What If GPS Stood for “Galactic Positioning System”?

NASA scientists demonstrate how to navigate in space using signals from distant pulsars

6 min read

NICER Payload onboard the International Space Station
Illustration: NASA

At the American Astronomical Society’s 231st meeting, in Washington, D.C., earlier this month, Keith Gendreau, principal investigator for NASA’s Neutron-star Interior Composition Explorer (NICER) mission described something remarkable: the first successful demonstration of a system to use pulsars for navigation in space.

The basic idea is similar to what is done with the Global Positioning System (GPS) or other global satellite navigation systems. When you use GPS to find your way to Starbucks, you are depending on transmissions from an array of satellites whose positions are precisely known. The timing of the signals you measure can thus be used to deduce the position of the receiver. That works only if the receiver is on Earth or near Earth, however. If you wanted to visit a Starbucks in deep space, you have to find it by some other means.

Right now, deep-space navigation mostly depends on using radio signals sent from Earth to the distant space probe—signals that must be sent with giant antennas. The probe responds by sending a signal back. So it’s not hard to figure out range—overall distance—with good precision from how long a signal traveling at the speed of light takes to get to the probe and back. But angles are tougher to nail down. As a result, such position fixes degrade as you move away from Earth. Indeed, for critical operations, like insertion into orbit at the distances beyond Jupiter, space navigation done this way is especially challenging.

How then can spacecraft traveling far from Earth navigate precisely through the heavens? One possibility is to use pulsars as natural GPS beacons of a sort. To understand how that would work, though, you first need to know a little something about pulsars.

The first pulsar was discovered in 1967, when radio astronomers Jocelyn Bell Burnell and Antony Hewish, using a radio telescope that they had cobbled together at the University of Cambridge, detected oddly regular pulses coming from a distant celestial object. Initially, they dubbed the signal LGM-1, an acronym that stood for “Little Green Men.” While they didn’t seriously believe they had uncovered signals from intelligent extraterrestrials, they were at a loss to otherwise explain the phenomenon.

Soon afterward, other pulsar signals were found, and a model emerged for how these pulsating radio signals might arise naturally.

Pulsars appear to be neutron stars with strong magnetic fields. The poles of the magnetic field do not coincide with the star’s rotation axis, so they rotate with the star, channeling electromagnetic energy in a direction that sweeps rapidly across the sky like the beam from a lighthouse. When the Earth is momentarily illuminated with some of that energy, a pulse is received by the radio astronomer’s antenna. And those pulses repeat at regular intervals, controlled by the rate of spin of the neutron star.

In 1974, George S. Downs working at NASA’s Jet Propulsion Laboratory proposed the possibility of using radio pulsars for spacecraft navigation. He suggested that positions could be determined to within 150 kilometers with a suitable radio antenna and something like 24 hours’ worth of measurements.

The idea of using pulsars for space navigation got a boost in 1981, when other researchers at the Jet Propulsion Lab (JPL) suggested using X-ray pulsars instead of radio pulsars for this purpose. Doing so would allow a more compact antenna to be employed on the spacecraft and offered the potential for higher-precision fixes.

The following year a new class of these enigmatic objects was recognized—so-called millisecond pulsars. The first to be found had a repetition interval of just 1.6 milliseconds, indicating that the body that created it spun at more than 38,000 rotations per minute. The signals from millisecond pulsars are particularly regular, rivaling the oscillators in atomic clocks in their stability.

Then in 1993, millisecond X-ray pulsars were discovered. These potential sources of navigation signals offered both “stable clocks” and the ability to receive their signals with compact X-ray antennas rather than large radio dishes. A proposal came that very year to test the possibility of using millisecond X-ray pulsars for space positioning using the X-ray sensor on the ARGOS satellite, which was slated to be launched later in the decade.

The ARGOS experiment was carried out in 1999, although it didn’t reveal much. But the idea of using X-ray millisecond pulsars for space navigation continued to be investigated. For example, Suneel Sheikh studied it as a Ph.D. student at the University of Maryland. And researchers at the U.S. Naval Research Laboratory, located near Washington, D.C., mounted a DARPA-funded project to further develop this technology. And like all good space tech, the approach was given an upper-case moniker—XNAV—even though at that stage it was still just an idea.

Fast forward to November of 2016 when China launched its X-Ray Pulsar Navigation-1 satellite. With it, Chinese researchers had hoped to be able to demonstrate the use of X-ray pulsars for navigation. But according to Jason Mitchell, who manages the pulsar-navigation project at NASA’s Goddard Space Flight Center, the Chinese satellite lacked sufficient sensitivity—or so it would appear from results that Chinese researchers published in June of 2017. “They spent most of their time looking at the Crab pulsar, which is quite bright,” says Mitchell.

About the time those Chinese results were coming out, NASA launched the NICER X-ray telescope. With it, Gendreau, Mitchell, and others at NASA hoped to mount a successful demonstration that X-ray pulsars could be used for navigation. That experiment, which has its own acronym, SEXTANT (for Station Explorer for X-ray Timing and Navigation Technology), was piggybacked on the NICER mission, which mostly has scientific rather than engineering objectives.

The relevant observations took place in October and November of last year, when the NICER telescope, which is perched on the International Space Station, carefully measured signals from a small number of millisecond X-ray pulsars. “Our navigation goal was 10 kilometers,” says Mitchell. “We were able to do quite a bit better in less than 8 hours.”

How you translate pulsar measurements into a position fix is straightforward, at least conceptually. Consider the following thought experiment:

Imagine for a moment that your spacecraft is stationary. (That’s, of course, never going to be the case, but it makes it easier to visualize things.) Observe a known millisecond X-ray pulsar by pointing your X-ray telescope in its direction. Mark the time when one pulse is, say, at its peak intensity.

That pulse was emitted long ago from the distant pulsar and then spread outward through space. If you knew the exact time was when it was first emitted and the exact time you had received it, you’d be able to deduce how long it had been flying through space. Multiplying by the speed of light would provide you with a measurement of your distance from the pulsar.

All you would know, though, is that—your distance from the pulsar. So you could be anywhere on a giant spherical surface centered on the pulsar. And remember, the pulsar is emitting pulses at regular intervals, and you can’t distinguish one from another when you receive it. So, really, you could be on any of a large set of spherical surfaces concentrically arranged around that pulsar.

Because the pulsar is far, far away, these spherical surfaces don’t curve much in your vicinity: To you, they are essentially planes, positioned at regular intervals in space. And because you don’t know which pulse you had measured in the first place, there are many such planes, all parallel to one another, that you might in principle be located on.

If that is all the information you had, you wouldn’t know which of these surfaces you were on or which point on a surface you occupied. But you can now aim your X-ray telescope at other well-studied millisecond pulsars in different parts of the sky, and each would provide a similar set of parallel surfaces your X-ray telescope could possibly be on.

The key here is that there is only one place in space that is on a surface of possibility for each pulsar. So that’s the spot where you must be located. “That’s the way GPS works, too,” says Luke Winternitz, technical lead for the SEXTANT project at Goddard. More correctly, there would be only one solution that would be reasonable.

And that’s exactly what Winternitz and his NASA colleagues found, using several dozen sets of measurements of four carefully selected pulsars. Initially, they processed the data on the ground and were delighted to discover that they could calculate an accurate position. Later they were able to process the measurements on board the NICER instrument itself to produce a position solution. “Our mission requirement is to demonstrate this on board in real time,” says Winternitz.

I find this a stunning accomplishment. It suggests that future spacecraft, even ones exploring the fringes of the solar system, should be able to determine their positions autonomously to within a few kilometers. After decades of musing about this approach, some clever people have finally figured out how to make practical use of a constellation of navigation beacons fortuitously arranged around the galaxy by Nature herself.

It’s quite incredible, really. Indeed, it’s so incredible that some might see in this accomplishment hints that there could be more at work than nature here. Even before these measurements were taken by NICER, a group of researchers at the Free University of Brussels led by Clément Vidal explored whether these X-ray millisecond pulsars could have been arrayed around the galaxy on purpose. In a preprint titled “Pulsar Positioning System: A quest for evidence of extraterrestrial engineering,” they examine that albeit remote possibility.

While I’m far from convinced, it’s a fun notion to ponder—that some pulsars could be the handiwork of Little Green Men after all.

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