A CDM (Conjunction Data Message) is the standardised alert issued when screening predicts two space objects will pass dangerously close. It states who, when and how close — time of closest approach, miss distance, relative velocity, collision probability and each object's uncertainty — in a machine-readable format.
Understanding CDM
Reading a CDM like an operator
The headline miss distance is the least useful number in the message. Operators look first at the probability of collision and its trend across successive CDMs: a Pc that climbs as covariance shrinks means the geometry is genuinely tight; one that collapses means the early alarm was uncertainty, not danger. Next they check the component breakdown — radial separation is the most trustworthy, because along-track errors dominate both objects' uncertainty. Then the data quality: how fresh is the secondary object's tracking, and is the primary's state based on the operator's own GPS ephemeris or on external tracking alone? Only with that context does the decision — manoeuvre, wait for the next message, or stand down — get made.
From screening to CDM to manoeuvre
| Stage | Typical timing | What happens |
|---|---|---|
| Catalogue screening | Continuous | All objects propagated, pairs checked against screening volumes |
| First CDM issued | ~7–3 days before TCA | Operators alerted; risk usually still uncertainty-dominated |
| Refined CDMs | Daily → hours before TCA | New tracking tightens covariance; Pc trend watched |
| Decision point | ~1–2 days before TCA | Manoeuvre planned if Pc stays above threshold (commonly 1-in-10,000) |
| Post-manoeuvre screening | After burn | New trajectory rescreened — the avoidance itself must be screened |