The J2 Relativistic Effect and Other Periodic Variations in the Galileo Satellite Clocks

The frequency of atomic clocks flying on board the satellites of global navigation satellite systems, as can be observed on the ground, shows periodic variations of different origins. One of them is related to the J2 relativistic effect due to the Earth oblateness, whose amplitude is now measurable thanks to stable clocks like Galileo's passive hydrogen masers. Orbital estimation errors are also translated into periodic frequency variations, whose amplitude changes in time and which are superimposed to the J2 signal. In this work we analyze the data of a Galileo satellite clock by using two different techniques, with a twofold objective: to characterize the most evident periodic variations affecting the apparent clock frequency; to estimate the amplitude of the J2 relativistic effect for a comparison with its theoretical value. This also represents a validation of the J2 correction that should be applied to the space clocks' data, in order to improve the timekeeping/positioning performance of global navigation satellite systems.