Dead reckoning with an ADCP and current extrapolation

Summary form only given, substantially as follows. For many years, oceanographers have navigated vessels by dead-reckoning techniques using raw frequency shift data from a bottom-tracking Doppler sonar velocity log (DSVL) and heading data from a gyro-compass. These data have been combined through the standard Janus equations to yield the horizontal velocities of the vessel. A more accurate three-dimensional velocity can be computed by implementing a matrix formulation of the basic Doppler equations. With this method, the accuracy can be increased even more by incorporating pitch and roll data from the vessel and information about misalignments of the transducers within the DSVL. With the advent of bottom-tracking acoustic Doppler current profilers (ADCP), additional data are available to increase the robustness of this matrix formulation. These data are especially useful during time periods when the ADCP has lost beam lock on the ocean bottom but has retained beam lock on the water column. In the most basic mode, frequency shift data from the ADCP can be used to give the through-the-water velocity of the vessel by water-tracking on a reference layer. Unfortunately, this method is biased by the currents in the reference layer. A more accurate through-the-water velocity is computed when several reference layers are used simultaneously. In this case, the errors introduced by the current are averaged over a larger portion of the water mass beneath the vessel. Finally, if the ADCP is both bottom-tracking and water-tracking, the effects of local currents on the vessel motion can be computed for a sliding data window. When the ADCP loses beam lock on the bottom, the effects of the current can be added to the water-tracking velocity to extrapolate an over-the-ground velocity.