A historical review of atomic frequency standards used in space systems

The remarkable navigational accuracy currently enjoyed by the users of satellite navigation systems is in major part due to the excellent performance of the on-board atomic frequency standards (AFSs). Since the laboratory demonstration of a Cesium AFS in the 50's the performance and reliability of AFSs have significantly improved. Currently Cesium (Cs) and Rubidium (Rb) AFSs are on-board many satellite systems-Navigational Satellites GPS (Global Positioning System-USE) and GLONASS (Global Navigation Satellite System-Russia) and the Communication Satellite MILSTAR (Military Strategic and Tactical Relay-USA). To test the predictions of General Theory of Relativity a hydrogen maser clock was flown in space. In all, we estimate that the total number of space-borne AFSs is no more than several hundred-by all accounts a relatively small sample size for any accurate reliability studies. The manufacturing technology of space qualified AFSs has vastly improved in the last three decades and the significant improvement in performance is primarily due to the maturity of the electronics industry. In this paper we present a historical review of the AFSs used in space systems. We will briefly review the unique requirements for space qualification for frequency standards and outline the performance characteristics of different AFSs which are presently on-board various satellite systems. We also present a brief discussion of the advanced AFSs for potential future space applications.