National Physical Laboratory

Atomic Clock Ensemble in Space (ACES)

NPL is involved in the European Space Agency's Atomic Clock Ensemble in Space (ACES) mission to conduct tests of fundamental physics and novel atomic clock and time transfer technologies.

International Space Station (courtesy of NASA)
International Space Station (courtesy of NASA)

The ACES mission is scheduled for launch by the European Space Agency (ESA) in 2016 for installation on the International Space Station (ISS) and has a planned lifetime of 18 months with a possible extension to three years. The ACES payload will include two atomic clocks, namely PHARAO, which is a microgravity caesium primary frequency standard based on laser-cooled caesium atoms, and the Space Hydrogen Maser (SHM). In addition to this, there will be a MicroWave Link (MWL) and a European Laser Timing (ELT) optical link enabling space-to-ground and ground-to-ground comparisons of clocks.

With support from the UK Space Agency, NPL has secured funding from the Department for Business, Innovation and Skills (BIS) to acquire a fixed MWL ground terminal for installation at NPL. This will enable NPL to receive the ACES ultra-precision timing signal transmitted by the ACES payload.

NPL has also secured funding to lease a dedicated optical fibre link between NPL and the International Space Innovation Centre (ISIC) in Harwell, which can be used for experiments involving the ACES timing signal.

The scientific objectives of ACES cover three important areas as outlined below.

Tests of fundamental physics

All tests of General and Special Relativity have so far agreed with the predictions within experimental uncertainty. ACES will enable tests to be carried out to levels not previously possible in order to search for violations of the theory. These will include tests of the gravitational red-shift, time variation of the fundamental physical constants and a search for any anisotropy of the speed of light. The MWL and ELT links will be used to achieve these through space-to-ground clock comparisons.

Tests of next generation space and ground-based clocks

Accurate clocks in space are essential for many applications in navigation, geodesy or fundamental physics. The ACES timing signal will be produced by combining the performance of the PHARAO caesium primary frequency standard with that of the active hydrogen maser SHM. This timing signal will be up to 100 times more stable than the signals produced on-board the GPS and Galileo satellites. ACES will also allow comparisons of the next generation of ground-based optical atomic clocks at National Measurement Institutes, such as NPL.

Tests of a high performance time and frequency transfer link

The ACES timing signal will be transferred to laboratories on the ground using the microwave MWL and optical ELT links. The MWL is one of the key technology demonstrations and is expected to be a factor of between 20 and 50 better than existing microwave technologies.

Project team

Related research areas

Last Updated: 12 Dec 2014
Created: 22 May 2013

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