Measurement of the coherence and fading of long-range acoustic signals

A series of underwater acoustic experiments has been performed to measure the effects of fading and partial coherence upon array performance after long-range transmission through the ocean. Measurements were made of the complex demodulates of signals that were generated by a bottom-fixed source off Eleuthera operating at 367 Hz and received by the hydrophones of a bottom-fixed array in the Bermuda area about 700 nmi away. The fading spectra of the signals showed a strong modulation at the ocean wave frequency of 0.08 Hz that is a distinctive characteristic of surface scattering. Slower variations were observed in the data that are probably due to changes in the scattering by the inhomogeneities of the sound velocity structure of the ocean caused by temperature change. The scattering strongly affected the coherence properties of the multipath signal and there was a severe degradation in the signal gain of the array that was obtained with plane-wave steering. The degradation was due to both the distortion of the average signal wavefront from planarity and the reduced level of coherence over the aperture of the array. The severity of these two effects was found to increase for averaging times sufficiently long to include the slower changes that were found in the scattering.