Event parameter

Primary and secondary objects: The primary and secondary object distinction is taken from Space Track conjunction data messages (CDMs). This means that the primary object will not always be your own satellite.

Probability of collision: The probability of a collision as calculated by Space-Track or the UKSA. The method of calculation is shown in each event page under the ‘Probability of collision calculations' accordion.

Time of closest approach: The time, in UTC, that the distance between two objects is predicted to be the smallest.

Miss distance: The Consultative Committee for Space Data Systems (CCSDS) describes the miss distance as "the norm of the relative position vector. It indicates how close the two objects are at the time of the predicted encounter."

Monitor your satellites uses the RTN coordinate framework to describe miss distances. Radial is the unit vector in the radial direction pointed outward from the centre of the central body. Transverse or in-track is the unit vector perpendicular to the radial vector in the direction of the spacecraft velocity. Normal or cross-track is the unit vector normal to the satellite’s inertial orbit plane (in the direction of the satellite’s angular momentum) that completes the right-hand coordinate frame.

Total miss distance is the exact distance between the two objects in 3D space. It is the combination of the vectors of radial, in-track and cross-track distance.

Event history: The event history includes all updates to an event over time. This will include all CDMs and UKSA data.

Data sources

Ephemeris: Ephemerides are cartesian vectors providing position and velocity, and optionally accelerations. More information about ephemerides and .oem files can be found at the CCSDS recommended standard for orbit data message.

Monitor your satellites provides a user-friendly system to help identify which CDMs contain SPECIAL ephemeris data.

Special ephemeris files represent a possible new orbit which assume a manoeuvre such as orbit raising, relocation, or COLA, will occur and are submitted to Space-Track for planning purposes using SPECIAL in the ephemeris file name.

Operational ephemeris files denote the current operational plan for the satellite (this may include manoeuvres which have been confirmed and are about to be committed). 

By labelling which CDMs include SPECIAL information, users can easily access the data they need to plan their operations.

SSN: The United States space surveillance network.

ESA DISCOS: The European Space Agency’s Database and Information System Characterising Objects in Space.

TLE (two line elements): A two-line element set is a data format encoding a list of orbital elements of an Earth-orbiting object for a given point in time, the epoch.

Data received: The timestamp in UTC of when the data source was received.

Observations: The number and timespan of observations of the objects are shown to reflect the reliability of the data.

Object information

All object information within Monitor your satellites is sourced from Space-Track's satellite catalogue. UKSA orbital analysis is supplemented with wider data sources listed in 'Conjunction analysis process and sources'. Any fields that are 'pending' are yet to be connected to a source.

NORAD ID: An object in space's catalogue number, assigned by the United States' Space Command. Unknown objects are given an ID in the 80,000s.

Object type: This can be payload, rocket body, or unknown.

Manoeuvrability: This is based upon data that operators provide to Space-Track. Note that this information may be out-of-date or inaccurate.

Further definitions can be found in the Terms of Service.