An Introduction to Satellite Based Atomic Frequency Standards

A major part of satellite based navigation systems is the atomic frequency standard (AFS). The first satellite navigation was realized in the 1960´s, with the US Navy´s navigation satellite system known as TRANSIT. The TRANSIT satellites were launched with quartz crystal oscillators (XOs) for stable and precise frequency generation. In 1964 the Navy started the TIMATION program, a predecessor to GPS. The TIMATION developmental satellites (TIMATION-1 and -2) used high performance XOs and time referenced ranging signals. In 1974, TIMATION-3 extended the ´state-of-the-art´ in satellite navigation by orbiting very precise AFSs. The superior frequency stability of the AFSs made satellite navigation a practical system to operate. This pioneering work provided the stimulus for developing reliable AFSs for space applications. This paper discusses AFSs on current and upcoming navigation systems. The two current systems are the Russian Global Navigation Satellite System (GLONASS) Global Positioning System (GPS). The upcoming navigation systems with AFSs are: the Galileo system, China´s Beidou (a.k.a. Compass) satellite positioning system, and Japan´s quasi-zenith satellite system (QZSS). Other systems with AFS are introduced. These including the Gravity Probe-A (GP-A) experiment, the military strategic and tactical relay (Milstar), the Advanced EHF (AEHF) program, the navigation experiment (NAVEX), the Cassini- Huygens mission, the cesium clock in the primary atomic clock in space (PARCS) mission and the Projet d´Horloge atomique par refroidissement d´atomes en orbite (PHARAO) project. The future of AFSs are discussed including subminiature Rb, smart clock technology, optically pumped cesium, coherent population trapping (CPT) technology. The Advanced Technology Atomic Frequency Standard (ATAFS) program and the DARPA Chip Scale Atomic Clock (CSAC) program are mentioned as well at the hydrogen maser and the developments with trapped ion, optical, and cold atom clocks.