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.


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 !





3 thoughts on “CPA and TCPA Alarms Explained

  1. Pingback: Sneak preview of NavLink for Android on Sony Xperia Z2 Tablet | Digital Yacht Ltd

  2. Pingback: nmea 2000 etc - SailNet Community

  3. Pingback: Conecte un receptor AIS a un producto de Android

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s