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X-ray sextant

Mar 1, 2018
An illustration showing the X-ray sextant mounted on the International Space Station.

An illustration showing the X-ray sextant mounted on the International Space Station.

Courtesy NASA

Recently, the U.S. Naval Academy resumed the instruction of celestial navigation to midshipmen after cutting that field of study in the early 2000s. Nobody expects celestial navigation to unseat GPS from its premier position as the bedrock of modern navigation, but celestial nav can be an effective and reliable backup. Now, NASA is testing a type of celestial navigation system for use on future voyages — voyages deep into the solar system. And in a nod to the history of navigation, NASA even fooled around with the program’s acronym to produce an evocative name: Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT.

The need for a new way of navigating through interplanetary space for both manned and unmanned robotic probes is pressing because current approaches have big drawbacks. As NASA explains on the SEXTANT website, “NASA’s plans for distant exploration demand breakthrough navigation tools.” Future manned missions to Mars will require precise navigation — especially if SpaceX president Elon Musk is able to carry through with his ambitious plans to shuttle people to Mars and possibly beyond. Plus, there are compelling reasons to visit the outer planets, such as Jupiter’s moon Europa and Saturn’s moon Enceladus, both of which may have deep subsurface oceans.

GPS is Earth-centered and not useful for navigation once astronauts (what will SpaceX crewmembers be called — Xtranauts?) are on their way to Mars or for an unmanned probe headed for Europa.

One way to perform this guidance task is to use optical equipment to sight on the light coming from stars. This method has been used for decades in providing guidance for ballistic missiles. Such an approach has drawbacks, however, in that it only confirms the orientation of the spacecraft but not its position. Inertial navigation units can be used, but inertial units accumulate error that gets larger over time.

Another way is to do the navigation from Earth. Spacecraft are tracked with Earth-based radar, which is used to determine the spacecraft’s position. Then the position is sent to the spacecraft via radio — a technique commonly used for space probes to the outer planets. This method also has problems in that the spacecraft is not autonomous but relies on the data link to Earth. And that tracking and radio relay effort uses a sizeable chunk of NASA communications assets, meaning they can’t be used for other tasks. Again, from NASA: “The best current capabilities are resource-intensive and degrade as explorers recede from Earth. At Mars, they yield cross-range spacecraft positions to a few kilometers but impose scheduling burdens on the Deep Space Network (DSN). For critical applications, such as orbit insertion at Jupiter and beyond, the current state-of-the-art is pushed to its practical limit.”

A much better system would be one that allows the spacecraft to perform its own position calculations. In the hope of developing this autonomous approach, NASA put together the SEXTANT program.

Instead of using visible light from stars, the SEXTANT idea makes use of X-rays coming from a special type of star called a pulsar that can be used like a set of radio beacons to get a fix position. A pulsar is a dense star that has experienced a massive gravitational collapse and has an intense magnetic field. These bodies are so dense that a teaspoonful would weigh billions of tons on Earth. As a pulsar rotates, it produces waves of electromagnetic radiation, including X-rays. Some pulsars are extremely stable in their X-ray output and these millisecond pulsars can be used as timing sources and for position finding.

For the SEXTANT program, NASA built a unit that detects X-rays from these pulsars. The receiver measures the arrival times of X-ray pulses from multiple pulsars and then a sequence of measurements is stitched together into a position solution. Much like sextant observations of different stars in celestial navigation, this X-ray technique uses pulsars from different angles to get a similar position fix with good geometry.

NASA placed the SEXTANT unit on the International Space Station in June 2017 and a team led by the Goddard Space Flight Center in Greenbelt, Md., conducted tests of this X-ray navigation technique (called XNAV).

The tests revealed that the navigation concept is viable. “This demonstration is a breakthrough for future deep space exploration,” said SEXTANT project manager Jason Mitchell, an aerospace technologist at Goddard. “As the first to demonstrate X-ray navigation fully autonomously and in real time in space, we are now leading the way.”

According to NASA, researchers were hoping to show that the technique could locate the space station to within a 10-mile radius as it orbited the Earth at 17,500 mph. The system was able to achieve 10-mile accuracy. Mitchell reported that “a good portion” of the data showed positions accurate to within three miles.

So perhaps in the future, the tradition of the marine sextant will live on in the space navigation techniques pioneered by NASA’s SEXANT program.

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