There are two general types of vessel tracking systems – transmitters and loggers. Transmitters use satellite tracking to continuously transmit vessel locations. The Automatic Identification System (AIS) and Vessel Monitoring System (VMS) are both transmitters. Loggers record location and transmit later. Lots of new technologies are continuously being developed. These include spot trackers with satellite communications ability, options with AIS and satellite communication capability, such as Hali from ORBCOMM, and solutions using cellular phone technology.
AIS and VMS are two different and complementary vessel tracking systems that Global Fishing Watch uses to track global fishing activity. AIS is required for vessels over a certain size by international law, is publicly available and was designed as a collision avoidance tool for vessels. VMS requirements are at the national level. Data have traditionally been proprietary to the national government and VMS has generally been used to track vessels authorized to fish within the exclusive economic zone (some of these may be foreign flagged) and to track the national distant water fleet. AIS systems allow one way communication from the vessels to the satellite or terrestrial receivers they are sending to, while VMS allows back and forth communication. The other key difference between AIS and VMS is in temporal resolution – VMS systems are set up to broadcast locations at consistent intervals from 10 minutes to a few hours, depending on the system, while AIS broadcasts every few seconds, but the number of positions we receive varies depending on satellite reception.
Other satellite detection technologies allow us to generate snapshots of where vessels may be, even if we can’t track and identify them. Three of these technologies are Optical, Visible Infrared Imaging Radiometer Suite (VIIRS) and Synthetic Aperture Radar (SAR). Optical imagery allows us, on cloud free days, to count vessels at sea. Coastal areas are currently imaged frequently by medium resolution optical systems. VIIRS satellite uses highly sensitive optical sensors to see lights at night, allowing us to identify vessels that use light to attract catch, such as most industrial squid vessels and some types of purse seines. SAR uses radar technology to identify the presence of vessels and has the advantage that it is not limited by clouds.