Optical frequency references for space applications

Abstract

Frequency references have made an enormous contribution to scientific research and technological advances. The impact on everyone’s life is demonstrated with every navigation handheld device or smartphone. The primary frequency standard nowadays (since 1967) is based on the caesium atomic clock, developed 1955. In space applications, the two commonly used technologies are the passive H-maser and the rubidium frequency references. The current development achieves fractional frequency instabilities at the 10-18 level. With this performance, the definition of the SI second will be adapted within a few years. Furthermore the great stability opens up new and better measurements, ranging from fundamental science to life sciences and navigation. A variety of future space missions rely on the availability of high-performance optical clocks with applications in fundamental physics, geoscience, Earth observation and navigation and ranging. Examples are the gravitational wave detector LISA (Laser Interferometer Space Antenna), the Earth gravity mission NGGM (Next Generation Gravity Mission) and missions, dedicated to tests of Special Relativity, e.g. by performing a Kennedy- Thorndike experiment testing the boost dependence of the speed of light.