Conjunction Assessment
Conjunction Assessment (CA) is the process of identifying any close approaches between two Resident Space Objects (RSOs). RSOs include operator payloads, derelict payloads, rocket bodies, debris, and other unknown tracked objects. The CA process involves gathering data into RSO catalogs, performing catalog screenings, and assessing collision risk through a variety of metrics. Conjunction Data Messages (CDMs) are then sent to the owners and operators (O/Os) of the RSOs in the conjunction.
The CA Process​
Collecting Data and Building RSO Catalogs​
Catalogs are typically created through commercial- and government-owned Space Surveillance Networks (SSNs), running 24/7 to provide accurate measurements of positions and velocities for all RSOs. The measurement results are collected, and the current state of each RSO is deterimined through orbit determination (OD). From these initial states, high-fidelity numerical propagation is used to predict future states of the RSOs, and multiple-day predictions of the RSO trajectory are saved as ephemeris data into catalogs.
Operators of active RSOs also use their onboard GPS measurements to provide high-accuracy state information, which can also be propagated into predicted O/O ephemeris data.
Catalogs may also be created via analytic SGP4 libraries, which use initial states from OD, periodic functions, and force models to provide low-accuracy ephemeris information. SGP4 ephemeris is not considered accurate enough to perform CA.
Performing Catalog Screenings​
Catalogs are updated at least once per day, as trajectories become increasingly uncertain the farther forward they are propagated. In a catalog screening, primary RSOs are compared against all other RSO trajectories. A series of analytic and numerical filters are then applied to locate any future close approaches between RSOs. This catalog screening process can be done between on-orbit RSOs, or be done for launch vehicles and on-orbit RSOs.
The screening process yields a list of close approaches for each primary and secondary RSO pair. The time of closest approach (TCA) is typically found through interpolation of the RSO trajectories, and a relative vector of the miss distance at TCA is reported. This vector is often rotated into the frame of the two RSOs, providing the geometry of the conjunction through nominal radial, in-track, and cross-track distances.
A primary RSO is the operator's (your) asset, whether it is on-orbit or a launch vehicle.
A secondary RSO is the object in conjunction with the primary (or primaries). Secondaries can be debris, other operator assets, or entire catalogs. For constellations with more than one asset, both primary and secondary may belong to the same organization.
Conjunction Reports​
A Conjunction is a close-approach event between an RSO or launch vehicle, and an on-orbit object in the space domain. For a given Screening, a Conjunction is reported for each instance at which the miss distance between Primary and Secondary objects violates the Screening's Threshold Radius.
Assessing Collision Risk​
Once identified, each conjunction is assessed by a set of metrics to understand which conjunctions pose a threat to the space environment. The nominal miss distance is not absolute truth, however. All measurements include uncertainty, and the uncertainty grows as the RSO trajectories are projected into the future.