What is a SONAR? 
Sonar (originally an acronym for SOund Navigation And Ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect 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 of acoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. continue..

Sonar is a system that uses transmitted and reflected underwater sound waves to detect and locate submerged objects or measure the distances underwater. It has been used for submarine and mine detection, depth detection, commercial fishing, diving safety and communication at sea. The Sonar device will send out a subsurface sound wave and then listens for returning echoes, the sound data is relayed to the human operators by a loudspeaker or by being displayed on a monitor. As early as 1822, Daniel Colloden used an underwater bell to calculate the speed of sound underwater in Lake Geneva, Switzerland. This early research led to the invention of dedicated sonar devices by other inventors. continue..

Single-beam sonar usually consist of a combined transmitter / receiver element which usually is mounted pointing straight down toward the bottom. It occurs also other applications, such as forward mounted to detect obstacles or up mounted in underwater vehicles, for example, to detect water or for operations under the ice. Such sonar has a relatively large opening angle, preferably 15-30 °, which leads to the footprint of the base is large, and one loses when details of the terrain. These these units can operate on different frequencies, and 38 KHz have been a common frequency, combining good range with relatively small transducers. Low frequencies causes the sound needs something into the ground. This is exploited by special sonar that is designed to look down in the first few meters below the bottom. These are called bottom penetrating sonar.
Multi-beam sonar continue..

Lewis Nixon first invented the Sonar listening device in 1906. Sonar is used to transmit and reflect sound waves under the water. It can be used to find items that have sunk underwater. continue..

Sound navigation and ranging, or sonar, is an umbrella term for technology and methods that use sound to detect objects, usually underwater, much the same way bats rely on sound for catching food in the air. The basic principles on which sonar relies are that sound moves at a steady rate through a given medium, such as air or water, and that certain types of objects produce certain types of sounds. With this knowledge, calculations can determine the distance to an object and identify it with reasonable certainty. The Navy mainly uses sonar to detect vessels, torpedoes and mines. Commercial applications for sonar include navigation, mapping and locating fish. continue..

Echolocation, also called biosonar, is the biological sonar used by several kinds of animals. Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects. Echolocation is used for navigation and for foraging (or hunting) in various environments. Echolocating animals include some mammals and a few birds; most notably microchiropteran bats and odontocetes (toothed whales and dolphins), but also in simpler form in other groups such as shrews, one genus of megachiropteran bats (Rousettus) and two cave dwelling bird groups, the so called cave swiftlets in the genus Aerodramus (formerly Collocalia) and the unrelated Oilbird Steatornis caripensis. continue..

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