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Sonar (sound navigation and ranging) is a technique that uses sound propagation under water to navigate or to detect other watercraft. There are two kinds of sonar, active and passive.
Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections of the pulse. To measure the distance to an object, one measures the time from emission of a pulse to reception. To measure the bearing, one uses several hydrophones, and measures the relative arrival time to each in a process called beamforming .
The first active sonar technology was originally called ASDIC after the "Allied Submarine Detection Investigation Committee".
The pulse may be at constant frequency or a chirp of changing frequency. In a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total energy were emitted. The chirp signal is sent over a longer interval, therefore the instantaneous emitted power will be reduced, which simplifies the design of the transmitter. In general, long-distance active sonars use lower frequencies. The lowest have a bass "BAH-WONG" sound.
The most useful small sonar looks roughly like a waterproof flashlight. One points the head into the water, presses a button, and reads a distance. Another variant is a "fishfinder" that shows a small display with shoals of fish. Some civilian sonars approach active military sonars in capability, with quite exotic three-dimensional displays of the area near the boat. However, these sonars are not designed for stealth.
When active sonar is used to measure the distance to the bottom, it is known as echo sounding.
Active sonar is also used to measure distance through water between two sonar transponders. A transponder is a device that can transmit and receive signals ('pings') but when it receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transponder transmits an interrogation signal and measures the time between this transmission and the receipt of the other transponder's reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two transponders. This technique, when used with multiple transponders, can calculate the relative positions of static and moving objects in water.
Some marine animals, such as whales and dolphins, use echolocation systems similar to active sonar to locate predators and prey. It is feared that sonar transmitters could confuse these animals and cause them to lose their way, perhaps preventing them from feeding and mating. A recent article on the BBC website (see below) reports findings published in the Journal Nature to the effect that military sonar may be inducing the whales to experience decompression sickness (and resultant beachings).
High-powered sonar transmitters can kill marine animals. In the Bahamas in 2000, a trial by the US Navy of a 230 decibelThe decibel is a "dimensionless unit" (like percent) that is a measure of ratios on a logarithmic scale. Usually, it is ten times the base-10 logarithm of the ratio. It's not an SI unit, although the International Committee for Weights and Measures (BIPM) transmitter in the frequency range 3 to 7 kHz resulted in the beaching of sixteen whales, seven of which were found dead. The Navy accepted blame in a report published in the Boston Globe on 1/1/ 20022002 is a common year starting on Tuesday (see link for calendar). 2002 was the first palindromic year since 1991 and the last until 2112. 2002 was also designated: International Year of Ecotourism and Mountains National Science Year in the United Kingdom. However, at low powers, sonar can protect marine mammals against collisions with ships.