OPTICAL CLOCKS IN FUTURE GLOBAL NAVIGATION SATELLITES

The primary payload of a navigation satellite is its clock that is indispensable to perform precise ranging meas-urements. Navigation and positioning performance de-pends, among others, on the quality of the frequency generator. The first Hydrogen maser is in orbit with GIOVE-B. With further progress of clock technology a future GNSS system could be equipped with optical clocks, thereby making available a network of fre-quency standards of utmost stability in space. Today, epoch-wise synchronisation of satellite and re-ceiver clocks is an integral part of precise carrier phase positioning. As a consequence only relative positions are obtained with high accuracy and reference sites with well-known coordinates are needed to retrieve absolute position. Synchronised space clocks, on the other hand, allow for absolute precise positioning in single receiver mode. Similarly precise orbit determination will profit from the increased stability of the observation system. Simulations for the latter are presented in the paper. Optical clock technology can be divided into two main implementations. Single ion clocks have a higher tech-nical maturity while a stable frequency is obtained in a shorter integration time with optical lattice clocks. A trade-off between the different types of optical clocks involves not only performance but also readiness for space qualification of the optical clock components