page_title: Sonar
section_title: Scattering
hierarchical_section_title: Sonar / Performance factors / Scattering
caption_attribution_description: English: A large round metal target is within 2 metres of a sonar, trying to detect it in bubbly water. The upper panel shows the image from standard sonar: scatter from the bubbles clutters the image, hiding the true target. The lower panel shows how TWIPS (Twin Inverted Pulse Sonar) can readily find the target in the same bubbly water. From Leighton, T.G., Finfer, D.C., White, P.R., Chua, G-H. and Dix, J.K. (2010) Clutter suppression and classification using Twin Inverted Pulse Sonar (TWIPS), Proceedings of the Royal Society A, 466, 3453-3478 (doi: 10.1098/rspa.2010.0154)
context_page_description: Sonar is a technique that uses sound propagation 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 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. Sonar may also be used for robot navigation, and SODAR 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 to extremely high. The study of underwater sound is known as underwater acoustics or hydroacoustics.
  The first recorded use of the technique was by Leonardo da Vinci in 1490 who used a tube inserted into the water to detect vessels by ear. It was developed during World War I to counter the growing threat of submarine warfare, with an operational passive sonar system in use by 1918.
context_section_description: When active sonar is used, scattering occurs from small objects in the sea as well as from the bottom and surface.  This can be a major source of interference. This acoustic scattering is analogous to the scattering of the light from a car's headlights in fog: a high-intensity pencil beam will penetrate the fog to some extent, but broader-beam headlights emit much light in unwanted directions, much of which is scattered back to the observer, overwhelming that reflected from the target ("white-out").  For analogous reasons active sonar needs to transmit in a narrow beam to minimize scattering.
   
  The scattering of sonar from objects (mines, pipelines, zooplankton, geological features, fish etc.) is how active sonar detects them, but this ability can be masked by strong scattering from false targets, or 'clutter'. Where they occur (under breaking waves; in ship wakes; in gas emitted from seabed seeps and leaks etc.), gas bubbles are powerful sources of clutter, and can readily hide targets. TWIPS (Twin Inverted Pulse Sonar) is currently the only sonar that can overcome this clutter problem.
   This is important as many recent conflicts have occurred in coastal waters, and the inability to detect whether mines are present or not present hazards and delays to military vessels, and also to aid convoys and merchant shipping trying to support the region long after the conflict has ceased.
caption: Comparison of Standard Sonar and TWIPS in finding a target in bubbly water. Adapted from ref.[36]
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