NICER / ISS Science Nugget for November 17, 2017
Autonomous X-ray Navigation Successfully Demonstrated
In a technology first, the SEXTANT team has successfully demonstrated fully autonomous, real-time X-ray pulsar navigation (XNAV) on-board the NICER mission. SEXTANT's primary demonstration goal, or Key Performance Parameter (KPP), sought to achieve an orbit determination error of less than 10 km worst direction, taken as 17 km of Root-Sum-Squared (RSS) 1σ error, using up to two weeks of X-ray pulsar observations. A future 'stretch' goal will seek to reach an error of less than 1 km worst direction, or 1.7 km RSS 1σ.
On 2017-11-09, the SEXTANT and NICER team developed and loaded a millisecond pulsar-rich observation schedule informed by lessons learned through recent analysis of navigation performance in numerous ground processing evaluation campaigns using NICER flight data. At 20:33:27 UTC, the SEXTANT flight software was initiated, and the GEONS navigation filter was initialized with a degraded state – i.e., position and velocity knowledge – designed to diverge rapidly from the truth state without X-ray pulsar observations. During the following 2.2 day autonomous experiment, the SEXTANT flight software significantly surpassed the primary KPP by reaching <10 km RSS error in approximately 7.5 hours and remaining well below that threshold for the duration of the experiment. A total of 78 measurements were generated from the X-ray "beacon" pulsars J0218+4232 (33), B1821-24 (30), J0030+0451 (13), and J0437-4715 (2).
In the accompanying figure, the flight system's predicted performance without measurements, represented in blue, rapidly diverges from the initial state, as expected. The autonomous flight software performance, while processing measurements, is shown in green and initially displays error growth similar to the no-measurement case. Within 7.5 hours, the real-time error reaches and remains below 10 km RSS for the duration of the experiment.
This successful demonstration firmly establishes the viability of X-ray pulsar navigation as a new autonomous navigation capability to enable and enhance deep-space exploration anywhere within the Solar System, and potentially beyond. The current flight results will be used to update and tune the flight software, as well as the ground segment software, for a second navigation experiment later in the NICER nominal mission, where SEXTANT's pulsar timing models will be developed using only NICER data.