Analysis of Vernier interferometers for sonar bathymetry

Underwater bathymetry applications are based on the estimation of the echoed target spatial coordinates in order to map the sea bottom. A quick and reliable approach consists in carrying out the interferometry method. This direction-finding technique estimates the arrival angle of backscattered signals through the propagation time delay between two close sensors. This time delay can be determined from the phase difference between signals received by each sensor, but its value is limited to a 2 pi-length interval due to the computation of the complex argument operator. As the effective phase difference may exceed this range, according to spacing between sensors, a 2 pi rotation counter may be introduced within the direction-finding signal processing in order to remove the phase ambiguity. This paper introduces the Vernier principle in order to remove the 2 pi-phase ambiguity, enhancing the main assets and analyzing the best interferometer configuration to minimize the drawbacks. Mastering this technique allows the use of individual phase-angle samples for both sidescan sonars and multibeam echosounders, thus increasing the target detection capabilities by bringing out an image-like bathymetric resolution.