Measurement of Tidal Currents Using an Autonomous Underwater Vehicle

Small autonomous underwater vehicles (AUVs) offer advantages in ease of deployment and recovery, covertness, and minimal logistical requirements for operation; however, speed and battery capacity can limit their effectiveness in areas where there are strong currents. While measurement of currents from these platforms is enabled by use of Doppler velocity logs (DVLs) with profiling capability, the quality of these current measurements needs evaluation. A trial was conducted in the Narrows channel of Halifax Harbour in October 2019 with coinciding tidal current measurements using fixed acoustic Doppler current profilers, drifters, and an AUV equipped with a profiling DVL. Output from a high-resolution tidal current model, Halifax Finite-Volume Community Ocean Model, has also been compared. The intercomparison of the current measurements and model output shows good agreement and highlights the complex structure of the tidal flow in the harbor channel. Evaluation of the AUV/DVL current measurements shows that adequate smoothing is achieved using shorter spatial/temporal averaging windows than other investigators have reported. A spatial (alongtrack) averaging bin size of 20 m, or equivalently 13 1-Hz profiles at 3-kn speed, reduces the standard deviation of the current magnitude measurements to a stable level. A shorter averaging scale allows fine enough detail in current mapping to be useful in harbor and littoral environments with strongly localized flows.