AIS signal loss is the apparent disappearance of a target, defined as the end of received or displayed reports from that vessel. This does not mean the ship has physically vanished or deliberately switched off its transponder; rather, the graphical display may show a lost-target symbol at the latest reported position when reports from a target within a preset distance are no longer received.
Shipborne AIS functions within VHF/FM range. The International Maritime Organization cites 20 to 30 nautical miles as a typical value at sea, depending on antenna height, but this is not a guaranteed range. Local conditions, terrain, and station placement affect reception, while repeaters can extend coverage.
Not every vessel carries AIS, and fitted equipment may be switched off in limited circumstances, so the system does not present a complete traffic picture. If AIS detects its own malfunction, the built-in integrity test provides an alarm and the unit should stop transmitting. A failed or absent connected sensor can instead produce a not-available data value.
