Sonar was, and still is, the primary means for locating submerged submarines. It relies on the fact that water is a good conductor of sound energy (much better than air) and that the steel hull of a submarine is an excellent reflector of sound.
The term sonar
originally referred specifically to what is now called active sonar.
This was preceded by passive sonar or sound detection, which
used sensitive hydrophones (underwater microphones) to detect the noise
generated by the engines and screws of a submarine. A
suitable directional hydrophone or set of hydrophones could determine
the bearing to the submarine. Determining the distance is
more difficult, typically requiring knowledge of the surface ship's
course and speed and some guesswork regarding the submarine's course
and speed. The fact that most surface escorts moved at a higher speed
than most submerged submarines helped, but it was still difficult to
determine the range with enough accuracy to score a kill with the very
short-range antisubmarine weapons available, such as depth charges. Active sonar
overcame this difficulty, because it is possible
to accurately measure how long it takes for a powerful sound pulse to
travel from the surface ship to the submarine, be reflected, and arrive
back at the surface ship.
Detecting a submarine with hydrophones was quite difficult, and detecting it with sonar was all but but impossible. Sonar was highly directional. This allowed sonar to get a good bearing on its target, but it also limited the usefulness of sonar for search, since it took several seconds to listen for a return on a single bearing. The sonars of the Pacific War were thus fire control systems rather than search systems, with effective search sonars not becoming available until 1946. Range was also limited, rarely exceeding 3000 yards (2700m) even under the most favorable conditions. Sonar was generally ineffective at speeds over about 10 knots, requiring "sprint and drift" tactics in which the antisubmarine warship had to periodically slow almost to a stop to make best use of its sonar. Sonar could not determine depth with any accuracy and was unable to track a target immediately underneath it. Thus, a submarine could sometimes evade a depth charge attack by maneuvering sharply just as the attacker passed overhead and lost sonar contact. The depth charge explosions themselves blinded sonar, and a submarine that survived a depth charge attack could sometimes break contact behind the "wall" of sonar interference created by the depth charges.
The British developed the "creeping attack" to overcome these
deficiencies. One antisubmarine ship would maintain sonar contact with
the sub while guiding a second antisubmarine ship that closed in at low
speed with its own active sonar turned off. This form of attack was
extremely difficult for the submarine to evade.
Later in the war, high-frequency sonar was developed that was capable of detecting objects as small as contact mines. Such sonar was fitted to American submarines, which allowed them to penetrate minefields protecting the Sea of Japan. This was a considerable shock to the Japanese, who had assumed that the Sea of Japan was a safe haven for their merchant shipping.
Towards the end of the war, the United States developed the
sonobuoy, a small battery-powered sonar that could be dropped from an aircraft on the
suspected location of a submarine and radio its findings to the
aircraft. Early sonobuoys were plagued with difficulties with
background noise, and the device did not get into operational service
in time to help sink a single submarine.
Wavelength. The wavelength was an important indicator of a sonar's capability. High-frequency (short-wave) sonar had limited range but could detect small objects such as mines.
Pulse width. This is the duration of a single pulse from a sonar transmitter. The shorter the pulse, the better the distance resolution. However, achieving a short pulse width is technically challenging, and it is more difficult to put a lot of power into a very short pulse.
Pulse repetition frequency. This is the frequency at which pulses are transmitted. A low pulse repetition frequency is necessary for long-range sonars, to avoid "aliasing" (mistaking a distant target for a nearby one, or vice versa.) However, a higher pulse repetition frequency makes for a more sensitive sonar, ceteris paribus.
Power. This is the average transmitted power. The greater the power, the greater the sensitivity of the sonar.
Range. This is a direct measure of the distance at which an approaching target will typically first be detected. Larger targets are naturally easier to detect.
Accuracy: This is a measure of how precisely the direction and range of a target can be determined.
References
National
Technical Information Service AD-A033 812 (1976, accessed 2010-2-3)
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