High-resolution interferometry for multibeam echosounders

Two main approaches are met today in swath bathymetry sonar systems. Interferometric sidescan sonars use the phase difference between two (or more) close receivers to estimate the arrival angle of instant signal. Multibeam echosounders (MBES) use beamforming methods (prior to amplitude or phase detection) in order to obtain an intrinsic spatial resolution and to improve the received signal to noise ratio (SNR). Here we present a comparison between the interferometry method and high-resolution techniques. Experimental results obtained from data recorded with a Simrad EM300 30-kHz multibeam echosounder are presented, showing that both methods provide a similar DOA estimation.

I. Introduction

While interferometry potentially provides the finest available information by measuring the phase difference between two receiving arrays at the time-sampling rate, classical processing used by multibeam echosounders (MBES) takes into consideration just the zero-crossing instant of the phase difference between two “sub-beams” measured from half-arrays in order to estimate the two-way propagation time for the angular direction, corresponding to beam-steering. The main drawback of this robust and widely-spread technique is that the beam angular width and repartition actually limit the spatial resolving ability of the system. In this paper, we take into account not only the zero crossing instant of the phase difference, but also all samples of the phase difference belonging to the beam footprint. This yields an improvement of the spatial resolution with the counterpart of increasing the seafloor relief variance. At the same time, it makes possible to decrease the number of formed beams.