New Atomic Clock in Space

Not that I needed anything more exact than I've already got, but...

Atomic Clock Ensemble in Space (ACES)

ACES will be located on a platform (bottom right) outside Columbus that points towards Earth for good access to ground terminals distributed around the globe

The fact that time can be measured very precisely - far better than any other physical parameter - is a technological asset of great importance. This is the essence of ACES, which will test a new generation of atomic clock in space.
One of the most exciting microgravity physics experiments ever conceived, ACES is accredited with the highest scientific merit. ACES will test a new generation of microgravity cold-atom clock in space. PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite) and the Space Hydrogen Maser (SHM) will be characterised and compared with each other and national frequency standards worldwide using a dedicated microwave link. The ultimate performance of PHARAO in microgravity will be explored and a number of fundamental physics experiments will be performed.
The 'cold atom' clock, PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), developed by CNES (F), and the SHM (Space Hydrogen Maser), developed by the Observatory of Neuchatel (CH), will be characterised and compared in a microgravity environment. Experiments include high-precision time and frequency transfer, atmospheric propagation, high-precision geodesy, global network synchronisation and fundamental physics.
ACES is a complex payload involving state-of-the-art instruments and subsystems. The atomic clocks are extremely sensitive to their operating environment, so the particularly harsh environment of space provides new challenges to the clock and payload designs. Thermal and electromagnetic sensitivity places particularly severe constraints on the payload.

Mock-up of the ACES payload

PHARAO uses six orthogonal laser beams to cool caesium atoms to a few µK. The combination of these slow atoms and their low acceleration in microgravity allows observation times significantly longer than on Earth, providing better stability and accuracy of the frequency.
The SHM is a miniaturised (35 kg) Active Hydrogen Maser offering extremely good medium-term frequency stability. At its heart is a single sapphire crystal resonator that dielectrically loads a microwave cavity. It will be frequency-locked to PHARAO for long-term accuracy and Time & Frequency (T&F) distribution. It will also serve as a reference clock for onboard characterisation of PHARAO. The ACES clocks are compared in orbit using a Frequency Comparison and Distribution Package (FCDP). They are also compared to terrestrial clocks using the T&F capability of the MicroWave Link (MWL) which in particular has a low phase noise compatible with a clock stability of 10-16. Key design parameters take into account errors introduced by the ionosphere, troposphere, multiple paths, Doppler rejection, etc.
The payload is controlled by an eXternal PayLoad Computer (XPLC). Power control and distribution is by the Power Distribution Unit (PDU). ACES will be mounted on a Columbus External Payload Adaptor and launched attached to a dedicated carrier aboard a Space Shuttle. The Station's robot arm will transfer it to nadir (Earth-pointing) position on the Columbus External Payload Facility. ACES will operate in orbit for 18-36months.
The first 6 months will be dedicated to characterising and evaluating the clocks and the T&F link, followed by its utilisation phase, including a demonstration of T&F distribution involving users around the world.
A number of important experiments will be performed across several scientific domains, including a demonstration of high-precision time and frequency transfer, atmospheric propagation, high-precision geodesy and global network synchronisation. The fundamental physics experiments include:
Gravitational Frequency Shift (Einstein Effect)
A source of radiation in a gravitational potential appears to an observer in a different gravitational potential to be shifted in frequency. The Einstein Effect can be determined with a relative uncertainty of 3 x 10-6, a factor 25 improvement.
Time Variation of the Fine Structure Constant
The fine structure constant (á) characterises the strength of the electromagnetic interaction. According to Einstein's Equivalence Principle, fundamental constants should not vary with time, but some modern theories predict the existence of some that do. Clocks using different atoms will be compared via ACES over time. An improvement of a factor 100/year is anticipated.
Test of Special Relativity
A number of theories predict violations of special relativity. ACES is expected to improve previous limits by about an order of magnitude by comparing the space and ground clocks continuously during several Station passes.
The payload prime contractor, EADS in Friedrichshafen (D), leads an industrial consortium for the design, development and delivery of the flight payload.

http://www.spaceflight.esa.int/projects/index.cfm?act=default.page&level=12&page=829

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