Contact:  pacome.delva[AT]obspm.frLocation:  Paris, France
Test of the gravitational redshift using Galileo satellites 5 and 6
Directed by: Pacome Delva and Peter Wolf
Observatoire de Paris, CNRS, Universite’ Pierre et Marie Curie
Systemes de Reference Temps-Espace SYRTE, Paris
Einstein’s General Relativity (GR) predicts that time flows differently for two clocks that have a relative speed and are placed in different gravitational potentials. It is therefore possible to test GR by comparing two clock frequencies. The most precise test of the gravitational redshift to date has been realized with the Vessot-Levine rocket experiment in 1976, also named the Gravity Probe A (GP-A) experiment. The gravitational redshift was verified to 1.4 x 10^(-4) accuracy.
We propose to use the on-board atomic clocks of the Galileo satellites 5 and 6 to look for violations of GR . These two satellites were launched on August, 30th 2014 and, because of a technical problem, the launcher brought them on an elliptic orbit. An elliptic orbit induces a periodic modulation of the gravitational redshift while the good stability of recent GNSS clocks allows to test this periodic modulation to a very good level of accuracy. The Galileo 5 and 6 satellites, with their large eccentricity and on-board H-maser clocks, are hence perfect candidates to perform this test. Contrary to the GP-A experiment, it is possible to integrate the signal on a long duration, therefore improving the statistics.
The proposed approach to reach an improved test of the gravitational redshift requires an accurate knowledge of the frequency of the satellite clock as it orbits the Earth. These data are made available by several Analysis Centers (ACs) of the International GNSS Service (IGS) in the framework of the Multi-GNSS-EXperiment (MGEX). Moreover, we plan to use satellite housekeeping data and Satellite Laser Ranging (SLR) data in order to improve the knowledge of systematic effects. The candidate will have to collect and analyze the data, modelize systematic effects, and use sophisticated data analysis in order to calculate robust limits on the parameters of the GR violation.
Strong skills in numerical computation and data analysis are necessary. A good knowledge of GNSS is strongly recommended.
The position provides a 16 months appointment that can start at beginning of november, and preferably before then end of the year 2015. Applicants should send their CV and two recommendation letters to Pacôme Delva (pacome.delva[AT]obspm.fr).
 Delva, P., A. Hees, S. Bertone, E. Richard, et P. Wolf. Test of the gravitational redshift with stable clocks in eccentric orbits: application to Galileo satellites 5 and 6. arXiv:1508.06159, 2015.