EXACT Research Significance

GPS-Denied Navigation

Global Navigation Satellite Systems (GNSS) is a term used to collectively describe satellite-based positioning and timing systems. The Global Positioning System (GPS), operated by the United States Department of Defense, is perhaps the most well-know and widely used GNSS. The sub-meter accuracy of its position solution has revolutionized space operations. Its timing function is the de-facto “heart beat” for clocks in use by DoD authorized users in support of communication and network applications which require oscillators in devices to be synchronized to better than a micro-second accuracy.

GNSS

However, it is well documented that GPS has vulnerabilities which could be exploited to deny its use on orbit. While the availability of advanced signals on the newer GPS satellites (e.g., M-Code on GPS III) will make dealing with malicious jamming and interference easier, it will not eliminate the threat. As such, it is the practice of Positioning, Navigation, and Timing (PNT) system designers to design and field multi-sensor systems which are not reliant on a single sensor or system for operation.

This has been the impetus for increased interest in using signals of opportunity (SOP) to enhance the robustness of PNT systems. SOPs are signals that were not conceived for PNT purposes but can be used opportunistically to generate a navigation or timing solution. SOPs can be natural (e.g., Earth’s magnetic field, sferics) or artificial (e.g. HDTV signals). EXACT’s primary mission is to use X-ray emissions from pulsars as SOPs for clock synchronization.

Space Weather

Due to its simplicity, high timing precision, and energy range, the payload can also be used for solar physics measurements. EXACT’s secondary mission will be to leverage this versatility and make solar activity measurements. In this role, EXACT will serve as a pathfinder for a set of CubeSats that can provide continuous, long-term, solar high-energy X-ray monitoring, and can, with serendipitous co-observations, perform groundbreaking new science in the study of solar flare-accelerated electrons, helping to understand the basic generation of the powerful solar sources of space weather.

Some flares produce coronal mass ejections (CMEs), in which a massive amount of plasma is thrown off the Sun; these are the origins of the most extreme space weather events that, if Earth-directed, produce high radiation levels in near-Earth space.  It is not known how flares accelerate electrons and ions up to such high energies, but it is known that accelerated electron populations produce high-energy (“hard”) X-rays via the bremsstrahlung process. Furthermore, previous observations have confirmed the existence of sub-second hard X-ray bursts which provide a powerful diagnostic tool for nonthermal particle acceleration. However, observations of these pulsations have been incredibly sparse over the years. EXACT seeks to observe accelerations on a sub-second timescale in order to better inform models of solar flares and CMEs.

Cartoon of a solar flare, showing magnetic field lines reconnecting in a \