A team of researchers from the University of Illinois at Urbana-Champaign have found a way for travelers across the solar system to determine exactly where they are, without needing help from ground observers on Earth. . They refined the pulsar navigation technique, which uses X-ray signals from distant pulsars, in a manner similar to how GPS uses signals from a constellation of specialized satellites, to calculate an exact position.
Navigate in space
Before you can follow a course in space, you need to know your location and orientation. Current spacecraft can only discover one of these things independently. It can find its orientation, or the direction the spacecraft is pointing, quite easily. On-board cameras can search for bright stars or the Sun and use them as a reference.
But position is a much more difficult problem to solve. On Earth, or even in Low Earth Orbit (LEO), you can measure the distance to nearby reference points and then refer to a map. But it doesn’t work in deep space where landmarks are too far away to measure. Instead, networks of tracking stations on Earth monitor radio signals from the spacecraft. They calculate its distance by measuring the delay and combine it with the direction from which the signal arrived to accurately calculate its position in space. The ground station can then transmit this information to mission control or to the spacecraft itself.
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“We can use star trackers to determine the direction a spacecraft is pointing, but to know the precise location of the spacecraft we rely on radio signals sent between the spacecraft and Earth, which can take very time consuming and requires the use of oversubscribers. infrastructure, like NASA’s Deep Space Network,” said Zach Putnam, a professor in the Illinois Department of Aerospace Engineering.
This proven standard system works, but involves a trade-off. As the number of active space missions increases, access to communications infrastructure becomes increasingly contested. And as we send spacecraft further into space, the round trip times for signals will become longer. This means that navigation measurements will take longer and longer. Future space missions, crewed or unmanned, will eventually need to be able to navigate on their own, without guidance from Earth. Fortunately, X-ray pulsar-based navigation (XNAV)which works on similar principles to GPS, could be the answer.
Pulsars are rapidly rotating neutron stars; the last remnants of dead stars in a supernova explosion. With their rapid rotations and strong magnetic fields, they generate powerful beams of radiation, sweeping across the sky. Individual pulsars spin at different speeds, ranging from several times per second to hundreds of revolutions per second. Each pulsar has a unique signature, which makes them identifiable even with a very simple radio receiver.
Putnam and his team have found a way to detect and process pulsar signals more efficiently. This allows a spacecraft to use a small antenna and a simple receiver to detect X-ray emissions from multiple pulsars. Because these signals are so consistent and predictable, the receiver can calculate exactly when a given pulse will arrive at a given location in the solar system. As each pulse travels through space, it forms a “wavefront” – a curved region of space marking all the places that particular pulse just arrived.
This wavefront is similar to a wave moving across the surface of a pond. If you have two waves traveling in the same pond, however, you get visible knots. These nodes mark the places where the different wavefronts intersect. Regions of space where X-ray pulses from two pulsars interact form a similar pattern of nodes. The more additional pulsars you add, the rarer these nodes become, until you can accurately pinpoint your location in the solar system to within five kilometers.
The work done by Putnam and his team focused on the computer algorithms needed to predict how the wavefronts of known pulsars will interact at a given location. Their goal is to find the most efficient way to perform these calculations, with the least computing power.
“We used the algorithm to study which pulsars we should be observing to narrow down the number of candidate spacecraft locations in a given domain,” Putnam said.
According to their work, the best results are obtained by using signals from pulsars with small angular separation — pulsars that appear to be close to each other in the sky — and which pulsate more slowly. They also confirmed that adding more pulsars improves accuracy. This is easier than trying to improve signal quality so that future spacecraft can use XNAV with cheaper and simpler radio receiving equipment.
You can learn more in their study, “Characterization of candidate solutions for X-ray pulsar navigation”. This article was originally published in IEEE Transactions on Aerospace and Electronic Systems.