Hybrid synthesizers in space: Galileo's CMCU

Galileo satellite navigation system employs as master navigation signal a 10.23 MHz tone generated from several 10MHz atomic clock references. The Clock Monitoring and Control Unit (CMCU) synthesizes dais navigation signal from one of the four embarked atomic clocks. The CMCU has to comply with very stringent requirements in order not to degrade the frequency stability of the atomic clocks and deliver an extreme spectral purity acting as a filter. As the atomic clocks are oscillating in free mode, the CMCU synthesizer has to be able to correct the frequency deviations in steps of 0.0001 parts per billion. In order to comply with these requirements, the CMCU implements a hybrid synthesizer built by a Phase Locked Loop (PLL) whose auxiliary frequency is generated by direct digital synthesis (DDS) with a granularity compliant with the requirements. This paper describes CMCU emphasizing the hybrid synthesizer concept and showing the results obtained in the engineering qualification model (EQM) and in the first two flight models.