We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.

We study the requirements on orbit determination compatible with operation of next generation space clocks at their expected uncertainty. Using the ACES (Atomic Clock Ensemble in Space) mission as an example, we develop a relativistic model for time and frequency transfer to investigate the effects of orbit determination errors. For the orbit error models considered we show that the required uncertainty goal can be reached with relatively modest constraints on the orbit determination of the space clock, which are significantly less stringent than expected from "naive" estimates. Our results are generic to all space clocks and represent a significant step towards the generalized use of next generation space clocks in fundamental physics, geodesy, and time/frequency metrology.

The ACES (Atomic Clock Ensemble in Space) mission is an ESA -CNES project with the aim of setting up onboard the International Space Station (ISS) several highly stable atomic clocks with a microwave communication link (MWL). The specifications of the MWL are to perform ground to space time and frequency comparisons with a stability of 0.3 ps at one ISS pass and 7 ps at one day. The ACES mission has applications in several domains such as fundamental physics, metrology or geodesy.The raw measurements of the ACES MWL need to be related to the scientific products including all corrections (relativity, atmosphere, internal delays or phase ambiguities) and considering all terms greater than 0.1 ps when maximized. In fact, the mission aims at extracting physical variables (scientific products) such as clock desynchronisation, electron content in the ionosphere (TEC), or range (instantaneous distance between the stations) from the code and phase measurements on ground and in space and auxiliary data (orbitography, internal delays, atmospheric parameters, ...).To this purpose we have developed the complete model of the time transfer at the required 0.1 ps level. We have then developed in parallel two softwares: from the raw measurements.During the mission only the algorithm (2.) will be used, but the program (1.) is necessary for testing purposes. The two softwares are kept as independent as possible (different programming languages, different algorithms ...) to ensure a maximum efficiency of such tests. We provide some details on the software and the tests, considering different cases from the simplest to the more complex and realistic situation using real ISS orbitography data and MWL measurement noise from the MWL engineering model.The phase ambiguity removal of carrier phase measurements is performed by the algorithm and its success strongly depends on the noise of the observables. We have investigated the statistics of cycle slips which appear during this operation using experimental data obtained from the tests of the MWL engineering model. We present two novel methods which allow the reduction of the cycle slip probabilities by a factor greater than 5 compared to the standard method.