QUANTIFICATION OF THE OCEANOGRAPHIC CONTRIBUTORS TO TIME-VARIANCE IN THE UNDERWATER ACOUSTIC CHANNEL DURING THE DALCOMMS1 EXPERIMENT

Abstract

Underwater acoustic communication is a key enabler for civilian and military applications such as ocean sampling networks, offshore exploration, pollution monitoring and underwater surveillance (Akyildiz et al., 2005). Being able to understand and model the impact of time-varying environmental properties on the communication link could improve ray-tracing simulation fidelity which is currently one of the few tools available to test, analyze, and compare underwater communication schemes and performances. The DalComms1 sea trials took place in the summer of 2017 in an effort to better understand the impact of the environment on the underwater acoustic link performance, to test spread spectrum modulation techniques and to validate signal processing algorithms. The experiment was conducted on the Scotian Shelf and included transmissions of channel probing sequences, such as low frequency modulated and pseudo-random noise sequences, at ranges between one and ten kilometers. Other instruments such as an acoustic data current profiler, a conductivity-temperature-depth profiler, and a surface roughness measuring instrument complemented environmental data, including surface wind velocity, significant wave height and dominant wave period obtained from a nearby weather buoy and visual observations. The processing of the received signals allowed the extraction of important performance metrics such as estimated channel impulse responses, Doppler and delay spreads, as well as coherence times. Ray-tracing simulations through BELLHOP revealed an acceptable degree of agreement between the simulated deterministic result and the measurements. The behavior of the underwater acoustic channel was then investigated using a series of simulations with varying sound speed profiles, surface roughness, and relative motion between the source and receiver. The resulting means and variances were compared to the sea trial results and the series of simulations also allowed for the determination of the underwater acoustic channel’s sensitivity to a variety of environmental perturbations. The result is a better understanding of the channel’s variability and the importance of its contributors.