Bistatic sonar Most sonar systems are monostatic , in that the transmitter and receiver are in the same place . Bistatic sonar describes when the transmitter and receiver ( s ) are separated by a distance large enough to be comparable to the distance to the target . Bistatic vs monostatic . Propagation ( transmission ) loss . This is a loss in sound level which happens while the sound pulse travels from projector to target and from target to receiver . There are 3 different mechanisms causing transmission loss : spherical ( or cylindrical in shallow water ) spreading , absorbing and scattering by ocean media inhomogeneities . Transmission loss ( TL ) is proportional to range , ( the longer the sound travels the more the loss ) , and to sound frequency . In monostatic sonar the sound first travels from projector to target , then the same way back from target to receiver , so two - way loss is just 2TL , where TL is one - way loss . In bistatic sonar the total loss ( in decibels ) is a sum of TL ( from projector to target ) and TL ( from target to receiver ) . Dead zone . In monostatic sonar , the first thing the receiver can hear is the sound of the transmitted ping . This sound level is very high , and it is impossible to detect the echo during the ping duration τ . That means targets are undetectable within the circle of Cτ / 2 radius , where C is sound speed in water . This area is usually referred to as “ dead zone ” . If the sonar is close to the surface , bottom or both , ( which may happen in shallow water ) , the dead zone may be greater than Cτ / 2 due to a high level of reverberation . In bistatic sonar , the travel distance from projector to target and from target to receiver is R = R + R . As the projector is separated from receiver by R distance , first R / C seconds after the ping starts , the receiver is just waiting . After that time , it receives direct signal from the projector ( often referred to as “ direct blast ” , ) which lasts Cτ seconds . So the sonar cannot detect targets within the ellipse R = Rpr + Cτ , as shown at the picture . High level reverberation in the projector area does not affect the dead zone . Target scattering pattern . Targets do not reflect the sound omni - directionally . The mechanism of sound reflection ( or scattering by the target ) is complicated , because the target is not just a rigid sphere . Scattered sound level depends on the angle β from which the target is ensonified by the projector , and it also varies with angle scattering direction α ( refer to local target axes Z { x , y } ) . These angles are often referred to as aspects . This scattered sound level vs ( α , β ) function is called the scattering pattern S ( α , β ) . Direction of maximum echo ( maximum of S ( α , β ) ) also depends on target shape and inner structure . So sometimes the best ensonifying aspect is not the same as the best receive aspect . This leads to the bistatic solution . Target scattering becomes even more complicated if the target is buried ( or semi - buried ) into sea bottom sediments . ( That happens to sea mines , waste containers , shipwrecks , etc . ) In that case , the scattering mechanism is effected not by target features only , but also by sound wave interaction between the target and surrounding bottom . Specific classes of bistatic sonars . Backscattering and forward scattering . In monostatic sonar the receiver is listening to the echo which is reflected ( scattered ) right back from the target . Bistatic sonar can work in two ways : by utilizing either the target backscattering or forward scattering . Backscattering bistatic sonar is the sonar in which the bistatic angle φ is less than 90° . Forward scattering is the physical phenomena based on Babinet ' s principle . Forward scattering bistatic sonar is the sonar in which the bistatic angle φ is greater than 90° . Pseudo - monostatic sonar . This is the sonar with a small bistatic angle . In other words , both the range from projector to target R and from target to receiver R is much greater than the distance from projector to receiver R . Multistatic sonar . This is the multi - node system with more than one projector , receiver or both . Applications . Long range surveillance . For coastal surveillance , a large receive array of hydrophones is usually deployed close to the shore and connected with cables to a land - based processing center . To enable long range target detection ( far away from the shore ) , one can use a powerful mobile projector , deployable from the ship . A system of this kind exploits the idea of “ bringing the projector closer to area of interest and getting the transmission loss down ” . Large area surveillance with a single projector and a net of receivers . A system of this type is multistatic . It exploits the idea of “ cover the area of interest with a sparse net of receivers and ensonify the whole area with a powerful projector ” . Receive nodes may be sonobuoy s ( with radio communication link to a processing center ) or autonomous underwater vehicles ( AUVs ) with an acoustic communication link . The example is GOATS project , using AUVs as receive nodes . Low frequency towed sonar . The lower the frequency , the less the transmission loss absorbing and scattering components . On the other hand , the lower the frequency , the larger the size of directional projector and receive array . So the ship - deployable long range sonar is a low frequency bistatic towed array sonar with spatially separated projector and receive array . The example is LFATS towed sonar . Buried object detection . To detect a buried object , the transmit ping must penetrate into the bottom . That requires a powerful and highly directional projector . Next , a directional receiver should be placed at the point where the “ target + surrounding bottom ” reflection is the best . This is a bistatic system . The example is SITAR project , developed to find objects like toxic waste containers and mines . The principal advantages of bistatic and multistatic sonar include : The principal disadvantages of bistatic and multistatic sonar include :