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CPA and TCPA Alarms Explained

CPA and TCPA Alarms Explained

AIS CPA Alarm Indicators

One of the best features of the Automatic Identification System (AIS) is the ability on some chart plotters or navigation software to set alarms that warn you if and when you will get too close to another vessel. These collision avoidance alarms are a godsend and can save you so much time and stress when navigating across a shipping lane or in bad weather.

There are basically two alarms; Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA). Most systems will allow you to enter your own CPA and TCPA values for when the alarm will be triggered and so you need to decide how close you want to let other vessels get to you (CPA) before an alarm sounds and also how much time you need to take avoiding action (TCPA). For instance, you might decide that 0.25NM is plenty close enough for large vessels to pass by you and you want to have 15 minutes notice so that you have time to take avoiding action and/or call them on the radio.

The image below shows the “imaginary” guard zone around the yacht that is set by the CPA alarm value (in this case 1NM) and also the distance the fast moving powerboat will be when the TCPA alarm goes off 5 minutes before the CPA situation occurs.

CPA+TCPA

Prudent use of the CPA and TCPA alarms will definitely make your voyages safer, but do familiarise yourself with how your particular system works and know how to turn off the alarms when you are approaching busy harbour entrances and rivers, otherwise they will keep going off and drive you crazy !

 

 

 

 

4 Comments

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    […] App provides full AIS functionality including CPA and TCPA alarms – click here to see our previous post on these […]

  2. nmea 2000 etc - SailNet Community says:

    […] CPA and TCPA and sound an alarm when the trigger values have been reached. More on CPA/TCPA here CPA and TCPA Alarms Explained | Digital Yacht Ltd In a NMEA 2000 network devices that have information will broadcast this onto the network and […]

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    […] CPA y TCPA alarmas (Haga clic aquí para leer la guía que explica las alarmas), los objetivos de colores AIS y también la opción de […]

  4. From Visby to Turku - The Naked Sailor says:

    […] try to resolve it ourselves then and there. Our 4G radar worked nicely and we could get rough CPA (Closest Point of Approach) info from that as well. Our main worry was that if we don’t see them (on the AIS), they […]

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