Bistatic sonar

Bistatic sonar is a sonar configuration in which transmitter and receiver are separated by a distance large enough to be comparable to the distance to the target. Most sonar systems are monostatic, in that the transmitter and receiver are located in the same place. A configuration with multiple receivers is called multistatic.

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 farther 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_pt (from projector to target) and TL_tr (from target to receiver).

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_pt + R_tr . As the projector is separated from receiver by R_pr distance, first R_pr /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 τ 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.

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 affected not by target features only, but also by sound wave interaction between the target and surrounding bottom.

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°.

This is the sonar with a small bistatic angle. In other words, both the range from projector to target R_pt and from target to receiver R_tr is much greater than the distance from projector to receiver R_pr.