Relativity and Timing in X-ray Pulsar Navigation

XNAV is a technology demonstration involving many organizations that will use photons from X-ray pulsars for navigation and spacecraft attitude determination. This paper summarizes relativistic effects in the context of XNAV. It also characterizes the primary task in the time domain of realizing an on-board master clock that time-tags detected X-ray photons with sufficient accuracy to permit meaningful navigation solutions. XNAV must first estimate the periods of uncatalogued X-ray pulsars to determine suitable candidate pulsars for navigation. This task will use an efficient search algorithm to determine the pulsar period from a sensor aimed at the pulsar. As a part of this search and catalogue task, an accumulator that integrates photon counts will compute average counts per sampling time interval, in time bins that are small compared to the pulsar's period. This operation is dubbed the pulsar profiler function. It is intended to build a reference or standard profile of a chosen pulsar for later use. The search and catalogue need to be sufficient for navigation based on times-of-arrival of pulsar signals in real time vs. the on-board reference clock. Operationally, the timing module locates in time the highest peak (or other defined phase center) in the group velocity of received, periodic plane-wave pulses from catalogued pulsars. The goal is to permit navigation accuracy approaching 100 m. This will be accomplished by cross-correlation of catalogued profiles to incoming profiles based on X-ray sensor data collected in real time