| Estimating the target direction of arrival(DOA)is an important application of sonar,and is carried out commonly using beamforming approach.It is well known that for a straight horizontal line array(HLA)or slightly curved HLA(CLA),one can only estimate the arrival angle of the signal with respect to the array axis using onedimensional conventional beamforming.In ocean waveguide,such as shallow water,sound propagates via many multipaths normally confined in a vertical plane.To the receiver,the signal arrives with a deterministic azimuth and different elevation angles for separate multipaths.For a HLA or CLA,when the signal arrives in near end-fire directions of the array,different multipaths arrive in different beams,known as the split beam problem.In this case,the beam angle is the angle between the multipath and array axis and is not the same as the true bearing angle thus causing errors of bearing estimation.Split beam problem has been observed when applying one-dimensional conventional beamforming to the data collected from two curved HLAs deployed during the SWellEx96 experiment.In this paper,to avoid the split beam problem,we apply two-dimensional beamforming to the CLA by treating it as a planar array and estimate the azimuth and elevation angle separately at the same time.Since the CLA has a small curvature,it yields poor resolution in angle estimation.To improve the beam resolution,the existing one-dimensional deconvolved conventional beamforming is extended to two-dimensional deconvolved conventional beamforming which can be used for a planar array with an arbitrary shape.And this method is extended to CLA.The simulated data carried for SWellEx96 experiment and the experimental data both show that compared with a straight HLA,CLA can be used to estimate the two-dimensional arrival angles.And the two-dimensional deconvolved method shows a much narrower beam in both the azimuthal and vertical angle distribution with much lower level sidelobes.Also,the split beam problem found in one-dimensional beamforming output when the source appears in near end-fire directions is resolved using the proposed deconvolved method.Once the target bearing estimation measured on two CLAs is obtained,the multi-array beamforming method is utilized to locate the source in x-y plane.The simulated results show that the localization errors based on the two-dimensional deconvolved method are smaller than that of the two-dimensional conventional method. |
