To keep GPS constellations from drifting, look to the moon

Global positioning systems are a key feature in everyday life for much of the world’s population. Whether it’s ensuring a ship delivers its goods safely or helping you avoid a road closure on your commute home, GPS can help.

But the system has a weakness—it relies on ground stations that could suffer from blackouts, natural disasters or cyberattacks. And if those ground stations go offline for long enough, the whole system can begin to “drift.”

Aerospace engineers have been attempting to counteract this problem for years, but so far with little success.

A new paper from a research team led by Xia Lin and Baojun Lin at the Shanghai Engineering Center for Microsatellites, published in Satellite Navigation, explains a potential answer to this problem—link the GPS satellites with some in orbit around the moon.

Earth-only links cannot anchor positions

Understanding the problem requires some explanation of orbital mechanics. In addition to their links with ground stations, traditional Earth navigation constellations use a technique called time-division multiple access to keep in contact with their neighboring satellites, measuring relative distances accurately down to the centimeter level. While that’s useful for determining how far apart the satellites are, it can’t establish their absolute positions in space.

Earth’s gravity around its rotational axis is nearly symmetrical, so all of the satellites in an “orbital shell” can drift relative to their positions on the ground simultaneously. This drift, known to orbital dynamicists as an orientational rank deficiency, slowly introduces position errors into the system if left unchecked by ground stations. And if those ground stations are offline for days or weeks at a time, those position errors could grow to significant distances.

Aerospace engineers have tried several other solutions to this problem, including star trackers, pulsar counters and “forecasting” an orbit ahead of time. But none work with the precision needed to keep the global GPS system functioning at the level it’s expected to. So the researchers turned to a novel solution—the moon.

Lunar orbits provide a reference

More precisely, the solution involves satellites orbiting the moon, in what is called cislunar space. Critically, they have to be in orbit around the moon rather than Earth to make sure the “drift” that affects the Earth-bound constellations can be noticed and corrected.

For their lunar satellites, the researchers chose elliptical lunar frozen orbits (ELFOs), the orbital paths that NASA’s LunaNet and ESA’s Moonlight projects are planned to use in support of missions at the moon’s south pole.

Less drift in the simulated network

To test their idea, the researchers set up a 60-day simulation using real data from 24 operational BeiDou-3 medium Earth orbit (MEO) satellites and a simulated cross-link with four ELFO orbiters. They found that, left entirely to traditional Earth-only cross-links with no ground station correction, the system drifted around 7.85 m (25.8 feet) over the course of two months.

That drift decreased significantly when they used trajectory prediction, dropping to just 0.6 m (2 feet) by day 60 if the satellites predicted how their orbits were changing. But the best results came from the simulated joint Earth–moon network, with just 0.35 m (1.1 feet) of drift over the course of the two-month experiment.

An added benefit of this setup is that the correction works both ways. The Earth-bound satellites can offer corrective positional data to the moon-bound ones. Though the correction wasn’t quite as good, the four simulated ELFO satellites deviated by only 2.26 meters (7.4 feet) from their expected positions. Having well-positioned lunar infrastructure in place is critical to planned crewed operations there. And this sort of system could both solve a problem we have on Earth and provide those future explorers with the precise positioning they will need on the lunar surface.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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