What is the difference between radar and sonar?

Sonar, an acronym for Sound Navigation and Ranging, is a detection system based on the reflection of underwater sound waves, just as radar is based on the reflection of radio waves in air. The sonar emits ultrasonic pulses using a submerged radiating device. It listens with a sensitive microphone, or hydrophone, for reflected pulses from potential obstacles or submarines. The sonar is used by airplanes and ships each deploying a different type of sonar. 

Airplanes use a device called a sonobuoy, consisting of a hydrophone mounted on a floating buoy. Spin-offs from the development of sonar technology include acoustic oceanography, the study of ocean properties using a variety of acoustic means, and an imaging or remote-sensing technique using computer analysis to study the data collected when acoustic signals are passed through an object.

Radar is an electronic system, used to locate objects beyond the range of vision, and to determine their distance by projecting radio waves against them. Also known as Radio Detection and Ranging it helps in the location of distant objects. The object's position in space, its size, shape, velocity and direction of motion can be determined. All radar equipment consists of a transmitter, an antenna, a receiver and an indicator. It employs a high-frequency radio transmitter to send out a beam of electromagnetic waves, ranging in wavelength from a few centimetres to about 1 m, by means of an antenna. It concentrates the waves into a shaped beam pointing in the desired direction. Objects in the path of the beam reflect these waves back to the transmitter forming an echo signal.

The antenna collects the energy contained in the echo signal and delivers it to the receiver. Through an amplification process and computer processing, the radar receiver produces a visual signal on the screen of the indicator, essentially a computer display monitor. The radio waves travel at about 300,000km/sec, or at the speed of light.

Source : The Hindu