# CDMs

A Conjunction Data Message (CDM) is a representation of a close approach between two satellites. (see the CCSDS spec).

When two trajectories have a close approach, a Conjunction Data Message (CDM) will be generated from the trajectory position and covariance information. The CDM is returned in either JSON or KVN format (default JSON), will have a UUID4 unique identifier, and can be retrieved from the `GET api/v1/cdm/{id}`

endpoint.

Each CDM will be assigned to an event, based on the participating objects and the TCA.

Collision probability is computed with the two-dimensional Alfano collision probability method: 'A Numerical Implementation of Spherical Object Collision Probability' by S. Alfano. (https://doi.org/10.1007/BF03546397)

It is possible for our collision probability calculation to fail (e.g., if one of the trajectories supplied a non-positive semidefinite covariance). In this case, a CDM will be generated with NULL collision probability, with the comment field: "COMMENT_FAILED_COLLISION_PROBABILITY_CALCULATION_ALFANO = Collision probability calculation failed."