Extended Towed Array Measurement: Beam-domain phase estimation and coherent summation

PAssive Synthetic Aperture (PASA) is a technique of increasing the length of a small array artificially through array towing motion and signal processing. This allows the user to obtain the advantage of a long array while maintaining the maneuverability of a short array. In recent years, there have been keen interests in improving the ETAM technique to overcome its disadvantage. In this paper, the Extended Towed Array Measurement (ETAM) technique [1] for passive synthetic aperture processing is investigated. It is shown that the overlap-correlator of ETAM is not effective in the presence of more than one source. A new approach of the overlap-correlator is proposed. This approach performs the phase correction factor estimation in the beam domain by correlating the beamformed output of the overlapped sections of the array. Using this approach, the technique is applied to simulated data and is shown to successfully detect two sources using only a single frequency band of processing. In addition, the new technique performs coherent summation of the synthetic aperture in the beam domain in a sub-aperture sense, instead of beamforming only after the array has been phase compensated and fully extended. The former approach ensures reduced sidelobes due to the effect of windowing at each sub-aperture beamforming step. This study has resulted in a beam domain PASA technique that overcomes original ETAM´s problem of incorrect phase compensation in the presence of multiple sources.