The cryogenic sapphire oscillator (CSO) is compared to a commercial (Datum Inc.) active hydrogen maser whose frequency is also regularly compared to caesium and rubidium atomic fountain clocks in the laboratory .
Tests of relativity using a microwave resonator
NAVEX in which special and general relativity (TD and the gravitational red-shift) were tested by comparing a caesium clock in a space shuttle in a low, almost-circular, orbit around the Earth with a similar, synchronised, clock at a ground station.
Proposal for a Satellite-Borne Experiment to Test Relativity of Simultaneity in Special Relativity
Results are used to estimate the atom density shift in a caesium fountain clock.
Bayesian inference of a negative quantity from positive measurement results