Two estimates
CPA is the estimated distance at closest approach and TCPA is the estimated time until that point. Both use available data and an extrapolation of motion.
Why estimates change
A manoeuvre, delayed data, position errors or a target that stops transmitting can change the estimate. An alarm attracts attention; it is not guaranteed detection or an instruction to manoeuvre.
Configure and cross-check
Set thresholds for the situation and consult the system manual. Maintain an appropriate lookout, use other means and follow applicable collision regulations.
An interpretation example
A small CPA several minutes away is an estimate based on current data and movement. If either vessel changes course or speed, the prediction changes. A favourable CPA does not rule out other vessels without AIS or current data.
Before setting the alarm
Check own position, updates, units and course and speed sources. Consult the manual to understand whether the threshold uses distance, time or both. Maintain a lookout and use other available means.