AIS signal loss is checked by verifying range limits, equipment status, and display symbols to confirm target disappearance. 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. This gap alone does not confirm the cause.
Shipborne AIS works within VHF/FM range; IMO guidance 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. 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 an AIS target does not establish the absence of a vessel; lookout, radar and all other safety-relevant information remain necessary to check for potential hazards that are not visible on the electronic display. Cross-checking these sources ensures accurate verification of the situation.
