Time, frequency and angular dispersion modelling in the underwater communications channel

This paper considers the propagation of a communications signal through the ocean environment, and in particular, the time and frequency dispersion imposed on the signal during its passage through the environment, for dynamic transmitter-receiver geometries. Dispersion of the propagating signal is governed by the inhomogeneous sound speed within the ocean, together with multipath reflections from the surface and from the sea bed. Additional fluctuations are imposed by the motion of the ocean itself; for example from tidal currents and internal waves. In order to represent all of these effects, a new model of acoustic propagation through the complex ocean environment has been developed. This is based on a 3D ray trace through a moving ocean, where the sound speed can display strong variation both with depth and in the horizontal plane. The ray tracer is extended to incorporate diffraction effects. The ray trace is particularly unusual in allowing dynamic transmitter and/or receiver geometry, with both platforms allowed to follow arbitrary trajectories. The modeling is deterministic, so that the results are reproducible and dominant factors contributing to the dispersion may be assessed. The model is used to examine the time varying channel transfer function and scattering function in a number of scenarios. Of particular interest is the case of a shallow water zone, where severe multipath conditions are encountered-around 20 distinct eigen-rays are found, many of which travel through the underlying sediment. The influence of this environment on the signal dispersion is examined