Submarines are unique as military machines in their degree of stealth. In all forms of conflict at sea, invisibility and the unpredictable nature of the ocean yields great advantages to the submarine. For example: the stealth of submerged SSBNs is widely agreed to be a factor that helps to deter the use of strategic nuclear weapons in a crisis.
The ocean environment, while providing good protection for the submarine from antisubmarine forces – whether above, on, or below the surface – also makes it difficult for the submarine to use the depths for its own benefit. The variations of temperature, salinity, and currents over time and locale, changes in bottom contour and composition, the effects of matter suspended in the water, noises caused by nature and human activities – all create a complex medium in which it is very difficult for the crew of a submarine to tell what is happening around them. Nonetheless, a submarine’s advantage lies in its ability to use the veil of seawater around it to choose its opportunities to attack or evade.
Warfare involving submarines may take place in virtually any ocean area, under any kind of conditions, and therefore with a wide variation in sensor performance. Sonar may detect a particular submarine or surface ship at hundreds of miles in one environment and at a few thousand yards in another. Nonacoustic detection systems such as magnetic anomaly detectors may be seriously affected by the occurrence of certain types of solar storms. Some of these changes in time or space are predictable, and some can only be described with statistics.
Some generalizations about the behavior of passive acoustic detection can illustrate the importance of environmental conditions. Several factors are important for passive acoustic detection: how quickly sound intensity decays over distance traveled and the amount of noise present. The ability of the sonar system to discriminate between noise and a submarine signal may also depend on the local environment Conditions associated with relatively good and relatively poor detection can be outlined at the risk of oversimplifying a very complex physical problem.
GOOD CONDITIONS FOR DETECTION
The deep ocean is generally one of the best environments for sound transmission, and in areas where shipping is remote and winds are low, detection ranges may be relatively great. The main shipping lanes between North America and Japan, and North America and Europe are more noisy than many other regions, but under good conditions, very noisy targets have been detected at ranges of hundreds of miles.
Many shallow water areas transmit sound more efficiently when oceanographic conditions can support propagation paths that are totally refracted. Such conditions may obtain during the winter, when lower surface temperatures do not cause sound to refract strongly toward the bottom, or in particular geographic areas such as straits where the water is stratified in such a way as to create totally refracted paths.
The central, deep Arctic can be a very favorable environment for detection when the ice cover is nearly continuous, the temperature is stable so that the ice is not forming stress cracks, and the wind is not strong.
POOR DETECTION CONDITIONS
Even when favorable conditions exist at some point in time and space, they can erode rapidly. Changes in the bottom type can change transmission characteristics over a few tens of kilometers. Fronts, such as those associated with the Gulf Stream, can create shadows in which sound from a point on one side of the front is refracted away from areas on the other side. Even a heavy rain shower can undercut detection performance by rapidly increasing the noise level over a broad range of frequencies.
Submarines are hardest to hear when their sounds do not propagate well through the ocean and when there is a great deal of noise present. Shallow water is generally a poor transmission medium because sound reflects from the surface and the bottom many times over its transmission range. At each bounce, sound is scattered in many directions and absorbed, especially by the bottom. In addition to attenuating the sound more rapidly, these repeated scatterings tend to make the sound less coherent, degrading the performance of arrays of sonar receivers. The transmission of sound through shallow water is particularly poor in the summer, when the higher sound speed at the warm surface causes particularly strong refraction of sound into the bottom. Even deep water can be so-called bottom limited if surface temperatures are high enough.
The Mediterranean Sea combines large changes in salinity and temperature over depth to produce very difficult detection conditions. Because of the large number of different nations with naval forces in that sea, the undersea picture can be particularly sensitive in a crisis. For example, at one point during the 1973 Arab~Israeli Crisis, the Commander of a U.S. aircraft carrier was completely surprised by a foreign submarine that surfaced nearby, in spite of the large ASW component associated with carrier battle groups. As it happened, the submarine was Israeli.
Coastal waters have the greatest concentrations of shipping, particularly near ports and harbors, and therefore some of the highest levels of noise. This shipping noise creates an acoustic thicket because it can be similar to submarine noise and both are concentrated in frequencies below a few hundred cycles per second. The poor transmission characteristics of the coastal waters can actually mitigate the effect of high shipping concentrations from distant locations, since the noise itself is highly attenuated. However, many submarine versus submarine scenarios involve the use of passive sonar relatively near a hostile port, where detection ranges would be reduced.
The Arctic region contains a wide diversity of ocean acoustic conditions, including all combinations of shallow water, deep water, open water, ice~covered water, high and low wind speeds. In general, the Soviet continental shelf, which extends over 500 mile from the shore, is characterized by poor detection conditions: depth of less than about 1000 feet; broken ice that grinds together; relatively high wind speeds that can disturb the ice; and, near the Soviet Arctic ports, high shipping noise levels.
Deep water in the lower latitudes does not always transmit sound well. If a ridge such as the one in the North Atlantic lies in the transmission path, the sound may not propagate nearly as well as it would in the absence of such a ridge. This is true even if the top of the ridge rises no higher than several thousand feet from the surface, because sound in deep water travels via long refracted paths that reach great depths, and when those deep refracted paths are cut off, much of the sound energy is lost. In some circumstances, however, that same ridge can cause signals to bounce into refracted paths and improve detection conditions.
LIMITS OF THE OCEAN ENVIRONMENT
The sea masks the presence of submarines in a number of ways. First, seawater is virtually opaque to most electromagnetic radiation. The exceptions to this rule — blue light and very low frequencies — are currently being used or investigated for communication to submarines, but do not appear to hold much promise for detection.
Sound energy, at frequencies below those corresponding to the highest octaves on a piano keyboard, travels with relatively low losses through seawater. Navies make use of this fact by detecting the sounds that submarines produce using passive sonar, or sounds they reflect using active sonar that generates a strong “ping.”
The effectiveness of passive sonars depends on five basic variables: the loudness of the enemy submarine — often called the source level; the loudness of the environmental noise background; the loss of sound intensity over distance; the ability of the sonar receiver to “listen” in a specific direction and shut out noise from other directions; and the ability of the signal processor to detect a weak signal in noise. The first four factors determine the signal-to-noise ratio that enters the signal processor, which in turn determines whether or not a signal is present with given probabilities of detection and false alarm.
Except for the source level, each of the variables above are themselves influenced by the ocean environment, and two of these are purely environmental parameters. The ambient noise level is the sum of noises from distant shipping, wind, waves, ice, organisms, and other sources. The Joss of sound intensity over the transmission range is determined partly by the geometric spreading law, partly by energy absorbed as molecular components of seawater undergo compression and relaxation, and partly by refraction, reflection, and scattering in the water column and its boundaries.
The other two parameters influenced by the environment are sonar array directionality and the detection of signals in noise. These variables can be thought of as being related to the design of a particular system, while being limited by the environment. The limits are imposed by the random variability in the ocean, both in time and space, and at many scales.
The directionality of an array, and its corresponding ability to shut out noise arriving from all directions other than the one specified, depends on several factors. The performance of the array increases with the size of the array, the number of hydrophones, and the method of processing the data. The gain is limited, however, by irregularities in the ocean which distort the sound waves in a random fashion. Improvements in the array gain have been attained at the price of a great increase in computer processing requirements, but these improvements have been small.
The threshold at which a signal buried in noise can just be detected depends on the ratio of signal to noise power in the frequencies of the signal, and on the statistical properties of the signal and the noise. The sounds of machinery and propellers generally have components that fall into narrow frequency bands. To the extent that all the energy of these sounds is confined to very narrow bands, they can be more easily detected against the background of noise. Once again, however, the variability of the ocean imposes a certain degree of limitation by smearing the energy over a wider bandwidth over the course of its propagation through the ocean. In addition, the submarine signal itself may vary, which has the same effect of spreading the signal energy over a wider band.
One of the most important features of the ocean that determines how sound travels is the profile of sound speed over depth. Changes in the speed of sound govern the refraction of sound as it travels through the sea, and this refraction governs the rate at which sound is attenuated over distance. In the deep water of the latitudes below the Arctic, sound tends to propagate along a depth layer where sound speed is a minimum, resulting in relatively good transmission. In the deep water of the Arctic, cold surface temperatures cause sound to refract upward and scatter off the rough undersurface of the ice.
From this discussion it becomes clear why submarine quieting has such a fundamental impact on antisubmarine warfare: the unpredictable, uncontrollable environment has at least a major influence on every other variable in the passive sonar equation and efforts to improve the sonar system will always meet with sharply diminishing returns.
With submarines becoming quieter, and the environment forcing limits on detection of faint signals in noise, the detection range of individual sensors is bound to decrease. These two facts in conjunction (not simply the quieting of submarines) will alter the military assessment of submarine forces. The new look at submarines wiU create incentives to adopt new approaches to ASW, or to revive ideas that have been tried in the past, but that were discarded because other, cheaper alternatives were available to detect louder submarines of that era.
Some of the technical directions in which submarine detection could evolve include the use of many small distributed sensors to detect over a wide area. This is a way of avoiding the limitations of array gain and signal processing by simply reducing the distance between the sensors and the submarine. H the sea floor were covered by sensors, then a submarine would never be more than a few miles from one of them. The technical problem becomes one of making sensors and connecting cables that are affordable in the numbers that would be needed.
Other means of surveillance might include small arrays that could be covertly placed by a submarine near the ports of its adversary. Using different physical principles, nonacoustic methods of detection from air or space are the subject of intense scrutiny. These means would be of particular concern if their functioning could threaten the confidence of the nuclear weapon states in the survivability of their sea based nuclear forces.
Thus, an important set of choices may confront the major military powers in the future regarding sea-based strategic nuclear forces. The obscurity of the ocean environment provides a measure of security in the sense of providing a relatively safe, stable haven for these submarines. To the extent that a hypothetical future surveillance system allows the nuclear nations to peer under the waves on a global scale, it may create some military problems while solving others.
Submarines will continue to be potent naval platforms for large and small nations, and will figure in international military affairs across the spectrum of violence. The submarine’s ability to use the environment to its own advantage will consequently be an important element in the development of armaments.
Tom Stefanick
SUBMARINE FORCE. U.S. ATLANTIC FLEET
The mission of Commander Submarine Force, U.S. Atlantic Fleet is to maintain combat ready strategic and attack submarines. COMSUBLANT, unlike other Type Commanders, is also an operational commander. The current Submarine Force is made up of three Submarine Groups and ten Submarine Squadrons consisting of 31 strategic and 56 attack submarines. The Submarine Force consists of 2,500 officers and 29,000 enlisted personnel. Today’s Submarine Force operates in all oceans of the world, including the Atlantic, Pacific, Arctic, and Indian Oceans, as well as the Mediterranean Sea.
