The formation of a synthetic aperture with towed hydrophones

Abstract

Passive‐synthetic apertures were formed with experimental data from the coherent summation of subapertures formed from 1–4 hydrophone groups at successive time samples. The results show that the coherent signal gain for synthetic apertures with lengths up to 95 λ was a function of the number of successive time samples (Nts) described by α 10 log(Nts) with α∼0.7. The hydrophones were towed with a speed‐to‐wavelength ratio of 0.168 s−1 at a depth of 26 λ. A calibrated moored source was at a depth of 37 λ in 3200 m of water at distance‐to‐wavelength ratio of between 20 and 30 × 103 from the receiver. The stable environment was characterized by a sound‐speed profile with a negative gradient of − 0.130 s−1 to a sound channel axis of 150 m and a positive gradient of + 0.016 s−1 to a critical depth of 1825 m. The resulting propagation was RR and RSR with losses of between 90 and 100 dB. These results show that for these environmental‐acoustic conditions the synthetic apertures can be formed by the coherent summation of phase corrected subaperture beams over successive time samples as long as the synthetic aperture length is less than the single path coherence length and the synthetic aperture processing time is less than the signal's temporal coherence length. The results presented represent an extension of earlier work [R. Fitzgerald, J. Acoust. Soc. Am. 60, 752‐753 (1976); R. Williams, J. Acoust. Soc. Am. 60, 60–73 (1976)] by the demonstration that coherent gain can actually be achieved with resolution such that multipath vertical arrival angle differences can be resolved.