Editing Sonar (section)

== Performance factors ==
The detection, classification and localisation performance of a sonar depends on the environment and the receiving equipment, as well as the transmitting equipment in an active sonar or the target radiated noise in a passive sonar.

=== Sound propagation ===
Sonar operation is affected by variations in [[sound speed]], particularly in the vertical plane. Sound travels more slowly in [[fresh water]] than in [[sea water]], though the difference is small.  The speed is determined by the water's [[bulk modulus]] and [[mass]] [[density]].  The bulk modulus is affected by temperature, dissolved impurities (usually [[salinity]]), and [[pressure]]. The density effect is small.  The [[speed of sound]] (in feet per second) is approximately:

:4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet)) + salinity (in parts-per-thousand ).<!-- Needs rephrasing, imperial system is not an international standard! -->

This [[empirical]]ly derived approximation equation is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters there is often a marked change, called the [[thermocline]], dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, because a sound originating on one side of the thermocline tends to be bent, or [[refraction|refracted]], through the thermocline. The thermocline may be present in shallower coastal waters. However, wave action will often mix the water column and eliminate the thermocline. Water [[pressure]] also affects sound propagation: higher pressure increases the sound speed, which causes the sound waves to refract away from the area of higher sound speed. The mathematical model of refraction is called [[Snell's law]].

If the sound source is deep and the conditions are right, propagation may occur in the '[[SOFAR channel|deep sound channel]]'. This provides extremely low propagation loss to a receiver in the channel.  This is because of sound trapping in the channel with no losses at the boundaries.  Similar propagation can occur in the 'surface duct' under suitable conditions.  However, in this case there are reflection losses at the surface.

In shallow water propagation is generally by repeated reflection at the surface and bottom, where considerable losses can occur.

Sound propagation is affected by [[Absorption (acoustics)|absorption]] in the water itself as well as at the surface and bottom.  This absorption depends upon frequency, with several different mechanisms in sea water.  Long-range sonar uses low frequencies to minimise absorption effects.

The sea contains many sources of noise that interfere with the desired target echo or signature.  The main noise sources are [[wave]]s and [[shipping]].  The motion of the receiver through the water can also cause speed-dependent low frequency noise.

===Scattering===
{{See also|Deep scattering layer}}
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.
[[File:Undersea bubble clouds.jpg|thumb|Bubble clouds shown under the sea. From ref.<ref name="Leighton_et_al_Nature_2018">{{cite journal |author1=Leighton, T.G. |author2=Coles, D.C.H. |author3=Srokosz, M. |author4=White, P.R. |author5=Woolf, D.K. |title=Asymmetric transfer of CO2 across a broken sea surface|journal= Scientific Reports |year=2018 |volume=8 |issue=1 |pages=8301 |doi=10.1038/s41598-018-25818-6 |pmid=29844316 |pmc=5974314 |bibcode=2018NatSR...8.8301L}}</ref>]] 
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;<ref name="Woolf+Thorpe_1991">{{cite journal |author1= Woolf, D.K. |author2=Thorpe, S.A.  |title= Escape of methane gas from the seabed along the West Spitsbergen continental margin  |journal= J. Mar. Res.  |year=1991 |volume=49 |issue=3| pages= 435–466 |doi= 10.1357/002224091784995765 }}</ref> in ship wakes; in gas emitted from seabed seeps and leaks<ref name= "Westbrook_et_al_2009">{{cite journal |author1=Westbrook, G.K. |author2=Thatcher, K.E. |author3=Rohling, E.J. |author4=Piotrowski, A.M. |author5=Pälike, H. |author6=Osborne, A.H. |author7=Nisbet, E.G. |author8=Minshull, T.A. |author9=Lanoisellé, M. |author10=James, R.H. |author11= Hühnerbach, V. |author12=Green, D. |author13=Fisher, R.E. |author14=Crocker, A.J. |author15=Chabert, A. |author16=Bolton, C. |author17=Beszczynska-Möller, A. |author18=Berndt, C. |author19=Aquilina, A.  |title= Escape of methane gas from the seabed along the West Spitsbergen continental margin  |journal= J. Mar. Res.  |year=2009 |volume=36 |issue=15 |pages= L15608|bibcode= 2009GeoRL..3615608W |doi= 10.1029/2009GL039191 |url= http://nora.nerc.ac.uk/id/eprint/164607/1/2009gl039191%2Baux.pdf |doi-access=free }}</ref> etc.), gas bubbles are powerful sources of clutter, and can readily hide targets. TWIPS (Twin Inverted Pulse Sonar)<ref name="Leighton_et_al_2010_TWIPS">{{Cite journal | doi = 10.1098/rspa.2010.0154| title = Clutter suppression and classification using twin inverted pulse sonar (TWIPS)| journal = Proceedings of the Royal Society A| volume = 466| issue = 2124| pages = 3453–3478| year = 2010| last1 = Leighton | first1 = T. G.| last2 = Finfer | first2 = D. C.| last3 = White | first3 = P. R.| last4 = Chua | first4 = G. – H. | last5 = Dix | first5 = J. K.|bibcode = 2010RSPSA.466.3453L | s2cid = 111066936|url=http://resource.isvr.soton.ac.uk/staff/pubs/PubPDFs/2010%20Leighton%20et%20al%20(PRS)%20Clutter%20suppression%20and%20classification.pdf}}</ref><ref name="Leighton_et_al_2012_PRS_dolphins">{{Cite journal | doi= 10.1098/rspa.2012.0247| title = Do dolphins benefit from nonlinear mathematics when processing their sonar returns?| journal = Proceedings of the Royal Society A| volume = 468| issue = 2147| pages = 3517–3532| year = 2012| last1 = Leighton | first1 = T. G.| last2 = Chua | first2 = G. H.| last3 = White | first3 = P. R.| url= http://resource.isvr.soton.ac.uk/staff/pubs/PubPDFs/2012_Leighton_et_al_%20(PRS)_Do_dolphins_benefit.pdf|bibcode = 2012RSPSA.468.3517L | s2cid = 109255100| doi-access = free}}</ref><ref name="Leighton_et_al_2011_JASA_TWIPS">{{Cite journal| pmid = 22088017| year = 2011| last1 = Leighton| first1 = T. G.| title = Clutter suppression and classification using twin inverted pulse sonar in ship wakes| journal = The Journal of the Acoustical Society of America| volume = 130| issue = 5| pages = 3431–7| last2 = Finfer| first2 = D. C.| last3 = Chua| first3 = G. H.| last4 = White| first4 = P. R.| last5 = Dix| first5 = J. K.|url=http://resource.isvr.soton.ac.uk/staff/pubs/PubPDFs/Pub12669.pdf| doi = 10.1121/1.3626131| bibcode = 2011ASAJ..130.3431L}}</ref> is currently the only sonar that can overcome this clutter problem.[[File:TWIPS picture 2.png|thumb|Comparison of Standard Sonar and TWIPS in finding a target in bubbly water. Adapted from ref.<ref name="Leighton_et_al_2010_TWIPS"/>]] 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.<ref name="Leighton_et_al_2010_TWIPS"/>

===Target characteristics===
The sound ''reflection'' characteristics of the target of an active sonar, such as a submarine, are known as its [[target strength]].  A complication is that echoes are also obtained from other objects in the sea such as whales, wakes, schools of fish and rocks.

Passive sonar detects the target's ''radiated'' noise characteristics.  The radiated [[Frequency spectrum|spectrum]] comprises a [[continuous spectrum]] of noise with peaks at certain frequencies which can be used for classification.

===Countermeasures===
''Active'' (powered) countermeasures may be launched by a vessel under attack to raise the noise level, provide a large false target, and obscure the signature of the vessel itself.

''Passive'' (i.e., non-powered) countermeasures include:
* Mounting noise-generating devices on isolating devices.
* Sound-absorbent coatings on the hulls of submarines, for example [[anechoic tiles]].