A test of special relativity using GPS: preliminary results

A novel test of the second postulate of Special Relativity (the universality of the speed of light) has been performed using data from hydrogen masers and caesium clocks on the ground and on board two GPS satellites (PRN 15 and 28). The clocks were compared via carrier phase measurements of the GPS signal using an AOA TTR4P time receiver at the BIPM and AOA Turbo-Rogue geodetic receivers at a number of stations of the IGS (International GPS Service for Geodynamics) spread world-wide. This experiment is sensitive to a possible anisotropy of the one-way speed of light in any spatial direction, and on a non-laboratory scale (baselines ⩾20000 km). The results presented here set an upper limit on the variation of the speed of light of δc/c<9×10^-9 but should be considered as preliminary to a more complete data set using a larger number of stations and, more importantly, all 24 GPS satellites when the signals are no longer degraded by Selective Availability