Inverse - Square Law Experiment in Space

After presenting a very brief outline of this flight program, this paper will dwell in more detail on the key challenges that must be met for successful flight. These include the comparison of the frequency of the PARCS cesium clock with Earth-bound cesium clocks using the GPS carrier-phase method, the implementation of the phase modulation scheme needed to minimize the impact of space-station vibrations on the cesium clock, the evaluation of the cesium spin-exchange frequency shift, the design of shutters for the cesium beam, and the acquisition of tracking and acceleration data essential to measurement of the relativistic frequency shift and transfer of the realization of the second to Earth. Among advances to be reported in this paper are the development of the new concept of phase modulation and detailed analysis of the limitations of the carrier-phase frequency-comparison system. Preliminary designs for several key components of the flight package will be presented and discussed. PARCS successfully completed its Science Concept Review in January 1999 and its Requirements Design Review in December 2000, and is now moving toward its Preliminary Design Review.