UNDERWATER DETECTION AND TRACKING SYSTEMS
OBJECTIVES AND INTRODUCTION
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 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
For many reasons it may be necessary for ships/aircraft and submerged submarines to communicate with each other. Of paramount importance is the safety of the submarine and its crew. During exercises the ship can advise the submarine when it is safe to surface. Should an emergency arise aboard the submarine, the ship can be so informed. Exercises can be started and stopped, or one in progress can be modified. Attack accuracy can be signaled by the submarine to the attacking ship. A number of devices exist which facilitate these communications. All incorporate either one or two way voice or tone/sound signal generation, utilizing sonar or sonobuoy type equipment.
The range and quality of transmission varies with water conditions, local noise level, and reverberation effects. Under optimum sonar conditions, however, communication between ships should be possible at ranges out to 12,000 meters. Under the same conditions, submarines achieve a greater range. If the submarines are operating in a sound channel, the communication range may be many kilometers greater than that achieved by ships. Local noise, caused by ship's movement through the water, machinery, screws, etc., can reduce the range to less than half the normal range. Severe reverberation effects may also cause a significant reduction.
Because of the characteristics of underwater sound transmission, the amount of data/information that can be communicated through sonic means is severely limited. The primary and most reliable means of communications between submarines and other forces/command structure is with various radio systems that range in frequency from ELF (extremely low frequency) to UHF satellite. The major disadvantage, however, is the requirement to be at or near periscope depth to facilitate use of various antennas during communication periods. Because of this it becomes apparent that during tactical operations, the only viable form of emergent communications to a submarine is through sonic signals/voice.
Sub-surface communication in the electro-optic spectrum has generated much research and development effort. Future systems may utilize laser communications allowing short duration, submerged exchanges.
ASW is an extremely varied and multi-faceted warfare area. Every scenario has its own unique set of conditions, with many different types of sensors being utilized encompassing many varied equipment, personnel and expertise. An attempt to examine all the factors that influence success in ASW is beyond the scope of this text. However, the following discussion will focus on the performance factors centered around shipboard sonar systems.
Sonar Performance Factors
Table 9-1 is a summary of factors affecting sonar performance and an indication of who or what controls each factor. If the controlling items from the right side are grouped, tactical sonar performance might be considered to be represented by
Tactical Sonar Performance = Sonar design + ship design +
ship operation + sonar operation + target design + target
operation + sonar condition + sonar operator training +
tactical geometry + environment
These controlling items may be further grouped as follows:
1. Items not under the control of the ASW commander:
own sonar design
2. Items under the control of the ASW commander (ship and force respectively):
own sonar conditions
own sonar operator training
own sonar operation
3. Items the ASW commander can measure:
sonar figure of merit
Several of these factors have been discussed earlier, and some additional ones will now be covered.
As has been previously stated, the environment is variable, which contributes to the degree of difficulty of the ASW detection problem. As counter to its variability, the speed profile of the environment is measurable, but it is measurable to both the hunter and the hunted. The understanding and exploitation of that which is measured will probably determine the victor, or at the minimum, determine who holds the edge in detectability vs. counterdectability in the encounter. It is generally accepted that the advantage belongs to the one who gains the initial detection in the ASW encounter.
Table 9-1. Factors Affecting Passive Sonar Tactical Performance
Factor Controlled by
Target Signature Target Design Characteristics
Target Machinery Quieting
Self-Noise Own-Ship Speed (Flow Noise)
Own-Ship Machinery Noise Field
Ambient Noise Environment (Sea State, Wind, Shipping
Noise, Sea Life)
Directivity Own Sonar Array Design
Own Sonar Operating Condition
Own Operator Training & Condition
Detection Threshold Own Sonar Design
Own Sonar Operating Condition
Own Operator Training & Condition
Transmission Loss Environment (Sound Paths Available,
Bottom Loss, Layer Depth, Ocean Area)
Attenuation Own Sonar Design (Frequency)
Additional Factors Affecting Active Sonar Tactical Performance
Source Level Own Sonar Design (Power Available)
Target Strength Target Aspect
Reverberation Environment (Sea Structure)
Own Sonar (Source Level)
Some of the measurable environmental factors that will affect own-ship detectability by the enemy and own-ship capability to detect him are listed below:
sound paths available
shallow water vs. deep water
seasonal variation in area (wind, temperature, etc.)
local transient phenomena (rain, afternoon effect, etc.)
currents in area of operations
Understanding of the sound paths available is paramount in assessing the counterdetectability situation. This is based on past marine geophysical survey data, correlated with current bathymetry (BT) or sound velocity profile (SVP) measurements. The direct sound path is readily determined from current BT or SVP measurements. The viability of the bottom-bounce sound path is determinable by survey data on bottom reflectivity and bottom ab-sorption loss. The existence of convergence zones is normally based on ocean depth and knowledge of prior surveys. In the ab-sence of prior surveys, ocean depth is an acceptable basis for pre-dicting the absence or presence of a convergence zone. However, even if sound paths prediction is in error, assuming the enemy has the capability to use the sound paths most advantageous to him is a sound tactical decision.
With respect to detection in shallow water, it has already been indicated that sonar performance may be enhanced because the ocean bottom and surface boundaries act as a duct for the chan-neling of sound. Since no precise quantitative measure of the expected improvement is available, the ASW tactical commander could view the shallow water problem as one in which ambiguities may be created by him to mask the real composition and precise presence of his forces. One can even conceive of his active sonars being used in a bistatic mode, i.e., using one ship's sonar as an ensonifying source, and any other ship's sonar or sonobuoys being used as re-ceiving or detecting sensors. Bistatic geometry creates problems in establishing precise target location. On the other hand, an in-dication of target presence may tip the tactical balance, or at least provide the tactical commander with alternative courses of action, no matter how the target presence was established.
The depth of the bottom of the surface layer of water is a great determinant of sonar performance from a hull-mounted surface-ship sonar because the target submarine may choose to proceed below the layer. As was dicussed earlier, cross-layer detection is u-sually limited in range because of the refraction or bending of the sound rays. Shallow layers favor the submarine because by going deep below the layer, he is frequently able to detect the surface ship's radiated noise when its active sonar transmission is trapped in the surface layer and/or refracted sharply downward. The tac-tical answer to this situation from the escort point of view is to vary the vertical angle of transmission of his sonar projector so as to penetrate the layer, or to deploy a variable-depth sensor be-low the layer.
Predictions of seasonal variation in the area of operations, based on previous surveys and observations of wind action and temperature profile, are basic data for the operational planner. When the ASW force is in situ, the tactical commander should val-idate or verify the predictions by regular periodic measurements of the ocean temperature or sound-speed structure. The length of the period between measurements should be based on observations of cur-rent weather phenomena affecting sound propagation conditions, i.e., wind, time of day, etc. It is particularly important to be aware of the surface-heating effect of the sun and of the fact that in areas of little mixing of the sea by wind-driven waves, a pos-itive temperature gradient may be developed between mid-morning and mid-afternoon that could seriously degrade surface-ship sonar per-formance. Under these conditions, hourly measurements of the environment are in order. Similarly, when conducting operations in the vicinity of currents like the Gulf Stream or the Labrador Cur-rent, where "fingers" of water with marked differences in temper-ature to that of adjacent waters is common, the tactical commander should consider hourly measurements.
Acoustic Emission Control Policy (EMCON)
Acoustic counter detection of active sonars can be accomplished at ranges much greater than that of the active sonar. Therefore, it is safe to assume that an adversary cannot be surprised when active systems are used unless transmission, for purposes of final pre-cision target location, occurs immediately prior to weapon firing. At the same time, it is very important to limit other acoustic e-missions to limit an adversary's ability to detect, identify, and target friendly units by sophisticated passive methods. Acoustic emission control (acoustic EMCON) is a method of limiting emitted noises and of using them to create an ambiguous acoustic environ-ment, thus forcing an adversary to come to very close range to re-solve his fire control and identification problems prior to firing a weapon. Ships and submarines are designed with noise reduction as a primary consideration. Operating procedures can be modified to control acoustic emissions, including practices such as turn count masking (where a multi-engine ship would operate her main engines at different RPM to confuse an adversary as to its actual speed). Other methods might include the use of sprint and drift tactics to vary the composite radiated noise signal level generated within a group of ships. Acoustic Countermeasures are covered in more detail in Chapter 11.
SUBMARINES AND ANTISUBMARINE WARFARE IN THE FUTURE
No one can say just what warfare will be like in the future, but it can be conjectured clearly enough to show that submarines will have much to do with determining who has overall command of the sea.
Due to the fact that the principals of sound propagation through the water remain physical constants, in some respects ASW of the future will resemble that of World War II. There will be great differences, however, primarily attributed to the vast improvements in submarine and shipboard quieting technologies. Figure ( ) provides a historical perspective on the relative improvements in source levels of U.S. and Russian submarines. It is clear passive sensing of modern submarines is becoming difficult at best. With no control over target source levels and very little control over ambient noise, it becomes incumbent on systems designers and operators to maximize passive detection thresholds and directivity indices, along with continual implementation of self noise level improvements.
Continued emphasis will be placed on development of new sensor types and radical new concepts applied to current technologies. Some examples of innovation on the research and development forefront include:
- Fiber optic towed array: allowing higher data rates, and longer, lighter cables
- Satellite and aircraft based laser detection: Use of the electro-optic spectrum for ASW detection is receiving much attention. Implementation remains in the distant future, however.
- Low frequency active towed arrays: With variable length and depth cables and longer range active sonar sources, the tactical disadvantage of active ASW revealing ownship position is reduced.
Antisubmarine warfare is waged by surface, airborne, undersea, and shore-based forces, each with its own unique capabilities. Me-thods of detecting submarines include sonar, radar, electronic sup-port measures, and magnetic anomaly detection. Of these, sonar is the most widely used and is capable of the three basic functions of echo-ranging (active), listening (passive), and communications. The device in a sonar for acoustic to electrical (and vice versa) energy conversion is the transducer. Simple versions of transduc-ers, designed for listening only, are called hydrophones. Active sonar transducers fall into two basic types, searchlight and scan-ning, with the latter being the most useful for tactical detection. Other types of sonars include variable-depth sonar, high-resolution sonar, towed array sonar systems, sonobuoys, echo sounders, and communications systems. The phenomenon of doppler shift due to a moving target is useful for target classification. Tactical con-siderations in the employment of sonar are grouped into those that the ASW commander can control, those that he cannot, and those that he can measure. The concept of EMCON is as applicable to acoustic emissions as it is to radio and radar. The submarine is a formid-able adversary, and sonar remains the best method of detecting it.
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