Qualification and frequency accuracy of the space-based primary frequency standard PHARAO

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.