A high-definition image of Earth taken by Japan’s Kaguya lunar orbiter in November 2007. Credit: JAXA/NHK
A new era of lunar exploration is upon us, with dozens of lunar missions planned for the coming decade. Europe is at the forefront here, helping to build the Gateway lunar station and the Orion spacecraft – which will bring humans back to our natural satellite – as well as developing its large logistical lunar lander, known as Argonaut. Since dozens of missions will operate on and around the Moon and will need to communicate together and determine their positions independently of Earth, this new era will require its own time.
Therefore, space organizations have begun to consider how to keep time on the Moon. Having started with a meeting at ESA’s ESTEC technology center in the Netherlands last November, the discussion is part of a larger effort to agree on a common ‘LunaNet’ architecture covering lunar communications and navigation services.
Artist’s impression of a lunar exploration scenario. Credit: ESA–ATG
Architecture for joint lunar exploration
“LunaNet is a framework of mutually agreed upon standards, protocols and interface requirements that allow future lunar missions to work together, conceptually similar to what we did on Earth for the shared use of GPS and Galileo,” explains Javier Ventura-Traveset, ESA’s Moonlight Navigation Manager, who coordinates ESA’s contribution to LunaNet. “Now, in the lunar context, we have the opportunity to agree on our interoperability approach from the very beginning, before the systems are actually deployed.”
Timing is a crucial element, adds ESA navigation systems engineer Pietro Giordano: “During this meeting at ESTEC, we agreed on the importance and urgency of defining a common lunar reference time that is internationally accepted and that all lunar systems and users can refer to. A joint international effort is now being launched to achieve this.”
On the 20th day of the Artemis I mission, Orion captures the Moon during its flyby. The image was taken by a camera mounted on the European Service Module solar array wings, on December 5, 2022. Credit: NASA
Until now, each new mission to the Moon has operated on its own time scale exported from Earth, with deep space antennas used to keep onboard chronometers synchronized with Earth time while facilitating two-way communication. However, this way of working will not be sustainable in the future lunar environment.
Once completed, the Gateway station will be open to astronaut accommodation, supplied regularly NASA Artemis launches, progresses to a human return to the lunar surface, culminating in a manned base near the lunar south pole. Meanwhile, several unmanned missions will also be in place – each Artemis mission alone will release several lunar CubeSats – and ESA will retire its Argonaut European Large Logistics Lander.
Artist’s impression of the Lunar Gateway, a habitat, refueling and research center for astronauts exploring our moon as part of the Artemis program. Credit: NASA/Alberto Bertolin
These missions will not only be on or around the Moon at the same time, but they will also often interact – potentially relaying communications to each other, conducting joint observations, or conducting rendezvous operations.
Moonlight satellites on the way
“Looking forward to the future of lunar exploration, ESA is developing, through its Moonlight program, a lunar communication and navigation service,” explains Wael-El Daly, Systems Engineer for Moonlight. “This will enable missions to maintain connections to and from Earth and guide them on their way around the moon and on the surface, so they can focus on their core tasks. But Moonlight will also need a common shared timescale to get missions associated and to facilitate position determinations.”
ESA’s Moonlight initiative involves expanding satellite coverage and communications links to the Moon. The first phase involves demonstrating the use of current satnav signals around the Moon. This will be achieved with the Lunar Pathfinder satellite in 2024. The main challenge will be to overcome the limited geometry of satellite navigation signals all coming from the same part of the sky, together with the low signal strength. To overcome this limitation, the second phase, the core of the Moonlight system, will see dedicated lunar navigation satellites and lunar surface beacons providing additional distance sources and extended coverage. Credit: ESA-K Oldenburg
And Moonlight will be joined in lunar orbit by a similar service sponsored by NASA – the Lunar Communications Relay and Navigation System. To maximize interoperability, these two systems should use the same time scale with the many other manned and unmanned missions they will support.
Fixation time to fix position
Jörg Hahn, ESA’s Chief Galileo Engineer and also Advisor on Lunar Time Aspects, comments: “Interoperability of time and geodetic reference frames has been successfully achieved here on Earth for Global Navigation Satellite Systems; all today’s smartphones are able to make use of existing GNSS for to calculate a user position down to a meter or even decimeter level.
Image of the far side of the Moon taken on flight day six of the Artemis I mission from the Orion spacecraft’s optical navigation camera. Credit: NASA
“The experience of this success can be reused for the engineering long-term lunar systems to come, although stable timekeeping on the Moon will throw up its own unique challenges – such as accounting for the fact that time passes in a different way. rate there at due to the Moon’s specific gravity and velocity effects.”
Setting global time
Accurate navigation requires strict timing. This is because a satellite navigation receiver determines its location by converting the time it takes multiple satellite signals to reach it into a measure of distance – multiplying the time by the speed of light.
Your satellite navigation receiver needs at least four satellites in the sky, their built-in clocks synchronized and orbital positions monitored by global earth segments. It picks up signals from each satellite, each with a precise time stamp.
By calculating how long it takes each signal to reach your receiver, the receiver builds a three-dimensional picture of your position – longitude, latitude and altitude – relative to the satellites. Future receivers will be able to track Galileo satellites in addition to US and Russian navigation satellites, providing meter-scale positioning accuracy almost anywhere on or even off Earth: satellite navigation is also widely used by satellites.
Credit: ESA
All terrestrial satellite navigation systems, such as Europe’s Galileo or the US’s GPS, run on their own distinct timing systems, but these have fixed offsets relative to each other down to a few billionths of a second, and also to UTC Universal Coordinated Time global standard.
The replacement for Greenwich Mean Time, UTC, is part of all our daily lives: it is the timing used for internet, banking and aviation standards, as well as precise scientific experiments, maintained by the Paris-based Bureau International de Poids et Mesures (BIPM) ).
Galileo is based on a worldwide time reference called Galileo System Time (GST), the standard for Europe’s satellite navigation system, kept close to UTC with an accuracy of 28 billionths of a second. Accurate timing enables accurate ranging for position and navigation services, and their dissemination is an important service in itself. Credit: ESA
BIPM calculates UTC based on input from collections of atomic clocks maintained by institutions around the world, including ESA’s Technical Center ESTEC in Noordwijk, the Netherlands, and the ESOC Mission Control Center in Darmstadt, Germany.
Lunar Chronology Design
Among the current topics under debate is whether a single organization should be similarly responsible for setting and maintaining the lunar time. And also whether the lunar time should be set on an independent basis on the Moon or kept synchronized with the Earth.
A mosaic of our Moon’s south pole showing locations of large craters, with images taken by NASA’s Lunar Reconnaissance Orbiter. Credit: NASA/GSFC/Arizona State University
The international team working on the subject will face significant technical challenges. For example, clocks on the Moon run faster than their terrestrial equivalents—gaining about 56 microseconds, or millionths of a second, per day. Their exact speed depends on their position on the Moon and ticks differently on the lunar surface than from orbit.
“Of course, the agreed time system must also be practical for astronauts,” explains Bernhard Hufenbach, member of the Moonlight Management Team from ESA’s Directorate of Human and Robotic Exploration. “This will be quite a challenge on a planetary surface where each day in the equatorial region is 29.5 days long, including freezing fortnight-long lunar nights, with the whole Earth only a small blue circle in the dark sky. But after establishing a working hours system for the Moon, we can continue to do the same for other planetary destinations.”
Finally, to work properly together, the international community will also need to settle on a common ‘selenocentric reference frame’, similar to the role played by the International Terrestrial Reference Frame on Earth, enabling consistent measurement of precise distances between points on our planet. . Appropriately adapted reference frames are essential ingredients in today’s GNSS systems.
“Throughout human history, exploration has indeed been a major driver of improved timekeeping and geodetic reference models,” adds Javier. “It is certainly an exciting time to do so now for the Moon, which is working towards defining an internationally agreed time scale and a common selenocentric reference that will not only ensure interoperability between the various lunar navigation systems, but will also promote a large number of research opportunities and applications in cislunar space.”