Sonar (originally an acronym for SOund Navigation And Ranging) is a technique that uses sound propagation (usually underwater, as insubmarine navigation) to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels. Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes. Sonar may be used as a means ofacoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. Sonar may also be used in air for robot navigation, and SODAR (an upward looking in-air sonar) is used for atmospheric investigations. The term sonar is also used for the equipment used to generate and receive the sound. The acoustic frequencies used in sonar systems vary from very low (infrasonic) to extremely high (ultrasonic). The study of underwater sound is known as underwater acoustics or hydroacoustics.

SODAR (SOnic Detection And Ranging), also written as sodar, is a

meteorological instrument used as a wind profiler to measure the scattering of sound

waves by atmospheric turbulence. SODAR systems are used to measure wind

speed at various heights above the ground, and the thermodynamic structure of the

lower layer of the atmosphere.

Sodar systems are like radar (radio detection and ranging) and lidar (light radar)

systems except that sound waves rather than radio or light waves are used for

detection. Other names used for sodar systems include sounder, echosounder and

acoustic radar.[1]

Radar is an object-detection system that uses radio wavesto determine the range,

altitude, direction, or speed of objects. It can be used to detect aircraft,

ships, spacecraft,guided missiles, motor vehicles, weather formations, and terrain.

The radar dish (or antenna) transmits pulses of radio waves or microwaves that

bounce off any object in their path. The object returns a tiny part of the wave's

energy to a dish or antenna that is usually located at the same site as

the transmitter.

Radar was secretly developed by several nations before and during World War II.

The term RADAR was coined in 1940 by the United States Navy as

an acronym for RAdioDetection And Ranging.[1][2] The term radar has since

entered English and other languages as a common noun,losing all capitalization.

The modern uses of radar are highly diverse, including air and terrestrial traffic

control, radar astronomy, air-defense systems, antimissile systems; marine radars to

locate landmarks and other ships; aircraft anticollision systems; ocean

surveillance systems, outer space surveillance

and rendezvous systems; meteorological precipitation monitoring; altimetry and flight

control systems; guided missile target locating systems; and ground-penetrating

radar for geological observations. High tech radar systems are associated

with digital signal processing and are capable of extracting useful information from

very high noise levels.

Other systems similar to radar make use of other parts of the electromagnetic

spectrum. One example is "lidar", which uses ultraviolet, visible, or near infrared light

from lasers rather than radio waves.

Lidar (also written LIDAR, LiDAR or LADAR) is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. Although thought by some to be an acronym of Light Detection And Ranging,[1]the term lidar was actually created as a portmanteau of "light" and "radar".[2][3] Lidar is popularly used as a technology to make high-resolution maps, with applications ingeomatics, archaeology, geography, geology, geomorphology, seismology, forestry,remote sensing, atmospheric physics,[4] airborne laser swath mapping (ALSM), laser altimetry, and contour mapping.