We evaluate the microwave lensing frequency shift of the microgravity laser-cooled caesium clock PHARAO. We find microwave lensing frequency shifts of δν/ν = 11 × 10 −17 to 13 × 10 −17 , larger than the shift of typical fountain clocks. The shift has a weak dependence on PHARAO parameters, including the atomic temperature, size of the atomic cloud, detection laser intensities, and the launch velocity. We also find the lensing frequency shift to be insensitive to selection and detection spatial inhomogeneities and the expected low-frequency vibrations. We conservatively assign a nominal microwave lensing frequency uncertainty of ±4 × 10 −17 .

The flight model of the laser-cooled cesium atomic clock, PHARAO, has been qualified for operation in space. The clock has passed the vibration, thermal and electromagnetic compatibility tests required to fly in low Earth orbit (400 km). On the ground, the clock realized a typical frequency stability of 3.0 × 10 −13 τ −1/2 with an estimated accuracy of 2.3 × 10 −15 . Frequency comparisons with the SYRTE primary frequency standard FOM agree within their stated accuracies. Because PHARAO is optimized for the longer interaction times possible in microgravity, we expect a frequency stability of 1.1 × 10 −13 τ −1/2 and a frequency accuracy of 1.1 × 10 −16 for operation in space. The clock has been delivered to the European Space Agency for the assembly of the ACES payload and is scheduled to be launched into space in 2021.