Delay-Doppler resolution performance of large time-bandwidth-product linear FM signals in a multipath ocean environment

Abstract

Active sonar systems that transmit large time-bandwidth (TW)-product linear frequency modulated (LFM) waveforms and receive echoes from targets of unknown range and speed can suffer considerable correlation losses that cannot be predicted from conventional (narrow-band) ambiguity function theory. As is well known, the theory can be modified to include the effects of Doppler distortion on large TW-product signals by correlating the received signal against a reference that is a time-compressed version of the transmitted signal. In this article, the effects of multipath (or target highlight structure) and Doppler on the correlation process for rectangular-weighted large TW-product LFM waveforms are examined. Gaussian-weighted waveforms are also considered to examine sidelobe behavior. It is shown that in a multipath environment, the correlator output peak does not generally occur at the correct Doppler reference channel. This is due to the constructive/destructive interference of the summation of complex delay-Doppler autocorrelation functions associated with each return. A summation technique that identifies the appropriate Doppler reference channel is proposed; this technique allows the target parameters to be estimated if the signal-to-noise ratio is sufficiently high.