AIS (Automatic Identification System) sends a ship's position, course, and speed via VHF to other ships in the vicinity. This way any ship knows if it is on collision course with somebody else. However, VHF has a range of only 25 nm. As soon as I am out of range from a land-based receiver, Ntombifuti won't show up on the map anymore. In that case I should still be visible on my YB Tracker.