Accurate and ultra-stable atomic clocks have been recognized as the critical equipment for the precision Global Navigation Satellite Systems (GNSS). SpectraTime (SpT) and T4Science (T4S) are space and ground clocks manufacturers of Rubidium Atomic Frequency Standard (RAFS) and Active & Passive Hydrogen Maser (HM) for various navigation systems (European, Chinese and Indian) and other programs. From Dec. 2005 to the beginning of 2012, both clock technologies have years of flight heritage through four Galileo and 11 Beidou satellites. Almost 90 SpT RAFS flight units and 25 Passive HM Physics Package flight units have been manufactured and characterized. As for ground application, more than 17 T4S Active HMs are involved in different ground segment worldwide, and one passive HM is in progress in the frame of a ground development program. This paper describes for space RAFS and HM the onground performances and life-time tests, as well as onboard achieved clock performances. A short overview of the ground GNSS timing reference segment with its active Masers and associated disciplining algorithms will be given. Even these standard Rubidium and maser technologies have been proven to be highly reliable and robust those could be subject to perturbations and could exhibit some anomalies, especially when exposed to single event radiations, magnetic field perturbations etc…. With those elements in hands, a presentation of novel onboard techniques to generate highly robust timing signal directly from the satellite onboard ONe CLock Ensemble (ONCLE) is presented. Performances achievements in presence of perturbations, and frequency jumps are also shown allowing a continuous and uninterrupted operation of the satellite navigation signals.
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