What will cause the ARPA to emit a visual alarm, audible alarm, or both?

The scenario where a tracked target enters your preset CPA-TCPA limits is what prompts the ARPA (Automatic Radar Plotting Aid) to emit a visual alarm, audible alarm, or both. CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach) are critical parameters for collision avoidance. When a target crosses these thresholds, it's indicative of a potential risk of collision, thus triggering the alarms to alert the operator. This is essential for maritime safety, allowing the operator to take necessary action to avoid an incident.

The other scenarios, while they involve target tracking and proximity, do not necessarily result in alarm activation according to the parameters that trigger protective measures within the ARPA system. For instance, acquiring a target that simply enters a guard zone might not be alarming if the CPA-TCPA limits are not violated, and losing a tracked target for just one radar scan doesn’t inherently warrant an alarm unless it poses a further navigational risk. Additionally, a target being initially detected within a guard zone does not trigger alarms unless it subsequently engages the CPA-TCPA limits or presents a real threat of collision.