A team of researchers at the University of Illinois Urbana-Champaign have found a way for travelers through the Solar System to work out exactly where they are, without needing help from ground-based observers on Earth. They have refined the pulsar navigation technique, which uses X-ray signals from distant pulsars, in a way similar to how GPS uses signals from a constellation of specialized satellites, to calculate an exact position .
Navigating through space
Before you can navigate a course through space, you need to know your location and orientation. Current space craft can only discover one of these things independently. It can find its orientation, or the direction in which the space craft is pointing, fairly easily. Onboard cameras can search for bright stars or the Sun, and use them as a reference.
But position is a much harder 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 this doesn’t work in deep space where landmarks are too far to measure. Instead, networks of tracking stations on Earth monitor radio signals from the spacecraft. They calculate it’s distance by measuring the time delay, and combine this 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 learn the precise location of the spacecraft, we rely on radio signals sent between the spacecraft and the Earth, which can take a lot of time and requires use of oversubscribed infrastructure, like NASA’s Deep Space Network,” said Zach Putnam, professor in the Department of Aerospace Engineering at Illinois.
This standard, tried-and-tested 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 out into space, signal round-trip times will get longer. This means that navigational measurements will take more and more time to complete. Future space missions, crewed or otherwise, will eventually need to be able to navigate themselves, without guidance from Earth. Fortunately, X-ray pulsar-based navigation (XNAV), which operates on similar principles to GPS, could be the answer.
Pulsars are rapidly rotating neutron stars; the final remains of stars that died in a supernova explosion. With their fast spins and strong magnetic fields, they generate powerful beams of radiation, sweeping across the sky. Individual pulsars spin at different speeds, from as slow as several times per second, up to hundreds of revolutions per second. Each pulsar has a unique signature, which makes them identifiable with even a very simple radio receiver.
Putnam and his team have found a way to detect and process pulsar signals more efficiently. This lets a space craft use a small antenna and simple receiver to detect the X-ray emissions of multiple pulsars. Since these signals are so consistent and predictable, the receiver can calculate exactly when a given pulse will arrive at any 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 where that particular pulse has just arrived.
This wavefront is similar to a wave travelling across the surface of a pond. If you have two waves travelling in the same pond, however, you get visible nodes. These nodes mark the places where the different wavefronts intersect. The regions in space where the 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 the nearest five kilometers.
The work done by Putnam and his team has focused on the computing algorithms necessary to predict how the wavefronts from known pulsars will interact at any given location. Their goal is to find the most efficient way to do these calculations, with the least amount of computing power.
“We used the algorithm to study which pulsars we should observe to reduce the number of candidate spacecraft locations within a given domain,” said Putnam.
According to their work, the best results are found when using signals from pulsars with a low angular separation — pulsars that appear to be close together in the sky — and that pulse more slowly. They also confirmed that adding more pulsars improves precision. This is easier than trying to improve signal quality, so future space craft will be able to use XNAV with cheaper and simpler radio receiving equipment.
You can read more in their study, “Characterization of Candidate Solutions for X-Ray Pulsar Navigation”. This paper was published in IEEE Transactions on Aerospace and Electronic Systems.