Target Strength Measurements in Shallow Water

A submarine on patrol needs to know how far it can be detected by active sonar. In order for such sonar performance calculations to be useful the target strength (TS) of the submarine must be known. Measurements done in deep water indicate that TS can vary as much as 20 dB as a function of aspect angle. This corresponds to a doubling of the detection distance in a noise limited environment, and up to a ten-fold increase in a reverberation limited situations. Hence, it is crucial for a submarine to know its TS and the angular dependence of it, in case it is subject to Anti Submarine Warfare (ASW) with active sonar. The Royal Swedish Navy (RSN) has recognised the need to measure TS for its submarines, but has hesitated to initiate a measurement program due to the huge problems of accurate measurements in shallow water. Attempts to measure TS have been made, but with poor results, mainly due to the influence of surface and bottom scattering. However, FOI has now proposed a new procedure where we use modern signal processing and antennas to suppress unwanted echoes. The method uses a transmitter with both horizontal and vertical directivity, and a line receiver with a narrow lobe. Broad-band pulses are used in the measurements. The result is that unwanted echoes from surface or bottom objects can be suppressed, giving more accurate values for the measured TS. The problem with transmission loss is solved by mounting reference hydrophones on the submarine. By analysing the signal from these hydrophones also the aspect angle of the submarine is determined. We have also developed a computational tool for modelling and analysis of the measurements. The purpose of this tool is to support planning and configuration of TS measurement campaigns as well as post-trial analysis of data from such campaigns including, in particular, identification and elimination of environmental influences on the measurement results.