High resolution spectral analysis and the acoustic remote sensing of ocean current velocity

Doppler sonars use the perceived shift in carrier frequency between the outgoing pulse and the returning echo to make an estimate of scatterer radial velocity at several ranges of interest. Scatterers suspended neutrally buoyant in the water column are used as tracers for the remote sensing of water mass motion. Inherently, the problem is one of high resolution (distinguishing small differences in velocity) spectral estimation across segments (range bins) which are too short to provide the desired resolution via conventional methods (FFT) of spectral analysis. In this paper, the application of pth-order autoregressive time series analysis models to the estimation of the time-evolving Doppler sonar reverberation power spectrum is reported. First, with the aid of a reverberation model, it is shown that the spatial transfer function characteristics of a transducer have a significant influence on the time-evolving shape of the reverberation power spectrum. Thus, it is suggested that the use of first-order models for the purpose of Doppler shift estimation simply is inadequate. Then, the results of an exploratory analysis of pings from a 67 kHz Doppler sonar are presented which appears to confirm this caution.