introduction to antisubmarine warfare

While aboard an attack carrier, we in the air group were not too concerned with the submarine threat, even though the Soviet Navy had the world's largest submarine fleet and highly capable individual vessels. We felt there were lesser units around to take care of them. As it happened, however, the ability to find, track, and counter the Soviet submarine threat was the sine qua non for successful offensive operations, and the Navy as a whole took this very seriously indeed. The Naval Air Reserve provided both land and sea based search and patrol squadrons to assist the active duty forces, both during normal peacetime operatations as well as upon mobilization in time of active conflict..

When I left the active Navy in 1971, virtually all the flying billets in the Reserves were in these patrol squadrons, especially for Naval Flight Officers. So it looked like that would have to learn how to spell "ASW" if I wanted to continue flying. It turns out that this was not such and easy task, highly technical but quite fascinating.

The Physics of ASW

For an entry level but thorough introduction to acoustic propagation, see the FAS's entry on sonar propagation.

Two phsical facts dominate ASW tactics: electromagnetic radiation (visible light, infrared, and microwaves) does not penetrate through seawater very well; sound, especially low freequency, does so quite easily.

So we use sound to detect and track submarines. Like any large piece of machinery, acoustic energy is radiated at certain frequencies and intensities. For example, submarines need to generate electricity for all the various tasks aboard the craft, from preparing torpedoes for launch to heating a sandwich for lunch. This is called "hotel power." For US subs, which use standard 60 Hz power, the generator emits at the frequency. Soviet and European ships use the 50 Hz standard. Thus, the detection of a 50 Hz signal is a good indicator that the emitter may be Soviet.

Intensity is another question. Submarine designers try very hard to limit the level of emissions. The units used to measure sound intensity are typically pressure, measured on a decibel scale referred to micro Pascals. (The Pascal is the SI unit of pressure, or 1 newton per square meter.

One advantage of using the decibel units is that it eases the cacluations for the pressure at range after propagation though seawater.

Sound speed in the ocean varies with density. Density, in turn, is dependent on several factors:

• increases with depth, since density increases with pressure

• decreases with temperature

• increases with salinity

The acoustic sonobuoy is the principal sensor used in ASW. Basically radio-equipped microphones dropped into the ocean, they transmit the acoustic signal back to the aircraft. Sonobuoys are dropped in patterns to maximize the probability of detecting the designated target over the widest area. Once detected, special directional sonobouys are used to track the target and establ a course. Finally, active sonobuoys, which emit pings that measure the range to the target and ore most useful during the final stages of attack. Also during this phase, the plateform emplyes its magnetic anonaly equipment, which senses the distortion of the the earth's magnetic field due to the large amount of ferrous material in the submarine. This sensor is located in the rear "stinger" on ASW aircraft to isolate it from influences from the rest of the aircraft. Even so, range is quite short, but dection is a very strong confirmation that a successful attack could be attempted.

Acoustic energy is propagated in the ocean with relatively low loss over straight line paths. The vertical structure of the ocean (temperature, depth, salinity) and other features of the ocean make straight line propagation very rare. In deep water, for example, downward directed sound rays are refracted upward as the speed of sound increase with depth . This results in a focussing of sound at a specific range, about 28 Nautical Miles.