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We have developed a simple model that is able to describe and predict hysteresis behavior inside Mumetal magnetic shields, when the shields are submitted to ultra-low frequency (<0.01 Hz) magnetic perturbations with amplitudes lower than 60 μT. This predictive model has been implemented in a software to perform an active compensation system. With this compensation the attenuation of longitudinal magnetic fields is increased by two orders of magnitude. The system is now integrated in the cold atom space clock called PHARAO. The clock will fly onboard the International Space Station in the frame of the ACES space mission.

PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), which is being developed by the French space agency CNES, is the first Primary Frequency Standard (PFS) specially designed for operation in space. PHARAO is a main instrument of the ACES (Atomic Clock Ensemble in Space) mission. ACES is being developed by ESA and the payload will be installed on-board the International Space Station. The mission is based on comparisons with ground based clocks to perform fundamental physics experiments: gravitational redshift measurements, analysis of the stability of the fundamental constants and anisotropy of light. Planned duration of the mission is 18 months with a possible extension to 36 months. The frequency accuracy requirement for PHARAO is less than 3 x 10 -16 and the expected frequency stability is 10 -13 Tau -1/2. An engineering model of PHARAO has been constructed and fully tested to validate the clock architecture. Two sub-systems for the flight model FM) have been delivered: the cesium tube where the atoms are manipulated and the microwave source which generates the 9.2 GHz signals. The other sub-systems, the laser source and the computer, will be delivered this year. Some performances tests have been performed on the microwave source and on the cesium tube assembled with the other EM subsystems. The whole FM clock will be assembled in 2013.

CNES, LKB and SYRTE are developing a primary frequency standard, called PHARAO, which is specially designed for space applications. The clock signal is referenced on the frequency measurement of the hyperfine transition performed on a cloud of cold cesium atoms (∼1 μK). The transition is induced by an external field feeding a Ramsey cavity. In microgravity the interaction time inside the cavity can be adjusted over two orders of magnitude by changing the atomic velocity (5 cm•s −1-5 m•s −1) in order to study the ultimate performances of the clock. An engineering model has been assembled to validate the architecture of the clock. This model has been fully tested on ground for operation faults. Of course the clock performances are reduced by the effect of the gravity on the moving atoms. The main results are a frequency stability of 3.3 × 10 −13 t −1/2. The main systematic effects have been analyzed and their frequency uncertainties contribution is 1.6 × 10 −15. The clock has been compared with the primary frequency standard, the mobile fountain of SYRTE. The mean frequency shift is lower than 2 × 10 −15. The mechanical and thermal space qualifications have been carried out by testing a representative mechanical model of the clock and by using refined calculations. The design of the clock has been improved and now the flight model is being assembled. The PHARAO clock is a key instrument of the European ESA space mission called ACES. This mission is dedicated to perform space-time measurements in order to test some fundamental physics aspects.

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