Application of synthetic-aperture processing to towed-array data

A synthetic-aperture processing technique has been applied to towed-array hydrophone data and has demonstrated that significant increase in signal gain, improved angular resolution, and improved peak-to-sidelobe level ratios can be achieved. The synthetic aperture was formed by the coherent sum of subaperture beams at successive time intervals. When the signal spatial coherence was greater than the synthetic-aperture length and the signal stationary over the synthetic-aperture processing time, 85% of the achievable coherent processing gain was realized. This gain was described by (x−1)10 log(NTF), with x the coherent gain factor and NTF the number of sequential time samples. The coherent gain factor was found at 175 Hz to be 1.7±0.3 for synthetic-aperture lengths between 192 and 962 m, formed from subaperture lengths between 5 and 80 m over coherent processing times of less than 9 min. Beamwidths were found to be comparable to 58°, divided by the effective aperture length in wavelengths. Synthetic apertures, formed from subaperture lengths of 160 and 640 m, showed improvement when a tow-ship phase reference was employed and yielded x=1.4, compared to a maximum value of 2. The synthetic processing technique was also shown to function as a velocity filter sensitive to the relative velocity of the source and receiver. These results demonstrate that the formation of midfrequency synthetic-aperture arrays in the deep-ocean environment is feasible.