Results of a multi-static synthetic aperture sonar experiment

It has long been recognized that point targets in synthetic aperture sonar (or radar) imagery could be used to calculate the beamformer coefficients and the motion compensation functions for the system. One difficulty with this approach arises when no point targets are available. Another problem exists in the initial recognition of true point targets, or rather, the automatic separation of point targets from complex targets in the raw data. A multi-static synthetic aperture sonar (MSSAS) laboratory experiment, instructive to practical field applications, was conducted at Naval Surface Warfare Center, Dahlgren Division, Coastal Systems Station (CSS) Acoustic Test Facility (ATF) in December 2000. The experimental method and three dimensional imagery results are described. Practical field system designs suggested by this experiment are explored. Hyperbolic Frequency Modulation (HFM) transmissions in the 5-17 KHz band were sent to a vertical array that was four wavelengths high at 10 KHz. A horizontal aperture of 40 wavelengths (at 10 KHz) was synthesized. A bullet shaped shell, approximately 32 inches long, was used as the target. A simplified self-calibration technique is harnessed to simultaneously form the basic multi-static beamformer kernel, remove motion effects, and resolve the unknown distances among transmitter, receivers, and target.