4. Conclusion

Orbit determination is based on observations processed by real-time filtering or delayed-time smoothing methods. The quality of the estimate, measured by the covariance of the error, depends on many factors: type of measurements (directions, distances, speeds), observation frequency, fidelity of the dynamic model, filtering method, etc. For navigation satellites such as GPS, the accuracy achieved is a few meters in real time. For scientific satellites such as TOPEX, designed to measure sea levels, radial distance accuracy is down to a few centimetres in delayed time.

Precise orbitography is crucial for keeping operational satellites on station and for space surveillance: observation frequency to avoid losing a satellite or piece of debris, detection of maneuvers, collision risks, risks associated with re-entry... Observation resources and non-linear filtering techniques...