F. Gonzalez

The In-Orbit performances of GIOVE clocks

The Galileo In-Orbit Validation Element (GIOVE) is an experiment led by the European Space Agency (ESA) aimed at supporting the on-going implementation of Galileo, the European global navigation satellite system (GNSS). Among the objectives of the GIOVE Mission are the validation and characterization of the on-board clock technologies. The current baseline technologies for on-board clocks are the rubidium atomic frequency standard (RAFS) and the passive hydrogen maser (PHM). Both technologies have been validated and qualified on ground and are now being further validated in a representative in-orbit environment aboard 2 spacecrafts, GIOVE-A and GIOVE-B. This paper presents the results obtained in the frame of the GIOVE experimentation. The behavior and performances of the clock technologies on board both spacecrafts has been investigated and analyzed in terms of operation, frequency stability, and clock prediction error after more than 3 years of operation for GIOVE-A and almost one year for GIOVE-B. In addition, relativistic frequency shifts of GIOVE spacecrafts have been investigated.

Update on the In-Orbit performances of GIOVE clocks

The Galileo In-Orbit Validation Element (GIOVE) is an experiment led by the European Space Agency (ESA) aimed at supporting the on-going implementation of Galileo, the European Global Navigation Satellite System (GNSS). Among others, the objectives of the GIOVE Mission are the validation and characterization of the on-board clock technologies. The today baseline technologies for on-board clocks are the Rubidium Atomic Frequency Standard (RAFS) and the Passive Hydrogen Maser (PHM). Both technologies have been validated and qualified on ground and are now being further validated in a representative in-orbit environment on-board two spacecrafts, GIOVE-A and GIOVE-B. This paper presents the results obtained on both spacecrafts, after more than three years (GIOVE-A) and almost one year (GIOVE-B) of operation.

Clock strategy experimentation with GIOVE clocks

GIOVE satellites broadcast currently a clock model in the navigation message which targets a maximum one day validity time. The associated accuracy is limited by the inadequacy of the model to the clock drift for the RAFS and to the presence of harmonics in the phase. The perform ance maybe increased by optimizing the fitting intervals and including an additional harmonic term in the clock model estimation. The associated stochastic model to the clock prediction is as well limited by the harmonic inclusion. A meaningful stochastic model associated to the prediction can be defined in case the harmonic is taken into account in the estimation.

ACES: a time and frequency mission for the International Space Station

ACES (Atomic Clock Ensemble in Space) was selected by ESA as a candidate express pallet adapter payload. Part of the early utilisation phase of the International Space Station, Express Pallet Adapters are specific external locations dedicated to the accommodation of small instruments and experiments. ACES tests a new generation of micro-gravity clocks with frequency stability in the range 10/sup -16/ to 10/sup -17/. The ACES mission is outlined with emphasis placed on payload engineering.