By: European Space Agency (ESA) March 5, 2023
A new era of lunar exploration is on the rise, with dozens of lunar missions planned for the next decade. Europe is at the forefront of this race, contributing to the construction "Moon Gate" station" and the Orion spacecraft – which should return humans to our natural satellite – as well as the development of the large logistical lunar lander known as Argonaut. Because dozens of missions will operate on and around the Moon and will need to communicate together and broadcast their location independently from Earth, this new era will require its own time synchronization.
Accordingly, space organizations began to consider how to keep time on the moon. Having started at a meeting at ESA's ESTEC Technology Center in the Netherlands last November, the discussion is part of a larger effort to agree on an architectureLunaNetA joint venture that covers lunar communication and navigation services.
"LunaNet is a framework of mutually agreed standards, protocols and interface requirements that allow future lunar missions to work together, conceptually similar to what we have done on Earth to share GPS and Galileo," explains Javier Ventura-Travast, ESA's Director of Moonlight Navigation, who coordinates the ESA contributions to LunaNet. "Now, in the lunar context, we have an opportunity to agree on our interoperability approach from the start, before the systems are actually implemented."
Timing is a crucial element, adds ESA navigation system engineer Pietro Giordano: "During this ESTEC meeting, we agreed on the importance and urgency of defining a common lunar reference time, which would be universally accepted and to which all lunar systems and users could refer." A joint international effort is now underway to achieve this goal."
On the 20th day of the Artemis I mission, Orion photographs the moon during its flyby. The image was taken by a camera mounted on the solar array wings of the European Service Module, on December 5, 2022. Credit: NASA
So far, each new mission to the Moon has been run on its own timeline exported from Earth, with the deep space antennas used to keep onboard chronometers synchronized with Earth time and thus enable two-way communication. This way of working would not be sustainable however in the near lunar environment.
Once completed, the Moongate lunar space station will be open to astronauts, will receive supplies through regular Artemis launches, will advance toward a human return to the lunar surface, and will culminate in the establishment of a manned base near the lunar south pole. Meanwhile, a large number of unmanned missions will prepare the terrain. Each Artemis mission alone will release multiple lunar CubeSats, and the European Space Agency will land the large European logistics lander Argonaut.
These missions will not only take place on or around the Moon at the same time, but they will often communicate with each other - potentially communicating with each other, making joint observations or conducting rendezvous operations.
Moonlight satellites on the way
"Looking ahead to the lunar exploration of the future, ESA is developing through its Moonlight program a lunar communication and navigation service," explains Wal-El Daly, system engineer at Moonlight. "This will allow missions to maintain communications to and from Earth, and guide them on their way around the Moon and on the surface, allowing them to focus on their core missions. But even Moonlight will need a common timeline to link tasks and facilitate location corrections."
ESA's Moonlight initiative involves expanding the satellite's coverage and communication links to the Moon. The first phase involves demonstrating the use of the current stern 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 Stenn signals all coming from the same part of the sky, along 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 more diverse sources and extended coverage. Credit: ESA-K Oldenburg
Moonlight will be joined in orbit around the moon by a parallel NASA-sponsored service - the Lunar Communications Relay and Navigation System. To maximize interoperability, these two systems should use the same timeline, along with the many other manned and unmanned missions they will support.
Time correction for location correction
Jörg Hahn, chief engineer of the European Space Agency's Mikon Galileo satellite project and consultant 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 of today's smartphones are capable of making use of existing GNSS to calculate a user's location down to the meter or even decimeter level.
An image of the far side of the Moon taken on the sixth flight day of the Artemis I mission from the optical navigation camera of the Orion spacecraft. Credit: NASA
"The experience of this success can be reused for the next long-term technical lunar systems, although keeping stable time on the moon will present its own unique challenges - such as accounting for the fact that time passes at a different rate there due to the effects of the moon's specific gravity and speed."
Global time setting
Accurate navigation requires strict adherence to time. This is because the satnav receiver of the satellite navigation device (GPS - although the name is reserved for only one such system AB) determines its location by converting the time it takes for multiple satellite signals to reach it into distance measures - multiplying the time by the speed of light.
The satnav receiver on your personal devices needs at least four satellites in the sky. Their integrated clocks are synchronized and orbit positions are monitored by global ground segments. It receives signals from each satellite, each of which incorporates a precise time stamp. By calculating how long it takes for each signal to reach your receiver, the receiver builds a XNUMXD image of your location – 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 positional accuracy almost anywhere on Earth or even beyond it: satnav is also heavily used by satellites. Credit: ESA
All terrestrial satellite navigation systems, such as Galileo in Europe or the GPS of the United States, operate on their own unique timing systems, but these have constant offsets relative to each other up to a few billionths of a second, and they also conform to the UTC Universal Coordinated Time global standard.
UTC, the replacement for Greenwich Mean Time, is part of 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 global time reference called Galileo System Time (GST), a standard for Europe's satellite navigation system, which is kept close to UTC with an accuracy of 28 billionths of a second. Accurate timings enable a precise range of location and navigation services, and their distribution is an important service in itself. Credit: ESA
The BIPM calculates UTC time based on inputs from collections of atomic clocks maintained by institutions around the world, including ESA's ESTEC Technical Center in Noordwijk, Netherlands, and ESOC's Mission Control Center in Darmstadt, Germany.
Planning a lunar chronology
Among the current issues being discussed is whether one organization should be similarly responsible for setting and maintaining lunar time. And, should the lunar time be set independently on the moon or should it be kept in sync with the Earth?
A mosaic of our moon's south pole showing locations of major craters, with images taken by NASA's Lunar Reconnaissance Orbiter. Credit: NASA/GSFC/Arizona State University
The international team working on the issue will face considerable technical problems. For example, clocks on the moon run faster than their terrestrial counterparts - gaining about 56 microseconds or millionths of a second per day. Their exact rate depends on their position on the moon, and ticks differently on the surface of the moon than in orbit.
"Of course, the agreed time system will also have to be practical for astronauts," explains Bernhard Hoffenbach, a member of the Moonlight Management Team from ESA's Human and Robotics Research Directorate. "This will be quite a challenge on the planetary surface where in the equatorial region each day is 29.5 days long, including two weeks of frozen lunar nights, when the entire Earth is just a small blue circle in the dark sky. But having established a working time system for the Moon, we can continue to do the same for other planetary destinations.”
Finally, in order to work together properly, the international community will also have to settle for a common "selenocentric frame of reference", similar to the role played on Earth by the International Terrestrial Frame of Reference, which allows for the consistent measurement of precise distances between points around our planet. Custom reference frames are essential components of today's GNSS systems.
"Throughout human history, exploration has actually been a major driver for improving timekeeping and geodetic attribution models," Javier adds. "It is certainly an exciting time to do this now for the Moon, and to work towards defining an internationally agreed timeline and common selenocentric reference, which will not only ensure interoperability between the various lunar navigation systems, but will also foster a large number of research opportunities and applications in lunar space."
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