Passive synthetic arrays

Abstract

Passive‐synthetic apertures [W. Carey and N. Yen, J. Acoust. Soc. Am. Suppl. 1 75, S62 (1984)] were formed with experimental towed hydrophone data in a sound channel that supported RR and RSR transmission. These apertures were formed with lengths up to 95λ with coherent temporal processing gains approaching 0.75 of theoretical. These results and those of previous investigators [R. Fitzgerald, J. Acoust. Soc. Am. 60, 752–753 (1976); R. Williams, J. Acoust. Soc. Am. 60, 60–73 (1976)] indicate that synthetic apertures can be formed by the coherent summation of the phase‐corrected summation of either hydrophone or subaperture beams over successive time samples when the synthetic aperture length is less than the spatial coherence length and the processing time is less than the temporal coherence length. The evaluation of synthetic apertures requires comparisons with conventional and other high‐resolution techniques. Comparisons between conventional array processing and high‐resolution techniques [maximum entropy (ME) and maximum likelihood (ML) methods] are performed by use of the analytical expressions developed by A. T. Parsons (AUWE, TN 700/83) for the determination of the array, integration, and the net processing gains. Analytical comparisons between conventional and synthetic aperture arrays formed with either the same number of hydrophones or with the same effective length but a different number of hydrophones, show that, when the spatial processing gain exceeds the loss in integration gain, then the use of synthetic apertures is advantageous. [Work performed while at NORDA.]