Modeling weak fluctuations of undersea telemetry signals

Abstract

Numerical calculation of acoustic field perturbation expressions can be used to predict fluctuations after propagation through ocean sound-speed structures, but before the onset of multipath. The general form of the expressions for signal spectra or correlation functions allow numerical evaluation for an unlimited quantity of vector wave-number spectral models of refractive index. In order to help define the bounds of applicability of the theory, log-intensity fluctuation variances have been calculated for three major situations: ocean internal waves, ocean turbulence, and continuous strong large-scale turbulence. Propagation through ocean thermocline internal waves, realistically weak thermocline turbulence, and unrealistically strong turbulence show that scintillations of intensity can be predicted and understood to first order up to ranges of tens of kilometers, given the proper transmission geometry. Internal wave effects dominate over any effects from expected microstructure. Nonhorizontal transmission yields small fluctuations, but eventually refractive effects of the sound channel will contribute some additional spatial variability and multipath, complicating the use of the theory. Multipath due to the sound channel can exist at ranges where the random small-scale structures would contribute only small perturbations (no multipath from small structures)