GPS is ubiquitous on Earth. It controls everything from precision surveying to aircraft navigation. To realize our vision of lunar exploration with a permanent human presence, we need the same precision on the Moon.
It starts with an accurate clock.
The US National Institute of Standards and Technology (NIST) is currently developing a framework for the precision measurement of lunar time. This will pave the way for a lunar GPS that could provide the precise positioning required for lunar navigation and could also be useful for future space missions.
“The proposed framework underlying lunar coordinate time could eventually enable exploration beyond the moon and even beyond our solar system.”
Bijunath Patla, physicist, NIST
GPS works because it measures time with extreme precision. Each GPS satellite has an atomic clock. GPS receivers receive signals from several GPS satellites simultaneously and then determine their location based on the time it takes them to receive those signals. All global navigation satellite systems (GNSS), such as ESA’s Galileo system, work on the same principle.
The challenge, however, is to develop a lunar GNSS that can be precisely synchronized with the Earth-based GNSS. The key issue is relativity.
Einstein’s theory of relativity states that two clocks in different places will tick at different rates due to local gravity. An atomic clock on the surface of the moon would tick about 56 milliseconds per day faster than one on Earth because gravity is weaker there. For a personal GPS, this is not a big problem. But for precision activities like landing a spacecraft, the different clock rate is a problem.
Relativity also teaches us that people on Earth perceive time differently than people on the Moon. The gravitational effects of the Moon and the Earth in their orbit around the Sun can have a significant impact on navigation and communication.
NIST’s solution to these problems is “Master Moon Time.” It would establish a temporal reference point for one location on the Moon, and all other locations would reference it, similar to how UTC works on Earth.
The Lunar Positioning System (LPS) would consist of a network of high-precision atomic clocks at various locations on the Moon. A fleet of lunar satellites would also contain atomic clocks. All of these precision clocks would provide the timing signals needed for precise navigation.
Atomic clocks are precise because they are based on the vibrations of atoms, often caesium-133, but also elements such as rubidium or hydrogen. In fact, the official definition of a second is based on the vibration of caesium-133. Their accuracy is extreme: the most accurate clocks can measure time to within one second over a billion years.
Caesium-133 clocks can be heavy compared to other types of atomic clocks, so rubidium atomic clocks are often used in satellites. The GPS system most commonly uses rubidium, but caesium and hydrogen clocks are also used as needed. ESA’s Galileo system uses both rubidium and hydrogen clocks on the same satellite, with the rubidium clocks as backups.
“It’s as if the entire moon is synchronized with a ‘time zone’ that is adjusted to the moon’s gravity, rather than the clocks gradually losing their sync with Earth time,” said NIST physicist Bijunath Patla.
“This work lays the foundation for the introduction of a GPS-like navigation and timing system that would be useful to both near-Earth and ground-based users in lunar exploration,” said NIST physicist Neil Ashby.
NASA and its Artemis partners intend to eventually establish a permanent presence on the Moon, where there are local resources that can be used to further efforts, such as water ice and rare earths.
At this level of activity, the need for precise navigation is obvious. As all these activities become more complex, the need for reliable positioning and navigation becomes ever greater.
“The goal is to ensure that spacecraft can land within a few meters of their intended destination,” Patla said.
The moon will also eventually serve as a staging area or launching point for missions into the solar system. As this effort takes shape in the coming decades, precise timekeeping will be required to coordinate complex missions. The researchers say atomic clocks in satellites at the Lagrange points can be used to relay times between Earth and the moon.
“The proposed framework underlying lunar coordinate time could eventually enable exploration beyond the moon and even beyond our solar system,” Patla said. “Once humanity has developed the capability for such ambitious missions, of course.”
“This understanding also forms the basis for precise navigation in cislunar space and on the surface of celestial bodies and thus plays a crucial role in ensuring the interoperability of different positioning, navigation and timing systems from the Earth to the Moon and to the most distant regions of the inner solar system,” the authors write in their article.
