What is the timing relationship between the DVL internal clock and the time of the Doppler bottom tracking measurement?
The actual measurement of velocity occurs when the transmitted sound reflects from the bottom. We take our measurement from the midpoint of the reflected signal, so the time of the DVL measurement for practical purposes is one half of the transmit pulse duration plus the one way travel time to the bottom. The length of the transmit pulse is typically 30% of the two way travel time to the sea bottom, however it can be limited by the BX command setting. The travel time in water is dependant on depth and speed of sound in water. The DVL accepts external NMEA0183 heading data up to 10Hz or uses an internal compass. The DVL takes the latest heading sensor data from the buffer just prior to time tagging the ensemble and transmitting a pulse. Therefore external heading data used in the output data string typically occurs some time before the DVL time tag. The Doppler measurement occurs after the time tag as shown below: 1/ C Cos J ms/meter x (Altitude in meters + min (0.3 Altitude in meters,
