What is a collision avoidance manoeuvre?
A collision avoidance manoeuvre (CAM) is a propulsive burn executed by an operational satellite to alter its trajectory when space surveillance data predicts a high-risk close approach (conjunction) with another space object. The manoeuvre changes the satellite's orbit enough to move the predicted closest approach distance beyond the dangerous zone — typically ensuring the miss distance uncertainty ellipsoid is no longer threatening — while minimising propellant consumption and service disruption.
Decision process
When the US 18th Space Control Squadron (or a commercial SSA provider) issues a Conjunction Data Message (CDM) with collision probability exceeding the operator's action threshold, the operator's flight dynamics team evaluates the conjunction: accuracy of the tracking data, size of the uncertainty ellipsoid, conjunction geometry, and predicted CDM updates. False alarm rates are high — the vast majority of CDMs do not result in manoeuvres after evaluation. When a manoeuvre is warranted, the delta-v is calculated to move the spacecraft's position at closest approach time by a safe margin (typically several hundred metres to kilometres).
Coordination challenges
When two active satellites are on a conjunction course, each operator may independently decide to manoeuvre — creating a coordination problem: uncoordinated manoeuvres can create new conjunctions if both satellites move in the same direction. The current system relies on voluntary coordination between operators via direct communication or the 18 SPCS's notification system. Starlink's autonomous collision avoidance (executed without human review based on internal conjunction screening) performs thousands of CAMs per year — raising the need for better real-time coordination protocols as satellite populations grow.