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CDMs

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

When two trajectories have a close approach (defined by our screening volume), 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 UUID, and can be retrieved from the GET api/v1/cdm/{id} endpoint. CDMs can be streamed via the GET /api/v2/cdm endpoint. See the quick-start guide for a code sample.

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

State Sources and CDM Generation​

Starting in spring 2026, CDMs will be generated based on owner/operator submitted ephemerides and on Stargaze observations. That means that if an operator submits an ephemeris to the system, they may generate two CDMs, one based on an estimate of the secondary object based on an operator_ephemeris trajectory submitted to the API, and another based on Stargaze observations of the secondary.

Operators must use the GET /api/v2/cdms endpoint to receive Stargaze CDMs. The (deprecated) GET /api/v1/event and GET /api/v1/cdm endpoints will only return results based on owner/operator data.

This can be determined by querying the metadata associated with the CDM (via the api/v1/cdm/:id/metadata endpoint) or the associated trajectory (via the /api/v1/trajectory/:id/metadata endpoint) to see if the state_source field is operator_ephemeris or spacex_optical.

Each CDM contains an is_active field. When a CDM is first generated, this field is True. It will be marked False if either:

  • A newer CDM is generated for the same conjunction event with that combination of state sources
  • Newer trajectories with those state sources were screened, and no CDM was generated (because their closest approach was outside our screening volume).

Probability of Collision Calculations​

If both trajectories that generated a CDM contain covariance, the COLLISION_PROBABILITY field will be populated in the CDM. 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). Two-dimensional collision probability methods are computationally efficient but may not be accurate at very low relative velocities.

If covariance was not provided for both trajectories, or if the probability of collision calculation fails (e.g., if one of the trajectories supplied a non-positive semidefinite covariance) a CDM will still be generated, but with NULL in the COLLISION_PROBABILITY field, with the comment field: "COMMENT_FAILED_COLLISION_PROBABILITY_CALCULATION_ALFANO = Collision probability calculation failed." In this case, the CDM will still contain a relative miss vector for conjunction assessment.