Unified Underwater Acoustic Localization and Sound Speed Estimation for an Isogradient Sound Speed Profile

This article introduces an iterative technique aimed at jointly estimating target location and sound propagation speed in a 3-D underwater environment characterized by an isogradient sound speed profile (SSP). The developed algorithm is based on the modified polar representation (MPR) framework and offers a unified solution for underwater near and far-field target localization. This solution enables precise identification of the source position in the near field and estimation of the direction of arrival (DOA) in the far field. Prior MPR-based solutions typically assume sound propagation along a straight path at a known, constant speed within the medium, an assumption which fails in complex underwater environments, resulting in excessive distance estimation bias and thus impacting positioning accuracy. Our article resolves this concern by introducing compensation for the stratification effect of the SSP. The proposed algorithm first employs a constrained weighted least squares (WLSs) approach, incorporating fixed weight matrices and constraint terms to generate an initial solution. Subsequently, through stratification compensation and iterative refinement, the accuracy of source localization is enhanced. Extensive simulations, conducted using BELLHOP, demonstrate that the positioning algorithm can attain Cramér–Rao lower bound (CRLB) accuracy under moderate noise levels. Its superior performance in challenging underwater environments is further validated through comparative analysis with several existing algorithms.