Dual-transponder Precision Navigation System for Synthetic Aperture Sonar

The technical details of a dual-transponder, long-baseline positioning system to measure the sway of a free towed Synthetic Aperture Sonar (SAS) are presented. The sway is measured with respect to freely deployed, battery powered, transponders which sit stationary on the seabed connected via cables to floating buoys housing high-accuracy GPS timing receivers. A T/R switch allows a single hydrophone on each transponder to alternately receive and transmit linear FM chirp signals. The time of flight of the signals is determined by matched-filtering using a DSP and transmitted to the towboat for storage in real time using RF modems. The sway information is completely independent for each sonar ping and allows the deblurring of the SAS images by post processing. A Matlab simulation predicts a worst case sway accuracy of ±1.5 cm.