National Institute of Standards and Technology
Gaithersburg, Maryland

Jan. 20, 1998


Researchers at the National Institute of Standards and Technology soon will begin work on an advanced laser-cooled cesium atomic clock that will be placed on the International Space Station. The clock will be designed to take advantage of the lack of gravity to increase its accuracy. It will be used for experiments in fundamental physics, for studying the orbits of Global Positioning System satellites and for improving the realization of the definition of the second.

With funding from the National Aeronautics and Space Administration, the proposed work will involve interaction with clock projects at Stanford University and the European Space Agency. The Jet Propulsion Laboratory will be responsible for the clock's compatibility with the launch vehicle and the space environment.

The accuracy of Earth-bound cesium atomic clocks is limited by the length of time each cesium atom can be observed -- on Earth, gravity quickly removes the atoms from the observation region. In the microgravity environment of space, each atom can be observed for many seconds.

The space clock will use the latest techniques for cooling atoms with laser beams. Cold atoms provide a narrower resonance linewidth, which translates into better long-term stability for the clock. This allows a longer averaging time and greater sensitivity in observing small effects. The projected accuracy of the space clock will be at least 10 times better than the best Earth-based clocks.

The space clock will enable researchers to make gravitational red-shift measurements, test whether the speed of light is truly the same in all directions, and do other relativity experiments. It also will improve the determination of GPS orbital parameters, which will be useful for certain GPS applications, improve the determination of the duration of the second, and improve our ability to compare and synchronize clocks at various places on Earth.

The NIST project is led by Donald Sullivan, chief, NIST Time and Frequency Division, with Neil Ashby of the University of Colorado acting as co-principal investigator. Other NIST members of the team include Nobel Prize winner William Phillips (1997, physics), Leo Hollberg, Thomas Parker, Hugh Robinson, Steven Rolston and Fred Walls.

This NASA grant of $150,000 funds the first year of a five-year "Flight Definition Phase" of development, with funding of $200,000 per year for the other four years. The long-term schedule calls for the clock to be ready to fly in five to seven years.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.

The NIST Boulder Laboratories Website is at

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