High Resolution Sonar Signals in a Multipath Environment

Because active sonar systems are typically peakpower limited, and because sonar targets can often appear at speeds within the doppler spread of reverberation, pulses having large time-bandwidth (TW) products are called for. The resultant high resolution in range and doppler creates a potential problem because of multipath. Multipath is caused by multiple reflections from the two rough surfaces -- the ocean surface and the ocean bottom -- from which signals are reflected, refraction, eand by multiple reflections from the target itself. Due to motions of the echo ranging ship, the target, and the ocean surface, the multipath exhibits doppler spreading as well as time spreading. A model is postulated for the overall spreading of the signal seen at the receiver in which the magnitudes of time spreading and doppler spreading are represented by L and B. Detection performance of signals with large pulse length (T) and bandwidth (W) is derived as function of WL and BT. Performance is measured in terms of input signal level required to produce specified probabilities of detection and false alarm at the processor output. Two basic signal codes are considered -- random modulation in amplitude and/or phase (RM) and linear frequency modulation (LFM). Detection performance of a single correlator, tuned to one range and doppler, is given. Next, a processor is postulated consisting of a multiplicity of correlators covering the entire expected range and doppler spread of the signal, the outputs of which are combined incoherently.