powered by a large dry battery in a steel container supported in a toroid-shaped buoy. A hydrophone is suspended below the transmitter buoy by a suitable length of cable. The battery buoy is anchored, and the transmitter buoy is connected to it with a tie rope.

The transmitter buoy consists of a 53-gallon steel barrel, to which a cover, a tower and antenna assembly, and a tail pipe are secured. The transmitter is attached to the bottom of the cover of the buoy in such a manner that the radio equipment may be removed from the barrel by removing the cover. The tail pipe, which is weighted at the bottom to stabilize the buoy in a vertical position, is removable to facilitate handling and storage of the equipment.

The hydrophone is connected to the transmitter through its cable and a watertight connector located on the top of the buoy cover. This hydrophone, consisting of two sound-sensitive Rochelle-salt crystals, is suitably encased in a metal housing protected by a sound-transparent rubber sleeve.

Battery voltage is applied to the transmitter through a length of cable from the battery and a second watertight connector located on the top

  of the buoy cover and diametrically opposite the hydrophone connector. The battery cable follows the side of the transmitter buoy through two securing cable clamps and then is suspended in the water until it reaches the battery buoy. This cable is secured to the side of the transmitter buoy. The termination of the cable at the battery buoy is similar to that at the transmitter.

The battery buoy consists of eight drums welded around a larger battery container, in which the battery is sealed.

The buoy is suitably anchored and the transmitter is secured to it by lengths of wire rope.

The schematic diagram of the JM-4 sonobuoy transmitter is shown in figure 16-11. The r-f oscillator tetrode V105 is frequency modulated by triode V104 acting as a reactance modulator. Pentodes V101 and V102 comprise two resistance-coupled amplifier stages preceding the reactance tube.

The transmitter operates at any frequency in the range of from 70 to 90 megacycles. It is frequency-modulated and has an undistorted modulation width of ±75 kilocycles. The transmitter is provided with pre-emphasis so that the frequencies between 600 and 12,000 cycles per second are emphasized, while signals of other frequencies are amplified with lesser gain. The purpose of this pre-emphasis is to increase signal-to-noise ratio between 600 and 12,000 cycles per second because most of the useful frequencies are in this range. The audio range of the transmitter extends to 18,000 cycles per second.

The output circuit of the transmitter is coupled to a concentric line which feeds a quarter-wave ground-plane type of antenna.

The transmitter frequency and the modulation level are adjustable by means of two tuning controls and one level control, all of which are located on the top of the cover of the transmitter buoy. The r-f output is sufficient to operate over distances of more than 10 miles-depending on the sensitivity of the receiver and the type of antenna installation-at battery voltages as low as 200 volts.

The battery is of the dry cell type, and is designed to operate the transmitter for a continuous period of approximately 3 weeks, after which it should be replaced with a fresh battery.

Some disadvantages of these radio sonobuoys

 
292

FOLDOUT - Figure 16-10. -Model JM-4 radio sonobuoy equipment.

are that they are subject to damage by vessels colliding with them, they are likely to capsize and be put out of operation when the antenna becomes coated with ice, and they require frequent battery changes. The batteries are heavy and sometimes create a logistic problem.

On the other hand they can be installed quickly and easily in any depth of water, and they do not necessitate the long task of cable laying.

HERALDS

As previously stated, the herald is the most precise of the underwater sound detection devices, in that with it the operator is able to obtain exact bearings and ranges with the equipment. This information enables him to determine accurately the position of the target.

Model QBH Herald

The QBH herald equipment consists of a shore-station cabinet (figure 16-12), and a water-station unit (figure 16-13). The shore-station cabinet contains a receiver, a transmitter, and operating and training circuits. This console is connected by submarine cable to the water-station unit, which contains a crystal transducer and means for rotating and tilting the beam. The equipment operates from a 115-volt 60-cps source and requires about 700 watts.

The shore-station unit has three chassis. The bottom chassis, called the driver-amplifier unit, contains power amplifiers and rectifiers. The

  center chassis, called the training-control unit, contains training controls and indicators for the bearing and tilt of the beam. The top chassis, or receiver-indicator and driver-oscillator unit, contains receiver circuits, a range indicator, and a driver oscillator. Two range scales are provided-0 to 1,000 yards and 0 to 4,000 yards, with alternate keying provisions. The effective ranging capability of the equipment is about 4,000 yards. The duration of the ping is about 250 milliseconds (200 yards) on the 4,000-yard scale and 95 milliseconds (75 yards) on the 1,000-yard scale. A short ping of 30 milliseconds also is available for use on the 1,000-yard scale.

The water-station unit is located on the sea bottom. The watertight cylindrical housing is filled with castor oil. The crystal transducer, located at the top of the upper cylindrical housing, projects a beam of ultrasonic energy downward to a metallic plate, or acoustic mirror. The mirror is positioned by step motors to rotate or tilt the beam. These motors are located below the mirror in the lower part of the cylindrical housing.

The water-station unit can be located up to 5 miles offshore and at a depth not exceeding 300 feet. The location selected should be such that (1) the beam is unobstructed, (2) the tripod does not tilt more than 15°, and (3) the unit does not settle excessively.

The circuits of the equipment are very similar to shipboard echo-ranging equipment, as is the operation of the equipment. A block diagram of the QBH herald equipment is shown in figure 16-14.

 
Submarine Cables
 
INTRODUCTION

The installation of shore-operated harbor-detection equipment necessitates the use of a large quantity of submarine cables. The cost of the cable required usually exceeds that of the associated equipment, and the installation and maintenance of the cable require considerable skill.

The efficient functioning of the cable system depends on (1) the selection of cable of suitable design, (2) proper care and handling of the cable and of the associated underwater equipment during laying operations, (3) well-made cable splices, and (4) correct testing and maintenance procedure. Therefore, it is of the utmost importance

  that all personnel responsible for the installation and maintenance of harbor-detection equipment should become thoroughly familiar with standard submarine-cable practice.

The sole function of a submarine cable is to transmit electric currents underwater with the greatest possible efficiency. The core designs selected vary greatly, depending on the voltage, current, and frequencies to be transmitted and on the number and kind of circuits required. In addition to the core, protective coverings must be added (1) to protect the cable from abrasion and damage by the sea, and (2) to impart sufficient tensile strength to permit handling. These coverings

 
293

Shore-station cabinet of the QBH herald
equipment.
Figure 16-12.-Shore-station cabinet of the QBH herald equipment.

also vary considerably, depending on the conditions encountered. Therefore, most submarine cables are manufactured to meet the specific requirements of individual customers. Although individual designs vary, several well-established design techniques usually are followed. Hence, most cable designs are merely suitable combinations of standard components, assembled to produce a cable with the required characteristics.

Submarine cables may be divided broadly into two classes-sheathed and nonsheathed. In the former class the core is enclosed in a watertight lead sheath, thus permitting the use of a non-waterproof material like paper for insulating the individual conductors. In the latter class the lead sheath is omitted and the conductor insulation is of some material-such as rubber or thermoplastic compound-that retains its insulating properties when exposed to moisture.

Lead sheaths are easily damaged or cracked when subjected to rough handling or continuous motion caused by waves, and consequently they are used ordinarily only in sheltered waters. As underwater-detection cables frequently are laid under adverse conditions, lead sheaths and paper insulation are not deemed sufficiently reliable. Therefore, these instructions deal exclusively with cables of the nonsheathed class.

Most nonsheathed cables consist of the required number of conductors, individually insulated with rubber or thermoplastic compounds and suitably

  taped or braided. These insulated conductors are laid up spirally, filled, and taped to form a round core. In some cables a rubber or thermoplastic jacket is applied over this core. Although such jackets are essentially waterproof, jacketed cables are not considered to be in the sheathed class because minute amounts of water may in time penetrate the jacket, making the use of waterproof insulation on the individual conductors advisable. Jute or other bedding usually is applied over the taped or jacketed core; steel-wire armor is then laid on spirally for mechanical protection; and a final layer of jute, coated with tar and asphalt, is added to reduce corrosion.

TYPES

Harbor detection cables can be divided into three classes-(1) magnetic loop cables, (2) herald cables, and (3) hydrophone cables.

Several types have been designed for each class, depending on their function and the conditions under which they are installed.

Figure 16-15 shows three submarine cables- types 101, 102, and 113.

Water-station unit of the QBH herald
equipment.
Figure 16-13. -Water-station unit of the QBH herald equipment.

 
294

FOLDOUT - Figure 16-11.-Schematic diagram of JM-4 radio sonobuoy transmitter.

 
Model QAA Demolition-Team Sonar
 
DESCRIPTION

The model QAA equipment (figure 16-16) is a portable f-m sonar device that gives a qualitative indication of direction and distance of objects within a range of about 5 to 75 feet. A block diagram of the equipment is shown in figure 16-17. The low-frequency oscillator varies the frequency of the high-frequency oscillator. The output of the high-frequency oscillator then is applied to a quartz-crystal transducer, which sets up ultrasonic waves in the water. A portion of the transmitted sound wave is reflected by the object, and on striking the quartz-crystal transducer it generates a voltage, which is applied to a detector. The equipment uses a single crystal for transmitting and receiving. A small amount of the output of

  the high-frequency oscillator also is applied to the detector. The audio beat-frequency output of the detector is amplified through two stages and then applied to the headphones.

The frequency of the detector output depends on (1) the rate at which the high-frequency oscillator frequency is changed and (2) the distance to the object. The shorter the range to the target, the lower is the frequency of the beat note, because less time is required for the transmitted wave to be reflected back to the detector. Hence the nearer the oscillator frequency will be to that of the reflected wave at the time it is received.

CIRCUIT

Block diagram of QBH herald equipment.
Figure 16-14. -Block diagram of QBH herald equipment.
 
295

Submarine cables, types 101, 102, and 113.
Figure 16-15 -Submarine cables, types 101, 102, and 113.
The first tube, V1, is the low-frequency oscillator. It uses a resistance-capacitance feedback network to produce a frequency of 12 cycles per second. This frequency is used to sweep the high-frequency oscillator.

Tube V2 is a conventional reactance tube modulator. The high-frequency voltage from tank, Ll, is fed back to the grid of V2 by a 10 μμf capacitor. A high-frequency current will flow through C7, R8, and C6 to ground. This results in a voltage across R8 and therefore on the grid of V2 which is in quadrature with the tank voltage. V2 plate current will therefore be in quadrature with the tank voltage and the effect is that of a capacitor in parallel with the tank. This tube current, and hence the effective capacitance, is varied at a 12-cps rate by the signal from

  V1. Thus, the resonant frequency of the V3 oscillator tank is varied. The frequency varies

Model QAA portable sonar equipment.
Figure 16-16 -Model QAA portable sonar equipment.

 
296

Block diagram of the QAA portable sonar equipment.
Figure 16-17 -Block diagram of the QAA portable sonar equipment.
over a range of 5,000 cycles per second above and below the average frequency.

Tube V3, the high-frequency oscillator, is a typical electron-coupled oscillator. The plate

  circuit, which is isolated from the grid circuit by the screen grid, has only a minor effect on the frequency. The reactance tube, V2, is connected across the frequency-controlling portion of the
Schematic diagram of the QAA portable sonar equipment.
Figure 16-18 -Schematic diagram of the QAA portable sonar equipment.
 
297

AN/CRT-1A radio sonobuoy and hydrophone.
Figure 16-19. -AN/CRT-1A radio sonobuoy and hydrophone. A, Transmitting position; B, hydrophone.
circuit and varies the frequency in accordance with the signal from the low-frequency oscillator. The plate circuit produces a high voltage across the quartz crystal. The oscillator normally operates at an average frequency of 500 kilocycles.

Tube V4 is a combination detector and first audio-frequency amplifier. The detector is a

  diode. The network of resistors and capacitors (RIO, R12, R13, C14, and C15) between the diode plate and the control grid is a filter to suppress high frequencies in the detector output.

Tube V5 is the audio output tube. The audio output is coupled to the phones through a transformer.

 
298

 
Submarine Detection by Aircraft
 
Antisubmarine warfare is a vital part of the defense problem, and the detection of submarines by aircraft is a high-priority phase of the antisubmarine problem. Although radar is the basic detection device for surface vessels, it is ineffective against submerged submarines. In World War II, two devices were developed and used by aircraft for the detection of submerged vessels. These devices are the radio sonobuoy and the magnetic airborne detector (MAD).

TYPE AN/CRT-1A RADIO SONOBUOY

The AN/CRT-1A, designated ERSB (Expendable Radio SonoBuoy), is an expendable device that is dropped from airplanes or blimps by means of a small, self-contained parachute. It is used to pick up the underwater sounds of submarines and transmit them to the aircraft by radio. The ERSB is made up of (1) a cylindrical magnetostriction hydrophone and (2) an amplifier connected to an f-m radio transmitter. The sonic hydrophone, amplifier, and transmitter, together with a battery power supply, are incorporated in a waterproofed cardboard tube about 30 inches in length and 4 inches in diameter and weighing about 12 pounds. The transmitter and batteries

Interior view of the AN/CRT-1A buoy.
Figure 16-20. -Interior view of the AN/CRT-1A buoy.

  housed in an upper compartment, which is separated by a watertight bulkhead from the release mechanism, hydrophone, and cable in the lower compartment.

The transmitter operates on frequencies between 67 and 72 megacycles and has a maximum range of about 35 miles when the aircraft is at an altitude of 5,000 feet. The device has an operating life of from 2 to 4 hours after planting. After this period, a carbowax plug dissolves and permits the buoy to sink. In order to track a moving submarine, several buoys may be dropped in a pattern surrounding the known or suspected location of the submarine. A receiver, designated by type-number AN/ARR-3, is carried in the aircraft. The receiver has as many as 12 channels corresponding to the frequencies of the buoys. A high degree of automatic-frequency control compensates for any lack of frequency stability in the buoys.

A wooden cap, fitted with a rubber gasket and clamping screws, seals the top of the tube and serves as a mounting for the antenna and parachute assembly. This cap also contains the carbowax plug, to flood the mechanism at the end of its life. Four holes are cut through the wall at the upper end of the lower compartment to ensure flooding and to provide a cushioning effect by regular air release as the buoy strikes the water.

The bottom of the housing terminates in a cast-metal ring, which aids in stabilizing the buoy in the water and which provides a mounting for the hydrophone release mechanism. This mechanism consists of a spring arrangement, which holds the hydrophone firmly in place during shipping and handling but which automatically triggers on impact with the water and permits the hydrophone to drop to the limit of its 24-foot cable, as shown in figure 16-19, A. The hydrophone, figure 16-19, B, is a cylindrical magnetostriction unit that is wound on a nickel shell. Its construction permits the storing of the cable inside the hollow shell and effects a reduction in length of about 4 inches compared with earlier models.

Two photographs of the AN/CRT-1A transmitter are shown with the cover removed in figure 16-20. The r-f side is shown at the left and the a-f side at the right.

 
299

Schematic diagram of the AN/CRT-1A.
Figure 16-21 -Schematic diagram of the AN/CRT-1A.
The f-m transmitter utilizes five vacuum tubes, which provide approximately 90 db of audio-voltage gain and an effective r-f antenna radiation of about 0.1 watt. Frequency modulation was used in preference to amplitude modulation for three main reasons: (1) the signal-to-noise ratio, which is considered of vital importance because the receivers are always used in close proximity to aircraft engines, is better for frequency modulation; (2) frequency modulation provides precise automatic control of volume of all signals sufficiently strong to fall within the effective operating   range of the receiver; and (3) frequency modulation reduces the effects of interference between two buoys of the same frequency. This interference exists when extra buoys are dropped while tracking, and before the original buoys have ceased operating.

The AN/CRT-1A transmitter is mounted on a single rectangular plate, with the audio amplifier and the reactance tube on one side and the r-f circuit on the other. The mounting provides compactness and improved isolation between the a-f and r-f circuits.

 
300

AN/ASQ-1 MAD equipment.
Figure 16-22 -AN/ASQ-1 MAD equipment.
Freedom from microphonic noise is achieved by use of (1) four shockproof rubber mountings for the chassis plate and (2) separate rubber mountings for each tube socket. The whole transmitter assembly is enclosed in a transparent acetate tube for protection when the unit is withdrawn from the housing for installation of batteries.

The battery assembly consists of four parallel-connected standard 1.5-volt flashlight cells for filament voltage, and two series-connected 67.5-volt miniature batteries for plate voltage. Sufficient battery capacity is available for a continuous operating life of about 4 hours.

239276°-53-20

  The antenna is a 39-inch telescoping quarter-wave tube mounted on the buoy housing cap. About 9 ½ inches of the antenna are enclosed in a watertight insulating sleeve to avoid short-circuiting by waves. The antenna is coupled to the r-f amplifier tube by a tuned circuit. This tuned circuit matches the impedances of the antenna and transmitter and helps to stabilize operation by isolating the tuned transmitter circuits from the direct influence of any variations in antenna characteristics due to motion of the buoy.

The parachute is 24 inches in diameter and is orange-dyed. After the buoy is launched, the pack

 
301

Magnetometer of the AN/ASQ-1.
Figure 16-23. -Magnetometer of the AN/ASQ-1.
cover is torn loose by a static line attached to the plane. The antenna protrudes through a hole in the chute, and the pull on one of the shrouds withdraws a switch pin and turns on the transmitter. On reaching the water, the parachute settles about the antenna base.

The schematic diagram of the AN/CRT-1A is shown in figure 16-21. The oscillator is frequency-modulated by the reactance tube. The reactance tube is driven by two audio-amplifier stages.

MAGNETIC AIRBORNE DETECTOR

One of the ASW devices developed and used in World War II was the magnetic airborne detector (MAD). Detection equipments AN/ASQ-1, AN/ ASQ-1A, AN/ASQ-3, and AN/ASQ-3A were installed on naval airplanes and airships. The AN/ASQ-1 equipment is shown in figure 16-22.

The MAD equipment uses a magnetometer, which is a saturable inductor. The magnetometer (figure 16-23) is a coil of wire wound on some high permeability core such as permalloy. A d-c current is passed through the coil to balance out the earth's magnetic field. The core therefore is in zero field.

The inductor is energized by a pure 400-cps sine-wave current (for the AN/ASQ-1 and AN/

  ASQ-1A) or a pure 1,000-cps sine-wave current (for the AN/ASQ-3 and AN/ASQ-3A). The current saturates the core on both positive and negative swings. As the magnetometer is in zero magnetic field, the core is saturated equally on both positive and negative swings, and only odd harmonics of the exciting signal appear at the output. However, when the magnetometer enters a magnetic field, the operating cycle is not the same on both positive and negative swings, and even harmonics appear at the output.

In the AN/ASQ-3 equipment, a band-pass filter separates the 2,000-cps second harmonic that is produced when the magnetometer element enters a magnetic field such as that which might be produced by a submarine. This signal is amplified and recorded on a recorder.

In an airplane or airship the magnetometer element is mounted so that it is as far as possible from the field of the ship. The detector element can be mounted on (1) the end of streamer cable, (2) a wingtip, (3) a "stinger" tail, or (4) the bag of an airship.

Provisions are made for stabilizing and orienting the sensitive element around two axes. This element is in a gimbal mounting, and two gimbal motors are used to position the element.

 
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