The Atomic Clocks Ensemble in Space (ACES/PHARAO mission), which will be installed on board the International Space Station (ISS), uses a dedicated two-way Micro-Wave Link (MWL) in order to compare the timescale generated on board with those provided by many ground stations disseminated on the Earth. Phase accuracy and stability of this long range link will have a key role in the success of the ACES/PHARAO experiment. SYRTE laboratory is heavily involved in the design and development of the data processing software : from theoretical modelling and numerical simulations to the development of a software prototype. Our team is working on a wide range of problems that need to be solved in order to achieve high accuracy in (almost) real time. In this article we present some key aspects of the measurement, as well as current status of the software's development.
The Atomic Clock Ensemble in Space (ACES) is a fundamental physics mission of the European Space Agency (ESA) to be launched in August 2021. It relies on a high-performance clock onboard the International Space Station (ISS), a network of high-performance clocks on ground, a dedicated two-way microwave link (MWL) enabling space-to-ground and groundto-ground clock comparisons, as well as an optical link (ELT). PHARAO/SHM (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite/Space Hydrogen Maser), the clock onboard the ISS, has a relative frequency accuracy at the 10 !"# level, a relative frequency stability (Allan deviation) equal to 10 !"$ √ ⁄ ( being the integration time in seconds) and a time deviation of 12 picoseconds after one day of integration. The MWL is designed to reach a time deviation below 7 ps after one day of integration. While space-to-ground clock comparisons will enable precise tests of the gravitational redshift, tests of deviations fromGeneral Relativity at the 10 !# level, and tests of local Lorentz invariance at the 10 !"% level, ground-to-ground clock comparisons will enable a search of the time variation of fundamental constants with uncertainty at the 10 !"& level after one year. In this contribution, we review the mission set up with a particular emphasis on the MWL, discuss the simulation and data analysis software developed to investigate mission performance, focusing on its primary scientific objective: the test of the gravitational redshift.
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