Tow vehicle depth verification

NRL demonstrated extraction of accurate single beam and multibeam bathymetry from a towed vehicle designed to locate mines in the water column. However, biases were encountered in measuring the static pressure depth of the moving vehicle. Water depth is calculated by simply adding tow vehicle depth, measured by a pressure sensor, to the multibeam ranges from the seafloor, measured acoustically from the vehicle. Comparisons of the tow vehicle bathymetry with ground truth showed a shallow bias in depth. The pressure sensor was suspected based on previous experience measuring multibeam bathymetry on the unmanned semi submersible ORCA. There were two contributing sources of error. The first was pressure sensor calibration; the sensor needs to be "zeroed" before entering the water. The second and more interesting was that the sensor was measuring variations in pressure caused from vehicle velocity in addition to changes in vehicle depth. The challenge is measuring static depth from a moving vehicle. The acoustic signal from the first water surface bounce was used to measure the bias and correct for pressure sensor error. This paper describes the sensors, data and techniques used to compensate for the depth bias.