Shallow-water wideband low-frequency synthetic aperture sonar for an autonomous underwater vehicle

Synthetic aperture sonar (SAS) has attracted intensive attention for its property of high spatial resolution independent of range. However, the SAS performance in shallow water can be severely degraded due to multipath interferences. This paper presents a wideband low-frequency synthetic aperture sonar system carried by an autonomous underwater vehicle (AUV). With a small-size array of vertically displaced hydrophones, the so-called steered robust Capon beamforming technique is utilized to separate the direct echo from the multipath signals. The sector factorized back-projection (SFBP) algorithm developed for fast SAS image creation has been improved to be suited to arbitrary synthetic apertures. And the motion trajectory estimated by the AUV navigation system can be directly utilized for synthesizing a large aperture. The effects of Doppler mismatch on the SAS image can be neglected after motion-model-mismatch compensation is carried out. The prototype SAS system underwent a number of lake tests and data processing results verified the performance improvements on output image quality in terms of both ghost target removal and contrast enhancement.