[CA] [CA11] AIS false positive alarms

[heysteve]

I have an AIS700 with all the latest updates and notice that in high traffic areas I get lots of false positive alarms. I believe the reason is the alarm will activate if the traffic CPA is less than a configured range OR the TCPA is less than a configured interval. Do I understand this correctly?

The result of the OR condition is that I often get alarms for close traffic (small CPA) but with a TCPA in the hours range. For example, I recently passed behind an anchored freighter (about .1 nm) with a TCPA of over 4 hours and yet still got an alarm. As a result, I must set my CPA alarm distance setting to the minimum (.1 nm) when in fact I'd like to use a larger value.

Is this expected behavior? Is there some other way to prevent these nuisance alarms?

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum Steve,

Q1. I believe the reason is the alarm will activate if the traffic CPA is less than a configured range OR the TCPA is less than a configured interval. Do I understand this correctly?
A1. This is indeed correct.

Q2. Is this expected behavior? Is there some other way to prevent these nuisance alarms?
A2. This feature is detailed within Chapter 11.9 Dangerous targets alarm of the LightHouse 3.9 Advanced Operation Instructions. This indeed the expected behavior as the minimum CPA has been violated. The Time to reach safe distance configuration setting is simply configuring how much advance warning the operator may have. Unfortunately, close quarters operation is the nature of the beast many high traffic areas. Should the the AIS Dangerous Targets alarm be left on in such areas, then the results reported may be expected.