Layered ocean sound speed model improves acoustic seafloor positioning precision

GNSS-A positioning, which combines satellite positioning of a sea-surface platform with underwater acoustic ranging, is essential for monitoring seafloor tectonic movements and improving global reference frames. However, the ocean's sound speed varies with depth and time in complex ways, creating errors that have long limited positioning accuracy. Researchers have now developed a layered sound speed gradient model that captures how horizontal sound speed variations differ across depth layers. By jointly estimating seafloor transponder coordinates, time varying sound speed perturbations, and layer specific gradients, the new method significantly reduces positioning errors. In simulations, the approach reduced positioning errors from the decimeter scale to the millimeter range. Field experiments further demonstrated improved acoustic residuals, stable positioning repeatability, and strong consistency with independent GARPOS solutions, offering a more reliable tool for seafloor deformatio