Beidou considers including low-orbit satellites, can navigation no longer "get lost"?

  China News Weekly reporter/Yang Zhijie

  Published in the 1074th issue of "China News Weekly" magazine on December 26, 2022

  Turn on the GPS of the mobile phone, and the accuracy of navigation and positioning can reach 4.9 meters in an open environment.

The global positioning accuracy of China's latest-generation navigation system Beidou-3 is better than 4.4 meters.

But any positioning system is not a panacea. For example, the existing navigation system will still "get lost" in complex road conditions such as overpasses.

  Recently, at the press conference of the white paper "China's Beidou in the New Era", the China Satellite Navigation System Management Office stated that before 2035, China will build a new generation of Beidou system, which will include low-orbit satellites, and through the constellation fusion of high, medium and low orbits, provide A high-precision space-time network covering the whole world.

  According to different orbit heights, artificial satellites are divided into high-orbit geosynchronous satellites with an altitude of 36,000 kilometers from the ground, medium-orbit satellites with an altitude of 2,000-36,000 kilometers, and low-orbit satellites with an altitude of 500-2,000 kilometers.

Compared with medium and high-orbit satellites, low-orbit satellites are smaller in size and more in number. The satellite constellations formed are low-cost and strong invulnerability. They can also supplement and enhance the existing global satellite navigation system (hereinafter referred to as "GNSS").

  Satellites can be used for navigation, communication, remote sensing and other services.

In the past few years, commercial aerospace companies represented by SpaceX and OneWeb have entered the market, driving the accelerated development of low-orbit communication satellites.

Today, the combination of low-orbit satellites and navigation has also begun to become a race track for navigation companies and space agencies around the world.

On October 17, 2021, at the 2021 National Science Popularization Day "China Flying Dream" China Aerospace Science Popularization Tour Exhibition held in Guiyang City, Guizhou Province, people visited the "Beidou-3 Global Satellite Navigation System" model.

Picture/China New

Why is low-orbit navigation so popular?

  At present, GNSS such as GPS in the United States, GLONASS in Russia, Beidou in China, and Galileo in Europe are all gathered in the middle and high orbits.

  According to Yuan Hong, a researcher at the Aerospace Information Innovation Institute of the Chinese Academy of Sciences, an important reason why low-orbit navigation satellites are sought after is the inertial demand for continuous improvement of satellite navigation and positioning accuracy.

"Looking back at the development history of satellite navigation technology, its accuracy has been improved from the earliest hundred-meter level to the meter level, and now it can reach the real-time decimeter level. With the significant reduction in the manufacturing and launch costs of low-orbit satellites in recent years, people will wonder whether It can make the positioning accuracy of navigation satellites reach a finer centimeter level." He said.

  Satellite positioning has three indicators, namely speed, accuracy and integrity.

The existing GNSS is not perfect. Because the satellite is more than 20,000 kilometers away from the ground, the signal transmission to the ground will be weakened a lot. If it encounters occlusion, the positioning timeliness will also be slowed down. Moreover, GNSS cannot realize indoor and underwater positioning.

  "If low-orbit satellites can broadcast independent ranging signals, they will have independent positioning and navigation capabilities, which is a useful supplement to GNSS." Li Min, deputy director of the Analysis Center of the International Satellite Navigation Service Organization (IGS) of Wuhan University, told China News Weekly "Say.

  The satellite navigation system consists of three parts: navigation satellites, ground stations and user terminals.

Unlike GPS, which has ground stations all over the world, most of the ground stations of China's Beidou system are built in China, and need to achieve global observation and operation through inter-satellite links.

Many interviewees mentioned that low-orbit satellites can cover the whole world and can be used as aerial monitoring stations to improve the global service performance of Beidou.

The Beidou system is a satellite navigation system independently developed by China. The Beidou-3 system was officially completed and opened in July 2020 and provides services to the world.

  Generally speaking, at least 4 satellite signals are required to determine the position coordinates.

Low-orbit satellites are closer to the ground, and to cover global signals, the number of satellites required far exceeds that of medium- and high-orbit satellites.

The more satellites, the more accurate the positioning.

  If you want to further improve the accuracy of satellite navigation and positioning, you need to use navigation enhancement technology.

There are two types of existing navigation augmentation technologies: satellite-based augmentation and ground-based augmentation.

Ground-based enhancement, that is, building reference stations on the ground to assist in correcting satellite signal errors requires intensive deployment, and it is difficult to cover areas such as seas and deserts.

According to incomplete statistics, different departments in the United States have jointly built more than 2,000 reference stations around satellite navigation, and Beidou also has more than 2,000 ground-based augmentation systems in China.

  LEO navigation augmentation is a form of satellite-based augmentation.

"The traditional ground-based enhancement does not increase the signal source, but only enhances the information processing signal. The low-orbit navigation enhancement can increase the signal source and improve the positioning accuracy, which can play a role in some rescue scenarios such as mountain fires." Academician of the Chinese Academy of Sciences, China Li Deren, an academician of the Academy of Engineering and an expert in surveying, mapping and remote sensing, introduced to China News Weekly.

  Chen Ruizhi, director of the State Key Laboratory of Surveying, Mapping and Remote Sensing Information Engineering at Wuhan University, said, "If the transmission power of low-orbit satellite signals is increased, the single-layer roof space such as gymnasiums, high-speed rail stations, and airports can penetrate the roof, which can solve the problem of the Beidou system. Indoor positioning problem, which needs to be further verified in reality."

  At present, the high-precision single-point positioning technology used by GNSS generally takes 20 to 30 minutes to achieve centimeter-level positioning. If low-orbit navigation enhancement is used, the simulation verification takes about 1 minute.

  The biggest advantage of low-orbit navigation is not in civilian use, but in military and Internet of Things scenarios.

Lionel Rees, research director of the European Space Agency's low-orbit satellite PNT, once lamented that the successful application of navigation satellites "stimulates more demanding market demand in the next decade."

For example, with the emergence of unmanned vehicles, ships or drones, smart cities, industrial Internet of Things and other scenarios, the demand for satellite positioning is rising from the current meter level to the centimeter level, and it is necessary to obtain reliable indoor and outdoor positioning at any time. Signal.

  PNT (positioning, navigation, and timing), that is, positioning, navigation, and timing, is a key technology for human beings to know the space-time position.

PNT plays an important role in navigation and positioning, calibration time in daily life, and in fields such as national security.

  In 2021, the report "Analysis of More Resilient National PNT Capabilities" released by the American think tank RAND Corporation mentioned that the GPS system is facing multiple threats: nuclear war against space systems, extreme weather such as solar storms, large Smaller threats include interference with GPS signals in some areas or some cities.

  Yuan Hong told "China News Weekly" that if GNSS is destroyed in a special period such as war, low-orbit satellites are the only universal alternative.

The construction and launch costs of low-orbit satellites are much lower than those of medium- and high-orbit satellites. Even if some of them are destroyed, they can be quickly replenished and launched. "From this perspective, it is very necessary to build low-orbit navigation satellites."

A complex game of old and new technologies

  At the "China's Beidou in the New Era" white paper press conference, the spokesperson of the Beidou satellite navigation system said that the construction of a high-medium-low-orbit navigation constellation "has been well demonstrated technically, and the plan is also complete."

  According to Yuan Hong's understanding, after long-term research and demonstration, low-orbit satellite navigation will definitely be included in the national navigation and positioning service system.

However, what kind of service model and technical route to choose for low-orbit satellite navigation in the future must be carefully considered.

  The technology of the low-orbit navigation system is different from that of the medium-high orbit.

Take the space-time reference as an example. This is the core technology of the satellite navigation system. During satellite positioning, the time of the satellite, ground station, and user receiver must be synchronized to accurately calculate the distance, and the orbital position of the navigation satellite must also be accurately calculated.

  The space-borne atomic clock is an important device for space-time reference design. It can provide extremely stable time-frequency reference signals, but it requires extremely high precision and is very difficult to manufacture. It used to be the "stuck neck" technology of Beidou. In 2007, domestically-made atomic clocks were developed in China. .

  Yuan Hong and others pointed out in related research that the power consumption, size, quality and cost of atomic clocks on medium and high orbit satellites are difficult to apply to low orbit satellites.

How to rely on the existing GNSS to establish the space-time reference of low-orbit satellites is a hot research topic in the industry.

  In addition, frequency resources are the basis for human development of space services, but the frequency range designated by ITU for satellite navigation services is limited, and the existing navigation frequency bands are already very crowded.

"Compared with the navigation frequency band, the low-orbit communication business has more abundant frequency resources." Yuan Hong told China News Weekly that adding some navigation functions to the low-orbit communication business allows navigation to take some of the "cheaper" of communication. Solving the problem of shortage of navigation frequency resources in a disguised form is also a potential way out for the development of low-orbit navigation.

  "The frequency occupied by low-orbit communication can be understood as a train, and each car is loaded with communication services of different users. The space in the car that is not fully occupied by communication services can be interspersed with some navigation services to achieve certain navigation service functions. "Yuan Hong explained that the frequency resources occupied by low-orbit navigation will not necessarily significantly affect the quality of communication services.

  How to use navigation frequency resources, Yuan Hong introduced, one method is that the low-orbit navigation system broadcasts navigation signals at the frequency of existing GNSS signals, but this will have the risk of interfering with existing mid-high orbit GNSS signals; another method, Select a frequency band adjacent to the existing GNSS signal, and modify the antenna and radio frequency of the user terminal to adapt to the reception of low-orbit navigation signals.

At present, relevant domestic units have targeted the latter route.

  Some experts believe that the use of low-orbit navigation enhancements will compete fiercely with existing technologies.

Liu Jingnan, academician of the Chinese Academy of Engineering and director of the National Satellite Positioning System Engineering Technology Research Center, once said that the low-orbit satellite system is the future development direction of China's Beidou, but at the same time, the construction of existing infrastructure cannot be ignored.

The low-orbit satellite system must be supported by high- and medium-orbit navigation satellites and ground-based augmentation systems to ensure better positioning accuracy and stability.

  Li Deren mentioned that in response to the shortcomings of the existing GNSS, all countries are studying solutions.

One way is to enhance satellite navigation; the other is not to rely on satellite navigation system positioning, but to use vision, hearing and other sensors for positioning to form an intelligent service system integrating air, space and ground.

A new round of international competition

  Low-orbit satellites are not a new concept, but due to long-term high manufacturing and launch costs, they have not been developed.

  In 1963, the Meridian satellite system, the first-generation satellite navigation system developed by the U.S. Navy, operated in a low orbit at an altitude of about 1,100 kilometers, but the number was only 5 to 10. Accurate positioning took a long time and the accuracy was limited.

In the 1970s, the United States and the Soviet Union switched to medium-orbit satellites to achieve rapid positioning with fewer satellites, and only then did the GPS and GLONASS systems come into being.

  In 1998, low-orbit satellites received attention again.

In 1991, Motorola established its subsidiary, Iridium, to carry out the "Iridium Project", in which 66 low-orbit satellites provide ground communication services.

In 1998, the "Iridium Project" was officially put into operation, but it was defeated in the competition with 2G due to the high cost of equipment and communication.

After 2000, Iridium went bankrupt, was acquired and "resurrected with full blood". After that, it served the US military for a long time and gradually faded out of public view.

  Until 2015, SpaceX CEO Elon Musk announced the launch of the "Starlink Project", launching a constellation of low-orbit satellites to provide high-speed Internet access services covering the world.

Since then, the construction cost of low-orbit satellites has dropped significantly, from "luxury goods" to affordable "consumer goods".

  Yuan Hong said that the cost of manufacturing and launching low-orbit satellites is as low as one million dollars.

In addition, the navigation load of low-orbit navigation satellites draws on the mature technology of medium and high orbits, which further reduces the construction threshold.

  In 2016, Iridium demonstrated navigation service technology based on satellite timing and positioning, proving that low-orbit satellites can be used as an alternative to GPS, which has attracted global attention.

  Yuan Hong remembers that around this year, China began to demonstrate the possibility of including low-orbit satellite navigation services under the PNT system.

In 2018, Yuan Hong's team at the Aerospace Information Innovation Institute of the Chinese Academy of Sciences designed and developed the first Beidou signal enhancement test for low-orbit satellites in China.

In the same year, Wuhan University launched "Luojia-1", and China Aerospace Science and Industry Corporation and Aerospace Science and Technology Corporation successively launched the "Hongyun Project" and "Hongyan" low-orbit test satellites.

Since then, relevant domestic state-owned enterprises and commercial aerospace companies have successively launched low-orbit navigation constellation plans.

In 2021, the State-owned Assets Supervision and Administration Commission will fund the establishment of China Satellite Network Group Co., Ltd. (hereinafter referred to as "Star Network Group"), which will also take low-orbit navigation as an important business direction.

  In 2019, the United States completed the launch and deployment of the new-generation Iridium system.

In addition to communication services, the Iridium system can assist GPS to realize positioning and navigation in indoor and canyon areas.

After years of research on the concept of low-orbit navigation satellites, the European Space Agency recently announced further on-orbit demonstrations to complement the Galileo system.

  The development of low-orbit satellites has also made orbital resources more and more crowded, and will face increasing collision risks.

  A fierce space "enclosure movement" is underway in low Earth orbit.

SpaceX has launched more than 3,000 satellites in low orbit, and Musk's ambition is to launch a total of 42,000 satellites in the future.

In addition, there are many "players" such as OneWeb, Amazon, Boeing, and Galaxy Aerospace among the competitors.

Starnet Group also plans to launch more than 12,900 satellites in the future.

  "The original satellite launch orbit is high, the design is more sophisticated, and the life span is longer. Satellites like 'Starlink' may be eliminated in three to five years. If there are tens of thousands of satellites in low orbit, there may be more satellites in the future." Many satellites fail, and if they are not controlled, they will bring security risks." Kong Dejian, a researcher at the Aviation and Space Law Research Center of China University of Political Science and Law, told China News Weekly.

  "If two satellites collide and cause the satellite to disintegrate, multiple debris will be generated, which will further increase the risk of collision. Eventually, low-Earth orbit will be filled with debris, and all low-orbit systems will no longer have space to operate safely." Yuan Hong said.

  Lucinda King, space project manager at the University of Portsmouth in the United Kingdom, also mentioned in an interview with the BBC that if there is too much debris, "we may not be able to go through low-Earth orbit and enter the higher places where navigation satellites and telecommunications satellites are located." track."

  Kong Dejian introduced that at present, there are some management regulations and supervision guidelines for the space traffic management of low-orbit satellites in the world, but none of them have legal effect.

In 2017, the Inter-Agency Space Debris Coordination Committee composed of 11 space departments including the United States, Russia, Japan, Europe, and China drafted the "Statement on the Low-Earth Mega-Constellation" to deploy altitude, altitude interval, and orbital life in low-orbit constellations. , the number of satellites and other aspects, such as deploying at an altitude of 500 kilometers, will have the least impact on the space debris environment, etc.

This has guiding significance for countries to regulate the deployment and operation of low-orbit constellations.

At present, the United States and China have promulgated relevant management measures.

  "At present, the key to the development of low-orbit navigation is how to achieve better service performance at a lower cost. In the end, the solution that can be implemented may not necessarily be the 'optimal' technically, but it must be a solution that is recognized by all parties. .” Yuan Hong told China News Weekly.

  "China News Weekly" Issue 48, 2022

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