Definition

Closest Point of Approach (CPA). 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

Note: In the context of this article, CPA refers to the minimum distance between two aircraft and is not related to ACAS equipage.

Description

The adherence to separation standards is an essential part of aviation safety. In a surveillance environment, this is usually expressed in a minimum distance that is allowed between aircraft if no (insufficient) vertical spacing is provided.