The possibility of testing the Einstein Equivalence Principle (EEP) using Two Way Satellite Time and Frequency Transfer (TW) and a Software Defined Receiver (SDR)

The Einstein Equivalence Principle (EEP) requires that clocks in free fall, when observed by other clocks in free fall, show no frequency variations. To some approximation, clocks on the surface of the Earth can be considered freely falling in the solar gravitational potential, and one would expect to find no frequency variations between the clocks as the Earth's rotation modulates the potential experienced by each clock. A previous study [1] using GPS satellites yielded an upper limit to an EEP violation of <10(-3), parameterized as a fraction of the expected differential gravitational redshift of a clock pair not in free fall. Upper limits were also found using TW, but these were much higher than with GPS. We investigate the attainable limits of a search for EEP-violation using SDR-enhanced TW, and find a three-sigma uncertainty slightly higher than 10(-4) may be achievable, using optimistic assumptions.