The interplanetary navigation is based on microwave signal exchanges between the space vehicle and a ground station equipped with a high stability timing reference. Ranging and Doppler tracking resolutions are limited by the clock instabilities.
VLBI allows data from widely separated radio telescopes to be combined to get exceptional astronomical measurement resolution providing each radio telescope is equipped with an ultra-stable clock
In an atomic clock, a local oscillator (LO) is frequency locked on the resonance frequency of an atomic transition. The best current atomic clocks using cold atoms operate sequentially. During the period of cooling no information is available for the frequency of the LO which is then free-running. Consequently the overall clock frequency stability is degraded by the short-term instability of the LO and a stability better than 1x10-14 over 1-100s is required.
National Metrological Institutes have in charge to maintain the standards and to establish certifications for industrial products and laboratory instruments. The frequency stability is one of the major characteristics of the signal source that could impact on the security and performances of number of systems. A standard presenting frequency stability better than 1x10-14 over 1-1000s will notably increase the current calibration capability making it compatible with all current industrial requirements.
To estimate, without ambiguity the frequency performances of state-of-the-art oscillators and clocks requires an ultra-stable reference clock. Some industrial companies making high performance quartz oscillators are interested by a versatile frequency reference of less than 1x10-14 frequency instability.
Special Relativity is one basic pillar of modern physics. Each basic theory has to be tested as its best, because it is so fundamentally important. The best tests of Special Relativity are based on ultra-stable oscillators for which we are locking for a frequency variation as a function of the speed and orientation of the laboratory. The frequency fluctuations of the oscillator limit the measurement resolution.