Split-beam Range-gated Doppler Velocity Sonar for Operations at High Altitude above the Seabed

The long term accuracy (i.e., scale factor) of a Doppler Velocity Sonar for speed over seabed estimation is studied theoretically, using a simplified physical model. A classical 150 kHz piston DVL, which averages the Doppler phase over the entire beam footprint, is compared with the same piston operated in a new range-gated mode, to reduce the spread in echo strength which biases the estimation. The predicted absorption and terrain biases are lower than 0.1% for the range-gated DVL, whereas they vary between 0.3% and 1% for the classical DVL, depending on operating altitude and grazing angle. To exploit this benefit of range-gating, the direction of arrival of the echoes from the gate must be accurately estimated in the piston frame, e.g., the gate must be centered on the piston axis without bias. It is shown how this can be achieved by an improved Interferometric DVL design which splits the piston into at least two phase centers, and centers the gate using the zero-crossing of the differential Doppler phase. A 0.1% accuracy at more than 500m altitude above the seabed should therefore be achievable with the new InDVL instrument.