Space Surveillance and Tracking Background

Geospace is populated by millions of man-made objects in Earth orbit of which more than 13,000 with a size >10 cm are regularly tracked. The majority comprises of launcher bodies and other mission-related objects and fragments, with only about 10% tracked active satellites. Earth-orbiting objects constitute a possible threat, in space (collisions, adverse actions on space systems) and on ground (uncontrolled and high-risk re-entry).

Tracking involves detection of objects in space and determination of orbit state and covariance (uncertainty). An important operational issue concerns forecasting of the trajectories of decaying objects that eventually enter the lower and denser atmospheric layers. The object trajectory change depends on the thermosphere density at its precise location, i.e. on altitude, latitude, and longitude, all of which appear rapidly changing during the re-entry process. Consequently, the requirement to take risk reduction measures to avoid re-entry over populated areas requires accurate 3-D modelling of the thermosphere and 3-D forecasting of the object trajectory in the geographic sector concerned.