Lowered ADCP Techniques Applied to Real-Time Navigation of a Descending UUV

We investigate the use of ADCP current profiles as a navigation aid to a descending UUV. A UUV can have a position fix from GPS at the surface, and can use bottom track for dead reckoning once it has descended to within tracking range of the bottom. The challenge is to determine the vehicle's position at the bottom, by tracking its motion through the transition from surface to bottom. The use of earthreferenced current profiles during the descent can provide an estimate of the water reference frame and therefore an estimation of the vehicle motion. GPS position and velocity combined with current profiles measured at the surface provide the first earth referenced measurement of the water reference frame. During the descent, subsequent profiles provide profile gradients that can be used to extend the earthreferenced profile to the bottom. This extrapolation of earth referenced current profiles can provide the means to estimate vehicle motion below the surface before the vehicle has reached the bottom. Once bottom track is achieved the corrected reference for vehicle motion can be used to improve the estimate of position. Feeding the water layer estimates, and eventually the bottom track, into a relatively simple Kalman filter is one way to weight the estimates of water reference layer and bottom track appropriately. This solution also provides real-time position that will allow longer distances between position fixes.