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Image caption: NASA's "Lunar Network" architecture can provide navigation and communication services near the moon.

Our special correspondent Chen Yang

As the new round of lunar exploration and development plans heats up, the proposal to establish a lunar navigation system has also been put on the agenda. According to the Hong Kong "South China Morning Post" on July 14, Chinese scientists recently proposed a design plan to establish a communication and navigation constellation similar to the Beidou navigation system that can serve the entire lunar surface. Previously, the United States, Europe and Japan have all proposed their own lunar navigation plans. So what are the challenges of establishing a lunar navigation system?

Chinese scholars propose lunar navigation plan

The South China Morning Post quoted a paper published in the journal China Space Science and Technology in June as saying that the Chinese research team took into account various indicators such as relay communications, lunar navigation, constellation construction and maintenance costs, and proposed a near-moon space navigation constellation consisting of 21 satellites. The goal is to provide long-term, high-precision navigation for humans' long-term, high-density exploration activities on the moon.

The South China Morning Post said that currently, the US Global Positioning System (GPS), China's Beidou Satellite Navigation System, Europe's Galileo Satellite Navigation System and Russia's GLONASS Satellite Navigation System are mainly used on the Earth's surface or near-Earth space to provide global users with all-weather, high-precision positioning and navigation services. These global satellite navigation systems are usually composed of 20-35 satellites with an accuracy of several meters. Users can use the combination of signals from at least four satellites to accurately locate and obtain time information.

The lunar navigation scheme proposed by Chinese scientists is similar to this one, requiring at least four satellites to be visible to the target at the same time. The report said that the study also proposed a route for building a near-moon space constellation, which would gradually achieve navigation over the entire moon in three stages. The Chinese research team said that subsequent research would combine orbital parameter optimization technology to form a more systematic lunar navigation scheme.

The report mentioned that although the paper did not give a specific construction timetable, the preliminary plan for China's lunar exploration project and the plan for the international lunar research station have been announced before: China plans to send astronauts to the moon before 2030, and to build an international lunar research station with basically complete functions and basic supporting elements with the South Pole of the moon as the core before 2035, and to complete the expansion model before 2045.

The United States, Europe and Japan have all proposed lunar navigation plans

In recent years, the construction of near-moon space constellations has become a hot topic in aerospace research. With more and more lunar exploration and development activities, the idea of ​​establishing a lunar navigation system has been constantly proposed. In order to meet the navigation needs of the US "Artemis" manned lunar landing program, scientists at the Jet Propulsion Laboratory of the National Aeronautics and Space Administration (NASA) confirmed after calculations in 2020 that GPS satellite signals can still provide services near the moon 380,000 kilometers away. It is said that spacecraft in lunar orbit can obtain signals from 5-13 GPS satellites with a positioning accuracy of between 200 and 300 meters.

However, if you want to carry out large-scale exploration and development activities on the lunar surface, such positioning accuracy is far from enough. According to a report from the Japan Aerospace Exploration Agency (JAXA), the signal strength of the global satellite navigation system operating near the earth is only 1/30 of the ground signal strength near the moon 380,000 kilometers away, and due to the obstruction of the moon itself, these navigation signals cannot be received on the back of the moon. At the same time, the surface of the moon is covered with craters, and the weak navigation signals from the earth are easily blocked by hills and the edges of craters, so special relay communication satellites are needed to provide support. In terms of relaying lunar communications, China has already had successful experience. The Queqiao-1 and Queqiao-2 satellites launched in 2018 and 2024 respectively have provided relay communication support for many lunar exploration missions.

In 2020, NASA proposed the "Lunar Network" architecture to support the "Artemis" program, which can meet the communication needs between astronauts and spacecraft on and near the lunar surface, and also allow astronauts and rovers to obtain location and time information on the rugged lunar surface. To this end, NASA launched the "Lunar Communications Relay and Navigation System" in January 2022, planning to launch multiple satellites in lunar orbit. On June 28, 2022, the U.S. "Capstone" probe was launched. This microwave-sized probe was promoted as "the world's first lunar navigation satellite", and one of its missions is to verify and test new navigation technologies to reduce the risks of future missions.

Europe is also advancing its lunar communications service plan. In 2021, the European Space Agency (ESA) released the "Moonlight" plan, proposing to build a sustainable lunar shared communications and navigation system (LCNS) around 2027, and plans to launch the first "Lunar Pathfinder" satellite in 2025 to carry out technical verification in an elliptical frozen orbit.

In 2022, Japan's JAXA also proposed the "Lunar Navigation Satellite System (LNSS)", planning to deploy 8 satellites in the lunar elliptical orbit to provide relay communication and navigation and positioning services for probes in the lunar south pole region.

It is not easy to build a lunar navigation system

Although there are already four major global navigation and positioning systems near the earth, it is not easy to "move" them to the surface of the moon. According to the NASA website, the first thing that affects the construction of the lunar navigation system is the lunar time issue. NASA's latest calculations show that according to the "time dilation" effect of relativity, the lunar surface time is 57.5 microseconds faster than the earth's surface time every Earth day, and there is even a difference between the time on the lunar surface and the time on the lunar orbit. Considering that the timing accuracy of the global navigation and positioning system has reached the microsecond level, this impact of lunar time cannot be ignored. Moreover, the moon currently has no independent time standard. Each country's lunar exploration program uses its own time scale and converts it into "coordinated universal time." However, with more and more lunar exploration activities, the problem of inconsistent lunar time standards is becoming increasingly serious. In November 2022, representatives of global space agencies and academic organizations held a meeting at the European Space Agency's European Space Research and Technology Center to draft recommendations on how to define lunar time. Patrizia Tavira, director of the International Bureau of Weights and Measures's Time Department, said that if the official lunar time is not established, space agencies and private companies in various countries will act according to their own plans.

Secondly, NASA said that the four major global navigation and positioning systems are based on the Global Geodetic Observation System (GGOS), and only through the latter can unified and coordinated Earth surface coordinates and altitudes be obtained. However, there is currently no similar geographic coordinate system established on the lunar surface. The National Geospatial-Intelligence Agency (NGA) is advancing related measurements and establishing a "Lunar Reference System (LRS)", but this requires a lot of observation data. NASA admits that it is expected to be difficult to play a role in the "Artemis" program in recent years.

In addition, the lunar navigation plan will also be affected by the shape and mass of the moon itself during its implementation. For example, the Queqiao-2 relay satellite uses a large elliptical frozen orbit around the moon as its mission orbit. This is because the moon's shape and structure are irregular, and spacecraft flying close to the moon are affected by the moon's gravity and other factors, which can easily cause deviations in the flight orbit.

It is worth noting that the lunar navigation programs of the United States, Europe and Japan are planning to provide lunar enhanced navigation services through mutual cooperation and build international standards. The United States and Europe will also pilot the use of weak satellite navigation signals emitted by Earth probes to calculate the position on the moon starting in 2024. ▲