Inside GNSS Media & Research

JUL-AUG 2018

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Page 44 of 59 J U L Y / A U G U S T 2 0 1 8 Inside GNSS 45 Up to date, the most precise test of the gravitational redshift has been realized with the Vessot-Levine rocket experiment in 1976 ( Figure 10 ), also named as the Gravity Probe A (GP-A) experiment, where the gravitational red- shi was verified to a level of 1.4 x 10 –4 accuracy. is gravitational redshi measure- ment had not been improved nor repro- duced at this level since then. e analy- sis performed showed that proper tests with the two eccentric Galileo satellites could potentially provide a more accu- rate test of Einstein's General Relativity Gravitational Redshi prediction. In view of the above considerations and following the recommendations of the ESA GNSS Science Advisory Com- mittee (GSAC), two parallel studies were launched by ESA with SYRTE/Paris Observatory and ZARM/University of Bremen in 2015. e so called GREAT project (Galileo gravitational Redshi Experiment with eccentric sATellites) aimed at measuring the Gravitational redshi predicted by Einstein General Relativity eory with the highest pos- sible accuracy. ese tests have been per- formed during a period of almost three years and are now concluding. On the occasion of the ESA 6th Inter- national Colloquium on Scientific and Fundamental Aspects of GNSS / Galileo held in Valencia, Spain, from October 25-27 2017, the two parallel consortia presented some preliminary results and the methodology followed, aer having assessed more than 1,000 days of data from the two eccentric Galileo satellites and properly modelled systematic errors. A caref ul ana lysis of systematic effects was essential to calculate robust limits on the gravitational redshi. All systematic effects, potentially impact- ing the Gravitational redshi tests, were identified and analyzed. Effects acting on the frequency of the reference ground FIGURE 10 PHM clock used during NASA Grav- ity Probe A (GP-A) experiment in 1976

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