PARCS: NASA's laser-cooled atomic clock in space

The Primary Atomic Reference Clock in Space (PARCS) mission is designed to perform certain tests of relativity theory, to study the performance of individual GPS space-vehicle clocks, to study the dynamics of atom motion in microgravity, to advance the state-of-the art for space clocks, and to serve as a pathfinder for precision instruments based on laser cooling of atoms. After a brief overview of the project, this paper discusses the specific objectives of PARCS, describes the key subsystems, and discusses the systematic frequency shifts that limit the accuracy of the clock.