The planned mission duration is 18 months with a possible extension to 3 years. During the first 3-4 weeks in orbit, commissioning is performed (power on, warm up and subsystem activation). The functionality of the clocks and of the MWL is also assessed. Then, a period of 6 months will be devoted to the performance evaluation of the clocks. During this characterisation phase, a clock signal with frequency inaccuracy in the 10-15 range will be available to ground users.
In the microgravity environment of the ISS, caesium atoms can be interrogated by the resonant microwave field for about 1 s, permitting the PHARAO atomic resonance line-width to be reach sub-Hertz values. After optimisation, performances in the 10-16 range for both frequency instability and inaccuracy are expected. This corresponds to a time error of about 1 second over 300 million years.
In the second part of the mission (12 to 30 months), the clocks will be compared to a number of ground clocks operating both in the microwave and optical domain. In this context, ACES will perform worldwide comparisons of the best available clocks using different atoms or molecules with 10-17 frequency resolution, the results being used to test fundamental laws of physics and to challenge our knowledge of the Universe.
ACES will benefit metrology – the science of measurement – because highly accurate atomic clocks in space will help in the comparison of ground based atomic frequency standards, in the synchronisation of clocks and in the realisation of global time scales such as the International Atomic Time.
The space clock will also demonstrate a new technique called “relativistic geodesy” which, based on a precision measurement of the Einstein’s gravitational red-shift, will resolve differences in the Earth gravitational potential at the level of tens of centimetres.
GNSS receivers are widely used in geodesy and Earth monitoring. ACES will contribute to evolutions of the global navigation satellite network and support the use of precision clocks in major Earth science activities (radio occultation and coherent reflectometry).
Last update: 3 November 2011