USE OF X-RAY STAR HORIZON CROSSINGS TO ENHANCE PERFORMANCE OF X-RAY NAVIGATION

A previous report (Wood et al., AAS 20-124) described the feasibility of using horizon crossings of celestial X-ray sources for satellite navigation, a technique of autonomous, on-board navigation that could complement the previously developed and tested method for X-ray pulsar navigation known as XNAV (Mitchell et al., AAS 18-155). The AAS 20-124 paper evaluates strengths and weaknesses of XNAV, and theorizes that horizon crossings could be used to gain navigational information that is complementary to XNAV. Subsequently, we have developed the Horizon Crossing Navigational Method (HCNM), which uses the same X-ray instrument that was used to perform the first real-time on-orbit demonstration of XNAV, the Neutron Star Interior Composition Explorer (NICER) telescope aboard the International Space Station (ISS). In this paper, we present preliminary resultsof HCNM for two NICER horizon crossings, using 1-5 keV X-rays, and explain how this technique can be used in conjunction with XNAV to perform more effective X-ray navigation near planets. Depending on the brightness of an X-ray source, HCNM can provide in-track position corrections of less than one kilometer and even better depending on the X-ray star’s brightness, provided the model for X-ray absorption in the planet’s atmosphere is not the limiting factor. Whereas XNAV can yield precise navigational results in between planets, HCNM can yield precise results around planets. Together, these two methods can be used can be used for accurate autonomous navigation anywhere in space throughout our Solar System and beyond.