GPS time interval and state measurement for PARCS

A science-quality space GPS receiver is being studied for the primary atomic reference clock in space (PARCS) mission. The PARCS flight experiment is an International space station (ISS) payload that will conduct investigations into the laser cooling of atoms, time interval measurement, and fundamental physics. The receiver will make GPS carrier phase observations, to transfer the frequency measurements made by other PARCS subsystems to the ground and to determine the experiment´s precise position and velocity. The receiver is based on the Jet Propulsion Laboratory´s BlackJack radiometric instrument. This is a dual frequency, codeless design that is a veteran of multiple spaceflights. The major challenges for its use on PARCS derive from the ISS environment, for example, the antenna field of view, multipath sources, and potential electromagnetic interference. Simulations indicate that the restricted field of view will be the main limitation, and that the receiver antenna should be tilted away from the ISS structure by ∼30° for better results. The use of GPS ground networks and data analysis techniques to provide a total measurement system adequate to meet PARCS´ requirements will need to be examined further.