Location Tracking of Ocean-Current-Related Underwater Drifting Nodes Using Doppler Shift Measurements

Ocean exploration is typically interpreted with reference to the location of a data collecting node, e.g., when reporting an event occurrence, or the location of an object itself. Underwater tracking (UT) is somewhat different from tracking using RADAR or radio-frequency signals, due to irregularities of the ocean current and depth-varying sound speed in water. Sound-speed uncertainty also makes it challenging to incorporate Doppler shift measurements into UT. In this paper, we present a new UT scheme which considers the above challenges and utilizes spatial correlation of ocean current to estimate the drift velocity of the tracked node (TN) as a combination of the drift velocities of anchor nodes. We also offer two types of unbiased confidence indexes aimed to control the use of drift velocity estimation. To evaluate the performance of our UT scheme, we employ a hybrid simulator that combines numerical models for the ocean current and the signal-power attenuation in the ocean. We also report results from two sea trials conducted in the Mediterranean Sea and in the Indian Ocean. By tracking the sound speed, and utilizing Doppler shift measurements and drift velocity information of anchor nodes, accuracy is significantly improved.