Improved time delay estimates of underwater acoustic signals using beamforming and prefiltering techniques

Abstract

The cross‐correlation method for estimating the differential time delay for an underwater acoustic signal to arrive at two spatially separated receivers is described. The output amplitudes of the two receivers are cross correlated and the lag time at which the cross‐correlation function peaks provides an estimate of the time delay. Often the time delay estimates of the signal are corrupted by the presence of noise. By replacing each of the (omnidirectional) receivers with an array of receivers and then cross correlating the beamformed outputs of the arrays, it is shown that the effect of noise on the time delay estimation process is substantially reduced. Both conventional and adaptive beamforming methods are implemented and the advantages of array beamforming (prior to cross correlation) are highlighted using both modeled data (for tutorial purposes) and experimental data. The performance of the cross correlator is further improved by using various prefiltering techniques to process the experimental data. Each of the prefilters—Hannan‐Thomson, smoothed coherence transform, and phase transform—consists of a frequency‐domain weighting function that is applied to the cross‐spectral density function before the cross‐correlation function is computed using the inverse Fourier transform. The prefilters are observed to enhance the estimation of the time delay by reducing the ambiguity associated with detecting the peak value of the basic (unweighted) cross‐correlation function, which is oscillatory in nature.