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Primary Atomic Reference Clock in Space

The Primary Atomic Reference Clock in Space or PARCS is an atomic-clock mission once scheduled to fly on the International Space Station (ISS) in 2008, but cancelled to make way for the Bush Mars initiative. The mission, to have been funded by NASA, involved a laser-cooled caesium atomic clock, and a time-transfer system using Global Positioning System (GPS) satellites. PARCS was to fly concurrently with the Superconducting Microwave Oscillator (SUMO) a different sort of clock that will be compared against the PARCS clock to test certain theory. The objectives of the mission were to have been:

Experiment location

The proposed ISS location for the experiment was on the External Facility of the Japanese Experimental Module (JEM). This location afforded good views of the GPS constellation of satellites, needed for comparing space and ground clocks. In addition, the volume, available power, and coolant system were well matched to the mission requirements.


The microgravity environment of space allows slowing of atoms to speeds well below those used in terrestrial atomic clocks, providing for substantial improvement in clock accuracy. This very accurate space clock will be compared continuously to the SUMO oscillator, and these two clocks (being fundamentally different) will provide a test of "local position invariance." Comparisons between the space and earth clocks will yield a related, but important measurement of the gravitational frequency shift. Finally, the signals conveyed to the ground through the GPS time-transfer system will serve as a truly international time standard available to anyone on earth.

Institutions and people

PARCS is a cooperative effort between the following organizations:


Bill Klipstein of JPL was the Project Scientist, and Dave Seidel of JPL was the Project Manager. The Co-Principal Investigators were Don Sullivan and Bill Phillips of NIST, and Neil Ashby of the University of Colorado. John Lipa of Stanford University was the Principal Investigator for SUMO and John Dick of JPL serves was the Project Scientist for that program.


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