Autosync works by exposing the Receiver and Transmitter to GPS or GNSS signals at the start of the mission, and optionally at intervals during the mission. This exposure sets the timebase of the Receiver or Transmitter to match GPS time. Satellite acquisition normally occurs within 30 seconds of powerup. In the canonical case of an ROVL Receiver sited near the ROV operator and an ROVL Transmitter mounted on the ROV, the satellite acquisition will normally occur in the background while the ROV is being readied for launch and while the ROVL Receiver IMU calibration is being done.
The Receiver must be out of the water (or at the surface) and in view of satellites at the start of the mission. This is often the time when IMU calibration maneuvers are being done, so you may not ever find this to be a constraint. Once satellites are acquired, the Receiver can be put into the water. You can tell when satellites are acquired and the Receiver is ready to submerge in one of two ways: (1) monitor the $USRTH message, and when the Last Sync field is no longer “-1”, satellites have been acquired; (2) If using the CeruleanTracker application, a status indicator will show on the dashboard tab of the main window, and the $USRTH window will show the value of the Last Sync field. It’s a good idea to give the unit about 10 seconds after satellite acquisition before submerging it so the GPS can stabilize.
The Transmitter must be out of the water (or at the surface) and in view of satellites at the start of the mission. Once satellites are acquired, the Transmitter can be submerged. You can tell if the Transmitter has a satellite lock because the Transmitter will not start transmitting until satellites are acquired. You can hear the Transmitter click once per second and feel the ping impulse if you put your hand on the Transmitter. It’s a good idea to give the unit about 10 seconds after it starts clicking before submerging it so the GPS can stabilize.
Transmitter and Receiver satellite acquisition can be done independently, i.e., they do not have to happen at exactly the same time, nor in the same location. Either unit can be reset or power-cycled during the mission, and after satellites are acquired again, the system will resume at full accuracy.
Once submerged, satellite lock will be lost almost immediately, and the Receiver and Transmitter will each start using their internal timebase to calculate the distance between units. The internal timebase is not as stable as the satellite atomic clocks and thus will immediately begin to drift. Drift will result in a growing error in the calculation of slant range between units (just like in systems without autosync). The error will typically accumulate at roughly 0.5 meter (often less) of slant range error per hour out of satellite lock. See for an example of clock drift.
At any time during the mission, either the Transmitter or Receiver can be surfaced to reset the internal timebase back to GPS time. Satellite re-acquisition usually takes less than 15 seconds. Resetting only one of the two partially but not completely will reduce the total error in slant range. Resetting both is required to return the accuracy to the best possible. It is not necessary to reset both at exactly the same time, but as with the initial sync, each internal timebase will begin to drift immediately after loss of satellite lock.