X-ray Pulsar Augmented GPS Navigation in Cislunar Space

The growing international interest in missions to the lunar surface, lunar orbits and cis-Lunar space has created a need for autonomous navigation capabilities in regions far removed from the normal operating environments of LEO to GEO orbits.  These missions include commercial endeavors like Xplore and CLPS as well as NASA missions such as Biosentinel, Lunar Flashlight, EM-1, CapStone, the Lunar Gateway, and manned exploration missions  and have created a need for autonomous navigation capabilities in this area as a means of supplementing the critical Deep Space Network.  CrossTrac Engineering, in cooperation with its partners at Asterlabs,  and Cateni proposes to develop an autonomous navigation system for spacecraft operating in cis-lunar and lunar orbits based on the integrated observations of one or more X-ray pulsars in conjunction with measurements of weak GPS main beam and side-lobe signals.  While it has been established that GPS measurements can be used for orbit determination above the GPS constellation altitude, the performance of GPS-only based systems degrades with increasing distance from the Earth.  This is due to both a decrease in signal strength with distance and the fact that the available GPS spacecraft appear to be closer and closer together with increasing distance, reducing the geometric degree of precision (GDOP).  The signals from one or more highly stable X-ray pulsars can be used as a sort of GPS pseudolite to augment the GPS constellation.  These pulsars, having stabilities on the order of atomic clocks, are distributed across the galactic sky offering navigation beacons that are not collinear with the spacecraft-Earth direction.  This provides a greatly improved geometry of reference beacons to the navigation system, increasing the overall navigation accuracy and measurement bandwidth.  The XNAV/GPS instrument combines an X-ray detector and associated timing electronics, with a low signal GPS receiver (e.g. a Blackjack GPS receiver) to make the combined XNAV/GPS measurement.  Based on existing technologies, this combined instrument will provide navigation and timing solutions to spacecraft operating autonomously in cis-lunar space without the need for new infrastructure on or about the moon.