NASA X-Ray Navigation System Aims to Be a Galactic GPS for Space Exploration

The navigation system uses x-ray light emitted from pulsars the same way global positioning systems use atomic clocks, which could eliminate the need for costly ground-based guidance systems.

This animation shows how NICER scans the sky and highlights the mission’s main features. | NASA's Goddard Space Flight Center
This animation shows how NICER scans the sky and highlights the mission’s main features. | NASA's Goddard Space Flight Center

NASA has successfully tested a new autonomous X-ray navigation system that could provide deep-space missions with a cosmic version of GPS. The system is a duo of experiments installed on the exterior of the International Space Station. They monitor the clockwork-like pulses from a naturally occurring source: distant pulsars, which are rapidly rotating neutron stars that emit regular beams of light.

“We are using pulsars as beacons to navigate through deep space,” Keith Gendreau, principal investigator for NICER (Neutron-star Interior Composition Explorer) said during a presentation the results at the American Astronomical Society meeting, which was held this week in Washington D.C. “The pulses are comparable to the ‘ticks’ from an atomic clock, which forms the basis of GPS. So we use the pulsars in the same way we use atomic clocks in GPS satellites.”

NICER, along with its sister experiment called SEXTENT (Station Explorer for X-ray Timing and Navigation Technology) were brought to the ISS in June 2017 aboard SpaceX's eleventh Dragon cargo resupply mission.

For the test, conducted in November of last year, the research teams showed that millisecond pulsars — ones that rotate hundreds of times a second — could be used to accurately determine the location of an object moving at thousands of miles per hour through space.

“NICER is an X-ray telescope that is trying to understand the nature of ultra-dense matter by looking at the X-ray timing characteristics of neutron stars and pulsars,” Gendreau explained in the briefing. “SEXTENT is flight software that works within our computer on NICER. It looks at a subset of pulsars that we are studying for science purposes anyway, to do a new type of navigation that makes use of infrastructure of pulsars that is naturally available around our galaxy.”

This illustration shows the NICER mission at work aboard the International Space Station. | NASA's Goddard Space Flight Center

About 2,000 known pulsars are distributed across the sky, with about 200 known millisecond pulsars. While pulsars emit electromagnetic radiation in multiple wavelengths, the light beaming from pulsars is most visible in the X-ray spectrum.

Gendreau said the system is “very cool science” and will help humanity navigate and explore the galaxy. NASA says this technology will potentially “enable sustained human presence throughout the solar system, as well as enhance and enable science in the outer solar system and beyond.”

Since the early days of space exploration, spacecraft have used NASA’s Deep Space Network (DSN) to communicate with Earth and navigate through space. The European Space Agency has also developed a similar system called the European Space Tracking (ESTRACK) network.

These are both international arrays of giant radio antennas located approximately 120 degrees apart on Earth to ensure that any satellite in space is able to communicate with at least one station at all times. Navigators for each of the missions process the radiometric-tracking data received from the networks to determine the spacecraft’s position and velocity. They also use optical data, where the spacecraft takes a picture of the star background to help refine the spacecraft’s trajectory.

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For years, scientists and engineers around the world have been exploring the feasibility of using X-ray pulsars for spacecraft navigation. Where the DSN and ESTRACK networks require a spacecraft to communicate with ground-based systems, the use of pulsars would enable autonomous navigation — at least to some extent — on-board the spacecraft and minimize communications with Earth. Proponents say this would also offer the potential of lower mission operating costs due to the reduced need for ground infrastructure.

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

“The cost savings come in that you don’t have to use resources from the DSN as much,” he said. “And, in a sense, it is priceless because it will enable missions to extreme deep space, and greater accuracy for all missions.”

NICER’s mirror assemblies concentrate X-rays onto silicon detectors to gather data that probes the interior makeup of neutron stars, including those that appear to flash regularly, called pulsars. | NASA's Goddard Space Flight Center/Keith Gendreau

Gendreau said the technology is not complicated or expensive, as it is essentially an X-ray spectrometer installed on the spacecraft and it also could be used as science tool.

NICER is about the size of a washing machine and has 52 small X-ray telescopes and silicon-drift detectors for studying neutron stars and pulsars. For two days in November 2017, NICER monitored five pulsars. SEXTANT then used these measurements to triangulate the position of the ISS in its orbit around Earth. By measuring tiny changes in the arrival time of the signals, the system could autonomously calculate its own position in space. These measurements were compared with GPS readings from satellites in Earth orbit.

Gendreau said that within 8 hours, the system had calculated the spacecraft’s position below the goal of a 10-kilometer radius and it remained at that threshold for the remainder of the experiment. He said that is more accurate than what is currently available from DSN. The team hopes to push down the errors to 5 km or less in a subsequent test to be done later this year.

“The goal is to try and turn the ‘G’ in GPS from ‘global’ to ‘galactic,’” Jason Mitchell, SEXTENT project manager, said in a video about the experiment.

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