Lifetime of space Passive Hydrogen Maser

Accurate and ultra-stable space qualified atomic clocks represent critical equipment for the precision Global Navigation Satellite Systems (GNSS). The Passive Hydrogen Maser (PHM), with its excellent frequency stability performance, is the master clock for European Navigation satellite payload, and is the most stable clock ever flown for GNSS. Nine PHMs have been flying onboard Galileo satellites (GIOVE-B and four IOV satellites) since Apr. 2008. More than 35 PHM flight Physics Packages (PP) have been manufactured and characterized by Orolia Switzerland SA (Spectratime), under the industrial consortium led by Selex ES S.p.A. Besides radiation effects on electronic components, lifetime on PHM depends mainly on which of PP. In the frame of the “Lifetime Qualification of the PHM” supported by ESA, two Qualification Model (QM) units had been subjected to test under vacuum since 2008. After the first 1.5-year test period as reported in previously published papers [1][2], one QM has been extended for another 2-year lifetime test. This paper provides test results over the overall period of QM1, and gives further comparison and analysis of key PP parameters over 3.5 years of operation (or 4.1 years including the stay-alive period). The extended test enhances the on-ground test statistics and provides better confidences in the PHM lifetime evaluation, which shows the instrument capability to comply with the lifetime requirement of 12 years.