October 24, 2023



The goals of the ACES/PHARAO mission encompass both fundamental science and technology. It will focus on cold-atom timekeeping, manipulating cold atoms for the first time under conditions not found on Earth to conduct fundamental physics tests (relativity, possible time-variation of fundamental constants). It will also validate a set of new technologies for the space environment. The global scientific community can look forward to taking advantage of the stable frequency offered by ACES (Atomic Clock Ensemble in Space), using a ground station to acquire its reference time. Such aspects are set to prove increasingly important for future navigation and positioning systems, new wave-matter inertial sensors and fundamental physics experiments in solar orbit.

Thanks to ACES, comparing terrestrial atomic clocks in different places will soon be possible and very useful, especially if some of them become even more precise than PHARAO over the next decade. ACES/PHARAO will prove increasingly useful for comparing new ground clocks with any other clock on Earth.

PHARAO aims to:

  • Operate a laser-cooled caesium cold-atom clock in microgravity at a relative frequency stability of 10-13τ-½, where τ is the time in seconds. Averaged over ten days, a stability of 10-16 will be reached. The figure below shows the expected frequency stability of PHARAO on the ISS. In space, PHARAO will explore the domain of long-duration interactions between atoms and probing microwaves, whereas this can only be achieved for short periods in gravity conditions.
  • Distribute from ACES the optimized timescale from the combined SHM-PHARAO system to users on the ground, over a specific high-performance microwave link. The accuracy of this timescale will be 30 picoseconds over one day. This will be of interest to users in a range of fields including time-frequency comparison, where numerous research laboratories are working on the International Atomic Time (TAI) standard, and geodesy, very long baseline interferometry (VLBI) and atmospheric propagation of microwave signals.
  • Conduct fundamental physics experiments to obtain a new measurement of gravitational redshift with an accuracy of 10-6, i.e. an almost one-hundred-fold improvement over the Gravity Probe A experiment in 1976; to look for possible anisotropies in the speed of light at the relative level of 2 10-10; and to look for possible time (or space) variations in the fine-structure constant, one of the fundamental physical constants, by comparing PHARAO’s time with that of ground atomic clocks using atoms other than caesium.

Stabilité fréquence PHARAO
PHARAO frequency stability compared to the ones of
quartz oscillators, SHM & cryogenic UWA oscillator.


PHARAO mission phases

Once fully integrated with the ACES payload, the PHARAO clock will be placed in the U.S. spacecraft and lofted into orbit by the SpaceX Falcon-9 launcher. The Dragon spacecraft has the capability to transport pressurized payloads as well as payloads exposed to vacuum. ACES falls within the second category because it is designed to operate in the vacuum of space (it could easily be placed on a conventional satellite). Once the spacecraft is docked to the ISS, ACES will be grappled by a robotic arm and installed at a temporary location on the ISS. It will then be moved to its final position outside the Columbus module using the same arm. The installation of PHARAO/ACES will not require an extravehicular activity by astronauts. The payload is designed to operate for at least 18 months with a possible extension to three years.