Sound Navigation and Ranging

SONAR Full Form in English, letter by letter

SONAR is a technique that uses sound pulses and their echoes to detect objects and measure distances underwater.

SONAR Full Form in Indian Languages

Submarines, fish finders and the depth sounder on a fishing boat all run on SONAR: Sound Navigation and Ranging. Here is the physics behind it and where you meet it.

The SONAR full form and the basic principle

The full form of SONAR is Sound Navigation and Ranging. It is a technique that uses sound to detect objects and measure distances under water. The name was adopted by the US Navy in the 1940s, in the same pattern as radar.

The principle is simple. Send a pulse, wait for the echo, and multiply half the elapsed time by the speed of sound. Sound travels at roughly 1,500 metres a second in sea water against about 343 in air, so an echo that returns after 2 seconds means the object is about 1,500 metres away.

Radar cannot do this job. Radio waves are absorbed within a few metres of salt water, which is why ships and aircraft use radar above the surface and sonar below it.

The returning echo carries more than distance. Its strength and shape hint at what the object is made of, how large it is and whether it is moving away from you.

From ASDIC to towed arrays: a short history

The idea predates the name. After the Titanic sank in 1912, inventors looked for ways to detect icebergs with sound, and during the First World War French and British researchers, including Paul Langevin, built devices to find submarines.

The British called their equipment ASDIC. The first sets were fitted in warships late in 1918, too late to change that war. You will sometimes see a long expansion of ASDIC quoted, but there is no record of a committee by that name, so treat it as folklore.

After 1945 the name sonar replaced ASDIC in most navies, and depth finders based on the same echo principle reached fishing boats and merchant ships within a decade.

Modern sets are a long way from a single ping: towed arrays trailing kilometres behind a warship, phased arrays that steer their beam electronically, and hull-mounted systems that follow dozens of targets at once.

Active sonar and passive sonar

Active sonar transmits a ping and listens for the return, giving both range and bearing. The cost is that the ping tells everyone with a hydrophone roughly where you are.

Passive sonar only listens (propeller noise, machinery, the hum of engines) and gives bearing without range. Submarines prefer it for exactly that reason: listening keeps you hidden.

Navies use both: long passive arrays towed behind a ship for detection, an active ping when a final fix is needed, and sonobuoys dropped from aircraft to cover open water.

Fishing and survey vessels use the same two modes in simpler form. A fish finder pings downwards continuously, while a multibeam system sweeps a fan of beams to chart the seabed in strips.

Where sonar turns up in ordinary use

Anti-submarine warfare is the best-known application, but commercial ships use echo sounders to check depth under the keel, which is what stops a loaded vessel running aground.

Hydrographic surveys build the charts every ship follows, and offshore oil and gas work uses sonar to route pipelines and inspect platforms. In India, the Navy and the National Institute of Oceanography do survey work of this kind along the coast, and the Naval Physical and Oceanographic Laboratory in Kochi, a DRDO lab, develops sonar systems for the Navy. Survey ships pair each depth reading with a position fix from GPS, so the chart shows where the depth was measured, not only how deep it was.

Harbour security, underwater archaeology, marine research and counting fish stocks all depend on it, as does monitoring whale populations acoustically over whole ocean basins.

Some animals do the same thing naturally. Dolphins and bats (in air) locate prey by sending clicks and reading the echo, which is why sonar is often explained through them in school science.

Effects on marine life and the limits of sonar

Loud sonar is not harmless. High-intensity pulses have been linked to strandings of beaked whales during naval exercises, because those animals dive deep, hold their breath and react badly to unfamiliar sound.

Navies now set exclusion zones and ramp power up gradually, with observers watching for marine mammals before an exercise begins.

Sound does not travel in a straight line under water. Temperature layers called thermoclines bend it and create shadows where a target can hide, so performance changes with depth, season and current.

Ambient noise does the rest. Rain, shipping, waves and even snapping shrimp raise the background, so detection is always about pulling a signal out of a noisy ocean.

SONAR, Sound Navigation and Ranging, is time measurement with sound: send a pulse, listen, convert the delay into distance. The same principle runs a small fish finder and a naval towed array.

SONAR Full Form: Common Questions