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CHAPTER 14

SUBMARINE SONAR EQUIPMENT

Introduction

The primary method of sonar detection by a submarine is listening (chapter 13) because echo ranging discloses its presence. On a war patrol a submarine maintains a listening watch. When a target is detected, the bearing is determined by the listening equipment. When the sonar range to the target is desired, the echo-ranging transducer is trained to the target bearing and a single short ping is emitted. The echo-ranging equipment on a submarine is used most often for navigation and only as required for target ranging.

During World War II, combination sonar equipments that provided ranging, listening, and sounding were installed on submarines. The model WCA is such a combination sonar equipment.

  The model WFA sonar equipment, which has torpedo-detection and mine-detection circuits, was developed near the end of World War II. As the WFA does not have a sounding device, it is installed usually with an echo-sounding equipment, such as the model NGA. The WCA and WFA-1 are described in this chapter. The NGA is a sounding equipment for installation in submarines only. It provides a paper record as well as a rotating-light indication.

Capacitive and f-m scanning sonars, which have been developed since the war, are being installed on modern submarines. The model QHB-1 capacitive-type scanning sonar and the model QLA f-m scanning sonar also are described in this chapter.

 
Model WCA Sonar Equipment
 
The WCA sonar equipment was mounted on most submarines during World War II. It is now being replaced by scanning equipment. Because it is no longer being installed on modern submarines, the WCA sonar equipment is described only briefly.

The model WCA-2 equipment uses 3 transducers and 1 hydrophone in three separate housings, as shown in figure 14-1. The NM is a magnetostriction transducer. The QC magnetostriction transducer and the JK Rochelle-salt hydrophone are housed in one sound head, called the QC-JK. The QB is a Rochelle-salt transducer.

As shown in figure 14-2, the WCA-2 consists of three systems.

One system, the QC-JK, uses the combination sound head for echo ranging and listening. The QC magnetostriction element is used for echo

  ranging. The JK crystal hydrophone, which is more sensitive than the QC magnetostriction transducer, is used for listening only. The QC and JK units cannot be used simultaneously.

The second system uses the QB Rochelle-salt transducer for echo ranging. The QB transducer can be operated over a wider range of frequencies than the QC because a crystal transducer is less sharply resonant than a magnetostriction transducer of the same beamwidth.

The third system uses the NM transducer for sounding only. The NM system requires no training controls because the beam is directed vertically downward.

Although the combined equipment consists essentially of three separate complete systems, each of the three systems uses one or more units in common with one or both of the other systems.

 
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FOLDOUT - Figure 13-22. -Schematic diagram of the AN/UQC-1 equipment.

The three sound heads of the WCA-2 sonar equipment.
Figure 14-1. -The three sound heads of the WCA-2 sonar equipment.
 
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For example, the QB and QC-JK systems use the same range indicator, and the sounding and QC-JK systems use a common driver unit.

The units of the WCA are similar to standard units discussed previously. The indicator is the familiar rotating-light type used on early echo-ranging equipment. The sounding unit has its

  own indicator, which is a rotating-light type. The only major difference between surface-ship echo-ranging equipment and the WCA is the use in the WCA of a Rochelle-salt echo-ranging system. The remote training unit provides for slewing in either direction because the equipment is used for listening most of the time.
 
Model WFA-1 Sonar Equipment
 
The model WFA-1 is a searchlight type of echo-ranging and listening equipment. It can be operated in any one of the following modes: (1) Listening, (2) echo-ranging, (3) communication, (4) torpedo-detection, (5) mine-detection, and (6) monitoring own ship's noise.

Listening may be carried out over the frequency range of from about 200 cps to about 100 kc. Bearing is determined accurately by a bearing-deviation indicating (BDI) meter.

The frequency band for echo ranging is from 17.2 to 46 kc. The BDI circuits can be used with echo-ranging operation for high accuracy. Higher frequencies used in enemy waters give maximum secrecy and highest bearing accuracy, but shorter maximum ranges.

Telegraphic communication with other vessels equipped with echo-ranging equipment is made possible by the inclusion of a telegraph key that keys the transmitter. The frequency range for this mode of operation is the same as for echo ranging.

For torpedo detection, the transducer beneath the keel is rotated constantly. Any sound signal picked up is fed through the receiver to the range recorder, the stylus of which is synchronized with the transducer rotation, as explained later.

For mine detection, the equipment is operated as a short-pulse echo-ranging equipment using either the topside transducer or the transducer beneath the keel. In this mode of operation the transducer is trained over a restricted arc of 30° or 40° on each side of the bow. Mines and other small navigational hazards can be detected at ranges up to 600 yards. The short pulse produces the high resolution that is necessary to detect objects, such as mines at short ranges or in close proximity to one another.

  The WFA-1 equipment has two identical control stacks, one in the conning tower and the other in the forward torpedo room. The conning tower is the primary control point, but, in an emergency, control can be switched to the forward torpedo room. Figure 14-3 is an over-all pictorial diagram of the equipment.

Block diagram of the WCA-2 sonar equipment.
Figure 14-2. -Block diagram of the WCA-2 sonar equipment.

The WFA-1 has two sound head assemblies-one mounted on the deck, the other on the hull near the keel.

The topside sound head is mounted vertically on the main deck above the forward torpedo room

 
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and is rotated by the topside training mechanism. It is not retractable. This sound head consists of three individual units, as follows:

1. A low-frequency (sonic) hydrophone, which operates over a band of from 200 cps to 15 kc for listening.

2. An intermediate-frequency (ultrasonic) transducer, which operates over a band of from 17.2 to 35 kc for echo ranging, and over a band of from 12.5 to 35 kc for listening.

3. A high-frequency (ultrasonic) transducer, which operates over a band of from 35 to 46 kc for echo ranging and over a band of from 31 to 100 kc for listening. These frequencies are selected because the crystals that are used operate at optimum efficiency in these ranges.

The lower sound head is mounted on a hoist-train shaft and functions like any retractable searchlight transducer. It contains a single crystal transducer that operates over a band of from 22 to 32 kc, with most efficient operation at 27 kc

  for both echo ranging and listening. The sound head is spherical to prevent turbulence at high speeds and to keep water noise at a minimum.

For torpedo detection, the lower sound head is rotated at 12 revolutions per minute and the range-recorder stylus sweeps across the chart in about 4.6 seconds. Thus, each sweep of the stylus occurs in one complete revolution of the transducer. Stylus travel is synchronized with the transducer by a microswitch mounted on the transducer shaft. The interval for fly-back of the recorder stylus occurs while the transducer rotates through the sector from 170° to 190°. This sector is chosen for fly-back because during this interval the transducer sweeps across the ship's stern, and only unwanted sounds from the screws are picked up. The bottom scale on the recorder is graduated to indicate the bearing of all received sounds. Torpedo detection is strictly a listening function.

The receivers are of the sum-and-difference type and have BDI meters, as well as RCG circuits.

 
Model QHB-1 Capacitive-Scanning Sonar Equipment
 
The model QHB-1 is almost identical with the model QHB scanning sonar that is used on surface ships. The QHB-a is described in chapter 6 of this text. The QHB-1 differs from the QHB-a in the following respects: (1) the QHB-1 has a relative instead of a true PPI presentation-that is, the bearings of signals are referred only to the heading of the submarine; (2) it has a single-ping   keying feature as well as facilities for automatic keying; and (3) its transducer is more ruggedly constructed to withstand increased pressure without leaking. To prevent accidental keying, the keying control mechanism of the QHB-1 must be manually held in the closed position, either for "automatic" or repetition keying or for single-ping operation.
 
Model QLA F-M Scanning Sonar Equipment
 
DESCRIPTION

The QLA echo-ranging equipment is an f-m scanning sonar. It provides a plan-position indication (PPI) of underwater objects within sound range. It can be installed on submarines or surface vessels. In contrast to the searchlight-type sonar, the f-m sonar provides continuous area search coupled with the ability to detect very small objects.

Sonar echo-returns from vessels, wakes, sand banks, antisubmarine nets, and other submerged

  objects that reflect ultrasonic energy are presented both audibly and visually. The audible signals are tones. The visual signals are intensity-modulated spots on the oscilloscope PPI indicator. Figure 14-4, A, shows a surface ship entering a channel. Figure 14-4, B, shows the QLA indication aboard the ship.

The QLA equipment and the location of the major units are shown pictorially in figure 14-5. The major units are (1) the frequency-modulated oscillator, which generates the f-m signal; (2) the driver, which amplifies the f-m signal and drives

 
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Typical QLA indication.
Figure 14-4. -Typical QLA indication. A, Ship entering channel; B, indication aboard the ship.

the projector; (3) the sound head, which contains the projector and a hydrophone for receiving the echo returns; (4) the hoist-train mechanism, which raises, lowers, and rotates the sound head; (5) the receiver, which heterodynes the returning echoes

  with the oscillator signal; (6) the analyzer, which uses a series of 20 filters which are sequentially connected through electronic switch tubes to extract the frequency components of the heterodyned signal; and (7) the indicator, which intensity-modulates a cathode-ray tube beam in accordance with the output of the analyzer and which moves the beam to present a plan-position indication. The loudspeaker makes the returning echo audible, and the test oscillator is used to adjust the sweep of the frequency-modulated oscillator.

PRINCIPLES OF OPERATION

The functional block diagram of the QLA equipment is shown in figure 14-6. The f-m oscillator develops the carrier signal, which varies with time. At the beginning of an operating cycle the frequency is at a maximum of 46 2/3 kc, and at the end of the cycle (several seconds later) it is at a minimum of 36 kc. The frequency decreases uniformly from the maximum to the minimum value. At the end of an operating cycle the frequency returns abruptly to its maximum value and the cycle is repeated. The abrupt return or fly-back to initial frequency requires only a few milliseconds, during which time the projector is silenced or "blanked."

If frequency is plotted vertically and time horizontally, the result is a curve having a sawtooth pattern, as shown in figure 5-20. The downward slope of the sawtooth signal represents a decrease of frequency with time; the vertical line forming the left side of the waveform represents the fly-back from minimum to maximum frequency. The length or base of the sawtooth waveform corresponds to the time required for one operating cycle. The frequency at any instant is different from that at any other instant in an operating cycle, and the frequency changes linearly with time.

The QLA sound head contains a projector and a hydrophone. The projector transmits sound waves in a wide fan-shaped beam that has an arc of about 80°. The sound waves are reflected by any object in the beam of the projector. A small part of the reflected energy returns to the sound head as an echo, where it is picked up by the sharply directive hydrophone.

 
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FOLDOUT - Figure 14-3. -Over-all pictorial diagram of the WFA-1 equipment.

Major units of the QLA f-m sonar.
Figure 14-5. -Major units of the QLA f-m sonar.
 
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Range Measurement

During the time required for sound of a particular frequency to reach the target and to return as an echo, the frequency of the projected sound decreases. The longer the travel time, the greater is the decrease and the greater is the difference in frequency between the echo and the sound being radiated as the echo arrives. Figure 5-20 shows that the frequency difference, f, between echo and signal is proportional to travel time. It is evident, therefore, that the difference in frequency between a returning echo and the signal being transmitted when the echo is received, is proportional to the range of the reflecting object.

The QLA sonar receiver mixes the echo with the signal that is being transmitted and produces a beat frequency equal to the difference in frequency between the echo and the transmitted signal. This frequency difference is presented to the operator both audibly and visually. The audible indication is a musical tone of constant pitch in the loudspeaker; and the visual indication is a spot of light on the cathode-ray indicator.

The difference frequency can be any frequency between 0 and 10 2/3 kc. However, only frequencies between 500 and 2,000 cycles per second are analyzed and used to indicate range. This band was chosen for technical reasons, including ear sensitivity and filter-design considerations.

The analyzer resolves the frequencies between 500 and 2,000 cycles per second by use of 20 bandpass filters, 20 detectors, and an electronic switch. The receiver output is applied to all 20 filters (figure 14-6). The signal at the output of each filter depends on the frequency of the signal and, hence, on the range to the target. The filter outputs are rectified and applied to the intensity amplifier of the indicator. The electronic switch applies the output from filter 1 through filter 20 in sequence as brightening voltage to the oscilloscope. During this time the spot on the oscilloscope is moved radially outward from the center of the screen. Thus, for each of the 20 filters there is a corresponding radius on the screen. For example, a signal with a frequency of 500 cycles per second brightens the trace at a point 3/4 of an inch from the center; a signal with a frequency of 2,000 cycles per second brightens the trace at a point 3 inches from the center.

  Range Scale

The range scale is selected by changing the rate at which the f-m oscillator sweeps in frequency. The rate at which the oscillator sweeps determines the frequency difference corresponding to a given range. The greater the rate of change of frequency, the greater is the difference (number of cycles per second) representing a given range.

The oscillator can be swept at five rates. Thus, the operator can select one of five range scales. The rates of sweep are such that range scales of 300 feet, 300 yards, 600 yards, 1,200 yards, and 3,000 yards are available. The periods of sweep corresponding to these ranges are 0.67 second, 2.0 seconds, 4.00 seconds, 8.00 seconds, and 20 seconds, respectively.

Bearing of the Indicator Sweep

The bearing of the hydrophone determines the angular displacement of the sweep on the indicator. As shown in figure 14-6, the hydrophone training mechanism operates through a sine potentiometer and a sweep generator and orients the trace so that the trace is at the same angle (with respect to the vertical) that the hydrophone axis is with respect to the heading of the ship. The indication of any echo on the screen therefore appears in a position corresponding to its relative bearing

Maximum Scanning Rate

The maximum angular rate of speed at which the QLA sonar can scan depends on the maximum range for which the equipment is being operated. The projector transmits sound into the water over an arc of 80°-that is, 40° on each side of the hydrophone. Thus, a particular target is in the field of the projector for about 40° of its rotation before it is received by the hydrophone. The sound head must rotate less than 40° in the' time required for the sound to travel to the maximum range and back.

The maximum useful speed of rotation of the sound head (in revolutions per minute) is approximately 5,000 divided by the range in yards. On short ranges the speed is limited by the characteristics of the electrical system to about 10 revolutions per minute. In a particular installation the choice of speeds is dictated by the service intended. The speed of rotation at long ranges can

 
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Functional block diagram of the QLA f-m sonar.
Figure 14-6. -Functional block diagram of the QLA f-m sonar.
 
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be increased if the search angle is limited to about 50°; the whole area can then be covered by allowing this search angle to progress from one sweep to the next.

DOPPLER EFFECT

The QLA sonar uses the frequency of an echo in determining range. Therefore, any doppler causes an error in the measurement of range. The error

  is 75 cycles per second (or one channel on the screen of the cathode-ray tube) for each 2 ½ knots of relative range rate. The error causes the measured range to be too long when closing, and too short when opening, the target. In ranging on moving vessels, the larger part of the echo comes from the wake. The doppler effect in this case is due largely to own ship's doppler.
 
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