AIS signal loss alerts operators that expected reports have stopped while the cause remains unknown. This status indicates a gap in data rather than a confirmed absence of the target.
On a graphical AIS display, a lost-target symbol typically appears at the latest reported position when reports from a target within a preset distance are no longer received. The system flags this interruption to support situational awareness, prompting operators to verify the contact through other means.
The absence of an AIS target does not establish the absence of a vessel; lookout, radar and all other safety-relevant information remain necessary. Not every vessel carries AIS, and fitted equipment may be switched off in limited circumstances, so AIS 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. Operators must treat these indicators as prompts for corroboration rather than definitive proof of a vessel's status.
