What are the conditions that cause an ARPA alarm?

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ARPA alarms are designed to warn when something about target tracking could affect safety. The conditions that trigger an ARPA alarm are when a target is first detected inside the guard zone, when a tracked target is lost, or when the ARPA system itself fails. The guard zone is a user-defined safety perimeter around your vessel; a target entering that zone signals potential close-quarters risk and requires attention. If a target that you were tracking disappears, you lose reliable information about its course, which could hide a threat and thus prompts an alarm. If the ARPA system has a fault, the plotting and tracking cannot be trusted, so an alarm alerts you to switch to manual tracking or diagnose the issue. The other options don’t fit typical ARPA behavior: AIS data inconsistencies aren’t an ARPA alarm condition, distance thresholds aren’t used to trigger ARPA alarms, and your own speed exceeding a threshold isn’t an ARPA alarm—ARPA focuses on target tracking status rather than your vessel speed.