PNT Roundup: Exploring X-ray navigation in space

Neutron-star Interior Composition Explorer, or NICER, is an external attached payload on the International Space Station. (Image: NASA Goddard Space Flight Center)

A team of engineers at the U.S. National Aeronautics and Space Administration (NASA) has demonstrated fully autonomous X-ray navigation in space — a capability that could enable robotic spacecraft to navigate beyond the edges of the solar system.

The experiment, Station Explorer for X-ray Timing and Navigation Technology (SEXTANT), showed that millisecond pulsars could be used to accurately determine the location of an object moving at thousands of miles per hour in space, functioning in a way similar to GPS.

The system provides a new option for spacecraft to autonomously determine their locations outside Earth-based global navigation networks because pulsars are accessible in virtually every conceivable flight regime, from low-Earth to deepest space.

The SEXTANT demonstration used the 52 X-ray telescopes and silicon-drift detectors that make up NASA’s Neutron-star Interior Composition Explorer (NICER), an external attached payload on the International Space Station.

The size of a washing machine, NICER studies neutron stars, which emit radiation across the electromagnetic spectrum. Incredibly dense — one teaspoonful of neutron star matter would weigh a billion tons on Earth — these objects would collapse into black holes if compressed any further.

Pulsars. The SEXTANT experiment focuses on a particular type of neutron star: pulsars, highly magnetized, rotating neutron stars. Their electromagnetic radiation can be observed only when the beam of emission points toward Earth, thus their pulsed appearance. The short, regular rotational period produces a precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. These predictable pulsations can provide high-precision timing information similar to the atomic-clock signals supplied through GPS.

Demonstration. A demonstration in November 2017 selected four millisecond pulsar targets — J0218+4232, B1821-24, J0030+0451 and J0437-4715 — and directed NICER to orient itself so it could detect X-rays within their sweeping beams of light. These millisecond pulsars are so stable that their pulse arrival times can be predicted to accuracies of microseconds for years into the future.

During the two-day experiment, the payload generated 78 measurements to get timing data, which the SEXTANT experiment fed into its onboard algorithms to autonomously stitch together a navigational solution that revealed the location of NICER in its orbit around Earth. The team compared that solution against location data gathered by NICER’s onboard GPS receiver.

“For the onboard measurements to be meaningful, we needed to develop a model that predicted the arrival times using ground-based observations provided by our collaborators at radio telescopes around the world,” said Paul Ray, a SEXTANT co-investigator with the U.S. Naval Research Laboratory. “The difference between the measurement and the model prediction is what gives us our navigation information.”

The goal was to demonstrate that the system could locate NICER within a 10-mile radius as the space station sped around Earth at slightly more than 17,500 mph. Within eight hours of starting the experiment on Nov. 9, the system converged on a location within the targeted range of 10 miles and remained well below that threshold for the rest of the experiment. In fact, a good portion of the data showed positions that were accurate to within three miles.

GPS-level accuracy on the order of a meter or less is not necessary when navigating the far reaches of the solar system, where distances between objects measure in the millions of miles. “In deep space, we hope to reach accuracies in the hundreds of feet,” said Mitchell.

The team will now focus on updating and fine-tuning both flight and ground software in preparation for a second experiment later in 2018. The ultimate goal, which may take years to realize, is to develop detectors and other hardware to make pulsar-based navigation readily available on future spacecraft.

To advance the technology for operational use, teams will focus on reducing the size, weight and power requirements and improving the sensitivity of the instruments. The SEXTANT team now also is discussing the possible application of X-ray navigation to support human spaceflight.

If an interplanetary mission to the moons of Jupiter or Saturn were equipped with such a navigational device, for example, it would be able to calculate its location autonomously, for long periods of time without communicating with Earth.

“This successful demonstration firmly establishes the viability of X-ray pulsar navigation as a new autonomous navigation capability,” said project manager Jason Mitchell. “We have shown that a mature version of this technology could enhance deep-space exploration anywhere within the solar system and beyond.”