DVL [1] stands for Doppler Velocity [2] Log. The DVL [1] is a sensor that reads the speed of the vehicle in the water, using the sound waves that bounce off the sea floor. This measurement includes not only forward velocity [2], but also the rate of travel in all the vehicle axis movements. The DVL [1] needs a “bottom lock”, or a read from the sea floor, to calculate the velocity [2] and return it to the listening device.