SECCION CARTELES PAGADOS
ACEPTACION DE HERENCIA
The possible detection range of particular equipment should be known so that a tactician will then have a measure of the sonar's capability and a feel for what the sonar can do in a given tactical situation. Unfortunately, with no change in basic sonar configuration its detection capability measured in terms of range can increase or decrease severalfold simply because the ocean itself has changed. To state this another way, sonar equipment can only be designed to detect the arrival of a certain sound energy intensity. The range from which that sound intensity arrives is highly dependent on how much energy was lost en route, and therefore detection range alone is a poor measure of sonar capability. A better measure is the ability of the sonar to detect a certain level of sound energy intensity just outboard of its receiver. The key to this better measure of performance is to separate the sonar from the ocean in which it must operate. Only then can sonar capability be discussed in terms of the unchanging sonar hardware as distinguished from the ever-changing ocean.
The better measure for sonar capability is called figure of merit (FOM), and it equals the maximum allowable one-way transmission loss in passive sonars, or the maximum allowable two-way transmission loss in active sonars for a detection probability of 50 percent. Therefore, solving equations (8-28) and (8-30) for transmission loss, we get
Passive FOM = SL - NL + DI - DT (8-32)
Active FOM = SL + TS - NL + DI - DT (8-33)
This combination of terms is probably the most used performance parameter for sonars, and it is important to understand just what it means. The FOM of a sonar system is the maximum transmission loss that it can tolerate and still provide the necessary detection probability as specified by DT. FOM is improved by raising the source level (this can be accomplished by increasing the transmitted power in the active case or finding a noisier target in the passive case), decreasing the ambient noise level, increasing the absolute value of the receiving directivity index, and decreasing the detection threshold. The value of figure of merit is that, with no knowledge of the intervening propagation path between a ship and a target, a
quantitative comparison of two different sonars can be made. The figure of merit may be utilized for comparing the relative performance of two passive sonars provided the calculations of the comparative figures of merit are made for the same frequency. The difference in FOM's represent additional allowable propagation loss that can be sustained by the sonar having the higher FOM and still make a detection; thus a longer detection range results.
8.10.1 Prop Loss Curves
Naturally, to the tactician, detection ranges are of prime importance, and although FOM can be interpreted in terms of range, it can be done only if the propagation losses involved are known. The measure of propagation loss (total transmission loss) in many parts of the ocean has proven that not only does sound propagation change with frequency and location but with season as well. Thus, the sound velocity profile will determine the propagation path(s)
available, which along with frequency and location will determine transmission loss versus range. Therefore, in order to convert FOM to range, one must have a propagation loss curve for the frequency or the sonar (s) concerned and for the area and season of the year for which range prediction is desired.
Figure 8-33 is a typical propagation loss curve for the waters off Iceland in the summer for a frequency of 2 Khz. Note that the propagation losses for the three sound paths previously described are plotted. Multiple bottom bounce losses were measured and mul-tiple convergence zones were estimated.
Assume three different passive sonars with the following values of FOM at 2 Khz: 80, 95, 105. Based on Figure 8-32, the range in Kyds at which 50% probability of detection is predicted for each sonar for each sound transmission path in the Iceland area is as follows:
Sonar (FOM) Direct Bottom Bounce Convergence Zone
A 80 7.5 None None
C 105 36 90 160
Note that the sonar with the higher figure of merit permits the use of sound paths (bottom bounce and convergence zone) not available to the sonar with relatively low figure of merit. The higher the FOM the longer the detection range for a given path. Propagation loss curves can be made from a ray tracing program or actual measurements and smoothed data.
In summary, sonar performance is the key to ASW success, and figure of merit is the key to sonar performance. With knowledge of his sonar's FOM, a commanding officer can ensure that his equipment is peaked, and also predict detection ranges against possible enemy targets. The war planner can do likewise for either a real or hypothetical enemy. Because the changing ocean results in dramatic changes in propagation loss versus range, to state a sonar's capability in terms of range is only half the story, and may even be misleading. Using figure of merit, however, the sonar with the higher FOM will always be the better sonar when comparing sonars in the same mode.
8.10.2 Figure of Merit Sample Problem
Your sonar is capable of either passive or active operation. You are operating in the shipping lanes with a sea state of 2. Water depth is 200 fathoms. Using the following information, you must decide which mode to use. Intelligence information indicates that the threat will be a Zebra-class submarine (all dB are reference 1Pa).
Target parameters:
radiated noise source level 100 dB
radiated noise frequency 500 Hz
target strength 15 dB
Sonar parameters: Active Passive Source level 110 dB - Frequency 1.5 kHz - Self-noise at 15 kts 50 dB 50 dB Directivity index 10 dB 8 dB Detection threshold -2 dB 3 dB
In order to determine which sonar to use, it is necessary to calculate the FOM and total transmission loss for each mode.
First, calculate the total transmission loss for each mode. Since the desired target detection range of 10 km is much greater than the water depth of 200 fathoms, we will use equation (8- 24) for cylindrical spreading:
TL = 10 log r + r + A
The quantity A is assumed to be zero since no information is available. The absorption coefficient () is calculated by substituting signal frequencies for each mode into equation (8- 21):
= .036f2 + 3.2 x 10-7 f2 (where f is in kilohertz)
Active mode Passive mode = .036(1.5)2 = .036(.5)2 (1.5)2 + 3600 (.5)2 + 3600 + (3.2 x 10-7)(1.5)2 +(3.2 x 10-7)(.5)2 = 2.32 x 10-5 = 2.6 x 10-6 TL = 10 log(10,000) + 2.32 TL = 10 log(10,000) + 2.6 x x 10-5(10,000) 10-6(10,000) TL = 40 + .232 TL = 40 + .026
TL = 40.232 (one way) TL = 40.026dB (Total TL)
2 x TL = 80.464dB (Total TL)
Note that TL depends only upon the detection range and the frequency of the signal.
Next calculate the FOM using equation (8-33) for the active mode and equation (8-32) for the passive case. Note that the only values that must be determined are the noise levels using the Wenz curves and nomograms for the different frequencies.
Active Mode Passive Mode
AN(shipping) = negligible AN(shipping) = 57dB
AN(sea state) = 58dB AN(sea state) = 61dB
Self-Noise = 50dB Self-Noise = 50dB
Using the nomogram, combine the signals for each mode:
58-50 = 8dB 61 - 57 = 4 dB
NL = 58 + .65 (from nomogram) AN = 61 + 1.5 (from nomogram)
NL = 58.65 dB AN = 62.5 dB
62.5 - 50 = 12.5 dB
NL = 62.5 + .25 = 62.75 dB
Note that when three noise signals are involved, a two-step signal-combining process is required. The resultant is always added to the higher signal level. Determining the FOM is now a simple matter of substituting the calculated and given values into the appropriate equations:
Active Mode Passive Mode
FOM = SL + TS - NL + DI FOM = SL - NL + DI - DT
FOM = 110 + 15 - 58.65 + FOM = 42.25 dB
10 -(-2)
FOM = 78.35 dB
Compare the FOM of each mode with the total TL for each mode to determine which mode is optimum for this target. The FOM for the active case is less than the total TL. Therefore, the active mode will give your ship less than 50% probability of detection. The FOM for the passive case is greater than the total TL. Therefore, the passive mode will give a greater than 50% probability of detection, which means that the passive mode should be used.
8.11 SUMMARY
Of all the energy forms available, sound, even with its inherent disadvantages, is the most useful for underwater detection of submarines. It travels as a series of compressions and rarefactions at a speed equal to the product of its frequency and wavelength. The pressure of the wave can be expressed as a function of both time and its distance from the source.
Acoustic power, called its intensity, is a measure of the amount of energy per unit area and has the units watts/m2. Acoustic pressure is expressed in micro-pascals. To make comparisons easier, both are normally converted to the logarithmic decibel system. The speed of sound in the sea is related to the bulk modulus and density of the water, which are affected by the temperature, pressure, and salinity. Temperature is the most important of these
environmental factors, and therefore the thermal structure of the ocean is of significant tactical importance. The tracing out of sound paths in water is known as ray theory and is governed by Snell's Law. Various unique propagation paths can be identified according to the thermal structure of the water, but in practice such paths are a complex combination of simpler structures. Actual sound propagation through the sea is subject to geometric
spreading and attenuation, both of which decrease the acoustic intensity at the receiver. The active and passive sonar equations are an expression of various factors determined by the equipment, the medium, and the target, which lead to an overall measure of sonar performance called figure of merit.
Cheney, R. E., and D. E. Winfrey, "Distribution and Classification
of Ocean Fronts," NAVOCEANO Technical Note 3700-56-76. Washington, D.C.: GPO, 1976.
Chramiec, Mark A. Unpublished lecture notes on Figure of Merit and
Transmission Loss, Raytheon Company, 1983.
Cohen, Philip M., "Bathymetric Navigation and Charting", United
States Naval Institute, Annapolis, Maryland, 1970.
Commander, Naval Ordnance Systems Command. Elements of Weapons
Systems. NAVORD OP 3000, vol. 1, 1st Rev. Washington, D.C.:
GPO, 1971.
Corse, Carl D. Introduction to Shipboard Weapons, Annapolis, MD:
Naval Institute Press, 1975.
Cox, Albert W., "Sonar and Underwater Sound", Lexington Books,
Duxbury, Alyn C. The Earth and its Oceans. Reading, Massachusetts:
Addison-Wesley Publishing Company, Inc., 1971.
Heppenheimer, T. A., "Anti-Submarine Warfare: The Threat, The
Strategy, The Solution", Pasna Publications Inc., Arlington, Va.
1989.
Honhart, D. C. Unpublished class notes on Acoustic Forecasting,
Naval Postgraduate School, 1974.
King, L. F., and D. A. Swift. Development of a Primer on Underwater for ASW. Master's thesis, Naval Postgraduate
School, 1975.
Kinsler, L. E., and A. R. Frey, Fundamentals of Acoustics, 2nd ed.
New York: John Wiley and Sons, 1962.
Myers, J. J., C. H. Holm, and R. F. McAllister, eds. Handbook of
Ocean and Underwater Engineering, New York: McGraw-Hill Book
National Defense Research Committee, "Principles and Applications
of Underwater Sound", Washington, D.C., 1976.
Naval Education and Training Command. Sonar Technical G3 2(u),
NAVEDTRA 10131-D, Washington, D.C.: GPO, 1976.
Naval Operations Department, U.S. Naval War College, Technological
Factors and Constraints in System Performance Study-Sonar
Fundamentals. Vol I-1, 1975.
Naval Training Command. The Antisubmarine Warfare Officer (U).
NAVTRA 10778-C. Washington, D.C.: GPO, 1973.
Operations Committee, Naval Science Department, U.S. Naval Academy,
Naval Operations Analysis, Annapolis, MD: U.S. Naval Institute,
1968.
Sollenberger, R.T., and T. R. Decker, Environmental Factors Affecting Antisubmarine Warfare Operations. Master's thesis,
Naval Postgraduate School, 1975.
Sverdrup, H. U., M. W. Johnson, and R. H. Fleming. The Oceans.
Englewood Cliffs, N.J.: Prentice-Hall, Inc. 1942.
Urick, R. J., Principles of Underwater Sound, 2nd ed. New York:
McGraw-Hill Book Company, 1975.
U.S. Naval Oceanographic Office. Ocean Thermal Structure Fore-
casting. SP 105 ASWEPS Manual Series, vol. 5, 1st ed., by R. W.
James. Washington, D.C.: GPO, 1966.
Chapter 9 UNDERWATER DETECTION AND TRACKING SYSTEMS
UNDERWATER DETECTION AND TRACKING SYSTEMS
OBJECTIVES AND INTRODUCTION
Objectives
1. Be acquainted with capabilities and limitations of surface, subsurface, airborne, and shore- based ASW forces.
2. Be acquainted with the basic principles of magnetic anomaly detection and its advantages and disadvantages.
3. Be acquainted with the four basic types of transducers and the processes employed in energy conversions.
4. Know the significance of and be able to calculate values of transducer directivity and power for a flat transducer.
5. Know the major components and operation of active sonars, including scanning circuitry operation.
6. Know the major components and operation of passive sonars.
7. Know the advantages of VDS and TASS over hull-mounted sonars.
8. Be able to calculate linear passive array directivity and perform calculations required to determine angle of arrival of a signal.
9. Be acquainted with various other sonar types, such as high resolution, sonobuoys, acoustic navigation systems, and communication systems.
10. Understand the fundamentals of sound energy doppler and how it is used to determine target aspect and motion.
11. Be acquainted with basic considerations associated with the employment of sonar systems in ASW.
Antisubmarine warfare, with the exception of fixed systems such as arrays of underwater hydrophones, is waged by various mobile antisubmarine craft: surface, airborne, and undersea. It is imperative that the officers and men of each type of antisubmarine force understand the characteristics, capabilities, and limitations of the other types. Only by such knowledge can they fully understand the basic concept of modern antisubmarine warfare - the integration and coordination of all forces available. Each type has certain advantages and disadvantages, and maximum effectiveness can be achieved only by coordinating all types. The basic characteristics of each force that should be evaluated are its inherent capabilities and limitations, detection methods, fire control systems, and weaponry.
Magnetic Anomaly Detection (MAD)
Another method of detecting a submerged submarine is through the use of MAD equipment, which uses the principle that metallic objects disturb the magnetic lines of force of the earth.
Light, radar, or sound energy cannot pass from air into water and return to the air in any degree that is usable for airborne detection. The lines of force in a magnetic field are able to make this transition almost undisturbed, however, because magnetic lines of force pass through both water and air in similar manners. Consequently, a submarine beneath the ocean's surface, which causes a distortion or anomaly in the earth's magnetic field, can be detected from a position in the air above the submarine. The detection of this anomaly is the essential function of MAD equipment.
When a ship or submarine hull is being fabricated, it is subjected to heat (welding) and to impact (riveting). Ferrous metal contains groups of iron molecules called "domains." Each domain is a tiny magnet, and has its own magnetic field with a north and south pole. When the domains are not aligned along any axis, but point in different directions at random, there is a negligible magnetic pattern. However, if the metal is put into a constant magnetic field and its particles are agitated, as they would be by hammering or by heating, the domains tend to orient themselves so that their north poles point toward the south pole of the field, and their south poles point toward the north pole of the field. All the fields of the domains then have an additive effect, and a piece of ferrous metal so treated has a magnetic field of its own.
Although the earth's magnetic field is not strong, a ship's hull contains so much steel that it acquires a significant and permanent magnetic field during construction. A ship's magnetic field has three main components: vertical, longitudinal, and athwartship, the sum total of which comprises the complete magnetic field, as shown in figure 9-3.
The steel in a ship also has the effect of causing earth's lines of force (flux) to move out of their normal positions and be concentrated at the ship. This is called the "induced field," and varies with the heading of the ship.
A ship's total magnetic field or "magnetic signature" at any point on the earth's surface is a combination of its permanent and induced magnetic fields. A ship's magnetic field may be reduced substantially by using degaussing coils, often in conjunction with the process of "deperming" (neutralizing the permanent magnetism of a ship); but for practical purposes it is not possible to eliminate such fields entirely.
The lines comprising the earth's natural magnetic field do not always run straight north and south. If traced along a typical 200-kilometer path, the field twists at places to east or west and assumes different angles with the horizontal. Changes in the east-west direction are known as angles of variation, while the angle between the lines of force and the horizontal is known as the angle of dip. Short-trace variation and dip in the area of a large mass of ferrous material, although extremely minute, are measurable with a sensitive magnetometer.
The function, then, of airborne MAD equipment is to detect the submarine-caused anomaly in the earth's magnetic field. Slant detection ranges are on the order of 500 meters from the sensor. The depth at which a submarine can be detected is a function both of the size of the submarine and how close the sensor is flown to the surface of the water.
Improving MAD detection ranges have proved extremely difficult. Increasing the sensitivity of the MAD gear is technically feasible, but operationally, due to the nature of the magnetic anomaly, is not productive. The magnetic field of a source, such as a sub, falls off as the third power of the distance; hence an eight-fold sensitivity increase would serve merely to double the range. Additionally, magnetometers are non-directional; the physics of magnetic fields do not permit the building of instruments that would respond preferentially to a field coming from a particular direction. Hence, a valid submarine caused disturbance frequently is masked by spurious "noise". Also, the ocean floor in many areas contains magnetic ore bodies and similar formations of rock, which can confuse the signal. Further confusion comes through magnetic storms, which produce small but significant variations in the earth's field.
MAD equipment is primarily used as a localization/targeting sensor by aircraft with optimum employment being by helicopters considering their smaller turn radius. Additionally, fixed-wing ASW aircraft MAD configurations are fixed in the tail boom, and helicopters tow the sensor on a 25 - 55 meter cable below and behind the aircraft, which reduces "noise" caused by the
helicopter. Because of the relatively short detection ranges possible, MAD is not generally utilized as an initial detection sensor.
Visual Sighting
Visual sighting is the oldest, yet the most positive, method of submarine detection. Even in this age of modern submarines, which have little recourse to the surface, the OOD and lookouts of any ship should always be alert for possible visual detection of a submarine. Aircraft, even those not normally assigned an antisubmarine mission, can use visual detection methods to particular advantage as a result of their height above the surface.
Of particular note is the potential for periscope and periscope wake sighting, which in many tactical situations is a necessary precursor to an opposing submarine's targeting solution.
Additionally, ASW forces need to be aware of the potential for night detection of bioluminescence; the light emitted from certain species of dinoflagellate plankton when disturbed by a moving submarine hull and its turbulent wake. This blue-green light,
predominately in a 0.42 - 0.59 band, occurs in various water conditions and is most prevalent between 50 and 150 meters and where the water has steep temperature gradients. Visual detection in most cases requires a moon-less night and a relatively shallow target.
9.6.5 Echo Sounder
9.6.6 Acoustic Log
Communications
For many reasons it may be necessary for ships/aircraft and submerged submarines to