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 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 paper, the effects of multipath (or target highlight structure) and Doppler on the correlation process for rectangular weighted large TW-product LFM waveforms are examined. When the received echo contains no multipath, the correlator peak output is a maximum for the reference channel that is closest in Doppler to the target. However, in a multipath environment, the correlator output peak does not generally occur at the correct Doppler reference channel. The spreading of the cross-ambiguity function as a result of Doppler mismatch and multipath is calculated in terms of the delay-Doppler coordinates and magnitude of the extrema. It is shown that, under multipath conditions, correct estimates of target parameters can be recovered through integration of the correlator output.