Transmit-Signal Design and Processing Strategies for Sonar Target Phase Measurement

The majority of active sonar systems detect and classify a target based on the amplitude of the received echo strength or the induced Doppler shift. However, additional classification information is available from the phase shift introduced by some targets as a result of the acoustic boundary conditions. For example, reverberation returns from the sea surface and from the swimbladders of various fish introduce an additional phase shift that is not usually present in returns from the seabed or man-made targets. Techniques based on the use of sub-band correlators are presented for measuring the phase shifts associated with certain stationary and moving targets when insonified by broadband transmissions. The performance of the target-phase measurement technique is derived for transmit signals used in existing sonar systems such as continuous wave (CW), linear frequency-modulated (LFM), and hyperbolic frequency-modulated (HFM) pulses in the presence of target or platform motion. The use of HFM signal processing with sub-band correlators to measure the Doppler shift of very slowly moving targets is presented. Modifications to sinusoidal frequency-modulated (SFM) and HFM transmission types are proposed in order to maintain a good compromise between overall detection and target-phase classification performance, i.e., between range resolution and phase measurement. Field trial results are presented for a plankton classification sonar system and a collision-avoidance system designed for operation near the sea surface.