Matched field beamforming applied to sonar data


  • Tomas Lundström

Publish date: 2009-04-08

Report number: FOI-R--2731--SE

Pages: 77

Written in: English


  • conventional beamforming
  • nearfield
  • Bearing estimation
  • Distance estimation
  • linear array
  • spatial covariance matrix
  • matched field


Two metods for evaluating and improving plane wave beamforming have been developed. The methods estimate the shape of the wavefront and use the information in the beamforming. One of the methods uses estimates of the time delays between the sensors to approximate the shape of the wavefront, and the other estimates the wavefront by matching the received wavefront to spherical wavefronts of different radii. The methods are compared to a third more common method of beamforming, which assumes that the impinging wave is planar. The passive ranging abilities of the methods are also evaluated, and compared to a reference method based on triangulation. Both methods were evaluated with both real ans simulated data. The simulated data was obtained using Raylab, which is a simulation program based on ray tracing. The real data was obtained through a field-test performed in the Baltic Sea using a towed array sonar and emitted tones from a stationary source. The performance of the matched beamormers depends on the distance to the target. At a distance of 600 m near broadside the power received by the beamformer increased by 0,5-1 dB campared to the plane wave beamformer. At a distance of 300 m near broadside the improvement is approximately 2 dB. In general, obtaining an accurate distance estimation proved to be difficult, and highly dependent on the noise present in the environment. A moving target at a distance of 600 m at broadside can be estimated with a maximum error of 150 m, when recursive updating of the covariance matrix with an updating constant of 0, 25 is used. When recursive updating is not used the margin of error increases to 400 m.