Research strategies

In the baseline architecture (Kepler) eight equally spaced satellites populate three Medium Earth Orbit (MEO) planes; in each of these planes the individual spacecraft are bi-directionally linked to adjacent satellites. These two-way optical links allow separating uncertainties in the spatial distance from clock biases and providing frequency transfer and clock synchronization with high precision. Furthermore, the links provide absolute ranging with μm accuracy, thus enabling unprecedented precision in along-track and radial position estimates.
Each of the MEO satellites carries a high-performance cavity- and/or iodine-based optical clock characterized by an Allan deviation at the 10-15 level. The optical links operate on the clock frequency and achieve a high directivity of transmission for a given instrument aperture. The individual optical clocks in the same orbital plane are merged into one composite clock. The inter-plane synchronization between the three composite clocks is guaranteed by a smaller constellation of satellites in Low Earth Orbits (LEOs), also equipped with optical terminals, that alternately establish connections to MEO satellites and relay time, frequency and data information through the optical channel with bandwidth of 50 Mbps. The Kepler space segment will thus provide the most stable system time conceived so far.

Gravitational and non-gravitational forces acting on each spacecraft may be measured by high-performance inertial sensors and gyroscopes where atom interferometry is investigated as an option for future systems. The optical inter-satellite signal receiver/transmitter terminals may also provide attitude information with μrad-accuracy. The navigation signals transmitted to ground are L-band radio frequency signals. The relation between optical frequencies and the L-band radio frequencies, which serves as reference for the navigation payload, is established by frequency combs on each satellite.

A crucial final task of the Orbit Determination and Time Synchronization (ODTS) processing is the estimation of the satellite orbits, the spacecrafts’ attitudes, the composite time and clock offsets, inter-frequency biases, atmospheric signal propagation, and the locations of the monitoring stations. These parameters are jointly estimated in a comprehensive estimation process (e.g. Kalman filter or least-squares minimization) that includes the dynamics of the system as well as the complete set of measurements. All measurement errors as well as uncertainties in the time evolution of the system are appropriately modeled. The solutions yield Precise Point Positioning (PPP) results in near real-time and thereby enable a range of innovative applications.

ADVANTAGE will impact the future evolution of satellite navigation. The results will be transferred both to the academic world and to industries to ensure the utilization of the most promising options in future systems and applications.