Acoustic localization with multi-layer isogradient sound speed profile using TDOA and FDOA

In the underwater medium, the speed of sound varies with water depth, temperature, and salinity. The inhomogeneity of water leads to bending of sound rays, making the existing localization algorithms based on straight-line propagation less precise. To realize high-precision node positioning in underwater acoustic sensor networks (UASNs), a multi-layer isogradient sound speed profile (SSP) model is developed using the linear segmentation approximation approach. Then, the sound ray tracking problem is converted into a polynomial root-searching problem. Based on the derived gradient of the signal's Doppler shift at the sensor node, a novel underwater node localization algorithm is proposed using both the time difference of arrival (TDOA) and frequency difference of arrival (FDOA). Simulations are implemented to illustrate the effectiveness of the proposed algorithm. Compared with the traditional straight-line propagation method, the proposed algorithm can effectively handle the sound ray bending phenomenon. Estimation accuracy with different SSP modeling errors is also investigated. Overall, accurate and reliable node localization can be achieved.