The submariner who takes advantage of the ocean’s anomalies in the vicinity of his submarine and who knows the limitations of his enemy’s submarine detection gear can, seemingly, make the oceans more opaque for enemy ASW efforts. An appreciation of today’s means of detection and tracking of submarines then offers valuable clues for the tactics to be used to escape detection. To this end, this article will concentrate on identifying factors which affect the success or wide area antisubmarine search operations by acoustic and non-acoustic detection devices, while suggesting ways for a submarine to minimize their success.
Acoustic Methods for Wide Area Search
or all the signals relied on to find submarines, acoustic ones are the most prominent. Two types of acoustic signals are used in the detection or submarines, active sound bounced orr a submarine’s hull and radiated energy generated by a submarine — which includes a target submarine activating its own sonar or incidental noise produced in submarine operations from disturbing the water through submarine movement, propulsion noises, and the utilization of internal machinery such as motors, pumps and gears.
If the oceans are to be made relatively transparent for detection of a submarine’s radiated energy, improvement in passive detection devices anchored to the ocean floor or ones towed from surface ships, show the greatest promise.
For detecting a submarine by bouncing sound off its hull, high powered sonars are indicated. However, highly powerful sonars used in large area search have to contend with an extremely serious reverberation problem. The greater the power, the more a sonar produces multiple echos which drown out the return of desired signals. Moreover, the direction as well as the intensity of sound are affected by factors which vary markedly depending on geographic location as well as the time of day and year. These factors include: the concentrations or sedimentary inorganic particles, tiny sea organisms, detritus, schools of fish, gas bubbles, and other such objects which can scatter or absorb sound. In addition, there are horizontal or vertical boundaries in the water column which can duct, refract, block, or attenuate sound. The most significant of the horizontal boundaries are the separations between layers of sharply differing temperature characteristics.
The discontinuities represented by the boundaries between each layer coupled with the effect on sound direction or the pressure and temperature characteristics of each layer can cause some of the sound to be ducted or channelled between the boundaries. Thus, the hearing of submarine radiated noise can be excellent within a duct. However, submarine detection can be hampered if the target is in one layer while detection devices are in another.
Some sound which travels steeply enough to penetrate from one layer to another, while undergoing attenuation and refraction, can make it into the deep sound channel where it can travel horizontally for great distances — for thousands of kilometers with relatively little attenuation except for spreading and absorption.
Cutting across horizontal boundaries in the water column are vertical ones associated with ocean eddies, fronts, the interface between two currents, and the presence of underwater mountains and ridges. Such boundaries can stretch for hundreds to thousands of miles, and, as with horizontal boundaries, can affect submarine detection if the boat is on one side and the acoustic sensor on the other. For example, a submarine detected in the Labrador Current but crossing into the Gulfstream has been compared to a person going from an open field and “disappearing into an adjoining woods.”
No less important is the interweaving effect on sound velocity of the water’s temperature, pressure, and salinity. Decreases in each of these contribute to a decrease in velocity, and sound waves will bend or refract as much as 15° toward those water areas which permit slower speed. The bending can make for highly complex sound propagation paths and produce “shadow zones,” i.e. areas where sound does not penetrate. Thus, a hydrophone might not hear a submarine even though both were quite close and in the same temperature layer. Similarly, active sonar emissions might be bent away from, and thus not reflect against, a target even when the latter is near the sonar and again in the same layer.
In addition to shadow zones, the bending of sound waves cause the formation of “convergence zones,” and these can be beneficial to the detection of shallow submarines by sensors placed near the surface many kilometers away.
Yet, if a submarine is to be detected, ASW forces must contend with more than the sound transmission qualities of seawater. The world’s oceans are rather noisy. Ocean life, the actions of the wind on the surface, human uses and exploitation of the ocean and seabeds — all contribute to the generation of ambient noise which mask or mimic desired signals. It has been noted that ambient noise has worsened because of the greater frequency of ocean drilling and increased numbers of very large tankers and bulk carriers plying the oceans.
It may be argued that the more oceanographers and acousticians learn about the variability of ocean conditions, the more opaque the oceans are seeming to become. In contrast to viewing the oceans as a relatively stable mass of water, but turbulent at the surface and crisscrossed by great currents like the Gulf Stream, oceanographers have come to appreciate the ubiquity of eddies cutting vertically across the water column, disrupting horizontal transmission of sound even in the deep SOFAR Channel. It is not that oceanographers have been unaware of eddies, but rather that they have not appreciated how widespread they really are. Another basis for the opacity claim is provided by RADM R.A. Geiger: “Recent basic research has revealed that the ocean is quite complex, and in many respects is analogous to the atmosphere: it contains the oceanic counterpart of atmospheric weather. This oceanic “weather” consists of highs, lows, fronts, jets — which, relative to ocean climatology, travel quite rapidly. The sharp temperature gradients associated with this weather are known to cause rapid changes in sonar conditions and provide acoustic shadows that obscure an object from detection.”
Seeking out acoustic shadows is one obvious countermeasure a submarine can use to avoid detection. Others include; going slow and maneuvering gently and staying out of ocean temperature layers which can transmit sound to adversary acoustic systems.
Should the improbable occur — a very effective long-range acoustic detection system -there would still be the problem or false alarms which would require ASW forces to adjust their sensor in terms of probability of detection versus probability of false alarms. But to adjust a sensor so as to increase the threshold of acceptability of a signal, and thus eliminate false alarms, also risks losing true signals. Improved signal processing increases the probability of correctly sorting through incoming sounds. Still, ASW forces must always decide on the balance between detectability and false alarms. Making this balance may, however, do little to screenout signals deliberately generated by a wily adversary who realizes that decoys, jamming, and even physical disruption of adversary acoustic systems can be cost-effective.
Acoustics in the Arctic
In the Arctic, a submarine which remained under the ice might be particularly difficult to find. Most of the sounds produced by a submarine do not tend to travel great distances for two related reasons: (1) the entire composite spectrum of submarine-generated noise is confined to a frequency range between 10 and 1,000 Hertz or so; (2) the transmission of sound in the Arctic degrades rapidly with increasing frequency above 20 Hz. It is better than it would be in the ice free field out to some range and is poorer beyond. This is the result of opposing influences on the propagation. At short and moderate ranges, ducting due to the ice cover improves the transmission, but at long ranges the repeated encounters with the under-ice surface degrade it. In short, nearly the entire spectrum of submarinegenerated sound is in the frequency range which degrades rapidly.
Most of the degradation of under-ice sound is due to numerous ice ridges with keels extending downward from the ice canopy. Their spacing is generally random, some are quite deep, and they are important not only because they absorb sound but also because if a ridge is large enough it provides a submarine with a near perfect place to hide. By lying quietly against an undersea keel, the noise generated by the submarine would tend to be refracted upward against the ice. Should an adversary utilize sonar to search actively, the echos would reverberate against the surface. Hence, any coherent return would probably not distinguish the submarine from the overhead ice.
Non-acoustic Methods for Detecting Submarines
No system is yet operational for detecting a submarine in a broad ocean search — if the submarine refrains from an activity such as communicating externally, activating radars, or firing weapons serving to give away its position. The most widely applied nonacoustic sensor today is the magnetic anomaly detector, MAD. But the disturbance in the earth’s magnetic field by a submarine which is detectable by MAD is inherently short-ranged. Thus, even with a highly sensitive MAD detector, the system is more suited for narrow barrier and localization than wide-area search.
Compared to some acou~tic signals, none of the submarine-generated nonacoustic signals propagate far. As a result, if a nonacoustic system does become operational for searching large areas, it will probably involve observables which persist behind the submarine in its wake and are detectable by air or space-based sensors. The greater the persistence, the longer the spatial extent of the signal and the better the opportunity for detecting it. Similarly, the higher the sensor can be while operating, the more extensive is the area it can cover in a short period. The observables potentially detectable by air or space sensors appear to be: contaminants, thermal scars, wake turbulence and internal wave effects.
A submarine introduces a variety of products into the ocean or the atmosphere above. These result from the leeching of anti-fouling paint, the leaking of lubricants from propeller shafts, the dumping of wastes, the corrosion of a submarine’s hull and propellers, the formation of radioisotopes following the escape of neutrons from the nuclear reactor into the seawater, etc.
Most contaminants probably mix too rapidly and reduce themselves to background levels before they can spread far as a detectable phenomenon. Thus, they provide a better basis for localization than for wide-area search. Moreover, contaminants discharged into the submarine’s wake tend to be confined to the wake which usually remains at the depth stratum of the submarine.
Because the trace element detectors used to find contaminants must be brought into the immediate vicinity of the sub’s track, a submarine could release waste products when weather or intelligence suggested that no sensing unit would be present at the time of release.
Nuclear submarines ingest and then expel seawater used to cool the reactors. Consequently, great volumes of warm water are left in the submarine’s wake. Since the discharged water is warmer and less dense than its surroundings, it rises and can cause a thermal scar on the sea surface detectable with either infrared or passive microwave radiometers. One Soviet author suggests that a thermal scar anomaly on the surface could be on the order of .005° C. But to achieve a sensitivity for detecting such a scar an infrared radiometer would operate at an altitude of about 100 meters — causing coverage to be too small for wide-area search.
The utility of space-based infrared and passive wave radiometers is questionable in that the sensor may be too far from the ocean surface and travelling too fast to be able to pick up signals of interest. Present satellite-based IR systems are most promising since they can measure tenths of a degree Kelvin with input data averaged over one kilometer. They can do this with swath widths of 2,700 kilometers which allows for widearea search. But, true all-weather day-night seasurface temperature data from satellites must await development of high-resolution multispectral passive microwave radiometers. Then, a serious problem is the enormous number of “false alarms” that result from local temperature differences on the ocean’s surface that are generated by a myriad of mechanisms other than a submarine. E.g. natural currents can produce turbulence in the water which give rise to temperature differentials on the surface — while thermal anomalies might not make it to the surface.
Turbulence behind a submarine results from the turning of the propeller and the resistance of the seawater to the submarine’s passage. This resistance causes both turbulence in the layer of water adjacent to the bull and an associated shedding of vortices from the edges of rough spots, the sail, and appendages. The wake resulting from these effects can propagate to several kilometers astern of the submarine and persist for many minutes. Since the wake remains confined to the general vicinity of the submarine’s depth stratum, the only airborne sensor capable of detecting a fairly deep wake is a blue-green laser, because of its unique ability to penetrate the water from above, bounce against the wake, and return to the source. Such a laser would probably be carried initially on an airplane since spacecraft utilization requires the solution of tta host of technical problems.” One problem for the laser system is that a submarine which cruised deep enough might well avoid detection of its wake because laser beam penetration is likely to be only a few hundt·ed meters. But this estimate is only nominal since local water turbidity and clouds will greatly lessen how deep a laser will actually penetrate and return to its source. Whatever the laser’s depth penetration, moreover, its swath width would be limited since the beam would be thin and at near perpendicular incidence to the sea surface for effective penetration. This should seriously degrade the laser’s utility for wide-area search even though turbulent wakes ruay persist for extended periods.
An internal wave is a vertical oscillation of the water column. A moving submarine leaves behind it a wake of internal waves since the water displaced by the submarine’s movement fluctuates up and down as it seeks to return to its original density equilibrium. For three reasons internal waves have the greatest potential among the nonacoustic observables to serve as a basis for wide-area search. (1) Internal waves produced by nature can sometimes persist for days so those produced by a submarine might last for hours and the wake of waves stretch for tens of kilometers. (2) Internal waves produce effects on the surface of the ocean by modulating the short capillary waves superimposed on the ocean’s larger surface gravity waves. (3) The surface manifestations are detectable by what is termed a synthetic aperture radar. The significance of this radar arises from its ability, even at satellite altitudes, to detect the manifestations in great detail and precision. It also has the advantage — unlike infrared detection — of being able to sense through clouds and rain. However, those who focus on surface manifestations of internal waves agree that reliance on such manifestations is presently not feasible and remains “an open question.”
That there is no shortage of mechanisms for generating internal waves accounts for their ubiquity. These include surface ships, winds, surface waves, atmospheric pressure fluctuations, the movements of currents and tides over irregular bottom topography and the actions of currents at the interface between ocean density and temperature layers. In short, it is not surprising that a very high false alarm rate could attend reliance upon internal waves as submarine observables.
Spacecraft for the effective use of most ocean surveillance sensors fly at altitudes which do not exceed 1.000 km. Such low orbit satellites have a speed over the ground of about 25,000 km per hour and can circumnavigate the globe in roughly 90 to 100 minutes. Anti-satellite weapons however may put them in hazard. Still they can overfly regions where no manned aircraft would be risked except in special cases.
Spacecraft in very low orbits (such as 300 km) stay aloft only a few days before air drag effects on the fringes of the atmosphere cause them to fall towards earth. A higher orbit makes for longer endurance, but too high an altitude generally restricts the effective use of on-board sensors.
It often takes a long time before a satellite is positioned over a point of interest. A sensor which circles the earth 15 times daily in a near polar orbit between 500 and 700 kilometers, for example, has an extremely broad 2700 kilometer swath width. It would take 18 days for a sensor sweeping a narrower 148 kilometers across. By maneuvering a satellite from one orbit to another, the drag make-up system reduces the time otherwise required to have a satellite overfly a desired area. But as with the drag make-up, these maneuvers can be propellent intensive and therefore limited in number and extent.
In sum, the wide area search prospects of nonacoustic methods is based on: (1) the use of sensors operating high above the water and covering long distances in short periods and (2) the persistence of signals in the submarine’s wake. When one considers however the factors which affect the generation, location and persistence of the signals, the relative utility of air and space vehicles and the characteristics of sensors operating from such craft, then it is not surprising that there is no nonacoustic system yet operational for wide area search.
Simply operating deeply and slowly, for example would probably negate most ocean surface effect signals. Nonacoustic systems could also be jammed and their communications disrupted.
On the other hand, maximizing stealth or quiet operations in an environment of antisubmarine acoustic sensing systems tends to keep the oceans opaque for detection. Use of the ocean’s anomalies to reduce the effectiveness of an enemy’s active sonar capabilities is also indicated.
In effect, a better understanding of the nature of the oceans and a better appreciation of enemy ASW detection capabilities can guide the smart submariner into modes of operations which tend to make the oceans more opaque to the enemy — rather than increasing its transparency through new technologies in the enemy’s hands.
(This article has been excerpted from Antisubmarine Warfare in the Nuclear Age,by Donald C. Daniel, in ORBIS, Fall 1984, a journal of world affairs, with permission of the Foreign Policy Research Institute, Philadelphia, PA.)