AIS Manipulation by sending out false information and AIS signals received by land stations and other vessels. False collision warnings are triggered for vessels in the vicinity, and some even influence the automatic steering system coupled to the AIS.
In coastal waters, signals from freighters sometimes appear, although the vessels were not there.
Reasons may be:
Military forces have no interest in foreign forces navigating with extreme precision within their borders.
Pirates change the position data of the vessels so that international naval forces would be misled in the event of a distress call and would thus no longer be able to prevent the vessel's capsizing in time.
Vessels with components could conceal their identity and route so that the cargo could reach its destination without anyone being able to react.
It's not without danger. Vessels with AIS systems transmit their position, course, and speed every second. Electronic nautical charts use this data to generate collision warnings, and the system contributes to safety in shipping traffic.
FleetMon tries to filter out jumps from ships and clean up the system.
AIS signals are also actively falsified.
That leads to a complete failure of GPS for the systems in the vicinity of the jammer. In this case, no position will be transmitted by GPS because there is no contact with the satellites.