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“You have arrived at your destination.” This phrase, delivered with robotic cadence, is familiar to anyone who uses satellite navigation systems to guide them on sightseeing strolls, cross-country road trips, and all manner of journeys from point A to point B.
But what if your destination is literally out of this world? Could something like the Global Positioning Service (GPS), the most well-known satellite navigation system, be extended to globes that are not Earth?
The answer is not only “yes,” but “soon,” according to space agencies involved in a new push to establish a GPS-like constellation around the moon. NASA and its partners in Europe and Japan are developing lunar satnav concepts that could be deployed by the end of the 2020s. In July, China’s National Space Administration (CNSA) unveiled its plans for a constellation of 21 communications and navigation satellites to support its lunar aspirations.
“There hasn’t been a thrust to translate all the communication and navigation infrastructure that exists on Earth to anywhere else in the solar system—until now,” says Bijunath Patla, a theoretical physicist at the US National Institute of Standards and Technology (NIST) who has published research into the logistics of these efforts. “This is the time when people are thinking of such a leap in technology.”
This leap is propelled by a surge of planned activity and exploration on the moon in the coming years that will demand sophisticated logistics, including the type of position, navigation, and timing (PNT) systems that underpin practically all of our infrastructure on Earth. The NASA-led Artemis Program aims to send astronauts on surface missions at the lunar south pole, a goal that necessitates reliable lines of communication and precision location services. China also plans to land crews on the moon this decade, and a host of other governmental and corporate entities are dispatching robotic explorers to the lunar surface in the near future.
The commercial space sector is keeping an eye out for opportunities that could arise in a fledgling lunar economy, such as resource extraction, low-gravity manufacturing, scientific research, or tourism. Whereas past moon missions relied on basic relays to communicate and navigate, the future of lunar exploration requires a satellite system fit to cover our natural satellite—or, at least, parts of it.
“GPS has been a backbone of our economy here on Earth,” says Cheryl Gramling, an aerospace engineer who leads lunar PNT and standards development at NASA. “Agriculture, safety, rescue, finance, mining—all of those industries really rely on GPS.”
In a similar manner, you need that kind of infrastructure in place on the moon if you’re going to build a lunar economy, Gramling says. Having a lunar positioning system “will aid with landing systems, in situ resource utilization, and path-planning,” she says, adding: “We joke that in the future, you can plan your path over to LunaBucks for your morning coffee.” She explains that initially the focus will be on the lunar south pole, given the missions planned there, but that covering the whole of the lunar globe “might be a long-term objective.”
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The dream of moonwalking over to the local LunaBucks is indeed enticing, but there are some complicated puzzles that must be solved to ensure a lunar GPS system would work. For starters, scientists have to address the basic question: What time is it on the moon? The answer is not simple. Moon missions always account for the cycles of the lunar night and day, each of which lasts two weeks, but there is currently no standard timescale on the moon similar to the Coordinated Universal Time system that keeps clocks ticking in tandem here on Earth.
Precision timekeeping is the core innovation that enabled the rise of global navigation satellite systems (GNSS), a category that includes the US’s GPS, China’s BeiDou, Russia’s GLONASS, and Europe’s Galileo constellations. Satellites in these networks carry atomic clocks that resolve time within a few billionths of a second. Positions on Earth are calculated based on the transit times of satellite signals to ground receivers; a time measurement that is off by just one nanosecond produces a distance error of 30 centimeters. For this reason, high temporal accuracy is fundamental to accurate geolocation services provided by GNSS, and it will also be key to any future analogs on the moon.
There’s a catch, however: Clocks tick slightly faster on the moon than they do on Earth. The difference is a consequence of general relativity, which shows that the flow of time is slowed by massive objects. The moon is less massive than Earth, thus atomic clocks on its surface tick faster.
To confront this problem, Patla and his colleague Neil Ashby, who is also a physicist at NIST, calculated the “rate offset” between clocks on Earth and the moon in a study published last month in the Astronomical Journal. The pair found that clocks on the moon tick faster than those on Earth by about 56 microseconds each day. (As an aside, the relative motion of the moon has the opposite effect on clocks, slowing them down slightly, but this factor is not pronounced enough to cancel out the gravitational effects that speed up clocks.)
“A good clock on Earth will lose one second, plus or minus, every 14 billion years—the age of the universe,” Patla says. But on the moon, he adds, a clock will lose one second approximately every 50 years when compared to a clock on Earth, if not corrected. “If you can compare the scope, this is a big effect.”
The new work by Patla and Ashby has laid the foundation for a standard lunar timescale, similar to UTC on Earth, which will allow future orbital and ground nodes on the moon to ping out synchronized timestamps needed for precision PNT services.
The exact configuration of these future moon constellations, including the number of satellites and their specific lunar orbits, is still being decided. NASA, the European Space Agency (ESA), and the Japan Aerospace Exploration Agency (JAXA) are currently overseeing sister projects for GPS-like constellations that would involve commercial partners. These projects—NASA’s Lunar Communication Relay and Navigation System, ESA’s Moonlight Initiative, and JAXAs’s Lunar Navigation Satellite System—are all designed to adhere to an interoperable framework called LunaNet.
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“I refer to LunaNet as the big umbrella,” Gramling says. “It is an architecture that defines the standards that are going to be used for interoperable communications and position, navigation, and timing services. There’s a large effort underway to define those standards and document those in a LunaNet interoperability specification.”
“It’s a very different paradigm than Earth, where the US has GPS, Europe has Galileo, or Russia has GLONASS,” she adds. “Because we’re at early stages, the idea is that we have to work together as three partners that are involved so far in LunaNet, and assert one system among the three of us.”
In other words, while NASA, ESA, and JAXA work away on their separate projects for now, they plan to ultimately merge those ideas into a single operating system. The detailed plans for ESA’s Moonlight Initiative are helpful for picturing how a lunar GNSS constellation might ultimately shake out.
As currently envisioned by ESA, Moonlight would consist of at least five satellites, including a large communication satellite and four smaller dedicated navigation satellites, placed in special orbits to optimize coverage at the lunar south pole. This initial setup would provide 15 reliable and predictable hours of PNT services in the coverage area every 24 hours, but Moonlight is also designed to be scalable, meaning more satellites could be added to enlarge the service area or to support more complicated missions.
“Moonlight will provide an extraordinary paradigm shift in the field of exploration,” says Javier Ventura-Traveset, who serves as Moonlight navigation manager at ESA. “Instead of each lunar mission requiring their own complex communication and navigation systems with a heavy dependence on Earth-based support, thanks to Moonlight, future missions will have access to broadband communication services and GNSS-like navigation systems directly from lunar orbit, all under a service contract with a commercial provider.”
It’s unclear the extent to which China, or any other nations, might collaborate on existing lunar navigation constellations systems, or if the moon will end up with multiple versions of GNSS, similar to Earth. Earlier this summer, a team of scientists at the China Academy of Space Technology outlined a phased plan for a GPS-style constellation in the journal Chinese Space Science and Technology.
“China has expressed interest in developing lunar navigation infrastructure at several international forums and has already launched this year the Queqiao-2 satellite, a lunar communication relay satellite,” notes Ventura-Traveset. “Similar to ESA, NASA, and JAXA, it is likely that China will also develop its own lunar navigation constellation. At some of these international forums, China has also indicated an interest in pursuing international interoperability.”
The emergence of these multiple competing concepts has led some to wonder if have entered a new “space race” to establish the first lunar version of GPS. But Gramling doesn’t see it that way. “I just know that we are putting our heads down and working with our partners because we have missions that we have to support in the relatively near term,” she says. “We’re just trying to focus on making sure that, among the partners that we’re working on LunaNet, that we are assured of what services we're trying to provide and that we work together.”
Patla pointed out that last month, the International Astronomical Union, an organization that mediates a host of astronomical issues, voted on a resolution that emphasized cooperation in establishing a lunar timescale and other elements of lunar PNT systems.
“At least at the beginning stages, collaboration would be cheaper, and it would also benefit everyone,” Patla says. “But we don’t know how this will pan out.”
Updated 9-4-2024 14:20 pm BST: Cheryl Gramling’s job title was corrected.
