Impact of thermocline and seabed variability on underwater acoustic communications Free

Shallow water acoustic communications experiments were conducted near Kauai in July 2003 and near Capraia and Elba Islands in October 2003. All experiments took place in approximately 100‐m water depth but the oceanography and seabed types differed significantly. The Kauai site had a reflective seabed that was combined with highly variable oceanographic conditions that led to performance closely tied to source/receiver geometry. The Capraia site also has a reflective seabed but the winter conditions produced a more mixed water column with a weaker and less variable thermocline than Kauai. The Elba site had nearly the same oceanographic conditions as Capraia but the seabed is a highly lossy. In each of these experiments, signals were transmitted over many hours from fixed and moving platforms and were received at multiple ranges and depths using vertical arrays and single hydrophones. Extensive environmental measurements were made simultaneous to the acoustic transmissions (e.g., measurements of the water column temperature structure and surface wave heights). In this paper, the correlation between environmental factors and communications performance will be presented along with the predictions from modeling. The performance of both multi‐frequency shift keying (MFSK) and direct sequence spread spectrum (DSSS) signals will be discussed.