Aviation Electricity andElectronics—UnderseaWarfare (USW)

NAVEDTRA 14340

NONRESIDENTTRAININGCOURSE

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

PREFACE

About this course:

This is a self-study course. By studying this course, you can improve your professional/militaryknowledge, as well as prepare for the Navywide advancement-in-rate examination. It containssubject matter about day-to-day occupational knowledge and skill requirements and includestext, tables, and illustrations to help you understand the information. An additional importantfeature of this course is its references to useful information to be found in other publications. Thewell-prepared Sailor will take the time to look up the additional information.

History of the course:

� Apr 2003: Original edition released.

Published byNAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENTAND TECHNOLOGY CENTER

NAVSUP Logistics Tracking Number0504-LP-102-0293

TABLE OF CONTENTS

APPENDIX

CHAPTER PAGE

1. Acoustic Systems ............................................................................................................... 1-1

2. Magnetic Anomaly Detection (MAD) ............................................................................... 2-1

I. References Used to Develop the NRTC.................................................................... AI-1

II. Answers to Review Questions .................................................................................. AII-1

CHAPTER 1

ACOUSTIC SYSTEMS

INTRODUCTION

Undersea Warfare (USW), formerly known asAntisubmarine Warfare (ASW), includes the detection,localization, and identification of potentially hostileforces or objects below the surface of the sea. One ofthe major tasks of the Navy today is the detection ofconcealed submarines.

Detecting the stealthy submarine starts withmaintaining a tool kit of several different electronicsensors (fig. 1-1). Each sensor has specific applicationsthat counter different submarine operations. Many ofthese sensors complement and corroborate with eachother to enhance USW effectiveness.

Airborne USW sensors are divided into two basictypes—acoustic and nonacoustic. This chapter willintroduce you to acoustic sensors as currentlyemployed by naval aircraft. Acoustic systems includecomponents that detect, process, and displaysonobuoy-detected data for analysis. Nonacousticsystems include radar, navigation, MAD, ECM, ESM,and infrared detecting sets.

SONAR PRINCIPLES

LEARNING OBJECTIVE: Identify thefactors that affect the behavior of a sound wavein seawater.

The word sonar is derived from the initial letters ofSOund, NAvigation, and Ranging, and is used todescribe equipment that locates targets by transmittingand receiving sound energy propagated through water.

Each sound we hear is the result of an object thatvibrates. The vibrations are transmitted through amedium (for example, air, water molecules) anddetected by the sensor. For USW purposes, soundoriginates from either natural (for example, whales,rain, and so forth) or manmade sources (for example,submarines, shipping, drilling, sonar transducers). Thesound travels through the complex ocean medium andarrives at the hydrophone sensor (an underwatermicrophone). The particular sound waves of interest tothe sonar operator are the waves that leave the sonartransducer (an underwater speaker) and go out into thewater in search of a submarine. If the sound wave finds

1-1

ACTIVEACOUSTICS

PASSIVEACOUSTICSAEf0101

Figure 1-1.—Acoustic Sensors.

a target, it will return (reflect) as an echo. Dependingupon the vibration frequency and the signal strength (orvolume), some sounds can be transmitted longdistances underwater.

The sonar operator should know what factors canweaken the sound wave as it travels through water, whatfactors in the seawater determine the path and speed ofthe sound wave, and what factors affect the strength andcharacter of the echo.

FACTORS AFFECTING SOUND WAVETRANSMISSION LOSS IN SEAWATER

Signal strength lost during the wave's travelthrough the water is known as transmission loss. Assound travels underwater, the ocean absorbs the soundand reduces its loudness. Additionally, sound wavescan be bent or scattered in different directions andsound velocity can be affected by water temperatureand salinity.

Absorption and Scattering

Some of the sound energy emitted by the sourcewill be absorbed while passing through the water. Theamount absorbed this way depends on the sea state.Absorption is high when winds are great enough toproduce whitecaps and cause a concentration ofbubbles in the surface layer of the water. In thesecircumstances, part of any sound striking the surface islost in the air, and part is reflected in scatteringdirections in the sea. In areas of wakes and strongcurrents, such as riptides, the loss of sound energy isgreater. Therefore, echo ranging through wakes andriptides is difficult because of the combined effect offalse echoes, high reverberations, and increasedabsorption. Absorption is greater at higher frequenciesthan at lower frequencies. As a result, lowerfrequencies tend to travel farthest.

Sound waves are weakened when they reach aregion of seawater that contains foreign matter, such asseaweed, silt, animal life, or air bubbles. This foreignmatter scatters the sound wave and causes loss of soundenergy. The practical result of scattering is to reduceecho strength, especially at long range.

Reflection

Echoes occur when the sound wave hits an objector a boundary region between transmission mediums insuch a manner as to reflect the sound or to throw it backto its origin. Reflection of sound waves sometimes

happens when a wave strikes a medium of differentdensity from that through which it has been traveling.This will occur in cases where the two mediums are ofsufficiently different densities, and the wave strikes at alarge angle. This happens because the sound wavetravels at different speeds through the two differentdensities. For example, a sound wave traveling throughseawater is almost entirely reflected at the boundary ofthe water and air. The speed of sound in seawater isabout four times greater than the speed of sound in air,and the density of water is more than 800 times greaterthan that of air. Therefore, practically all of the soundwave will be reflected downward from the sea surface.

Likewise, sound waves bounce off the oceanbottom and reflect upward. If the ocean bottom is asmooth, hard surface, there is little signal loss. In casesof extremely deep waters (600 feet or more), the soundmay never strike the bottom! This happens because thewater pressure is so great that the sound velocityactually begins to speed up and forces the sound wavesto refract (or bend) back toward the surface.

Similarly, when a sound wave traveling through theseawater strikes a solid object like a submarine, thedifference in the density and the sound velocity in thetwo mediums is such that all but a small amount of thesound wave will be reflected. That portion of the wavethat strikes surfaces of the submarine perpendicular tothe wave will be reflected directly back to the origin asan echo.

Reverberation

When sound waves echo and re-echo in a large hall,the sound reverberates. Reverberations are multiplereflections. Lightning is an example of this from nature.When lightning discharges, it causes a quick, sharpsound; but by the time the sound of the thunder is heard,it is usually drawn out into a prolonged roar byreverberations.

A similar case often arises in connection withsonar. Sound waves often strike small objects in the sea,such as fish or air bubbles. These small objects causethe waves to scatter. Each object produces a small echo,which may return to the transducer. The reflections ofsound waves from the sea surface and the sea bottomalso create echoes. The combined echoes from all thesedisturbances are called reverberations. Since they arereflected from various ranges, they seem to be acontinuous sound. Reverberations from nearby pointsmay be so loud that they interfere with the returningecho from a target.

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There are three main types of reverberation, orbackward scattering of the sound wave. They are asfollows:

• From the mass of water. Causes of this type ofreverberation are not completely known,although fish and other objects contribute to it.

• From the surface. This is most intenseimmediately after the sonar transmission; itthen decreases rapidly. The intensity of thereverberation increases markedly with increasedroughness of the sea surface.

• From the bottom. In shallow water, this type ofreverberation is the most intense of the three,especially over rocky and rough bottoms.

Divergence

Just as the beam from a searchlight spreads out andbecomes weaker with distance, so does a sound wave.The farther the target is from the sonar transducer, theweaker the sound waves will be when they reach it. Thisis known as spreading or divergence.

Refraction

If there were no temperature differences in thewater, the sound wave would travel in a straight line.This happens because the speed of sound would beroughly the same at all depths. The sound wave wouldspread and become weaker at a relatively constant rate.

Unfortunately, the speed of sound is not constant atall depths. The speed of sound in seawater increasesfrom 4,700 feet per second to 5,300 feet per second asthe temperature increases from 30°F to 85°F. Becauseof the varying temperature differences in the sea, thesound wave does not travel in a straight line, but followscurved paths. This results in bending, splitting, anddistorting of the sound wave.

When the sound wave is bent, it is said to berefracted. A sound wave is refracted when it passesfrom a medium of a given temperature into a mediumwith a different temperature. An example of this is asound wave traveling from an area of warm water intoan layer of cold water. The sound wave will bend awayfrom the area of higher temperature (higher sound veloc-ity) toward the lower temperature (lower sound velocity).

As a result of refraction, the range at which asubmarine can be detected by sound may be reduced toless than 1,000 yards, and this range may changesharply with changing submarine depth.

FACTORS AFFECTING THE SPEED OF THESOUND WAVE IN SEAWATER

There are two main characteristics of seawater thataffect the speed of the sound wave traveling throughit—temperature (the effect of which is calculated interms of slopes or gradients) and salinity (the amount ofsalt in the water). Although the speed of a sound wave isalso affected by water pressure as depth increases, thedifference in speed is very small and has little effect forthe operator.

Temperature

Temperature is the most important factor affectingthe speed of the sound wave in seawater. The speed willincrease with increasing temperature at the rate of 4 to 8feet per second per degree of change, depending on thetemperature.

The temperature of the sea varies from freezing inthe polar seas to more than 85°F in the tropics. Thetemperature can also decrease by more than 30°F fromthe surface to a depth of 450 feet. Thus, the temperatureis the most important factor because of the extremedifferences and variations. Remember, the speed ofsound in water increases as the temperature increases.Except at the mouths of great rivers where salinity maybe a factor, the path of the sound wave will bedetermined by temperature.

Figure 1-2 shows what happens when temperatureincreases steadily with depth. When the surface of thesea is cooler than the layers beneath it, the temperatureincreases with depth, and the water has a positivethermal gradient. This is an unusual condition, but

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AEf0102

CO

NS

TA

NT

LY

INC

RE

AS

ING

TE

MP

ER

AT

UR

E

COOL WATER

WARM WATER

Figure 1-2.—The effect of a positive thermal gradient.

when it does happen, it causes the sound wave to berefracted sharply upwards.

When the sea gets colder as the depth increases, thewater has a negative thermal gradient. In this situation,the effect of temperature far outweighs the effect ofdepth, and the sound wave is refracted downward.

If the temperature remains the same throughout thewater, the temperature gradient is isothermal (constanttemperature). Refer to figure 1-3 as you read thefollowing text. The surface layer of water in the figureis isothermal, but beneath this layer the temperaturedecreases with depth. This causes the sound wave tosplit and bend upward in the isothermal layer anddownward below it.

When the temperature changes with depth, thesound wave bends away from the warmer water.

Under normal conditions the temperature structureof the sea is similar to that shown in figure 1-4. Thisstructure consists of three layers as follows:

1. A surface layer of varying thickness withuniform temperature (isothermal) or arelatively slight temperature gradient

2. The thermocline, which is a region of relativelyrapid decrease in temperature

3. The rest of the ocean, with slowly decreasingtemperature down to the sea floor

If this arrangement changes, the path of the soundwave through the water will change.

Layer depth is the depth from the surface to the topof a sharp negative gradient. Under positive thermalgradient condition, the layer depth is the depth of

maximum temperature. Above layer depth, thetemperature may be uniform, or a weak positive ornegative gradient may be present.

Layer effect is the partial protection from echoranging and listening detection, which a submarinegains when it submerges below layer depth. Reportsfrom surface vessels indicate that effective ranges onsubmarines are greatly reduced when the submarinedives below a thermocline, and that the echoes receivedare often weak and sound "mushy."

Salinity

The effect of salinity on sound speed in the sea isless than the large effect that temperature has on soundspeed in seawater.

There is a high mineral content in seawater. Thedensity of seawater is approximately 64 pounds percubic foot, while fresh water has a density of about 62.4pounds per cubic foot. This difference is caused by thesalt in the seawater. Salt content in seawater is calledthe salinity of water.

The overall effect of increasing the salinity is anincrease in the speed of the sound wave in the water.This means that as the sound travels through water of

1-4

AEf0103

COOL WATER

ISOTHERMAL

Figure 1-3.—Isothermal conditions.

TEMPERATUREUNIFORM ORCHANGINGSLIGHTLY

WITH DEPTH

SURFACE LAYERISOTHERMAL OR"MIXED LAYER"

WHENTEMPERATURE

UNIFORM

TEMPERATUREDECREASING

RAPIDLYTHERMOCLINE

TEMPERATUREDECREASING

SLOWLY

REGION BELOWTHE

THERMOCLINE

SEA BOTTOM

SEA SURFACE

AEf0104

Figure 1-4.—Normal sea temperature structure.

varying salinity, it travels faster through the water withmore salt content. Such a change in salinity isconsiderable at the mouth of a river emptying into thesea. Elsewhere, the difference in salinity is too small tochange the rate of travel of the sound wave significantlyand may be ignored.

DOPPLER EFFECT

When there is relative motion between the sourceof a wave of energy and its receiver, the receivedfrequency differs from the transmitted frequency.When the source of wave motion is moving towards thereceiver, more waves per second are received than whenthe source remains stationary. The effect at the receiveris an apparent decrease in wavelength and, therefore, anincrease in frequency. On the other hand, when thesource of wave motion is moving away from thereceiver, fewer waves per second are encountered,which gives the effect of a longer wavelength and anapparent decrease in frequency. This change in

wavelength is called the Doppler effect. The amount ofchange in wavelength depends on the relative velocitybetween the receiver and the source. Relative velocityis the resultant speed between two objects when one orboth are moving.

You have heard the term Doppler effect manytimes, but may not have known what the phenomenonwas. An example of this is what you hear at a railroadcrossing. As a train approaches, the pitch of the whistleis high. As the train passes you, the pitch seems to drop.Then, as the train goes off in the distance, the pitch ofthe whistle is low. The Doppler effect causes thechanges in the pitch.

Sound waves generated by the whistle werecompressed ahead of the train. As the waves cametoward you, they were heard as a high-pitched soundbecause of the shorter distance between waves. Whenthe train went by, the sound waves were drawn out,resulting in the lower pitch. Refer to figure 1-5 as youread the following explanation of Doppler effect.

1-5

AEf0105

RECEIVER

RECEIVER

RECEIVER

STATIONARY

FIXED

CLOSING

OPENING

CONDITION (1)

CONDITION (2)

CONDITION (3)

EMITTER MOTION INTERVAL

TRAIN

EXPANDED SOUND WAVES

SOUND WAVES FROMSTATIONARY TRAINCOMPRESSED SOUND WAVES

FROM MOVING TRAIN

A

B

Figure 1-5.—Doppler effect. (A) One-second audio signal; (B) One sine wave of the audio signal.

If you examine 1 second of the audio signalradiated by the train whistle, you will see that the signalis composed of many cycles of acoustical energy. Eachcycle occupies a definite period of time and has adefinite physical wavelength. (Because of spacelimitations, only every 10th wave is illustrated in viewA of figure 1-5.) When the energy is transmitted from astationary source, the leading edge will move out inspace the distance of one wavelength by the time thetrailing edge leaves the source. The cycle will thenoccupy its exact wavelength in space. If that cycle isemitted while the source is moving, the source willmove a small distance while the complete cycle is beingradiated. The trailing edge of the cycle radiated will becloser to the leading edge.

Figure 1-5, view B, shows the effect of relativemotion on a radiated audio signal. Notice thewavelength of the sound from the stationary emitter, asillustrated in condition (1) of view B.

In condition (2) of view B, the emitter is movingtowards the listener (closing). When the cycle iscompressed, it occupies less distance in space. Thus,the wavelength of the audio signal has been decreased,and the frequency has been proportionately increased(shifted). This apparent increase in frequency is knownas UP Doppler.

The opposite is true in condition (3) of view B. Theemitter is moving away from the listener (opening).The wavelength occupies more distance in space, and

the frequency has been proportionately decreased. Thisapparent decrease in frequency is known as DOWNDoppler. The factors that determine the amount ofDoppler shift are the velocity of the sound emitter, thevelocity of the receiver, and the angle between thedirection of motion of the receiver and the direction ofmotion of the sound emitter.

The Doppler shift works both ways. If you were onthe train and had listened to a car horn at the crossing,the pitch of the horn would have changed. The effect isthe same because the relative motion is the same.

The sonar equipment deals with three basic sounds.One of these sounds is the sound actually sent out by theequipment. The second sound is the reverberations thatcome from echoes returning from all the particles in thewater—seaweed, fish, and so forth. The third sound isthe most important one, the echo from the submarine.

The sound sent into the water (the actual ping) isseldom heard by the operator. Most of the equipmentblanks out this signal so that it doesn't distract theoperator.

Consider just one reverberation particle for amoment. A sound wave from the transducer hits theparticle and bounces back, just as a ball would if thrownagainst a wall. If the particle is stationary, it will notchange the pitch of the sound. The sound will returnfrom the particle with the same pitch that it had when itwent out.

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REVERBERATIONS HIGH10.1 kHz

REVERBERATIONS THE SAME 10.0 kHz

REVERBERATIONS LOW9.9 kHz

ONE PARTICLEONE PARTICLE

OUTGOING PING 10.0 kHz

UNDERWAY

AEf0106

Figure 1-6.—Transducer installed on a moving ship.

If the sonar transducer is stationary in the water andsends out a ping of 10 kilohertz (kHz), the particles allsend back a sound that has the same 10 kHz pitch. Nowsuppose that the transducer acquires forward motionand a ping is sent out dead ahead. It is just as if thetransducer were the oncoming train, and the particleswere occupants of the car. Remember that as the traincame forward, the pitch of the whistle sounded higherto the occupants of the car. In the same way, theparticles "hear" a higher note and reflect this highernote. Therefore, the sonar equipment will detect ahigher note than the one sent out. If the transducer inthis example is pointed dead astern, a lower note thanthe one sent out will be heard.

If the transducer is aimed perpendicular to thedirection of motion, the particles in the water will echothe same note sent out because the transducer is neithergoing toward the particles nor away from them. (Seefig. 1-6.)

Now consider the echo from the submarine, shownin figure 1-7. Again, the transducer is shown stationary.When the submarine is neither going toward nor awayfrom the transducer, it must be either stopped orcrossing the sound wave at a right angle. If it is in eithercondition, it reflects the same sound as the particles inthe water. Consequently, the submarine echo hasexactly the same pitch as the reverberations from theparticles.

Suppose that the submarine is going toward thetransducer, as shown in figure 1-8. It is as though thesubmarine is the train heading toward the car that isblowing its horn at the crossing. The pitch of the hornsounds higher as the train approaches the car. In thesame manner, the pitch of the transducer sounds higherto the submarine as it approaches the transducer.

The submarine reflects an echo of higher pitch thanthat caused by the particles in the water, which are notmoving. When the echo from the oncoming submarineis higher in frequency than the echoes from thereverberations, the Doppler is high. The opposite formof Doppler shift will occur when the submarine isheading away from the transducer. In this case, thepitch of the echo is lower than the pitch of thereverberations. (See fig. 1-9.)

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SUBMARINE ECHO 10.0 kHzREVERBERATIONS 10.0 kHzNO DOPPLER

OUTGOING PING 10.0 kHzAEf0107

SUBMARINE ECHO 10.0 kHzREVERBERATIONS 10.0 kHzNO DOPPLER

Figure 1-7.—Transducer supported by helicopter. Dopplereffect is absent when submarine is stationary or moves atright angles to the sound wave.

REVERBERATIONS 10.0 kHzSUBMARINE ECHO 10.1 kHz

OUTGOING PING 10.0 kHz

HIGH DOPPLER

REVERBERATIONS 10.0 kHzSUBMARINE ECHO 10.1 kHz

HIGH DOPPLER{

{

AEf0108

Figure 1-8.—Comparison of echo frequency and reverberationfrequency when submarine moves toward transducer.

OUTGOING PING 10.0 kHzAEf0109

REVERBERATIONS 10.0 kHzSUBMARINE ECHO 9.9 kHzLOW DOPPLER

REVERBERATIONS 10.0 kHzSUBMARINE ECHO 9.9 kHzLOW DOPPLER

Figure 1-9.—Comparison of echo frequency and reverberationfrequency when submarine moves away from transducer.

The degree of Doppler indicates how rapidly thesubmarine is moving relative to the transducer. Forexample, a submarine moving directly toward thetransducer at 6 knots returns an echo of higherfrequency than one moving at only 2 knots. Also, asubmarine moving at 6 knots directly at the transducerreturns an echo of higher frequency than one movingonly slightly at the transducer. Refer to figure 1-10.This figure shows 12 submarines traveling at variousspeeds and courses with respect to a stationarytransducer supported by the helicopter. Notice how theDoppler of each submarine is influenced by its speedand direction.

Doppler also makes it possible to distinguish thedifference between a wake echo and a submarine echo.Relatively speaking, the submarine's wake is stationary.Therefore, its wake returns an echo with a frequencydifferent from that of the Doppler shifted submarineecho.

PROPAGATION PATHS

Sound arrives at the hydrophone sensor by severalways called propagation paths. The three most commonsound paths are direct path (directly from the soundsource to the hydrophone sensor), bottom bounce(reflected from the bottom), or convergence zone(refracted from deep water).

Direct path sound propagation is normally limitedto short ranges. Sound transmission through the bottombounce path irregularly occurs at medium rangesdepending upon the reflective angle between the soundwave and the bottom. Convergence zone detection islimited to deep ocean waters and is found in narrowdetection rings (or annuli) at distant ranges. Eachtransmission path affects sound in distinct ways thatenables the aircrew to determine their follow-onlocalization tactics after initial acoustic detection.

Q1-1. What are the two main factors that affect thevelocity of sound in seawater?

Q1-2. A sound wave traveling from an area of warmwater into an area of cold water will beaffected in what way?

SONOBUOY-BASED SONAR SYSTEMS

LEARNING OBJECTIVE: Recognize themajor components and operating principles ofa typical sonobuoy-based airborne sonarsystem, to include the typical sonobuoyreceiver, spectrum analyzer, and the classifica-tions and operating principles of sonobuoyscurrently in use.

Fixed-wing aircraft and certain rotary-wingaircraft, such as the P-3C Orion and the SH-60B

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4 KNOTSNO DOPPLER

2 KNOTSNO DOPPLER

2 KNOTSSLIGHT HIGH DOPPLER

6 KNOTSMODERATE LOW DOPPLER

2 KNOTSSLIGHT HIGH DOPPLER

4 KNOTSSLIGHT HIGH DOPPLER

6 KNOTSSLIGHT HIGH DOPPLER

6 KNOTSNO DOPPLER

6 KNOTSMARKED HIGH DOPPLER

4 KNOTSMODERATE LOW DOPPLER

2 KNOTSSLIGHT LOW DOPPLER

AEf0110

4 KNOTSMODERATE HIGH DOPPLER

Figure 1-10.—Varying degrees of Doppler effect due to differences in course and speed of submarines.

Seahawk, use a sonobuoy-based sonar system insupport of their USW mission. In such a system,sonobuoys are dropped from the aircraft into an area ofthe ocean thought to contain a submarine.

The sonobuoys are usually dropped in patternsinvolving three or more buoys. The sonobuoys detectunderwater sounds, such as submarine noise and fishsounds. These sounds modulate an oscillator in theradio frequency (RF) transmitter portion of thesonobuoy. The output of the transmitter is afrequency-modulated (FM), very high frequency(VHF) signal that is transmitted from the antenna. Thesignal is received by a sonobuoy receiver in the aircraftand sent to the spectrum analyzer where the acousticinformation is extracted. By analyzing the detectedsounds, the USW operator can determine variouscharacteristics of the detected submarine. The use ofseveral sonobuoys operating on different VHFfrequencies in a tactical pattern enables the USWoperator to localize, track, and classify a submergedsubmarine.

Each sonobuoy type is designed to meet a specificset of specifications that is unique to that particularsonobuoy. Even though sonobuoys are produced bydifferent manufacturers, the specifications andoperational performance characteristics are the samefor all manufacturers. There are differences in themethods used for prelaunch selection of life and depthsettings from one manufacturer to another for the samesonobuoy types. These differences are found in theSonobuoy Instruction Manual, NAVAIR28-SSQ-500-1. You should refer to this manual prior tostoring, handling, or disposing of sonobuoys.

SONOBUOYS

The detection, localization, and identification ofsubmarines is the primary mission of the Navy'sairborne USW forces. The ability of the Navy tocomplete this mission is dependent upon the sonobuoy.The sonobuoy has undergone a great deal of change inthe past 25 years. These improvements have providedthe fleet with large numbers of very reliable sonobuoysthat perform various missions.

Using the basic sound (or sonar) theory,experimental sonobuoys were developed during WorldWar II to detect hostile submarines. Aircrews usedsonobuoys to hear the acoustic sounds radiated from asubmarine (passive sonobuoys) or to detect an echobounced off a submarine using an acoustic pulsegenerated by an acoustic source (active sonobuoy).

Additionally, special purpose sonobuoys weredeveloped to measure the water temperature profile andto communicate with submerged submarines.

Sonobuoys are expendable devices; they are neverrecovered after they are launched from the aircraft. Itmay sound like a costly endeavor, but in actuality, it isthe cheapest, fastest, and most reliable way to searchthe ocean. This inexpensive USW method not onlyconsiders the cost of the sonobuoy, but also theoperating costs for the aircraft. Before any sonobuoy isdeveloped, a careful analysis is always conducted todetermine the cost effectiveness versus the expectedperformance. An expensive buoy may performsuperbly, but if it costs too much, the Navy will never beable to buy sufficient quantities. In fact, these complexanalyses have in the past led to the cancellation ofseveral developmental sonobuoy programs.

USW aircraft can search the same area much fasterand with less chance of being attacked by the hostilesubmarine. By comparison, a naval vessel searching thesame area is limited by its own self-noise, search speed,and much greater operating costs for the ship itself.Additionally, friendly submarines and navalcombatants conducting a USW search must always beon guard for potential torpedo or missile attacks. Dollarfor dollar and performance for performance, sonobuoysare the most economical and efficient way to conductair USW.

A sonobuoy is a cylindrical metal tube ("can")about three feet long and nearly five inches in diameter(fig. 1-11). The sonobuoy normally weighs 20 to 39pounds, depending upon the type. It is encased within aplastic cylindrical housing called a sonobuoy launchcontainer (SLC).

1-9

Figure 1-11.—Sonobuoy.

Deployment

The sonobuoy is aircraft-deployable by any of threemethods: pneumatic, free-fall, or cartridge.

The SLC is loaded into special launch chutes foundon USW aircraft and launched by using either a smallexplosive charge (P-3 aircraft) or pneumatically(SH-60 helicopters). On some aircraft, a sonobuoy canbe gravity-launched from an open chute or door ("freefall"). Once released, the plastic SLC remains inside theaircraft launch chute. The now empty SLC remainswith the aircraft and is properly disposed of or recycledupon aircraft landing.

Sonobuoys are deployed from altitudesapproaching 30,000 feet at speeds approaching 370knots. Because descent velocities can exceed 120 feetper second, a descent-retarding device is used toincrease aerodynamic stability and to reducewater-entry shock. A parachute or a rotating-bladeassembly (rotochute) is used as the descent-retardingdevice. Because of the different descent characteristicsof the parachute and rotochute, do not intermix the two.With intermixed sonobuoys, the spacing of the tacticalpattern will not be right and submarines might bemissed.

Water Entry and Activation

Once a sonobuoy hits the water, it descendsapproximately 40-60 feet underwater until its battery isactivated by the seawater and initiates the deploymentor jettison of the various sonobuoy components. A CO2bottle inflates a plastic float. The plastic float returnsthe sonobuoy back to the surface where the attacheduplink antenna becomes energized. Jettisoning of thebottom plate allows the hydrophone and other internalcomponents to descend to the preselected depth. Mostsonobuoys are capable of deploying a hydrophone athousand feet below the ocean surface.

As shown in figure 1-12, a termination mass and/ordrogue stabilizes the hydrophone at the selected depth,while the buoyant sonobuoy section or float follows themotion of the waves. A section of elastic suspensioncable isolates the hydrophone from the wave action onthe buoyant section.

Most of the sonobuoys in the fleet today areequipped with seawater-activated batteries, whichprovide the power required for the sonobuoyelectronics. Data transmission from the buoys usuallybegins within 3 minutes after the buoy enters the water.In cold water and/or water with low salinity, the

activation time might be increased. Some sonobuoysnow have non-water-activated lithium batteries. It isquite an engineering feat to stuff these hydrophones,float bags, electronic processors, inflation bottles,environmentally friendly batteries, and hundreds of feetof cable all inside a 3-foot long, 5-inch diameter steel oraluminum can.

After several hours of operation, the sonobuoybattery fails and water-soluble seals dissolve, whichallows the sonobuoy to sink to the bottom. Allsonobuoy material is non-toxic and is carefullyspecified per international, federal, and localregulations to prevent adverse environmental impact.

Sonobuoy Frequency Channels

Certain sonobuoy designs are equipped with anelectronic function select (EFS) system. The EFSsystem provides each sonobuoy with a selectable99-channel capability. (Table 1-1 shows sonobuoychannels and their assigned frequencies.) EFS alsoprovides each sonobuoy with 50 life and 50 depthsetting selections. The operator must reset all threesettings any time any of the three are changed.

1-10

Figure 1-12.—Deployed sonobuoy.

Sonobuoy Classification

Sonobuoys are grouped into three categories:passive, active, and special purpose.

PASSIVE SONOBUOYS.—Passive sonobuoyslisten to underwater sounds. They rely on thesubmarine to produce characteristic sounds, eitherintentionally or unintentionally, that can be used fordetection, classification, and localization. This acousticsignature or fingerprint consists of sounds emitted bypropellers, various machinery equipment, electricalsystems, and active sonar emissions. By evaluating thesignature characteristics, the sonar operator aboard theaircraft (an Aviation Warfare Systems Operator) canclassify the sounds as either a submarine, a ship, orbiologics, such as a whale. Depending upon the type ofacoustic contact, the operator can further analyze thespecific sources that are detected.

LOFAR Sonobuoy.—Low frequency and ranging(LOFAR) sonobuoys detect sounds emitted by thesubmarine by using an omnidirectional (equalreception from all directions) hydrophone that has beenlowered from a passive sonobuoy. Data regarding thefrequency and amplitude of these sounds are thentransmitted by the sonobuoy antenna to the receivingstation, where the sound data is analyzed, processed,displayed, and recorded. The basic LOFAR systemplots the frequency of the sound waves against theintensity of their acoustic energy and against theduration of the sound emission.

The omnidirectional hydrophone used by LOFARsonobuoys has several advantages over the directionalhydrophones used in other sonobuoys. With afrequency range of 5 to 40 kHz, the amount of auraldata from a LOFAR sonobuoy can be four times theamount as that from a DIFAR sonobuoy, which has a

1-11

Ch. Frequency(kHz)

Ch. Frequency(kHz)

Ch. Frequency(kHz)

Ch. Frequency(kHz)

32 136.000 57 145.375 82 154.750 20 164.875

33 136.375 58 145.750 83 155.125 5 165.250

34 136.750 59 146.125 84 155.500 21 165.625

35 137.125 60 146.500 85 155.850 6 166.000

36 137.500 61 146.875 86 156.250 22 166.375

37 137.875 62 147.250 87 156.625 7 166.750

38 138.250 63 147.625 88 157.000 23 167.125

39 138.625 64 148.000 89 157.375 8 167.500

40 139.000 65 148.375 90 157.750 24 167.875

41 139.375 66 148.750 91 158.125 9 168.250

42 139.750 67 149.125 92 158.500 25 168.625

43 140.125 68 149.500 93 158.875 10 169.000

44 140.500 69 149.875 94 159.250 26 169.375

45 140.875 70 150.250 95 159.625 11 169.750

46 141.250 71 150.625 96 160.000 27 170.125

47 141.625 72 151.000 97 160.375 12 175.500

48 142.000 73 151.375 98 160.750 28 170.875

49 142.375 74 151.750 99 161.125 13 171.250

50 142.750 75 152.125 1 162.250 29 171.625

51 143.125 76 152.500 17 162.625 14 172.000

52 143.500 77 152.875 2 163.000 30 172.375

53 143.875 78 153.250 18 163.375 15 172.750

54 144.250 79 153.625 3 163.750 31 173.125

55 144.625 80 154.000 19 164.125 16 173.500

56 145.000 81 154.375 4 164.500

Table 1-1.—Sonobuoy Channel Allocations (Listed in Frequency Order)

maximum range of only 2400 Hz. This wide frequencyrange provides the best audio for aural listening. Sinceaural data is the only real-time acoustic data available tothe operator, this characteristic is essential.Omnidirectional hydrophones also provide improveddata in the lower frequency range (5 to 30 Hz) and areless susceptible to thermal noise (changes in watertemperature) and high shear currents associated withfronts, eddies, straights, choke-points, and littoralareas. In most situations, LOFAR sonobuoys are thebest sensor for initial detection.

DIFAR Sonobuoy.—The directional frequencyanalysis and recording (DIFAR) sonobuoy uses twohydrophone elements mounted perpendicular to eachother. These hydrophones measure differences in thepressure strength of individual sounds and convert themeasured pressure strengths into voltages that are thenused to determine from which direction the soundarrived relative to magnetic north (fig. 1-13). Thissignal is then transmitted to the aircraft via a VHFtransmitter, where it is processed and the bearing iscomputed. Subsequent bearing information from thebuoy can be used to pinpoint, by triangulation, thelocation of the sound or signal source. Today's aircrewsnormally use a combination of both DIFAR andDoppler information to determine the position of aconcealed submarine.

VLAD Sonobuoy.—The vertical line array DIFAR(VLAD) specification is CONFIDENTIAL and

impacts on the level to which this sensor may bedescribed in an unclassified document. We will usegeneric descriptions and material to provide theinformation necessary to understand the concept of theVLAD sensor.

As submarines become quieter, it is more difficultto separate ambient noise from the sounds of threatsignals. This serves to complicate acoustic USWdetection and tracking. A sonobuoy with a highersignal-to-noise ratio (S/N) counters this problem.

Distant shipping contributes to high levels of localambient noise in the lower portion of the frequencyspectrum, and the angle at which this noise arrives at adeployed hydrophone is within a narrow band of nearhorizontal angles. By eliminating or greatly attenuatingreception of horizontally arriving sound, a VLAD buoyreduces this major contributor to low-frequency noisewhile still being capable of receiving threat signals ofinterest from targets. In a like manner, high-frequencynoise from locally produced disturbances at the oceansurface, such as wind, waves, and rain, arrive at asonobuoy hydrophone at near vertical angles. Byeliminating or greatly attenuating reception ofvertically arriving sound, a VLAD buoy reducesundesirable high-frequency noise.

VLAD sonobuoys use beam-forming technology toreject reception of unwanted noise and to enhancereception of desired signals of interest. The

1-12

Figure 1-13.—Block diagram of the DIFAR sonobuoy.

beam-forming array of a VLAD buoy consists ofseveral hydrophones placed at fixed points along avertical line. Each hydrophone is tuned to apredetermined frequency band and will process onlyacoustic signals within that range. Beam-formingtechnology results in an acoustic reception pattern thatattenuates horizontally arriving low-frequency noiseand vertically arriving high-frequency noise. Thisenhances reception of sound from bottom bounce andsurface-reflected propagation paths that reveal thepresence of a submarine.

VLAD sonobuoys are designed to providelong-range detection of submarines operating in deepwater and are not intended to replace DIFAR as a nearfield detection sensor.

ACTIVE SONOBUOYS.—Active sonobuoysrely upon the principle of sending out an acoustic pulsethat strikes the hull of a submarine and is reflected backto the sonobuoy. The operating principles are similar tothat of radar, except sound waves are used instead of RFwaves. When the sound wave strikes an object, some ofthe energy reflects back to the source from which itcame.

Today's modern active sonobuoys use a principlecalled “extended echo ranging,” where the echo isdetected by one or more passive sonobuoys. Thissystem provides an exceptional search sensor fordetecting the ever-quieter submarine.

DICASS Sonobuoy.—The directional commandactivated sonobuoy system (DICASS) relies on anultrahigh frequency (UHF) command signal from theUSW aircraft to send an acoustic pulse, or ping. Uponreceipt of a valid radio signal from the aircraft, theDICASS sonobuoy transmits a sonar pulse in alldirections (omnidirectional). This acoustic pulsereflects off any obstructions in its path, and thesonobuoy directional hydrophones receive thereturning echoes. Data from received reflections istransmitted on the VHF uplink channel to themonitoring USW aircraft, where the aircraft acousticanalysis equipment processes range and bearinginformation for a precise fix on the submarine position.

The DICASS allows the aircraft to deploy thesonobuoy, but the buoy will remain passive untilcommanded to ping. This allows the aircrew to surprisethe submarine.

EER Sonobuoy.—The extended echo ranging(EER) sonobuoy transmits an acoustic energy pulse

into the water. This broadband pulse is incoherent innature. The pulse travels through the water until it isreflected off natural and man-made objects. When itstrikes the hull of a submarine, the pulse forms an echothat is detected by a passive receiving sonobuoy, suchas the VLAD. The EER sonobuoy provides long-range,active detection of submarines.

SPECIAL-PURPOSE SONOBUOYS.—Specialpurpose sonobuoys are designed for special USWfunctions. There are two types of special purposesonobuoys in use today—bathythermograph (BT), anddata link communication (DLC).

Bathythermograph Sonobuoy.—The BT sono-buoy is used to measure water temperature versusdepth. The water depth is determined by timing thedescent of a temperature probe. Once the BT buoyenters the water, the probe descends automatically at aconstant 5 feet per second.

The probe uses a thermistor, which is atemperature-dependent electronic component, tomeasure the temperature. The electrical output of theprobe is applied to a voltage-controlled oscillator. Theoscillator's output signal frequency modulates thesonobuoy transmitter. The frequency of the transmittedsignal is linearly proportional to the water temperature.The water temperature and depth are recorded on graphpaper that is visible to the USW operator. The sonobuoysignal is processed by the acoustic equipment onboardthe aircraft.

The BT buoy is normally the first sonobuoydropped in any USW mission. Its real time assessmentof the temperature profile allows the aircrew tooptimize the success in detecting the submerged threat.

Data Link Communication Sonobuoy.—TheDLC sonobuoy provides short-range, one-way acousticcommunications with friendly submerged submarines.A series of acoustic frequencies are transmitted inencoded sonic patterns at different depths to ensurethey are heard by the friendly submarine. The codedmessages are brief in nature, but once decoded, allowthe submarine to receive sufficient information toproceed without having to come to the surface toreceive radio communications.

SONOBUOY RECEIVERS

The sonobuoy radio receiver recovers the acousticinformation that the sonobuoy transmits. As indicated

1-13

in table 1-2, the currently used equipment may be anARR-78 or ARR-84, depending on the aircraft platformand its configuration. The receiver units listed aboveactually contain 20 or 4 individual receivers,respectively. Each subunit can operate simultaneouslyand independently on a channel selected by theoperator or a computer. Some receiver units also havecontrol inputs that enable selection of digital datareception modes or wide bandwidths for reception offuture sonobuoy systems. An indication of receivedsignal strength is provided by most units as well as therequired outputs (audio and signal strength) to drive theon-top position indicator (OTPI) system.

AN/ARR-78 Receiver

This receiver is intended to be configured in either a20-channel (16 acoustic receivers, 4 auxiliary) or a10-channel (8 acoustic receivers, 2 auxiliary) versionfor employment with a fixed-wing or rotary-wing USWaircraft. This system is currently used in aircraft such asthe P-3C Orion.

DESCRIPTION.—The ARR 78 is a solid-statehybrid single-frequency conversion receiving system.It receives, demodulates, and amplifies sonobuoyanalog and digital transmissions in the VHF band from

136 to 173.5 megahertz (MHz) and providesdemodulated outputs to analysis, recording, anddisplay equipments. The receiver operates in such amanner that any of the aircraft processing channels maybe used interchangeably. Each synthesized receiverchannel is capable of being tuned by computer ormanual control to any one of 99 VHF channels on 375kHz channel spacing in the narrowband mode or, in thewideband mode, operates on the channels with 750 kHzsuperimposed channel spacings.

COMPONENTS.—The ARR-78 receiver systemconsists of five units: a dual preamplifier, a receiverassembly, an indicator control unit, an RF status panel,and an OTPI control unit.

The RF preamplifier assembly contains twoidentical preamplifier modules and is capable ofdriving two receiver assemblies. Each preamplifieraccepts and amplifies FM signals in the VHF frequencybands with RF signal levels between 0.5 and 100,000µV. Each amplifier can withstand RF input levels of upto 0.5 V without incurring damage.

The receiver assembly consists of a receiver chassisassembly, 20 synthesizer tuned receivers (16 acoustic,4 auxiliary), a built in test equipment (BITE)module, dual power supplies, ADF preamplifier/RF

1-14

AN/ARR-78 AN/ARR-84

Platform P-3C SH-60B/F

Frequency Range 136-173.5 MHz(99 RF channels)

136-173.5 MHz(99 RF channels)

Channels 20 including OTPI channel 4

Control Mode Manual and computer (allfunctions, high speed Proteusserial words)

Manual and computer (allfunctions, high speed Proteusserial words)

BITE Manual or computer initiatedBIT via internal RF generatorand extensive monitor circuits.

Manual or computer initiatedBIT via internal RF generatorand extensive monitor circuits

Weight 114 lbs. 25 lbs.

Cubic Inches 3,560 1,125

Power Consumption 418 watts 100 watts

Units 1 Receiver1 Control/Indicator1 RF Status Panel1 Dual Preamp1 OTPI Control

1 Receiver1 Control/Indicator (4 channels)

Table 1-2.—Comparison of Sonobuoy Receivers Currently Used in the Navy

amplifier/multicoupler, reference oscillator, clockgenerator, and computer interface for control. Eachacoustic receiver is identical and capable of receivingFM and/or frequency shift key (FSK) modulated RFsignals (only 4 receivers are capable of FSK) of a levelbetween 0.5 and 100,000 microvolts (µV). The fourauxiliary receivers are capable of outputs for audiomonitor, RF monitor and OTPI functions.

The indicator control unit provides a centralizedcontrol and status display of the receiver modes. Eachindicator control unit command status word to or fromthe receiving system is comprised of a serial digitalword, fully compatible with the advanced signalprocessor (ASP) input/output (I/O) channel.

The RF status panel displays on a continuous basisthe control mode (computer/manual), RF channelassignment, and the IF level for each acoustic receiveractively being processed.

The OTPI control unit provides a remote control ofthe OTPI receiver. The operator of the control unit canmanually tune the OTPI receiver to any one of the VHFfrequencies.

AN/ARR-84 Receiver

This lightweight receiver is designed to accom-modate the weight restrictions of aircraft such as theSH-60 series helicopter.

DESCRIPTION.—The ARR 84 is a solid-statereceiver that receives and processes analog and digitaltransmissions from any of the 99 sonobuoy channels inthe VHF band between 136.0 and 173.5 MHz. Itcontains four receiver modules that operateindependently so that transmission from four differentsonobuoys may be monitored simultaneously. Eachreceiver module processes both analog and digital data,according to buoy type.

The receiver system provides analog and digitaldata outputs, retrieved from sonobuoy transmissions,for use by other equipment in the aircraft.

COMPONENTS.—The receiver system is ofmodular design and consists of seven sections: fourreceiver modules, a power supply assembly, BITEcircuits, and preamplifier circuits, which areelectrically connected and mechanically bolted to achassis.

The preamplifier circuits filter, amplify, and splitantenna signals before they are applied to the fourreceiver modules. The preamplifier circuits include the

antenna relay, filter circuit, amplifiers, a coupler circuit,and a four-way splitter. The sonobuoy antenna receivesall signals from all frequencies and is connectedthrough normally closed contacts of the antenna relayto the filter circuit, which allows only the 136 to 173.5MHz frequencies in the sonobuoy band to pass. Outputof the filter is amplified, and then fed through thecoupler circuit to the four-way splitter to provide theamplified sonobuoy signals (if present) on four separateelectrically isolated lines to each of the four receivermodules. Amplified signals from any sonobuoys withinline-of-sight range are connected to each receivermodule.

The receiver system contains four receivermodules. Each module is a complete receiver capableof receiving both analog and digital FM radio signals.All four receiver modules are identical and directlyinterchangeable. Each receiver module can be tunedand configured to amplify and process signals from anysonobuoy or the BITE circuit. Transmissions from up tofour different sonobuoys may be received andprocessed simultaneously by the receiver system. Thereceiver modules may be tuned via the remote controlunit (RCU) or through the aircraft computer.

ACOUSTIC DATA PROCESSING

When acoustic signals are received from deployedsonobuoys by the sonobuoy receiver, the data must beextracted and converted to a format usable by the sensoroperator. This task is accomplished by the AN/UYS-1single advanced signal processor (SASP) system.There are several versions of the UYS-1 systemcurrently in use, but all perform the same basicfunction.

Operating Principles

The SASP processes sonobuoy audio in active andpassive processing modes to provide long-range search,detection, localization, and identification ofsubmarines. The RF signals from the sonobuoys arereceived by the sonobuoy receivers and sent to theSASP. After processing, signals are sent to the displaysand the recorders for operator use. The SASP alsogenerates command tones for controlling the CASS andDICASS sonobuoys.

Components

The major components include the spectrumanalyzer (SA), power supply, and the control-indicator(SASP power control).

1-15

SPECTRUM ANALYZER.—The spectrum ana-lyzer is a high-speed signal processor that extractsacoustic target information from both active andpassive sonobuoy data. The SA determines frequency,amplitude, bearing, Doppler, range, and other char-acteristics for acoustic targets.

POWER SUPPLY.—The power supply converts115 volts ac into 120 volts dc operating voltages. The120-volt dc power is then converted to low-level dcvoltages for operation of individual circuits. A powerinterrupt unit protects the data against transient powerinterruptions that normally occur during airborneoperations.

CONTROL-INDICATOR.—The control-indicatorprovides SASP power control and monitoring. Aswitch-indicator labeled POWER ON/OFF controls thepower to the SA, the display computer (DCU), theCASS transmitter, and the displays. The AU/DCUCAUTION/OVHT indicator indicates the temperaturestatus in the SA and the DCU. The CAUTION sectionwill flash on when the thermal warning is activated ineither unit. The OVHT section indicates an overheat ineither unit. The STA OVHT indicator indicates anover-temperature condition exists at the sensor stations1 and 2 consoles. The OVERRIDE-NORMAL switchwill override the over- heat warnings for the sensorstations 1 and 2 consoles.

Q1-3. Sonobuoys that transmit an acoustic pulseand detect a resulting echo are classified aswhat type of sonobuoy?

Q1-4. Sonobuoys equipped with an EFS systemhave what capability?

Q1-5. What information can a DIFAR sonobuoyprovide that a LOFAR sonobuoy cannot?

Q1-6. Which of the following sonobuoys provideswater temperature data?

ON-TOP POSITION INDICATOR(OTPI) SYSTEM

LEARNING OBJECTIVE: Recognize thepurpose, major components, and operatingprinciples of an On-Top Position Indicatorsystem.

When sonobuoys are deployed from fixed-wingand/or rotary-wing aircraft, the aircrew must have away of determining the location of the deployedsonobuoy. The OTPI system provides a means oflocating deployed sonobuoys.

SYSTEM DESCRIPTION

The OTPI system operates in conjunction with thedirection finder (DF) antenna to provide sonobuoybearing with respect to aircraft heading (relativebearing). The OTPI receiver receives signals fromsonobuoys transmitting in the VHF range of 136,000 to173,500 megahertz (MHz) via the DF antenna. (TheSH-60 uses the AN/ARN 146 radio receiver, and theP-3 uses an auxiliary receiver housed inside theAN/ARR-78 sonobuoy receiver for OTPI functions.)

SYSTEM OPERATION

Sonobuoy signals received through the DF antennaare routed to the OTPI receiver. Receiver audio isrouted to an electronic control amplifier that providesdrive signals to the antenna. As the antenna scanstoward the sonobuoy, the drive signals from the receiverare reduced. When the antenna reaches the low point, ornull, the antenna stops scanning. The resulting positionindicates the direction of the sonobuoy.

Sonobuoy bearing is displayed on the number oneneedle of the aircraft horizontal situation indicator,HSVD, or equivalent indicator. The pilot can adjust thecourse of the aircraft to coincide with the location of thesonobuoy. When the aircraft flies over the sonobuoy,the number one needle will swing 180 degrees,indicating the sonobuoy is directly beneath the aircraft.

Q1-7. The OTPI system provides what informationto the USW pilot?

DIPPING SONAR SYSTEM

LEARNING OBJECTIVE: Recognize themajor components, operating principles, andmodes of operation of an airborne dippingsonar system.

Dipping sonar sensors, or dunkers, are retrievablesonar devices that are deployed from USW helicoptersor from some naval ships. The dipping sonar device canprovide both passive and active sonar detection ofsubmarines. Normally, they are used in the active mode,which generally provides longer ranges as compared toactive sonobuoys. Conversely, the noise associated withhelicopter or surface ship operations makes passivedipping sonar sensors less sensitive as compared topassive sonobuoys. One of the distinct advantages ofthe USW helicopter dipping sonar over sonobuoys isthat the Aviation Warfare Systems Operator running thedipping sonar can choose an optimum search depth tomaximize the performance of the dipping sonar. Once

1-16

active sonar contact is gained, the aircrew quicklyraises the dipping sonar, "jumps" the aircraft to a newposition (dip point), lowers the dipping sonar, andre-acquires the now-evasive submarine. Once a dippingsonar helicopter is in hot pursuit, the submarinecommander's job of evasion is extremely difficult.Adding a second dipping sonar helicopter into thechase makes the submarine commander's job nearlyimpossible! In addition to the passive and active sonarmodes of the dipping sonar, the system is used forcommunications and measuring the water temperatureprofile. The AN/ASQ-13F dipping sonar is the currentsystem used by naval aircraft, and it is used only in theSH-60F carrier-based helicopter (fig.1-14).

AN/AQS-13F SONAR SYSTEM

The AN/AQS-13F sonar set is a multimode,multifunction, airborne sonar system. It is used tolocate, classify, and track submerged targets. It canoperate in either active or passive mode, using deployeddipping sonar or sonobuoys. Target data from the sonartransducer is processed by the sonar receiver and sonardata computer, and is displayed on the azimuth-rangeindicator (ARI) and/or multifunction displays (MFD).

The following components make up the sonar set:

• IP-1607/ASQ-13F azimuth range indicator

• R-2433/ASQ-13F sonar receiver

• CP-1971/ASQ-13F sonar data computer

• TD-1421/ASQ-13F multiplexer

• C-12056/ASQ-13F dome control

• RL-299/ASQ-13F reeling machine

• MX-18320A/ASQ-13F cable and reel assembly

• TR-348/ASQ-13F sonar transducer

The following systems/components directly or in-directly interface with the sonar set:

• Sonobuoy receiver

• Intercommunication system controller

• Mission tape recording set

• Digital automatic flight control system(DAFCS)

• Tactical navigation set data processors, controldisplay units (CDU), and multifunction displays

• Horizontal situation video display (HSVD) set

1-17

Figure 1-14.—SH-60F dipping sonar.

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IP-1607/ASQ-13F Azimuth-Range Indicator

The azimuth-range indicator (ARI) (fig. 1-16) ispositioned at the sensor station and produces a visualrepresentation of range and bearing information,providing a means for the operator to track targets. The7-inch cathode-ray tube (CRT) is marked in 10-degreeincrements to aid in determining target bearing. Thereare four controls on the left-hand side of the indicatorfor operator comfort. The CURSOR INTENSITYswitch controls the brightness of the cursor. The CRTINTENSITY controls the brightness of the overallCRT. The VIDEO GAIN controls the level of the videosignal applied to the CRT. The AUDIO GAIN switchcontrols the level of the audio signal.

The right side of the indicator face contains a metercalled the RANGE RATE-KNOTS meter. This meterdisplays the opening or closing speed of the selectedtarget. The MTI THRESHOLD switch selects the rangerate threshold of targets to be displayed on the CRT.The DISPLAY switch selects either sonobuoy signalsor sonar signals to be shown on the CRT. To activate the

sonar set, press the POWER switch. This activates theentire system with the exception of the dome control.The TEST switch initiates the built-in test functionsand analyzes the results.

R-2433/ASQ-13F Sonar Receiver

The sonobuoy receiver (fig. 1-17) generates thetransmit signal and receives and processes sonic signalsfrom the transducer for display on the ARI. Thereceiver also provides audio to the intercommunicationsystem (ICS) for aural monitoring of acoustic signals.The R-2433 receiver should not be confused with theAN/ARR-84 sonobuoy receiver, which detects RFsignals from deployed sonobuoys.

CP-1971/ASQ-13F Sonar Data Computer

The sonar data computer (fig. 1-18) is aprogrammed array processor that provides improvedoperation of the dipping sonar and processes signalsfrom multiple passive and active sonobuoys.

1-19

Figure 1-16.—Azimuth-range indicator.

Figure 1-17.—Sonar receiver. Figure 1-18.—Sonar data computer.

TD-1421/ASQ-13F Multiplexer

The multiplexer (fig. 1-19) provides the electricalinterface between the sonar set units mounted in thehelicopter and the sonar transducer submerged in thewater. The multiplexer is located in the cabin area,parallel to the reeling machine.

C-12056/ASQ-13F Dome Control

The dome control (fig. 1-20) provides controls forraising and lowering the sonar transducer andindicators for monitoring the sonar transducer and

reeling machine. The dome control is located at thesensor station.

RL-299/ASQ-13F Reeling Machine andMX-18320A/ASQ-13F Cable and Reel Assembly

The reeling machine (fig. 1-21) is a hydraulic hoistthat raises and lowers the sonar transducer. It operatesfrom utility hydraulic power at a pressure of 3000pounds per square inch, requiring a flow ofapproximately 13.4 gallons-per-minute at maximumspeed. The reeling machine is located in the cabin area.It houses the cable and reel assembly (fig. 1-22), whichcontains a sonar cable 1550 to 1600 feet in length. Thesonar cable is a jacketed triax cable with an armor braidused as the strength component of the cable. The innerelectrical conductors allow for the passage of signalsbetween the transducer and multiplexer.

TR-348/ASQ-13F Sonar Transducer

The sonar transducer (fig. 1-23) generates andtransmits sonar pulsed (ping) energy or voice signalsinto the water. The sonar transducer also acts as alistening device, converting sound energy received inthe water into electrical signals.

The sonar transducer is connected to the wet end ofthe reel-cable assembly cable, which is mounted in thereeling machine. The transducer tail structure provideshydrodynamic stability during raising and lowering.The tail structure is seated or caged against the upper

1-20

Figure 1-19.—Multiplexer.

Figure 1-20.—Dome control.

stop plate of the transducer body during lower and isfree to move or is uncaged during raise by the dashpotassembly that is housed under a rubber bladder on thetop of the transducer body. Also contained in thedashpot assembly is a universal joint that allows freemovement of the transducer body when uncaged. Theuniversal joint also facilitates mechanical connection ofthe transducer to the sonar cable termination.

The center body of the transducer contains an outeracoustic array and an inner pressure vessel. The sonartransducer acoustic array consists of piezoelectricelements electrically connected in vertical staves forthe receive or hydrophone portion and circular rings forthe transmit or projector portion. It is surrounded by acastor oil-filled neoprene boot for proper acousticinterface with seawater. The boot provides little or nomechanical protection for the sensitive acousticelements and should be handled with extreme care.

The inner pressure vessel contains the transmitterassembly and an electronics assembly. Also containedwithin the pressure vessel are catalysts that combinehydrogen generated by the battery with oxygencontained in the air within the vessel to form water. Thevessel also contains desiccants to absorb the water. Theoxygen must be refreshed and the catalyst anddesiccant must be replaced periodically.

The electronics assembly provides the circuitsnecessary to send and receive signals via the sonarcable, to monitor various fault conditions, to conditionthe signals from the hydrophone, pressure, andtemperature sensors, and to drive the transmitter.

The transmitter provides the electrical signal to theprojector for the acoustic transmit pulse. The noseassembly contains a lead-acid battery that providespower for the transmitter during acoustic trans-missions.

WARNING

A lethal dc potential of up to 176 V is present onthe connector pins of the battery connector P1.During handling, do not touch the P1 connector pinsor battery plug receptacle pins. Do not allow anyforeign objects to fall into the nose assembly.

The battery is charged when the sonar power is on.Periodic charging is required to maintain the propercharge. When the transducer is removed from theaircraft, the transducer should be charged prior to

1-21

Figure 1-21.—Reeling machine.

AEf0122

Figure 1-22.—Cable and reel assembly.

AEf0123

Figure 1-23.—Sonar transducer.

removal. After removal, the battery should be dis-connected by removing the battery connector.

MODES OF OPERATION

The sonar set can be operated in a variety of modesand controlled from various components to achievespecific functions. Table 1-3 contains a list of the sonarset modes and a general description of the functionsthey provide.

AIRBORNE LOW-FREQUENCY SONAR

The SH-60R is the newest addition to the Navy’sfleet of helicopters. The SH-60R combines thetraditional mission areas of the SH-60B and SH-60Finto a more versatile and up-to-date weapons platform.One of the most drastic improvements is the airbornelow-frequency sonar (ALFS).

The airborne low-frequency sonar (ALFS)provides a dipping sonar with demonstrated

capabilities typically three to six times (square miles ofocean searched per hour) the ASQ-13F deep watercapability. ALFS provides longer detection ranges anda greater detection capability by using lowerfrequencies, less signal attenuation, longer pulselengths, improved processing, and increased trans-mission power. ALFS uses the Enhanced ModularSignal Processor, designated UYS-2A, for improvedsonobuoy processing capability.

Remanufacture of the SH-60B fleet has started andwill continue through FY 2009. Remanufacture of theSH-60F and HH-60H fleets will begin in FY 2004 andcontinue through FY 2012. The SH-60R is scheduled toreach operational capability in 2002.

Q1-8. What type of naval aircraft employs theAN/ASQ-13F dipping sonar system?

Q1-9. On a dipping sonar system, what componentgenerates and transmits sound energy into thewater?

1-22

Operating Mode Mode Description

Dipping sonar-active Uses submerged, actively pinging sonar transducer to locate, iden-tify, and track a target.

Dipping sonar-passive Uses submerged sonar transducer to listen to target noises.

Sonobuoy-active Uses CASS and DICASS sonobuoys to actively locate, identify, andtrack a target.

Sonobuoy-passive Uses LOFAR, DIFAR, and VLAD sonobuoys to locate targets bytheir noise signatures via returned up-link target data.

Underwater voicecommunication (UVC)

Uses submerged sonar transducer to transmit and receive voicesignals to and from other USW helicopters, submarines, or similarlyequipped surface ships.

Bathythermograph (BT)recording

Uses submerged sonar transducer or BT sonobuoy to record thedifference in water temperature vs. depth.

Table 1-3.—AN/AQS-13F Operating Modes

CHAPTER 2

MAGNETIC ANOMALY DETECTION (MAD)

INTRODUCTION

While vigilant lookouts carefully scan the turbulentocean surface for submarine periscopes and wakes,nonacoustic sensors (fig. 2-1) augment acoustic sensorsto locate potentially hostile forces below the surface ofthe sea. These sensors include radar (to detect exposedperiscopes and hull surfaces), electromagnetic systems(to intercept the radar emissions from submarines),infrared receivers (to detect the heat signatures ofsurfaced submarines), and magnetic anomaly detectors(MAD) (to sense small changes in the Earth's magneticfield caused by the passage of a submarine). Thischapter covers the MAD sensor and describes magneticdetection principles, magnetic noise compensation,helium magnetometer theory, and a typical airborneMAD system and its subsystems.

PRINCIPLES OF MAGNETICDETECTION

LEARNING OBJECTIVE: Recognize theoperation principles of magnetic anomalydetection.

Light, radar, and sound energy cannot pass from airinto water and return to the air in any degree that isusable for airborne detection. On the other hand, linesof force in a magnetic field are able to make thistransition almost undisturbed because the magneticpermeability of water and air are practically the same.Specifically, the lines of force in the Earth's magneticfield pass through the surface of the ocean essentiallyundeviated by the change of medium (from water to airor vice versa) and undiminished in strength.Consequently, an object under the water can bedetected from a position in the air above if the objecthas magnetic properties that distort the Earth'smagnetic field. A submarine has sufficient ferrous massand electrical equipment to cause a detectabledistortion (anomaly) in the Earth's field. Detection ofthis anomaly is the function of magnetic anomalydetection (MAD) equipment.

MAGNETIC ANOMALY

The lines comprising the Earth's natural magneticfield do not always run straight north and south. If

2-1

ACTIVEACOUSTICS

MAGNETICS

RADAR

AEf02001

PASSIVEACOUSTICS

Figure 2-1.—Nonacoustic sensors.

traced along a typical 100-mile path, the field twists atplaces to east or west and assumes different angles withthe horizontal. Angles of change in the east-westdirection are known as variation angles, while anglesbetween the lines of force and the horizontal are knownas dip angles (fig. 2-2). At any given point between the

equator and the magnetic poles, the relationship of theangle between the Earth's surface and the magneticlines of force is between 0 degrees and 90 degrees. Thisdip angle is determined by drawing two imaginary linestangent to the Earth's surface and to the line of forcewhere it enters the Earth's surface.

If the same lines are traced only a short distance,300 feet for instance, their natural changes in variationand dip over such a short distance (short-trace) arealmost impossible to measure. However, short-tracevariation and dip in the area of a large mass of ferrousmaterial, though still extremely minute, are measurablewith a sensitive anomaly detector. This is shown infigure 2-3. The dashed lines represent lines of force inthe Earth's magnetic field.

View A of figure 2-3 shows the angular direction atwhich natural lines of magnetic force enter and leavethe surface of the Earth. Note that the dip angles areconsiderably steeper in extreme northern and southernlatitudes than they are near the equator. View Brepresents an area of undisturbed natural magneticstrength. In views C and D, the submarine's magneticfield distorts the natural field as shown. The density ofthe natural field is decreased in view C and increased inview D. The natural dip angle is also affected, but onlyvery slightly.

2-2

800

300

DIP ANGLE

DIP ANGLE

MAGNETIC LINESOF FORCE

AEf02002

Figure 2-2.—Dip angles.

EARTH

EQUATOR

N

S

B

A

ASW AIRCRAFT WITH MAD

C D

AEf02003

Figure 2-3.—Simplified comparison of natural field density and submarine anomaly.

SUBMARINE ANOMALY

The maximum range at which a submarine may bedetected depends on the intensity of its magneticanomaly and the sensitivity of the detector. A mag-netometer is used as the detector in MAD equipment.

A submarine's magnetic moment (magneticintensity) (fig. 2-4) determines the intensity of theanomaly. It is dependent mainly on the submarine'salignment in the Earth's field, its size, the latitude atwhich it is detected, and the degree of its permanentmagnetization.

MAD equipment is very sensitive, but the sub-marine's anomaly, even at a short distance, is normallyvery weak. The strength of a complex magnetic field(such as that associated with a submarine) varies as theinverse cube of the distance from the field's source.That is, if the detectable strength of a field source has a

given value at a given distance and the distance isdoubled, the detectable strength of the source at theincreased distance will then be one-eighth of its formervalue. Therefore, from the foregoing, at least two factsshould be clear. First, MAD equipment must beoperated at a very low altitude to gain the greatestproximity possible to enemy submarines. Second, thesearching aircraft should fly at a predetermined speedand follow an estimated search pattern. This ensuressystematic and thorough searching of the prescribedarea so that no existing anomalies are missed.

ANOMALY STRENGTH

Up to this point, the inferred strength of asubmarine's anomaly has been exaggerated forpurposes of explanation. Its actual value is usually sosmall that MAD equipment must be capable ofdetecting a distortion of approximately one part in60,000. This fact is made apparent by pointing out thatthe direction of alignment of the Earth's magnetic linesof force is rarely changed more than one-half of 1degree in a submarine anomaly.

Figure 2-5, view A, represents a contour mapshowing the degree of anomaly caused by a submarine.The straight line is approximately 800 feet in length andrepresents the flight path of a searching aircraft throughthe area of the submarine anomaly. If the submarinewere not present, the undisturbed magnetic intensity inthe area due to its assumed natural characteristicswould be 60,000 gamma. (The gamma, symbolized bythe Greek letter ã, is a measure of magnetic intensity.)All variations in the field, when the submarine is

2-3

LONGITUDINALMOVEMENT

TRANSVERSEMOVEMENT

VERTICALMOVEMENT

AEf02004

Figure 2-4.—Submarine's magnetic moment.

MOVING PAPERRECORDER TAPE

STYLUS SWUNGBY AMPLIFIEDMAGNETOMETERSIGNAL

100’

SCALE

F

B

A

(A) (C)

(B)E

D

C+

00

2

45

-10

-20

-2

5 4 3 2 1 0 1 2 3 4 5

AEf02005

Figure 2-5.—(A) Degree of anomaly; (B) anomaly stylus; (C) sample anomaly record.

present, would then be above or below this naturalintensity. Therefore, 60,000 ã is the zero referencedrawn on the moving paper tape shown in figure 2-5,view C.

Refer to view A of figure 2-5. Starting with theaircraft at point A, where the anomaly is undetectable,the Earth's field concentration decreases to an intensityof -2 (59,998) at point B. Its intensity then increasesuntil a peak value of +45 is reached at point C. Fromthat point it decreases to zero at point D. Beyond pointD, another zone of what amounts to magneticrarefaction is encountered. The Earth's field is lessintense than its normal value. Consequently, anomalousvalues in this zone are considered as minus quantities.A peak minus intensity is reached at point E, andthereafter, the signal rises back to its normal, orundetectable, intensity at point F.

As the varying degrees of intensity areencountered, they are amplified and used to drive aswinging stylus, as shown in figure 2-5, view B. The tipof the stylus rides against the moving paper tape,leaving an ink trace. (Some recorders useelectrosensitive paper tape and a chain-driven stylus.Some other recorders use a cathode-ray tube (CRT)display.) The stylus is swung in one direction forpositive and in the opposite direction for negative. Themagnitude of its swing is determined by the intensity ofthe anomaly signal. Figure 2-5, view C, is a sample ofpaper recording tape, showing the approximate tracecaused by the anomaly in view A.

In the illustration just given, the search aircraft'saltitude was 200 feet. At a lower altitude, the anomalywould be stronger. Likewise, the anomaly would beweaker at a higher altitude.

Q2-1. The angle between an imaginary line tangentto the Earth’s surface and to the magnetic lineof force where it enters the Earth’s surface isknown by what term?

Q2-2. The gamma, the measure of magnetic inten-sity, is symbolized by what Greek letter?

MAGNETIC NOISE COMPENSATION

LEARNING OBJECTIVE: Identify sourcesof magnetic noise. Recognize the purpose andprocedures of magnetic compensation.

Any noise or disturbance originating in the aircraftor its equipment that could produce a signal on a MADequipment recorder is classified as magnetic noise.

NOISE SOURCES

In an aircraft, there are many sources of magneticfields, such as engines, struts, control cables,equipment, and ordnance. Many of these fields are ofsufficient strength to seriously impair the operation ofMAD equipment. Consequently, some means mustbe employed to compensate for "magnetic noise"fields. The noise sources fall into two majorcategories—maneuver noises and dc circuit noises.

Maneuver Noises

When the aircraft maneuvers, the magnetic field ofthe aircraft is changed, causing a change in the totalmagnetic field at the detecting element. The aircraftmaneuver rates are such that the signals generated havetheir major frequency components within the bandpassof the MAD equipment. Induced magnetic fields, eddycurrent fields, or the permanent field cause maneuvernoises.

The variations in the induced magnetic fielddetected by the magnetometer are caused by changes inthe aircraft's heading. This causes the aircraft to presenta varying size to the Earth's magnetic field, and only theportion of the aircraft parallel to the field is available formagnetic induction.

Eddy current fields produce maneuver noisebecause of currents that flow in the aircraft's skin andstructural members. When an aircraft's maneuvercauses an eddy current flow, a magnetic field isgenerated. The eddy current field is a function of therate of the maneuver. If the maneuver is executedslowly, the effect of the eddy current field is negligible.

The structural parts of the aircraft exhibitpermanent magnetic fields, and, as the aircraftmaneuvers, its composite permanent field remainsaligned with it. The angular displacement between thepermanent field and the detector magnetometer duringa maneuver produces a changing magnetic field, whichthe detector magnetometer is designed to detect.

DC Circuit Noises

The dc circuit noise in an aircraft comes from thestandard practice in aircraft design of using asingle-wire dc system with the aircraft skin andstructure as the ground return. The resulting currentloop from the generator to load to generator serves as alarge electromagnet that generates a magnetic fieldsimilar to a permanent magnetic field. Whenever the dc

2-4

electrical load of the aircraft is abruptly changed, thereis an abrupt change in the magnetic field at the detector.

COMPENSATION

Regardless of its source, strength, or direction, anymagnetic field may be defined in terms of three axialcoordinates. That is, it must act through any or all ofthree possible directions—longitudinal, lateral, orvertical in relation to the magnetometer detector.

Compensation for magnetic noises is necessary toprovide a magnetically clean environment so that thedetecting system will not be limited to the magneticsignal associated with the aircraft itself.

Experience has shown that the induced fields andeddy current fields for a given type of aircraft areconstant. That is, from one aircraft to another of thesame type, the difference in fields is negligible. Thesefields may be expected to remain constant throughoutthe life of the aircraft, provided significant structuralchanges are not made. In view of these factors, the

aircraft manufacturer provides compensation forinduced fields and eddy current fields.

Eddy current field compensation is usuallyachieved by placing the detector magnetometer in arelatively quiet magnetic area. In some aircraft, themagnetometer (detecting head) is placed at least 8 feetfrom the fuselage. This is done by enclosing thedetecting head in a fixed boom (fig. 2-6, view A).Helicopters tow the detector head by use of a cable (fig.2-6, view B).

Induced magnetic field compensation isaccomplished by using Permalloy strips. The aircraft isrotated to different compass headings, and the magneticmoment is measured. The polarity and the variation ofthe magnetic moment are noted for each heading, andPermalloy strips are oriented near the detectormagnetometer to compensate for field changes due toaircraft rotation. Additional compensation is needed forthe longitudinal axis, and is provided for by thedevelopment of outrigger compensators of Permalloynear the detecting element.

2-5

CAUTION

CAU

TION

+

B

A

DETECTOR HEAD ATEND OF BOOM

AEf02006

Figure 2-6.—(A) Stationary detector boom; (B) cable deployed towed detector.

Permanent field compensation must be done inthree dimensions rather than in two, and it isaccomplished by three compensating coils mountedmutually perpendicular to each other (fig. 2-7, view A).The aircraft is rotated in 5-degree and 10-degree stepsaround its three axes. Adjustment of the field strength isaccomplished by controlling the amount of directcurrent that flows through a particular coil. Figure 2-7,view B, shows a circuit for a single compensating coil.

Compensation for the dc magnetic field isaccomplished by using electromagnetic compensatingloops. The loops are arranged to provide horizontal,vertical, and longitudinal fields, and are adjusted to beequal and opposite to the dc magnetic field caused bythe load current. The compensating loops are connectedacross a variable resistor for a particular distributioncenter, and are adjusted to allow current flowproportional to the load current for correctcompensation. Different types of aircraft have severalsets of compensating loops, depending upon thenumber of distribution centers. In newer aircraft,production changes have been made to use groundreturn wires to minimize loop size.

The procedure for adjustment of the dccompensation system makes use of straight and levelflight on the four cardinal headings. For example,actuation of a cowl flap motor will cause dc field

changes representative of those caused by any nacelleload. The load is energized, the size and polarity of thesignal are noted, and the compensation control isadjusted. The load is re-energized, and thecompensation control is adjusted again. Adjustmentsare continued until the resulting signals from the dcfield are minimized.

Under ideal conditions, all magnetic fields tendingto act on the magnetometer head would be completelycounterbalanced. In this state, the effect on themagnetometer is the same as if there were no magneticfields at all. This state exists only when the followingideal conditions exist:

1. The aircraft is flying a steady course (nomaneuvers) through a magnetically quietgeographic area.

2. Electric or electronic circuits are not turned onor off during compensation.

3. Direct current of the proper intensity anddirection has been set to flow through thecompensation coils, so that all stray fields arebalanced.

To approximate these conditions, the compensationof MAD equipment is usually performed in flight, wellat sea. The equipment is compensated under operatingconditions, which closely resemble those of actualundersea warfare (USW) search flights.

From the foregoing, it should be clear that theobjective of compensation is to gain a state of totalbalance of magnetic forces around the magnetometer.Thereafter, any sudden shift in one of the balancedforces (such as an anomaly in the Earth's field force)upsets the total balance. This imbalance is indicated onthe recorder. Unfortunately, a shift in any of thebalanced forces will be indicated. Shifts in any of theforces other than the Earth's natural field are regardedas noise.

Q2-3. Magnetic noise sources fall into what twomajor categories?

Q2-4. The compensation of MAD equipment formagnetic noise is usually performed at whatlocation?

HELIUM MAGNETOMETER THEORY

LEARNING OBJECTIVE: Recognize thetheory and operation of helium magnetom-eters. Define Larmor frequency and themetastable state.

2-6

VERTICALAXIS

LATERALAXIS

LONGITUDINALAXIS

A

B

REWSW

COMPCOIL

DC V

AEf02007

Figure 2-7.—(A) Arrangement of compensating coils; (B)compensating coil circuit.

The detection of disturbances in the Earth’smagnetic field requires extremely sensitive equipment.Today’s naval aircraft currently employ heliummagnetometers to detect these anomalies. In order tounderstand how a MAD system works, the theory andoperation of the helium magnetometer must bediscussed.

MAGNETIC RESONANCE

Magnetic resonance is based on the theory ofatomic structure, which states "the atom is consideredto be a nucleus around which one or more electrons areorbiting." The nucleus has a positive charge becauseprotons are part of it. The electrons have a negativecharge that causes the atom to be neutral. The electronsand the nucleus have a spin and, because of the spin andthe charge, a magnetic moment results.

The electron also is spinning about its own axis(much like the Earth orbits around the sun once a yearand spins about its axis once a day). This spin causesthe electron to have a magnetic moment, much like asmall magnet, and to exhibit the characteristics of a tinygyro. As is the case with a mechanical gyro, a forceapplied to the electron causes it to precess, resulting inthe wobble motion of the electron's spin axis, as shownin figure 2-8, view A. Furthermore, the magneticcharacteristics of an electron make it possible tosubstitute a magnetic field force (Earth's magneticfield) for the mechanical force normally used to precessa conventional gyro. In addition to this, if a rotatingmagnetic field at a radio frequency (RF) is applied

perpendicularly to the main magnetic field, the electronprecesses further. The dash lines depict this condition infigure 2-8, view B, as an increased wobble motion ofthe electron's spin axis.

When the frequency of rotation of the magneticfield is adjusted until it is the same as the naturalfrequency of the particular material in use (helium inthis case), the deviation of the spin axis of the electronstends to increase and paramagnetic resonance isachieved. Electron paramagnetic resonance isresonance in which the electron is the only particleshifting energy states. This resonance occurs when theangular velocity of the rotating magnetic field isapproximately the natural spin axis wobble rate, orprecession rate, of helium electrons. The natural spinaxis wobble rate is also called the Larmor frequency. Asthe electrons are caused to precess more by the externalRF magnetic field, the amplitude of the precessionbecomes so great that the electrons jump to a higherenergy level, at which time light energy is absorbed bythe helium. Light is used initially to increase the energylevel of the electrons to a metastable energy state,which is a higher energy level with a much longerlasting duration than any other excited level. Heliumgas is one of the elements or materials that can assume ametastable energy level.

The ASQ-81 solid-state MAD system uses a beamof low-frequency light to periodically increase theenergy level of the electrons and orient the magneticmoments of the atoms and their electrons on a plane inthe direction of the light beam. This procedure is known

2-7

MAGNETICFIELD

SPIN AXIS

ROTATING MAGNETIC FIELD

SPIN AXIS

A BAEf02008

Figure 2-8.—Electron precession.

as optical pumping. The pumping action of the lightenergy causes the electrons to jump two energy levels;that is, from ground energy level (E0) to excited energylevel (E2). Excited energy state 2 (E2) has a very shortlifetime without external excitation. Since the light ispumping, the electrons tend to fall back toward thestationary state (E0), but must pass through themetastable excited state—energy state 1 (E1).

If the pumping action is made continuous, themajority of electrons at any one time will congregate atthe (E1) or metastable excited state. This is necessarybecause if resonance of unaligned ground state heliumwere attempted with an external rotating magneticfield, only a small percent of the gas would change stateand absorb light energy. This would make detection ofresonance difficult. However, with the helium's atomicsystem aligned and excited when resonance occurs, agreat majority of the atoms change state and an easilymonitored amount of light is absorbed. The metastablestate in the gas is necessary to have a relatively stablehigher energy level in a much larger number of atoms.This produces a much greater change when resonanceoccurs.

Optical energy is absorbed and released by theelectron during these energy level transitions. It shouldbe noted here that resonance can be achieved only incertain solids and liquids with loosely knit atomicstructures and in gases such as helium.

This discussion so far has concerned the energylevel change of the helium atom electrons by the use ofan external magnetic field (supplied through coils)oriented 90 degrees to the Earth's field. The externalfield rotates at the Larmor frequency of the electron,which was determined by the Earth's magnetic fieldstrength. Since the precession or Larmor frequency ofelectrons varies with magnetic field intensity at the rateof 28 hertz (Hz) per ã, monitoring the Larmorfrequency changes is a convenient method for detectingand measuring changes in the Earth's magnetic field.

However, a small change in the Larmor frequencyof electrons is difficult to measure directly. It is moreconvenient and accurate to make an indirectmeasurement by monitoring the definite increase inlight energy absorption that occurs at resonance. Theatoms then radiate some of this light energy as theLarmor frequency shifts away from the externallyapplied field's frequency, and resonance is lost. Thus,variations in the Earth's magnetic field strength arereflected by the changes in the quantity of light passingthrough the helium gas. Helium gas is contained in atransparent container called an absorption cell.

As shown in figure 2-9, view A, energy is absorbedin forcing electrons to jump from one orbit to anotherorbit at a greater distance from the nucleus. Figure 2-9,view B, depicts the atom giving up energy as theelectrons move back to their original orbit.

When the oscillator supplying the coils (resonanceoscillator) reaches the Larmor frequency, light isabsorbed. When the resonance oscillator comes off theLarmor frequency, light is emitted. Thus, the Larmorfrequency (which represents the Earth's magnetic fieldstrength) can be tracked by knowing the resonantoscillator frequency when the absorption cell isabsorbing energy.

As shown in figure 2-10, the light energy source forthe magnetic resonance magnetometer is a heliumlamp. The infrared (IR) detector, which is a verysensitive and trouble-free device, is used to track thelight energy level changes.

HELIUM MAGNETOMETER OPERATION

The necessary components for a magneticresonance magnetometer are shown in figure 2-10. Theexternal energy source, the helium discharge lamp,applies light energy to the transparent helium-filledabsorption cell. Helium atoms in the absorption cellabsorb or give up light energy in relation to slight

2-8

A B

AEf02009

Figure 2-9.—Helium atom (A) absorbing energy and (B) radiating energy.

changes in the Earth’s magnetic field. This causes theabsorption cell to go in and out of resonance. The IRdetector, mounted on the other side of the absorptioncell from the light source, picks up these energy levelchanges and produces an electrical output. The energy

level is low when the cell is resonant and high when it isnot resonant. The IR detector output signal can then beused to keep the resonance oscillator on or about theLarmor frequency of the helium in the cell regardless ofchanges in the Earth's magnetic field. A magneticanomaly can then be detected when the resonanceoscillator center frequency makes a typical swingwithin the bandpass relative to the flight envelope of theMAD-equipped type of aircraft.

If the resonance oscillator frequency were alwaysmaintained precisely at resonance, the IR output wouldsimply be a very low dc level. This low dc level wouldbe difficult to monitor and would not provide a phasechange above and below resonance. For this reason theRF energy applied to the resonance coils is frequencymodulated by 430 Hz. The resultant variation in themagnetic field forces the helium atoms in and out ofresonance around the null and provides a phasereference signal to a phase detection circuit. Figure2-11 depicts the phase reversal above and belowresonance. Note also that at resonance, which is thenormal operating point, the IR detector acts much like afull wave rectifier. Because of rectifying action, the IR

2-9

INFRAREDDETECTORS

HELIUM LAMP

RESONANCEFREQUENCY

COILS

FROMRESONANCEOSCILLATOR

HELIUMABSORPTION

CELL

AEf02010

Figure 2-10.—Essential parts of helium magnetometer.

RES CENTER FREQ

ARBITRARYABSORPTION

CELL BANDPASS

REF FM430 Hz

LOW LOW

RES CENTER FREQ

RES CENTERFREQ

ARBITRARYABSORPTION

CELL BANDPASS

REF FM430 Hz

REF FM430 Hz

RCF

RCF

ARBITRARTYABSORPTION

CELL BANDPASS

RCF

RCF

RESONANCE OSC

RESONANCE OSC

OSC BELOW FREQIR CELL OUTPUT430 Hz LAG SHIFT

OSC BELOW FREQIR CELL OUTPUT430 Hz LEAD SHIFT

LOW

HI

RESONANCE OSC

OSC ON FREQIR CELL OUTPUT

860 Hz

HIHI

AEf02011

Figure 2-11.—Resonance waveforms from a phase detection circuit.

output at resonance is a pulsating 860-Hz signal. Thedetector also acts like a discriminator in that thefrequency modulation (FM) swing of the resonanceoscillator is converted to an amplitude modulation(AM) output when near the resonant frequency of thehelium atoms in the absorption cell. This is truebecause its output goes positive with each increase inlight energy given off by the absorption cell.

Q2-5. When electrons are excited to an energy levelthat is higher in energy and longer lasting induration, this energy state is known as whattype of energy state?

Q2-6. The Larmor frequency of electrons varieswith magnetic field intensity at the rate of howmany Hz per ã?

Q2-7. What component of a helium magnetometerdetects the energy level changes of the heliumatoms and produces an electrical output?

TYPICAL AIRBORNE MAD SYSTEM

LEARNING OBJECTIVE: Recognize thecomponents and operating principles of atypical airborne MAD system.

2-10

+

+

+ +

+

+

AMPLIFIER-POWER SUPPLYAM-4535/ASQ-81 (V)

AMPLIFIER POWER SUPPLY

ELAPSED TIME

2 CB 12 AMP

2 CB 21 AMP

DET PWR INPUT PWR

BUILT- IN TEST

FAIL

DETECTOR AMP PWRSUPPLY

ALT COMP

FAIL FAIL

LAMP TEST OFF

1

2

3

4

5

12 13

Y1

910

2J1 2J22J3 2J4

2J5

2J6

POWER

SIGNAL

TO 115 VOLTS3 - PHASE POWER

DETECTING SET CONTROL

MAGNETICDETECTOR

OUTPUT TOTHE RECODER

UNIT FAIL

SYSREADY

M

A

G

N

E

T

I

C

D

E

T

E

C

T

I

N

G

S

E

T

B

A

N

D

P

A

S

3 2

1

OFF

OFF OFFPWR

CAL

REC ZERO

ALT COMP

ON

ONON

0.108

06

04

2.0

0.6

0.4

0.2

FS

0.1

0.2

0.4124

10

20

10

TST

P

USH

TO

DAMP

AEf02012

Figure 2-12.—Basic MAD system units.

The magnetic detecting set is a sensitive,metastable, helium magnetometer used to locate andclassify submarines by detecting a disturbance orchange in the normal Earth's magnetic field. TheMAD system currently used by all naval USWaircraft is the AN/ASQ-81 system. The basic platformincludes a magnetic detector, amplifier-power supply,and detecting set control. Figure 2-12 shows the

controls and indicators as they appear on theequipment.

Helicopters use the same basic AN/ASQ-81 equip-ment with the addition of special equipment designedto trail the detector behind the aircraft to minimize theeffects of magnetic fields generated by the helicopter.This special equipment includes the towed body, a reel-ing machine, and a reeling machine control (fig. 2-13).

2-11

1

2

3

45

6 7 89

10

11

12

13Y1

MAGNETIC DETECTINGTOWED BODYTB-623

REELING MACHINE CONTROLC-10556

DETECTING SET CONTROLC-10557

MAGNETIC DETECTING SETREELING MACHINERL-305

AMPLIFIER-POWER SUPPLYAM-4535

AEf02013

Figure 2-13.—Helicopter MAD system units.

MAGNETIC DETECTOR

The detection element includes six separate heliumabsorption cells and six IR detectors arranged in pairs,with the pairs oriented at 90 degrees to each other.The arrangement of the magnetometer is shown infigure 2-14. This configuration ensures that one ormore of the pairs is at least partially in line with theEarth's field regardless of aircraft attitude or directionof flight. The signals from all three detector pairs arecombined in a summing amplifier; thus, the final outputto the amplifier-power supply is not affected by aircraftmaneuvers.

Two helium discharge lamps provide light energyto the three absorption cell pairs in the magneticdetector. The lamps are ignited by a 52 kilohertz (kHz),1,500 volt supply (fig. 2-15). After ignition, the lampsare maintained in an ionized state by the 49.6megahertz (MHz) output of the exciter-regulator.

In addition, the magnetic detector unit includes apressure transducer and an altitude compensatorcircuit. The output of the pressure transducer varies thefrequency of a 5.4-kHz oscillator in the altitudecompensator at the rate of 1 Hz/ft of altitude change.The total swing of the oscillator is 5.0 to 5.8 kHz, which

2-12

ABSORTION CELLTOP

BOTTOM

BOTTOM VIEW

FWD

AFT

INFRARED DETECTORS

INFRAREDDETECTORS

INFRAREDDETECTORS

HELIUM DISCHARGELAMP FOR C1 - C2

ABSORPTIONCELLS

HELIUM DISCHARGELAMP FOR C1 - C2

HELIUM DISCHARGELAMP FOR A1 - A2

AND B1 - B2

ABSORPTION CELLS

ABSORPTIONCELLS ABSORPTION

CELLS

INFRAREDDETECTORS INFRARED

DETECTORS

HELIUM DISCHARGELAMP FOR A1 - A2

AND B1 - B2

C1

VIEW LOOKING FORWARD

C2

C1

C2

C1

B2 A2

AEf02014

Figure 2-14.—Magnetometer axis orientation.

HELIUMDISCHARGE

LAMP

HELIUMFILLED

ABSORPTIONCELL

RESONANCECOILS

IRDETECTOR430/860 Hz

PRE-AMPSUM AMP

0.6 TO 2.2 Mhz

IGNITIONASSEMBLY

52 KHz1500V

EXCITER/REGULATOR

49.6 Mhz

ALTITUDECOMPENSATION

SINGLE COAXIAL CABLE

5.4 KHzALTITUDE

COMPENSATIONAMPLIFIER/POWER SUPPLY

RECORDERHEAD

ADJUSTABLEHI/LO PASS

FILTERNETWORK

RECORDER CONTROL

DC

0.6 TO 2.2 MHz

LINEDRIVER

PHASE LOCKOSCILLATOR

FREQUENCYCONVERTER

PHASEDEMOD-ULATOR

RESONANCEOSCILLATOR

0.6 TO 2.3 MHz

AEf02015

MAGNETIC DETECTOR

5, 4 KHz ALT SIGNAL

Figure 2-15.—Simplified block diagram of a MAD system.

can compensate for a maximum of 400-foot rapidaltitude variations. Additional circuitry in theamplifier-power supply converts the altitude-inducedfrequency change to a varying dc level. At theoperator's discretion, this dc level may be used tocorrect the final signal output.

CAUTION

When maintenance is performed on or nearthe magnetometer head, special nonferrous toolsMUST be used. Additionally, any parts (nuts,bolts, or screws) that are replaced MUST be madeof a special nonferrous metal. Always consultthe maintenance instruction manual (MIM) forthe system being worked on before attemptingrepairs.

AMPLIFIER-POWER SUPPLY

A single coaxial cable carries all signals betweenthe magnetic detector and the amplifier-power supply.The signals on the coaxial cable include the 5.4-kHzaltitude signal, the 430/860-Hz IR signal, the 0.6- to2.2-MHz resonance coil excitation, and the 52-kHzignition signal. The 52-kHz ignition signal is monitoredfirst by system BITE (built-in test equipment) for itspresence and, with the latest configuration, is checkedapproximately 5 minutes later (maximum warm-uptime) for its absence. If the signal is not present initially,the system timer stops and shows a detector headfailure. Approximately 5 minutes later, the absence ofthe signal is checked. The absence of the signalindicates that the intensity of both helium lamps wassufficient to cause the system to switch from ignition tothe normal 49.6-MHz exciter signal. If this switchoverdid not occur, the system removes power to the detectorhead and illuminates the detector failure indicator.Bandpass filters at both ends of the coaxial cable routethe frequencies to their respective circuits.

The IR detector error signal (anytime 430 Hz ispresent, this represents an error between the Larmorfrequency and the resonance oscillator centerfrequency; 860 Hz indicates matched frequencies) isphase-detected in the phase demodulator to produce avariable positive or negative dc voltage. The variable dcis used to change the resonance oscillator frequency.This closed-loop action keeps the oscillator always atresonance as the Earth's field strength changes or as ananomaly is detected. When neither 430 Hz nor 860 Hzis coming back from the magnetic detector, the systemsenses this and causes the resonance oscillator to sweep

its entire range (0.6 MHz to 2.2 MHz). During the downsweep of the resonance oscillator, when 430/860 Hz isdetected, the sweep is stopped and the loop is closed.The resonance oscillator output is routed to theresonance coils via the line driver and thepreamplifier/summing amplifier. It is also applied tothe phase lock oscillator assembly.

The purpose of the phase lock oscillator is toreproduce the resonance oscillator frequency, retain themagnetic anomaly, and eliminate the 430-Hzmodulation signal. The oscillator is voltage controlledby a dc signal from the acquisition circuit until it lockson to the resonant frequency. After lock-on, the phasedetector provides the control necessary for tracking theresonant frequency. This tracking is similar to the waythe resonant oscillator tracks the Larmor frequency.

The frequency converter develops a variable dcsignal proportional to the frequency shift of the phaselock oscillator. The frequency converter also generatesa variable dc voltage proportional to the frequency shiftof the 5.4-kHz altitude compensation signal inputsupplied by the magnetic detector unit. If the operatorselects ALT COMP (altitude compensation) on thecontrol unit, a summary network combines the two dcsignals to compensate for magnetometer altitudechange effects. Two driver-amplifiers provide aprimary output to the detecting set control unit and anauxiliary output for test purposes.

The primary output is passed through a series ofhigh- and low-pass filters in the control unit. The filtersremove all extraneous frequencies and noise from thevariable dc except the anomaly signal. The filtered dcoutput drives the pen of a recorder to produce apermanent record of the submarine anomaly.

CONTROL BOX

The detecting set control box (fig. 2-12) containsthe operating switches and indicators for the MADsystem. Across the top of the faceplate are fiveindicators that indicate faults in the other units. Theindicator labeled 3 indicates a magnetic detector failurewhen lit. The indicator labeled 2 indicates amplifierfailure. The next two indicators indicate a control boxfault. The SYS READY (system ready) indicatorilluminates when the system is ready for operation.This indicator will blink during warm-up.

There are three toggle switches across the middleportion of the control box. The one on the right is thepower switch. This switch applies power to the system.The middle switch is labeled CAL. It selects the

2-13

calibration signal for use. The switch on the left islabeled ALT COMP. This switch is used to connect thealtitude compensator to the system.

The bottom portion of the control box contains fourknobs. The two on the left side are labeledBANDPASS. These knobs select the high and lowfrequencies. The knob labeled REC ZERO (recordzero) is a dual-purpose knob. Turning this knobcontrols the pen deflection on the recorder. Depressingthe knob inhibits system output. The bottom right knobis labeled ãFS, and is used to select one of ninesensitivity ranges (from 0.1ã to 40ã full scale) orself-test. In the TST (test) position, the self-testfunction will be initiated.

Q2-8. How many helium absorption cell/IR detectorpairs are contained in the detector element?

Q2-9. The signals between the magnetic detectorand the amplifier-power supply are trans-mitted through what type of cable?

Q2-10. The fault indicator on the detecting setcontrol labeled 3 indicates a failure in whatcomponent?

MAGNETIC COMPENSATOR GROUP

LEARNING OBJECTIVE: Identify the threemagnetic field terms associated with the com-pensation process. Recognize the componentsand operating features of the AN/ASA-65magnetic compensator group.

The AN/ASA-65 magnetic compensator group isused in conjunction with the AN/ASQ-81 MAD systemto reduce the effects of unwanted magnetic dis-turbances (anomalies) during MAD system operation.The MAD compensator generates opposing magneticfields to nullify noise interference caused by the searchaircraft.

MAGNETIC FIELD TERMS

Relative to MAD compensation theory, the wordterm refers to a magnetic field component. Permanentfield terms are designated by a single capital letter suchas T, L, or V, which stand for transverse, longitudinal,and vertical, respectively. These three axes, which arereferences to the three aircraft axes, remain fixedregardless of aircraft orientation with respect to the

2-14

AN/ASQ-81 (V)MAGNETIC DETECTIONSET SUBSYSTEM

TRANSVERSEMAGNETICCOMPENSATION COIL

VERTICALMAGNETICCOMPENSATION COILVERTICAL

COMPENSATIONSIGNAL

LONGITUDINALMAGNETICCOMPENSATION COIL

LONGITUDINALCOMPENSATIONSIGNAL

MAGNETICAIRCRAFTSIGNALS

CONTROLAND MONITORSIGNALS

STANDARDMAD SIGNAL

TRANSVERSECOMPENSATIONSIGNAL

ELECTRONICCONTROL AMPLIFIER

MAGNETOMETERASSEMBLY

CONTROL - INDICATOR

AEf02016

Figure 2-16.—Magnetic compensator group interface diagram.

Earth. Induced terms are designated by two capitalletters, which may be any combination of L, T, or V,such as LL, VL, or TT. The first capital letter designatesthe inducing aircraft’s structure axis, and the secondcapital letter designates the resulting field axis as seenat the detector head. The inducing axis may be differentfrom the resulting field axis at the detector head.Eddy-current terms are designated by two lowercaseletters, which may be any combination of the threebasic components in the same manner as induced terms,such as ll, vl, or tt.

The total interfering field at the sensor can beresolved theoretically into 16 terms comprising threepermanent, five induced, and eight eddy-current terms.Generating an opposing field at the detector containing16 terms, which cancel their interfering counterparts,could eliminate the interference. In practice, not morethan nine terms need to be opposed to compensate any

aircraft satisfactorily because not all the induced andeddy-current terms are significant.

AN/ASA-65 COMPENSATOR COMPONENTS

AN/ASA-65 magnetic compensator group compo-nents involved in the magnetic compensation of theAN/ASQ-81 MAD system include a few systemcomponent variations. The basic system contains acontrol-indicator unit, an electronic control amplifier(ECA), a magnetometer assembly, and compensationcoils. However, the upgraded ASA-65(V)4/5 includesthe addition of the compensator group adapter (CGA).As you read about the components, refer to figure 2-16and observe the relationship between the varioussignals and components involved. Refer to figure 2-17to see the physical appearance of the components com-prising the AN/ASA-65 magnetic compensator group.

2-15

3

3

32

1

45

67

8

0

4

4

5

5

6

6

78

9

9

10

11

12

13Y1

ELECTRONIC CONTROLAMPLIFIER

MAGNETIC COMPENSATING COILSMX-8897/ASA-651(V)

CONTROL - INDICATOR

COMPUTER MAGNETIC FIELDCP-1390/ASA-65(V)

MAGNETIC FIELD INDICATORID-2205/ASA-65(V)

T-V MAGNETIC COMPENSATING COILSMX-9058/ASA-65(V)

MAGNETOMER ASSEMBLYDT-355/ASA-65(V)

MOUNTING TRAYMT-6086/ASA-65(V) S-3AMT-6174/ASA(V) P-3C

MAGNETIC FIELD INDICATORID-2254/ASA-65 (V)

L MAGNETIC COMPENSATING COILMX-10079/ASA-65(V)

AEf02017

Figure 2-17.—Magnetic compensator groups AN/ASA-65(V)4 and 5.

Control-Indicator Unit

The control unit contains all the controls andindicators required for system operation and all theelements necessary for term adjustment. The controlunit provides the operator with a numeric indication ofcompensation potentiometer position.

Electronic Control Amplifier (ECA)

The ECA processes standard MAD signals fromthe MAD subsystem, operator compensation adjust-ments, and maneuver signals from the magnetometer.The ECA provides compensation currents, which aresent to the MAD boom compensation coils. The ECAprovides all necessary interconnections and receives allinputs to the system. It converts nine separate termadjustment outputs from the control-indicator to threecurrent outputs, which energize the magnetic com-pensating coils.

Magnetometer Assembly

The magnetometer contains three coils oriented tosense magnetic strength in each of the basiclongitudinal, transverse, and vertical aircraft axes. Thisresults in three output signals, which are sent to theECA as operating voltages for induced and eddycurrent compensation. In the upgraded ASA-65(V)4and 5 configurations, signals also go to the magneticfield computer. The computer calculates the differencevalue of the terms necessary to reduce themaneuver-related interference to a minimum.

Magnetic Compensation Coils

The L coil, MX-10079/ASA-65(V), and the T andV coils, MX-9058/ASA-65(V), generate magneticfields that oppose the aircraft-generated noise field forcompensation. These fields cancel or minimizesmagnetic fields interfering with MAD operation. Thecoils are located in the MAD boom. When installed, thecable ends of the coils should point to port (T), forward(L), and downward (V). Refer to appropriate MIMs forproper installation and alignment procedures.

ASA-65(V)4 and 5 Magnetic Field Computer

The addition of the CGA completely computerizesthe compensation calculation. Compensation isperformed simultaneously on all nine terms byperforming an aircraft maneuver pattern requiring onlya few minutes. The computer provides interconnectionbetween primary components of the CGA and receives

28 analog maneuver signals. These maneuver signalsare digitized for data processing to providelight-emitting diode (LED) drive voltages to themagnetic field indicator.

Magnetic Field Indicator

The indicator contains the controls and indicatorsnecessary to operate the CGA to perform semi-automatic compensation. The unit also displays termdifference figures, calibration voltages, and BITEcodes. The MODE control selects the various operatingconditions of the computer.

Q2-11. Magnetic field terms that are designated bytwo capital letters, such as TT, are what typeof terms?

Q2-12. The ECA of the AN/ASA-65 sends com-pensation currents to what component?

Q2-13. The CGA performs what function?

SUBMARINE ANOMALYDETECTOR (SAD)

LEARNING OBJECTIVE: Recognize thecapability and purpose of the ASA-64/ASA-71(SAD).

SAD works in conjunction with MAD, andincreases the capability to detect submarines throughevaluating MAD signals and separating submarineanomaly from magnetic noise. SAD also providesaudio and visual indications that a submarine type ofcontact has been made. The ASA-64 SAD system blockdiagram (fig. 2-18) indicates some of the signal flowand shows some of the typical circuitry within theASA-64. One of the outputs is a +15 volts dc to thethreshold adjust on the C-7693/ASA-71, which thenbecomes the preset threshold input to the ASA-64Threshold Detector U3. The system separates thesubmarine anomaly by selective filtering, full-waverectification, short-term integration, and correlation ofaircraft maneuvers through the recognition andmaneuver channels.

The MAD/SAD system interface is accomplishedby the AN/ASA-71 selector control group. The groupincludes the C-7693/ASA-71 selector control panel(A302) and the MX-8109/ASA-71 selector controlsubassembly (A303). The A302 selector control panelprovides controls for operating the A303 selectorcontrol subassembly. The A303 receives, processes,and distributes signals between the MAD/SAD systemand the central repeater system, ICS, and ADP system.

2-16

The A302 selects the signal to be routed through theA303, which is either the AUX, SAD, or MAD signal.

Q2-14. What are the functions of the ASA-64/ASA-71SAD system?

2-17

MULTIPLE SPIKEINHIBITCIRCUIT

1A2, Q9 - 011

PRESETTHRESHOLD

VOLTAGEFROM

G-7693/ASA-71

MAD SIGNALINPUT

FROM MADEQUIPMENT

RECOGNITION CHANNEL

0.4 H2 MARKSWITCH

1S1

FILTERAr1, AR2

1A4

DEMODULATORAND FILTER

U1

FULL WAVERECTIFIERAR3, AR2

1A4

1A3

RESETSWITCHQ1, Q2

INTEGRATORU2

BUFFERQ4, Q5 CLAMP

Q3

THRESHOLDDETECTOR

U3

OUTPUTAMPLIFIER

U4

1A2 (MODIFIED)CR1, R20Cr3, R22

Q6

1A2 (ALTERNATE)R20, R22

Q4OR

MARKDST

OSCILLATORQ1, Q2, Q3

1A2

SWITCHED 115V400 HZ

POWERSUPPLIES

MANEUVER SIGNALFROM MAD

INTERFACE MANEUVERPROCESSOR

+15V -15V

1A5

INHIBITSWITCH

1S2

RATEDETECTOR

U1

MANEUVER CHANNEL

1CB1CIRCUIT

BREAKER

MANEUVERRATE

ADJUST R2

THRESHOLDDETECTOR

U4

RECTIFIERU2, U3

OR

OR

R24, R40

SET

EXTERNALINHIBIT LIGHT

TESTLAMP

1A2 (MODIFIED)Q7, Q8, CR7

CR8, CR31, R32

RELAYDRIVER

RELAY

1A2 (ALTERNATE)Q7, CR7, R32

RESET

F/FQ6Q7

Q1

R22, R23R25, R26

TIMER Q2, Q3TIMING

ADJUST R1

MANEUVERINHIBIT

MONGSTABLEQ4, Q5

AEf02018

Figure 2-18.—ASA-64 system block diagram.

APPENDIX I

REFERENCES USED TO DEVELOP THISNONRESIDENT TRAINING COURSE

Although the following references were current when thisNonresident Training Course was published, their continuedcurrency cannot be assured. When consulting these references,keep in mind that they may have been revised to reflect newtechnology or revised methods, practices, or procedures.Therefore, you need to ensure that you are studying the latestreferences.

Chapter 1

SH-60F IETM, A1-H60CD-60F-00, Naval Air Technical Data & EngineeringService Command, San Diego, CA, 01 July 1999.

http://sonobuoy.crane.navy.mil/indexhome.htm, Naval Surface Warfare CenterDivision Crane.

Chapter 2

Integrated Sensor Station 1 and 2—Update III and Block Mod Upgrade ProgramWiring Data, Navy Model P-3C Aircraft, NAVAIR 01-75PAC-2-15.1, Naval AirSystems Command, Washington, D.C., November 2002.

AI-1

APPENDIX II

ANSWERS TO REVIEW QUESTIONS

CHAPTER 1

A1-1. The temperature and salinity of the water.

A1-2. It will bend toward the area of cold water.

A1-3. Active sonobuoys.

A1-4. The capability of selecting channel, life and depth settings.

A1-5. Submarine bearing.

A1-6. BT sonobuoys.

A1-7. Sonobuoy bearing information with respect to aircraft heading.

A1-8. The SH-60F.

A1-9. The transducer.

CHAPTER 2

A2-1. Dip angle.

A2-2. The Greek letter ã.

A2-3. Dc circuit noise and maneuver noise.

A2-4. In flight at sea.

A2-5. Metastable energy state.

A2-6. 28.

A2-7. The infrared detector.

A2-8. Six (6) absorption cell/IR detector pairs.

A2-9. A single coaxial cable.

A2-10. The magnetic detector.

A2-11. Induced terms.

A2-12. The ECA provides compensation currents to the MAD boom compensation coils.

A2-13. The CGA calculates compensation data.

A2-14. The ASA-64/ASA-71 SAD system separates submarine anomalies from magneticnoise and provides visual and audio indications that a submarine contact hasbeen made.

AII-1

ASSIGNMENT 1

Textbook Assignment: “Acoustic Systems,” chapter 1, pages 1-1 through 1-22. “Magnetic Anomaly Detection(MAD),” chapter 2, pages 2-1 through 2-17.

1-1. The acronym sonar is derived from whatwords?

1. Sound, navigation, and radar2. Sound, navigation, and ranging3. Sound, naval, and radar4. Sound, naval, and ranging

1-2. A sonar transducer can be compared to whichof the following items?

1. Microphone2. Amplifier3. Speaker4. Receiver

1-3. Signal strength lost as sound waves travelthrough water is known by what term?

1. Saturation2. Attenuation3. Radiation loss4. Transmission loss

1-4. Which of the following sea states causes thegreatest amount of absorption?

1. Whitecap waves2. Large swells3. Small swells4. Calm

1-5. Sound waves in seawater travel how manytimes faster than sound waves in air?

1. 82. 23. 104. 4

1-6. Which of the following sea floor surfaces hasthe least amount of signal loss?

1. Rocky2. Muddy3. Sandy4. Smooth and hard

1-7. Multiple reflections of a sound wave is referredto by what term?

1. Resounding2. Reverberating3. Scattering4. Rumbling

1-8. Which of the following statements best de-scribes the effect of divergence on soundwaves?

1. The wave spreads out and becomes weakerwith distance

2. The beam converges and becomes strongerwith distance

3. The beam divides with distance4. The beam width remains the same with

distance

1-9. What is the most important factor affecting thespeed of a sound wave in seawater?

1. Salinity2. Depth3. Temperature4. Frequency

1-10. An increase in the salinity of seawater has whateffect on the sound wave?

1. Speed increases2. Speed decreases3. Scattering increases4. Scattering decreases

1-11. According to the Doppler effect, when thesource of a sound wave is moving away fromthe receiver, what apparent change in the soundwave takes place?

1. The intensity increases2. The wavelength decreases3. The frequency decreases4. The frequency increases

1

1-12. What are the three most common propagationpaths for sound waves?

1. Reflective, refractive, and direct2. Direct, bottom bounce, and reflective3. Bottom bounce, surface bounce, and re-

flective4. Direct, bottom bounce, and convergence

1-13. What is the minimum number of sonobuoysnormally deployed during a USW mission?

1. 12. 23. 34. 4

1-14. Acoustic data received from a sonobuoy istransmitted to the sonobuoy receiver using (a)what frequency band, and (b) what type ofmodulation?

1. (a) VHF (b) FM2. (a) VHF (b) AM3. (a) UHF (b) FM4. (a) HF (b) AM

1-15. Sonobuoys that transmit an acoustic pulse anddetect a resulting echo are classified as whattype of sonobuoy?

1. Passive2. Active3. Ringing4. Delay

1-16. What are the three methods of deploying asonobuoy?

1. Pneumatic, free-fall, and cartridge2. Pneumatic, free-fall, and hydraulic3. Free-fall, hydraulic, and piston4. Cartridge, hydraulic, and piston

1-17. Sonobuoy descent-retarding devices are neces-sary for what reason?

1. To prevent ice build-up2. To allow electronic circuits to activate3. To reduce water entry shock4. To allow visual tracking

1-18. How are sonobuoys activated?

1. Atmospheric pressure switch during de-scent

2. Saltwater battery activation3. Command signal from the operator4. Preset timer

1-19. What happens to the sonobuoy after the batterydies?

1. It sinks to the bottom2. A recovery pinger activates3. A recovery float deploys4. It remains in place and decomposes

1-20. Sonobuoys equipped with an EFS system havewhat capability?

1. Unlimited battery life2. Electromagnetic pulse protection3. Frequency discrimination4. Selectable channel, life and depth settings

1-21. A LOFAR sonobuoy uses what type ofhydrophone?

1. Unidirectional2. Omnidirectional3. Hybrid4. Reactive

1-22. Under most situations, what type of sonobuoyis best for initial detection?

1. DIFAR2. LOFAR3. VLAD4. DICASS

1-23. What information can a DIFAR sonobuoyprovide that a LOFAR sonobuoy cannot?

1. Submarine bearing2. Submarine depth3. Submarine size4. Water temperature

1-24. Which of the following sonobuoys is designedfor long-range submarine detection in deepwater?

1. LOFAR2. DIFAR3. VLAD4. DICASS

1-25. DICASS sonobuoys operate on the sameprinciple as what other electronic system?

1. INS2. IFF3. TACAN4. Radar

2

1-26. DICASS sonobuoys are commanded to ping bywhat source?

1. A UHF signal from the USW aircraft2. A VHF signal from a surface ship3. A preset timer located within the sonobuoy4. A range proximity switch located within

the sonobuoy

1-27. An EER sonobuoy is typically used in con-junction with what other sonobuoy forlong-range detection?

1. DICASS2. VLAD3. LOFAR4. DIFAR

1-28. What are the two special-purpose sonobuoys?

1. BT and DIFAR2. BT and DLC3. DLC and DIFAR4. DLC and LOFAR

1-29. What information does a BT sonobuoy pro-vide?

1. Salinity2. Pressure versus depth3. Water temperature at sea surface4. Water temperature versus depth

1-30. The ARR-78 sonobuoy receiver is capable ofsimultaneous reception of up to what numberof sonobuoys?

1. 52. 103. 204. 99

1-31. The ARR 84 sonobuoy receiver is used in theSH-60 series helicopter for what reason?

1. Accommodation of weight restrictions2. Rotor nulling circuitry3. Higher signal to noise ratio4. Less sonobuoy monitoring requirements

1-32. What component of the sonobuoy-based sonarsystem extracts acoustic target informationfrom both active and passive sonobuoys?

1. Sonobuoy receiver2. Power supply3. Data display4. Spectrum analyzer

1-33. The OTPI system works in conjunction withwhat other aircraft system?

1. DF2. VOR3. TACAN4. ACLS

1-34. The OTPI system provides what information tothe USW pilot?

1. Distance to the sonobuoy2. Sonobuoy bearing with respect to true

north3. Sonobuoy bearing with respect to aircraft

heading4. Sonobuoy depth

1-35. Dipping sonar systems use what type of sonardetection?

1. Active only2. Passive only3. Active and passive4. Active and array

1-36. What distinct advantage does a dipping sonarhave over a sonobuoy based system?

1. Low operating cost2. Operator control of the search depth3. Longer operating time4. Search area increased

1-37. In addition to active and passive sonar modes, adipping sonar provides what other informa-tion?

1. Water salinity and temperature2. Water pressure salinity3. Communications and water temperature4. Communications and water pressure

1-38. The electrical interface between the sonar setunits mounted in the helicopter and the sub-merged sonar transducer is provided by whatcomponent of the ASQ-13F dipping sonar set?

1. Power supply2. Multiplexer3. Dome control4. Sonar data computer

1-39. What unit of the ASQ-13F converts soundenergy to electrical signals?

1. Receiver2. Computer3. Transceiver4. Transducer

3

1-40. A submerged sonar transducer is used totransmit and receive voice signals to and fromother USW helicopters, submarines, orsimilarly equipped surface ships during whatASQ-13F mode of operation?

1. BT2. UVC3. Active4. Passive

1-41. Angles of change in the Earth’s naturalmagnetic field in the east-west direction isknown by what term?

1. Dip angle2. Variation angle3. Equatorial angle4. Force angle

1-42. The maximum range at which a submarinemay be detected depends on the intensity of itsmagnetic anomaly and what other factor?

1. The sensitivity of the detector2. The size of the submarine3. The salinity of the seawater4. The temperature of the seawater

1-43. If the detectable strength of a magnetic fieldsource has a given value at a given distance andthe distance is doubled, the detectable strengthof the source at the increased distance will thenbe how much of its former value?

1. One-half2. One-third3. One-fourth4. One-eighth

1-44. Any noise or disturbance originating in theaircraft or its equipment that could produce asignal on a MAD equipment recorder is knownby what term?

1. Static voltage2. Magnetic noise3. Stray voltage4. Phantom noise

1-45. Maneuver noises caused by eddy current flowcan be minimized by taking what action?

1. Making slow aircraft maneuvers2. Making quick aircraft maneuvers3. Flying in a circular pattern4. Flying in a zigzag pattern

1-46. Any magnetic field may be defined in terms ofwhat number of axial coordinates?

1. 12. 23. 34. 4

1-47. What is the purpose of magnetic compensationin USW aircraft?

1. To provide a negative polar coefficient2. To provide a positive polar coefficient3. To counteract the Earth’s magnetic field4. To provide a magnetically clean environ-

ment

1-48. Compensation for an aircraft’s dc magneticfield is accomplished by using how manyelectromagnetic loops?

1. 12. 23. 34. 4

1-49. The natural spin axis wobble rate of aparticular atom is referred to by what term?

1. Deviation frequency2. Larmor frequency3. Wobble frequency4. Carver frequency

1-50. In the ASQ-81 solid-state MAD detector, theenergy level of the electrons is increased, andthe atoms are properly oriented by what pro-cedure?

1. Polar excitation2. RF induction3. Electromagnetic pulsing4. Optical pumping

1-51. What component or components of a heliummagnetometer detect the energy level changesof the helium atoms and produces an electricaloutput?

1. Absorption cell2. Helium lamp3. Infrared detector4. Excitation coils

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1-52. What are the main components of a basic MADdetecting set?

1. Magnetic array, field compensator, anddetecting set control

2. Magnetic array, magnetic detector, and de-tecting set control

3. Magnetic detector, amplifier-power sup-ply, and detecting set control

4. Magnetic detector, magnetic array, and de-tecting set control

1-53. When maintenance is performed on or near themagnetometer head, what type of tools must beused?

1. Nonferrous2. Stainless steel3. Tempered steel4. Iron

1-54. What is the approximate warm-up time for theAN/ASQ-81 MAD system?

1. 3 min2. 5 min3. 7 min4. 10 min

1-55. What fault indicator on the MAD control boxindicates an amplifier failure?

1. 12. 23. 34. 4

1-56. What is the purpose of magnetic compensa-tion?

1. To increase the intensity of the anomaly2. To match the magnetic field of the aircraft

with the anomaly3. To eliminate the magnetic field4. To reduce the effects of unwanted mag-

netic disturbances

1-57. What are the three magnetic fields relative toMAD compensation?

1. Transverse, longitudinal, and vertical2. Transverse, radial, and polar3. Radial, longitudinal, and horizontal4. Horizontal, longitudinal, and vertical

1-58. In practice, how many terms are needed tocompensate any aircraft satisfactorily?

1. 162. 123. 94. 3

1-59. The compensator group adapter (CGA) serveswhat purpose?

1. Allows manual compensation2. Provides a maintenance interface3. Allows modules to be interchanged4. Computerizes the compensation calcula-

tion

1-60. What is the purpose of the submarine anomalydetector (SAD) system?

1. Separates submarine anomalies from mag-netic noise

2. Replaces the MAD system3. Automatically identifies friendly sub-

marines4. Calculates submarine depth

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