The occurrence of minimum range between own ACAS aircraft and the intruder. Range at CPA is the smallest range between the two aircraft and time at CPA is the time at which it occurs.
Source: ICAO Doc 9863 Airborne Collision Avoidance System (ACAS) Manual
The adherence to separation standards is an essential part of aviation safety. In a radar environment, this is usually expressed in a minimum distance that is allowed between aircraft if no (or not sufficient) vertical spacing is provided.
Since the distance at the CPA is crucial in deciding whether or not a safety occurrence has happened it is important that it can be determined (a) accurately and (b) well in advance, so that corrective action is taken if needed.
The most popular means to obtain the CPA are:
Calculation using range and bearing (R&B) lines (described in the article Vectoring Geometry). This method results in higher controller workload compared to the alternatives.
Adjusting the lengths of the speed vectors and using the R&B lines to measure it directly. This method produces decent results but is limited by e.g.
the fact that the CPA needs to be determined manually (i.e. with an error).
the fact that a difference of a few pixels on the screen may result in half a mile difference in the measurement result (or more), depending on the range chosen. This adds another error.
Extending the speed vectors and trying to determine the CPA. It is obviously somewhere at the end of the fourth minute but the precise moment is difficult to pinpoint.