Abstract:

Harbor porpoises (Phocoena phocoena) generate ultrasonic echolocation signals in the 110--150 kHz frequency range and social calls below 20 kHz. The high-frequency ``clicks'' can be used to acoustically localize and track these animals in three dimensions at ranges within 500 m. In addition, the lower-frequency social calls may be used for longer-range localization. This presentation describes the techniques and methods for passively tracking harbor porpoises using these echolocation clicks and social calls. The approach is based on a time-difference-of-arrival (TDOA) direction finding and line-of-position (LOP) fixing from two arrays spaced 60-m apart. Each bottom-mounted array consists of four hydrophones in a 1-m tetrahedral configuration cabled 300 m to shore. Porpoise signal characteristics such as frequency, bandwidth, pulse rate, source level, and beamwidth and their effects on tracking are reviewed. Various components of the tracking system such as the hydrophone array, signal-conditioning electronics, underwater cable, recording, and postprocessing systems, and the tracking algorithms are described. Explicit localization solutions are presented for this array geometry and performance bounds are given. Tracking results are presented using both the low- and high-frequency signal types received from various ranges and bearings during a one-week experiment in the Gulf of Maine. [Work supported by NMFS and the URI/NOAA CMER Program.]