Global Navigation Satellite Systems receivers use pseudorange measurements for positioning and time determination. The control system uses the same measurements for estimating the satellite orbits, clock offsets and signal biases in a complex estimation process, which additionally involves the determination of atmospheric delays. In current systems, the separation of these different parameters imposes strong requirements on the stability of the satellite clocks and on the ground infrastructure. Several hundred ground monitoring stations are additionally used in high accuracy applications. The Kepler proposal follows a different approach. It re-uses the Galileo constellation of Medium Earth Orbiting (MEO) satellites, as well as its signals but complements them by a small constellation of Low Earth Orbiting (LEO) satellites and by selected optical Inter-Satellite Links (ISL). The LEO satellites are used to observe the signals transmitted by the MEO satellites from outside the atmosphere. The optical ISL connect all satellites in a multi-hop fashion. This enables a direct synchronization of the satellites at a level not achievable today and additionally does not require that the MEO satellites are equipped with clocks. The optical ISL furthermore provide ranges rather than pseudoranges for orbit determination as well as a broadband intra-system communication network. Terrestrial infrastructures become mostly obsolete with the Kepler approach. A single monitoring and control station remains necessary to maintain the alignment with earth-rotation, the synchronization to Universal Time Coordinate (UTC), and the capability of controlling the system.     «

Global Navigation Satellite Systems receivers use pseudorange measurements for positioning and time determination. The control system uses the same measurements for estimating the satellite orbits, clock offsets and signal biases in a complex estimation process, which additionally involves the determination of atmospheric delays. In current systems, the separation of these different parameters imposes strong requirements on the stability of the satellite clocks and on the ground infrastructure...     »