Unsynchronized Underwater Localization With Isogradient Sound Speed Profile and Anchor Location Uncertainties

Precise underwater localization has been very challenging due to the unsynchronized clock, stratification effect, and anchor location uncertainties. Although some localization methods have been proposed, they are all based on very strong and certain impractical assumptions, such as perfect time synchronization, constant underwater sound speed, or absolutely accurate anchor location information. In this paper, we investigate the underwater localization problem under more realistic conditions. By jointly considering the isogradient underwater sound speed profile (SSP) and anchor location uncertainties, an unsynchronized localization algorithm based on time-difference-of-arrival (TDoA) measurements will be presented. The target node initiates the localization process and anchors will respond by reporting their location with the corresponding time stamps. Due to the isogradient SSP, the linear relation between the propagation delay and distance no longer exists. Therefore, we will need to iteratively linearize the observation equations and update the localization result through Newton's method. As a benchmark for performance evaluation, we derive the Cramér-Rao lower bound (CRLB). We also compare the performance of the proposed scheme with other existing methods, in terms of localization accuracy and computational complexity. Comprehensive simulations are conducted, and the results show that the proposed method can achieve the CRLB and performs better than existing solutions.