"RATS", a Relative Acoustic Tracking System developed for deep ocean navigation

Long baseline (LBL) acoustic navigation techniques have traditionally been used to find the position of vehicles or towed systems in the deep ocean. LBL techniques compute the vehicle position by triangulation, based on measured acoustic ranges to fixed acoustic transponders. There is a considerable motivation to develop an accurate relative navigation system, whereby the subsea vehicle position may be calculated with respect to the surface ship, based on a range and bearing measurement, thus eliminating the need for bottom transponders. The vehicle's position may then be found in world coordinates, by adding the relative position of the vehicle to the GPS position found for the ship. The errors associated with relative navigation are primarily angular in nature, thus making it difficult to achieve sufficient accuracy at long ranges to satisfy the survey requirements. This paper describes RATS (Relative Acoustic Tracking System), which was developed by the Woods Hole Oceanographic Institution specifically for determining the position of the TOSS deep towed imaging vehicle system, operated by the Naval Oceanographic Office. RATS utilizes wide band signaling techniques with DSP (digital signal processor) implementation, combined with six axis motion compensation to obtain high accuracy relative positioning of the towed system with respect to the surface ship. A complete description of the system, as well as field results from deep ocean operations, is presented.