The ACES mission aims at operating ultra stable atomic clocks on board the International Space Station and comparing them to ground clocks located all around the world in the 2010-2011 time frame [1,2]. The expected performances of ACES space clocks (frequency stability and accuracy at the 10-16 level) and of the dedicated microwave link (time stability better than 6 ps in 1 day) will be of great interest for time and frequency metrology (comparison of primary frequency standards with a frequency resolution better than 10-16), for fundamental physics studies (tests of Einstein’s theory of general relativity, search for a drift of fundamental constants), and for other applications in positioning and geodesy. In this paper, we will present the main characteristics of ACES instruments and discuss the mission’s scientific objectives. An update of the development status of the mission will also be presented. Figure 1: the ACES mission. A cold atom cesium clock PHARAO (developed by CNES) combined with a space hydrogen maser SHM (developed by Neuchatel Observatory) provide an ultra-stable time and frequency reference on board the International Space Station. This reference is compared to an ensemble of ground clocks through a high performance two way time and frequency transfer system. [1] C. Salomon, C. Veillet, Proc. of the first symposium on the utilisation of the international space station, ESA special publication SP 385, 295, 1997. ACES: Atomic Clock Ensemble in Space [2] C. Salomon et al., “Cold Atoms in space and atomic clocks: ACES”, C. R. Acad. Sci. Paris, t.2 Série 4, pp. 1313-1330, October 2001

The ACES (Atomic Clock Ensemble in Space) experiment aims at the validation of new generations of atomic clocks onboard the International Space Station. To facilitate time determination and time transfer, it has recently been proposed to connect the ACES clocks with a GNSS receiver. With this background, the talk provides an overview of existing spaceborne GPS receivers and summarizes their performance and environmental characteristics. Besides dedicated space qualified receivers the possible use of commercial off the shelf (COTS) dual-frequency receivers is discussed. As an example, the tracking and navigation performance of the flight proven BlackJack/IGOR receiver is compared with a NovAtel OEM4-G2 and a Septentrio PolaRx2 for a representative low Earth orbit scenario. In addition, the results of preliminary thermal vacuum and radiation tests are presented. Given the fairly benign conditions inside the Columbus module, the use of a COTS GNSS receiver can provide a cost effective and well justified alternative to a fully space qualified device. It would also help to ensure access to the latest receiver technology for new GNSS signals such as Galileo or GPS L2C.