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.