Bistatic invariance for active sonar systems

Abstract

The concept of an invariant time‐frequency relationship for passive acoustic intensity has been established through the consideration of interfering normal modes traveling in a shallow water channel. This formulation has received considerable attention in the past and has been applied to the interpretation of passive lofargrams. For bistatic active geometries, the acoustic energy propagates along two distinct pathways: source‐to‐target and target‐to‐receiver. In recent work we proposed that this more complicated propagation may also exhibit an invariant structure, thus suggesting potential for improved target tracking through the application of invariant‐based track constraints. In this work, further exploration of this phenomenon is presented, with specific attention to the impact of the target scattering matrix that couples energy at the target between incoming and outgoing modes. Simulation results are presented for a variety of underwater environments to examine the effect on the invariant structure. Results are also presented and discussed from spectrograms produced from active sonar data obtained in shallow water environments.