This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
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.
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.
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