Coherent source direction estimation for three-row bathymetric sidescan sonars

In practical applications with bathymetric sidescan sonar, the multipath reflection and other directional interferences are the key limiting factors for a better performance. To deal with this problem, a multiple-row system can be used to resolve the multipath reflections from the direct signal instead of smoothing the measurements over some time and/or angle intervals as previously widely investigated. However, most current algorithms for signal direction-of-arrival estimation fail when the sources from different directions are coherent, which often occurs in a shallow water multipath environment. In the context of three-row bathymetric sidescan sonars, this paper proposes a coherent source direction estimator (CSDE) to resolve two sources from different directions. This method is compared with the ESPRIT-based approach using different sonar data models. The simulations show that CSDE works very well for highly coherent sources at the angular separation of 10° between two sources and at the signal-to-noise ratio above 10 dB while the ESPRIT-based method is quite robust for the incoherent source signal model. In addition, CSDE gives the source time series estimation on the basis of the direction estimation as well. Analyses to the interference field associated with the sidescan sonar show that a model with two or three simultaneously received signals describes even some shallow water fields very well. These preliminary results suggest that the combination of above two approaches could replace the currently used differential phase technique to get a better performance in shallow water without significant increase of the system cost