The Effect of Elevation Angle on Bearing Estimation for Array Beamforming in Shallow Water

Estimating the target direction of arrival is an important function of sonar, and is carried out commonly using beamforming. Conventional beamforming (CBF) has been used for target bearing estimation based on the direction of arrival of incoming signals but it suffers from fat beams and high-level sidelobes. Deconvolved conventional beamforming can achieve super directionality as a high-resolution beamformer and at the same time retains the robustness of conventional beamforming. When a signal arrives in near endfire directions of a horizontal line array (HLA) or slightly curved HLA, the signal energy may appear in split beams causing errors of bearing estimation. This has been observed in experimental data on a curved HLA, the north HLA, deployed during the SWellEx96 experiment. Beamforming the simulated data on this array in both azimuth and vertical angle domain, we show that both bearing and vertical angles can be well estimated using the deconvolved CBF approach. The observed target bearing wondering in near endfire directions in conventional bearing only beamforming (the split beam problem) is due to the periodically oscillating vertical angle as the source range changes. It shows that elevation angles of the multipath arrival can seriously affect the bearing estimation of a HLA in near endfire directions.