engleski

Gravity Field Determination Based on Relativity

Some 20 years ago, GPS for the first time opened the possibility for an easythree-dimensional positioning on the Earth. Although GPS Cartesian coordinates can be estimated with accuracy well below 1 cm in a very well defined international terrestrial system, they are purely geometrical and do not contain any gravity information. With the recent advances in distant frequency comparison methods and optical clocks reaching a relative frequency stability and accuracy in the 10-17 range, a new kind of relativistic geodesy based on Einstein’ s gravitational red-shift is proposed. As the clock frequency is sensitive to the gravitational potential rather than the local gravity acceleration, the information provided by clocks is of different nature. Near the Earth surface the Einstein effect amounts to a relative frequency shift of 10-16 per meter of elevation. Using ground optical clocks with 10-17 accuracy and adequate satellite time transfer link, the Earth potential difference between two arbitrarily distant clocks can be determined at the 10 cm level. Such a "relativistic geodesy" would nicely complement the current space geodetic missions CHAMP, GRACE and the coming GOCE mission in establishing the unified global height system and provide consistent link between space based and ground based gravity field determination. The ACES mission of the European Space Agency will provide a test bed for such relativistic geodesy. ACES is scheduled to fly onboard the International Space Station in 2014. The advancement status of the mission is presented as well as first measurements performed on the engineering models of the flight instruments. As ground optical clocks progress towards 10-18 accuracy, the geopotential determination at cm level appears feasible in the near future. Finally, we analyzed frequency transfer using the GPS satellites by estimating the so-called phase clocks. We show that for the best performing GPS satellite clocks we can clearly identify periodic relativity effects down to 70 ps with periods of 6 hours. Performance of the GPS satellite clocks are compared to the performance of the clocks in several timing labs with ACES clocks and first results in the relativistic gravimetry are presented. We show that relativistic gravitational potential differences can already now be determined with an accuracy of 1 m.

relativistic geodesy; optical clocks; satellite time transfer; gravity field determination

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