Channel Impulse Response Fluctuations at 6 kHz in Shallow Water

Abstract

A shallow water (∼100 m) experiment was carried out to measure the stability of forward transmissions at 6 kHz over a 6 km propagation path. The fixed source, fixed receiving array geometry enabled observing environmentally-induced fluctuations in the channel impulse response. The 64-element receiving array had an aperture of 12 m and thus a corresponding vertical angle of arrival resolution of ∼1°. Source transmissions were of duration 20 min and consisted of multiple subcomponents. Of particular interest here are the 2 kHz bandwidth, 1 s duration FM chirps which were transmitted continuously for 5 min and have been matched filtered to yield channel impulse response structure. In addition to CTDs taken in the region between the source and receiving array, a thermistor string at the receiving array site provided continuous measurements of water column temperature fluctuations. Discussed in this paper is the time-evolving structure of the channel impulse response observed from both indigvidual array elements as well as array beams which clearly show significant, environmentally-induced fluctuations over the 5 min duration of these transmissions. Also discussed is the relationship these fluctuations have to the characteristic ray path structure of the channel.