August 8, 2016


In an atomic clock, the duration of the coherent interaction between the atoms and the electromagnetic field is a fundamental limit to the resolution of the frequency measurement. This duration can be dramatically increased by using cold atoms and microgravity. Today, on Earth, the most accurate and stable atomic clocks make use of laser cooled caesium atoms.

A caesium clock operates with the hyperfine transition near 9.2 GHz used to define the SI unit of time, the second. Caesium atoms are cooled to a temperature of 1µK corresponding to an average velocity of about 7 mm/s. On Earth, a cold caesium clock operates in a fountain geometry. This geometry optimizes the interaction time between the atoms and the microwave field.

Due to gravity, this time cannot exceed 1 second for a reasonably tall fountain. By contrast, in microgravity conditions, the interaction time in PHARAO can increase up to 5-10 seconds with a simple and compact device. The principle of PHARAO is described in the figure on the left. It looks like an optically pumped caesium beam frequency standard but, in microgravity, the atomic velocity can be made 1,000 times smaller!

The PHARAO instrument is made of five sub-systems:

  • a vacuum tube also called caesium tube (TC), in which atoms are captured, launched, cooled, selected and detected after undergoing a microwave interaction within a microwave cavity. The caesium tube provides ultra-vacuum conditions throughout the atomic path and applies a constant and extremely uniform magnetic field along the atomic path, especially inside the microwave interrogation chamber. It includes also the ion pump high voltage supply (CV-THT), which is mounted separately on the ACES baseplate.
  • an optical bench also called Laser Source (SL), which provides the various laser beams necessary for the capture, launch, cooling down, atomic selection and detection of the atoms.
  • the microwave source (SH), which supplies the signal to drive interrogation and preparation cavities.
  • the on-board management unit (UGB), which processes the detection signal in order to command the frequency corrections to be applied to the microwave source in autonomous mode or transmitted to ACES-XPLC in the other operational modes. It also synchronizes the different phases of the atomic cycle, manages the acquisitions of measurements and the remote control systems in order to modify the functional parameters of the instrument.
  • the BEBA electronic unit which controls the caesium tube magnetic coils and acquires analogue signals issued by the caesium tube.

The Flight Software directs them all while decoding and operating the micro-sequences tables, distributing, for each launch of atoms cloud, the commands destined to the 5 sub-systems of PHARAO.

PHARAO units mass and volume

UnitMass (kg)Size (L x l x h, mm)Power (W)
Caesium tube (TC)44990 x 336 x 4445
Laser source (SL)20529 x 330 x 17840
Microwave source (SH)7300 x 270 x 11725
On-board management unit (UGB)6245 x 240 x 12026
BEBA1134 x 118 x 85 
(including harness, fixations and margin)
in its greater length