Atomic clock ensemble in space (ACES) data analysis

Abstract

The Atomic Clocks Ensemble in Space (ACES/PHARAO mission, ESA & CNES) will be installed on board the International Space Station (ISS) next year. A crucial part of this experiment is its two-way MicroWave Link (MWL), which will compare the timescale generated on board with those provided by several ground stations disseminated on the Earth. A dedicated Data Analysis Center (DAC) is being implemented at SYRTE -Observatoire de Paris, where our team currently develops theoretical modelling, numerical simulations and the data analysis software itself.In this paper, we present some key aspects of the MWL measurement method and the associated algorithms for simulations and data analysis. We show the results of tests using simulated data with fully realistic effects such as fundamental measurement noise, Doppler, atmospheric delays, or cycle ambiguities. We demonstrate satisfactory performance of the software with respect to the specifications of the ACES mission. The main scientific product of our analysis is the clock desynchronisation between ground and space clocks, i.e. the difference of proper times between the space clocks and ground clocks at participating institutes. While in flight, this measurement will allow for tests of General Relativity and Lorentz invariance at unprecedented levels, e.g. measurement of the gravitational redshift at the 3 × 10 −6 level.As a specific example, we use real ISS orbit data with estimated errors at the 10 m level to study the effect of such errors on the clock desynchronisation obtained from MWL data. We demonstrate that the resulting effects are totally negligible.

Gravitational redshift test with the future ACES mission

“…Once the MWL time transfer software was developed, the impact on time transfer was addressed numerically with the MWL software itself, with several levels of uncertainty of a real ISS orbitography file. It showed that below 1 km orbit error the impact is below specifications [17], confirming the estimates in [18] for the time transfer part.…”

“…In the ACES mission, a given ground clock in a laboratory equipped with a MWL ground terminal will be compared to PHARAO when ISS is in sight. The time transfer software for extracting the clock desynchronization from MWL raw data has been presented in [17].…”

“…The MWL data processing presented in [17] will provide for each clock a file with coordinate time tags (in UTC) and corresponding ground clock/PHARAO desynchronization, every 80 ms during each pass, with a new file for each day. However since no such real data are available yet, we simulate desynchronization data as will be provided by ACES MWL data analysis [17], as well as corresponding frequency data (discrete time derivative of the desynchronization data). For this we use the GR model of Equation (6), expressed in GCRS.…”

“…The MWL and its data analysis has been described in [17], which also provides an estimate of the effect of orbit determination errors on the time transfer model, but not on the determination of the frequency shift of the clock. The latter has been studied to some extent in [18].…”

Testing fundamental physics with photon frequency shift

“…A possibility along this line is offered by the hydrogen atoms for which the interaction between electron and proton magnetic moments induces a split in two levels of the 1s ground-state; this kind of atomic clock has been also used by Galileo [15] and ACES [16]. The sum of the angular moments of electron and proton spins, F = I + s, gives the singlet F = 0 and the triplet F = 1.…”