VMS (Vessel Monitoring System) data tracks vessels similarly to AIS, but this data has historically been restricted to government regulators or other fisheries authorities.
Several countries have decided to release their VMS publicly.
VMS systems transmit positions at set intervals (every ten minutes to just once every four hours), and some systems allow operators to increase transmission frequency when needed.
Some vessels broadcast both AIS and VMS. Because both systems include the same basic information (time, latitude, longitude, course, speed), they can be combined to give a higher resolution vessel track.
A Blue Box is installed on the ships. The ship determines its position by receiving GPS signals, which are sent from the blue box via satellite to the monitoring center. The state authority then controls the fishing area and port calls.