Time-delay estimation techniques applied to the acoustic detection of jet aircraft transits

Time-delay estimation techniques are used to observe the transits of jet aircraft. Noisy time-delay estimates are rejected by using a coincidence (or Boolean) detector. Cross correlating the beamformed outputs of two arrays of receivers leads to a significant improvement in detection performance when compared with the results of cross correlating the outputs of two single receivers. During the passage of a jet aircraft overhead, the differential time of arrival of the acoustic wave fronts at the receivers varies rapidly with time, and so the integration time for each time-delay estimate is required to be short. Also, the differential (Doppler) time scale compression between the received signals is required to be zero prior to cross correlation, otherwise the time-delay estimates will be in error. A digital time series interpolation technique, which is implemented using the discrete Fourier transform, enables the receiver waveform having the larger Doppler compression to be expanded (or time dilated) so that it has the same time base as the waveform from the other receiver. When the time scale factors of both receiver waveforms match, the cross-correlation function attains its global maximum value at a time displacement corresponding to the correct time delay. (C) 1999 Acoustical Society of America. [S0001-4966(99)03105-7].