GPS is ubiquitous on Earth, serving everything from precision surveying to aerial navigation. To realize our vision of lunar exploration with a sustained human presence, we'll need the same precision on the Moon.
It all starts with an accurate watch.
The United States National Institute of Standards and Technology (NIST) is developing a framework for precise lunar timekeeping. They are paving the way for lunar GPS, which could enable the kind of precise position determination needed for lunar navigation and could also contribute to future space missions.
“The proposed framework supporting lunar coordinate timing could eventually enable exploration beyond the Moon and even beyond our solar system.”
Bijunath Patla, Physicist, NIST
GPS works because it measures time extremely accurately. Each GPS satellite has an atomic clock. GPS receivers receive signals from several GPS satellites at once and then determine their location based on the time it takes to receive those signals. All global navigation satellite systems (GNSS), such as ESA's, Galileo systemThey work on the same principle.
But the challenge is to create a lunar GNSS that can coordinate precisely with terrestrial GNSS. Relativity is the sticking point.
Einsteinian relativity tells us that two clocks in different locations will tick at different rates because of local gravity. An atomic clock on the surface of the Moon would tick faster than one on Earth by about 56 milliseconds per day because gravity is weaker. This isn't a big problem for consumer GPS. But when it comes to precision activities like landing a spacecraft, the different clock speed is a problem.
Relativity also tells us that people on Earth experience time differently than people on the Moon. The effects of gravity from the Moon orbiting the Earth and from the Earth orbiting the Sun can have a pronounced effect on navigation and communications.
NIST's solution to these problems is “Master Moon Time,” which would set a time reference point for one location on the Moon, and all other locations would refer to it, similar to how the UTC works on Earth.
The Lunar Positioning System (LPS) would consist of a network of high-precision atomic clocks at various points on the Moon. A fleet of lunar satellites would also contain atomic clocks. All of these precision clocks would provide the time signals necessary for accurate navigation.
Atomic clocks are precise because they are based on the oscillations of atoms, often of caesium-133, but also of elements such as rubidium or hydrogen. Indeed, the official definition of a second is based on the oscillation of caesium-133. Their precision is extreme: the most precise ones can keep time to within a second for a billion years.
Caesium-133 clocks can be heavy compared to other types of atomic clocks, so satellites often use rubidium atomic clocks. The GPS system routinely uses rubidium clocks, but caesium and hydrogen clocks are also used, depending on requirements. ESA's Galileo system uses rubidium and hydrogen clocks on the same satellite, with the rubidium clocks serving as a backup.
“It's like having the entire Moon synchronized to a 'time zone' tuned to the Moon's gravity, rather than having clocks that gradually drift out of sync with Earth time,” said NIST physicist Bijunath Patla.
“This work lays the groundwork for the adoption of a GPS-like navigation and timing system that would serve both terrestrial and near-Earth users for lunar exploration,” said NIST physicist Neil Ashby.
NASA and its Artemis partners intend to develop a sustained presence on the Moon in the future. There are in-situ resources that can be used to power the project, such as water ice and rare earth elements.
With that level of activity, the need for accurate navigation is obvious. As the level of complexity of all that activity increases, the need for reliable location and navigation will become more acute.
“The goal is to ensure that spacecraft can land within a few metres of their intended destination,” Patla said.
The Moon will also serve in the future as a staging area or launching point for missions to the Solar System. As that effort takes shape in the coming decades, precise timekeeping will be needed to coordinate complex missions. Researchers say atomic clocks on satellites at Lagrange points can be used to transfer time between Earth and the Moon.
“The proposed framework supporting lunar coordinate time could enable exploration beyond the Moon and even beyond our solar system,” Patla said. “Once humans develop the capability for such ambitious missions, of course.”
“This understanding also underpins accurate navigation in cislunar space and on the surfaces of celestial bodies, thereby playing a critical role in ensuring the interoperability of diverse positioning, navigation and timing systems spanning from Earth to the Moon and the farthest regions of the inner solar system,” the authors write in their paper.
