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Conjunction Assessment

Conjunction Assessment (CA) is the process of identifying close approaches between two Resident Space Objects (RSOs), including active payloads, derelict spacecraft, rocket bodies, debris, and unidentified or uncorrelated objects. The CA workflow involves generating RSO catalogs, conducting catalog screenings, and evaluating collision risk. When a significant event is detected, a Conjunction Data Message (CDM) is generated and shared with relevant owners and operators (O/Os).

The CA Process​

Catalog Generation and RSO State Estimation​

RSO catalogs are derived from continuous tracking by government and commercial Space Surveillance Networks (SSNs), which provide state measurements for tracked objects. These measurements are processed through orbit determination (OD) to estimate each object's current state and associated uncertainty. The OD process yields both a nominal state and a covariance matrix, which is propagated forward alongside the trajectory to generate ephemerides and enable probabilistic conjunction risk assessment.

Operators with onboard GNSS-derived states can generate high-accuracy ephemerides for their own assets. These ephemerides are collected into an operational data catalog, which is screened independently or in parallel with SSN catalogs to support higher-fidelity conjunction assessment.

Some catalogs are composed of analytic SGP4 propagation based data. While SGP4 is sufficient for general awareness, it lacks the accuracy required for high-confidence conjunction analysis.

Catalog Screening​

Catalog screening compares predicted trajectories of primary RSOs against all other cataloged objects. Screenings may be applied to:

  • On-orbit conjunctions (e.g., active payloads and debris)
  • Launch trajectory screenings (e.g., launch vehicles vs. orbiting assets)

Screenings identify all predicted close approaches between object pairs. For each approach, the system computes:

  • Time of Closest Approach (TCA)
  • Miss Distance (MD) vector, expressed in relative coordinates
  • Probability of Collision (Pc) metrics, quantifying the expected risk

18 SDS screening intervals are every 8 hours, with increased frequency for elevated-risk events or during maneuver planning.

Primary vs. Secondary Designation
  • Primary: Operator-owned object under assessment (on-orbit or launch).
  • Secondary: All other objects screened against the primary.

Conjunction Reports​

A conjunction is any predicted event where the miss distance between a Primary and Secondary object falls below a predefined screening threshold. Each conjunction is documented with the objects' state and uncertainty information, TCA, relative geometry, and associated risk metrics.

Collision Risk Metrics​

  • Miss Distance (nominal): The closest point between trajectories assuming no uncertainty.
  • Probability of Collision: Computed using measurement uncertainty and object size.
  • Mahalanobis Distance: A statistical measure of how geometry aligns with uncertainty, used to assess Pc dilution.

Because trajectory predictions degrade over time, these metrics are updated regularly.

For details on how Satcat processes conjunction events and supports operational decision-making, see: