AIS signal loss is a display gap from range, carriage, switch-off, malfunction or lost-target behaviour.
Shipborne AIS works within VHF/FM range. The International Maritime Organization gives 20 to 30 nautical miles as a typical value at sea depending on antenna height, while repeaters can extend coverage and local conditions affect reception. Antenna height, terrain, stations and radio conditions affect coverage, so this is not a guaranteed range.
Not every vessel carries AIS, and fitted equipment may be switched off in limited circumstances, so AIS does not present a complete traffic picture. The absence of a signal may simply mean the vessel lacks the equipment or has exercised permitted switch-off rights.
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. A graphical AIS 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.
