Quantifying Long-Term Accuracy of Sonar Doppler Velocity Logs

A Doppler velocity log sonar measures relative velocity between the instrument and the bottom of a body of water by transmitting acoustic pulses that are scattered off the bottom. The scattered sound is received and the Doppler shift is measured. Like any other sensor, the data quality of a Doppler velocity log can be quantified by a variance and a bias. The goal of this work is to model error sources that do not average out over time; quantify those sources under various operating and environmental conditions; and derive a statistically significant range of expected long-term errors. The following error sources are analyzed: absorption bias, terrain bias, sidelobe coupling, beam alignment, clock drift, element spacing, and speed-of-sound error. A combination of analytical derivations and simulations, where the simulation results were fit to closed-form equations, were used to construct the long-term error model. Validity of the error model was demonstrated by tank testing Doppler velocity logs of different configurations under a range of operating conditions. We envision that the error model derived in this work will be of great use to both system designers that engage in design of new instruments and instrument users that wish to understand how to achieve high navigational accuracy during missions.