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Nasa develops GPS-like system for interstellar satellites based on X-rays

Nasa engineers have demonstrated fully autonomous X-ray navigation in space, which could allow the space agency to pilot robotic spacecraft to the far reaches of the solar system and beyond.

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

It works in a similar fashion to the GPS system that provides positioning, navigation, and timing services to users on Earth with a constellation of 24 operating satellites.

“This demonstration is a breakthrough for future deep-space exploration,” said SEXTANT project manager Jason Mitchell, an aerospace technologist at Nasa’s Goddard Space Flight Centre. “As the first to demonstrate X-ray navigation fully autonomously and in real-time in space, we are now leading the way.”

This technology provides a new option for deep-space navigation that could work in concert with existing spacecraft-based radio and optical systems.

Although it could take a few years to develop an X-ray navigation system that is practical for use on deep-space spacecraft, eventually it will allow them to autonomously determine their locations outside the currently used Earth-based global navigation networks.

“We’re doing very cool science and using the space station as a platform to execute that science, which in turn enables X-ray navigation,” said Goddard’s Keith Gendreau. “The technology will help humanity navigate and explore the galaxy.”

NICER, a Nasa observatory about the size of a washing machine that was installed on the International Space Station last year, is currently is studying neutron stars and their rapidly pulsating cohort, called pulsars.

Although these stellar oddities emit radiation across the electromagnetic spectrum, observing in the X-ray band offers the greatest insights into these unusual, incredibly dense celestial objects, which, if compressed any further, would collapse completely into black holes. Just one teaspoonful of neutron star matter would weigh a billion tons on Earth.

Although NICER is studying all types of neutron stars, the SEXTANT experiment is focused on observations of pulsars.

Radiation emanating from their powerful magnetic fields is swept around much like a lighthouse.

The narrow beams are seen as flashes of light when they sweep across our line of sight. With these predictable pulsations, pulsars can provide high-precision timing information similar to the atomic-clock signals supplied through the GPS system.

The SEXTANT team selected four millisecond pulsar targets and directed NICER to orient itself so it could detect X-rays within their sweeping beams of light.

The millisecond pulsars used by SEXTANT 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 on-board algorithms to autonomously stitch together a navigational solution that revealed the location of NICER in its orbit around Earth as a space station payload. 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 November 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, Mitchell said. In fact, “a good portion” of the data showed positions that were accurate to within three miles.

“This was much faster than the two weeks we allotted for the experiment,” said SEXTANT System Architect Luke Winternitz, who works at Goddard. “We had indications that our system would work, but the weekend experiment finally demonstrated the system’s ability to work autonomously.”

Although the ubiquitously used GPS system is accurate to within a few feet for Earth-bound users, this level of accuracy is not necessary when navigating to 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,” Mitchell said.

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