In Situ Array Calibration for Synthetic Aperture Sonar

The focusing of a synthetic aperture requires precise phase alignment of signals that are combined coherently to form an image. Synthetic aperture sonars are typically operated using multi-channel receiver arrays to maximize the achievable area coverage rate, which places strict requirements on calibration accuracy of the acoustic elements. Conventional sonar calibration is a time-consuming process requiring specialized facilities, and array calibration may change due to environmental conditions such as temperature and operating depth. In this paper, we describe an in situ calibration technique for an array installed in an autonomous underwater vehicle using backscatter from diffuse targets on the seabed as the calibration source. The technique includes compensation for vehicle motion and and range-variant corrections for bistatic geometry and phase curvature of the received wavefield. Results are presented for a modular receiver array with an installation-induced phase error that caused image blurring and grating lobes. The calibration technique quantified the installation offset and restored the imagery to its properly focused state. A simulation of the sonar impulse response is also presented to confirm that the measured phase error was the root cause of the observed image artifacts.