Status of the flight model of the cold atoms space clock PHARAO

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 × 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.