Tracking the Motion of Sea Ice by Correlation Sonar

Abstract

A prototype correlation sonar has been developed for the measurement of 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 numerical signal simulations, which justified the implementation of first-difference filtering of the signal prior to cross correlation. Four components of velocity are measured separately and combined in optimal least squares fashion. Controlled trials in air and water have clearly demonstrated the acquisition of useable interference patterns from simulated ice targets. The prototype has been deployed under moving sea ice during three winters in the Beaufort Sea, using four pulse intervals between 0.08 and 10 s that provided a speed range of 50 cm s?1. The observations have been evaluated in relation to independent ice speed and topography measurements by Doppler and ice-profiling sonars installed nearby. Useable data were obtained about 80% of the time over wide ranges in the speed and character of the ice target. Except for very low speeds, the estimates by correlation were noisy relative to Doppler 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 pattern across the antenna between pulses. This is consistent with a decreasing ability to track the interference pattern using a linear array of hydrophones when pattern displacement transverse to the array exceeds the pattern decorrelation scale. In future development of the correlation sonar for this application, it is recommended that the design of the transmitting and receiving arrays be modified to reduce the incidence of tracking failures, that all hydrophones in the receiving antenna be operated simultaneously, that the operating mode be converted from 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.