TRACKING THE MOTION OF SEA-ICE BY CORRELATION SONAR

A prototype correlation sonar has been developed for the measurement o f ice motion in polar seas. It operates in the very shallow-water mode as a two-pulse spatial correlation log. The design was guided by nume rical signal simulations, which justified the implementation of first- difference filtering of the signal prior to cross correlation. Four co mponents of velocity are measured separately and combined in optimal l east squares fashion. Controlled trials in air and water have clearly demonstrated the acquisition of useable interference patterns from sim ulated ice targets. The prototype has been deployed under moving sea i ce during three winters in the Beaufort Sea, using four pulse interval s between 0.08 and 10 s that provided a speed range of 50 cm s(-1). Th e observations have been evaluated in relation to independent ice spee d and topography measurements by Doppler and ice-profiling sonars inst alled nearby. Useable data were obtained about 80% of the time over wi de ranges in the speed and character of the ice target. Except for ver y low speeds, the estimates by correlation were noisy relative to Dopp ler determinations. This characteristic was traceable to the nature of the operating algorithm, to the design of the receiving antenna, and, at times, to the highly specular character of the echo from the ice. Only about 1% of data loss was attributable to reasonable failures of the operating algorithm. Within the remaining fraction, the incidence of loss increased with increasing displacement of the interference pat tern across the antenna between pulses. This is consistent with a decr easing ability to track the interference pattern using a linear array of hydrophones when pattern displacement transverse to the array excee ds the pattern decorrelation scale. In future development of the corre lation sonar for this application, it is recommended that the design o f the transmitting and receiving arrays be modified to reduce the inci dence of tracking failures, that all hydrophones in the receiving ante nna be operated simultaneously, that the operating mode be converted f rom a spatial to a temporal correlation concept, and that the dynamic range be extended. With these enhancements, the correlation sonar will be an effective tool for ice observation in polar seas.