ACES Mission: Scientific Objectives and Applications
Reina-Barragán, F.¹; Cacciapuoti, L.¹; Nasca, R.¹; Feltham, S.¹; Much, R.¹; Daganzo, E.¹; Dimarcq, N.²; Salomon, C.^3
1ESA/ESTEC (Netherlands); ²Syrtre (France); ³Laboratoire Kastle-Brossel (France)

Atomic Clock Ensemble in Space [ACES] is an ESA mission founded on the high performances of a new generation of atomic clocks operated in the microgravity environment of the International Space Station [ISS].

The heart of the ACES System is represented by two atomic clocks externally mounted on the Columbus module: the primary frequency standard PHARAO (“Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbite”), based on laser cooled samples of Cs atoms, and the active Space Hydrogen Maser [SHM]. These clocks will be locked and compared to each other using a specially developed Frequency Comparison and Distribution Package [FCDP]. The good short-term performances of SHM are combined with the long-term stability and accuracy of the PHARAO clock to generate an on-board ACES time scale with a fractional frequency instability and inaccuracy of a few parts in 1016. The on-board ACES reference signal will be world-wide distributed and compared with ground clocks through a high-performance two-way time and frequency transfer system operating in the microwave domain. The high stability of MicroWave Link [MWL] (0.3 ps over 300 s and 0.7 ps over one day) will allow ground-to-ground comparisons of atomic frequency standards reaching a frequency resolution of 10-17 after a few days of integration time.

Based on these comparisons, a number of fundamental physics experiments will be performed, and applications in different areas of research developed. ACES will test Einstein’s theory of general relativity, including an accurate measurement of the Einstein’s gravitational red-shift, search for time variations of fundamental constants, and tests of the Standard Model Extension. Specific applications in atomic time scales will be developed based on the ACES provided capability to compare primary frequency standards with a frequency resolution at the level of 10-17, synchronise ground clocks at 100 ps level and contribute to international time scales. In addition, ACES will demonstrate a new type of “relativistic geodesy” which, based on a precision measurement of the Einstein’s gravitational red-shift, is expected to resolve differences in the Earth gravitational potential at the 10 cm level. A GNSS receiver will be installed on the payload to ensure the precise orbit determination of the ACES clocks. The continuous monitoring of the GNSS clocks with respect to the ACES reference will significantly contribute to the improvement of the global navigation satellite systems [GNSS] and to the future evolution of these systems. ACES will also contribute to the monitoring of the Earth’s atmosphere through radio-occultation experiments, and by using the ISS as a sensor to retrieve its density and GNSS code-phase measurements as an estimator of the total electron content above the ISS.

The engineering model [EM] of PHARAO is presently under test. When driven in combination with a cryogenic sapphire oscillator, PHARAO reaches a fractional frequency instability of 2.3•10−13 at 1 s. The space hydrogen maser SHM, using the engineering model cavity, has already demonstrated an Allan deviation down to 1.5 10−15 after 104 s of integration time. The FCDP EM has been successfully tested in July 2006 and the EM electronics of the MWL flight segment is now completing functional and environmental testing; first results indicate that MWL will comfortably achieve its required performances. The ACES Ground Segment Requirement Review and the ACES Mission Preliminary Design Review were successfully conducted in 2006. ACES launch date is currently foreseen in 2014.

The ACES Mission concept, scientific objectives and potential applications will be outlined. The development status will be elaborated and the latest test results presented.