Design of Space-based PNT System Based on BeiDou Navigation Satellite System

XIE Jun, ZHANG Jianjun

1 Beijing Institute of Spacecraft System Engineering, Beijing 100094

2 China Academy of Space Technology, Beijing 100094

Abstract: BeiDou navigation satellite system (the BeiDou system) is the only PNT (Positioning, Navigation and Timing) resource in China that has coverage of the globe and near-Earth space and provides continuous high-precision, low-cost positioning, navigation and timing solutions for users in a unified spatio-temporal benchmark. As an important spatio-temporal benchmark transmission system, the BeiDou system is the most important resource for the national PNT system to provide a PNT capability under a unified spatial-temporal benchmark. This paper proposes the concept, composition and development model of the space-based PNT system design based on the Bei-Dou system with all its space characteristics, as well as the advantages of the system. It opens up a new direction for the construction of China's PNT system and expands a new horizon in the research of a PNT system in China.

Key words: BeiDou navigation satellite system, PNT, systems engineering theory

1 lNTRODUCTlON

The Positioning, Navigation and Timing (PNT) system,which is an important national infrastructure, utilizes multiple positioning and timing methods to provide a unified, accurate and reliable spatio-temporal information (time, location, etc.)for various users through standardized user interfaces, and realizes unified benchmark information for various information systems and platforms, meeting the demand for spatio-temporal benchmark information of any user at anytime and anywhere.

In recent years, with the completion of the construction of the BeiDou 2 navigation satellite system and the rapid development and cross-integration of aviation, ground, indoor and outdoor communications positioning technologies, China's PNT services are gradually infiltrating into many industries, and are gradually being introduced to the public. In terms of market expansion,China's PNT system has achieved certain progress and development in both the technical model and the public awareness. Several PNT information sources based on different principles have emerged. Except the BeiDou system, most of the systems providing PNT information source have relatively small scale, low degree of integration, and no complete service information chain, which have small market share and contribute less to China's PNT service. Therefore, one or several large-scale systems are needed to re-integrate the entire system in order to standardize China's PNT system as soon as possible. Thereupon, the majority of users can truly appreciate the more reliable and high-precision PNT services bringing us all kinds of functionality[1-2].

In November 2017, the Long March series carrier rocket was launched from the Xichang Satellite Launch Center, carrying thefirst and second satellites of the BeiDou 3 series and successfully put them into orbit, which signified the start of the global networking of BeiDou3. This enables the expansion of the service scope of the BeiDou system and the performance verification of a global RNSS service. At the same time, it will establish a stable hybrid constellation configuration, laying a good foundation for the construction of China's space-based information integrated network, and will become the core and cornerstone of China's PNT system. We hope to make China's PNT system bigger and stronger so to better serve the national economy. How to seize this opportunity, give play to the advantages, and take effective measures to develop the PNT service industry? Based on the establishment of a spacebased PNT system using the BeiDou satellite navigation system as its core, it enables a very real path for China's PNT applications to flourish. In the face of the multi-class PNT information source systems with fierce competition in the future, China Academy of Space Technology (CAST) must carefully analyze the conditions and environment that it faces,develop its strengths and circumvent weaknesses, giving full play to aerospace resources, seize opportunities, and adapt to local conditions to develop a viable space-based PNT system[3-5].

2 HOW TO RECOGNlZE SPACE-BASED PNT SYSTEM BASED ON BElDOU SYSTEM

In the design of the PNT system, we first consider the main single system, construct the system framework and interface, and then incorporate the relevant systems in a targeted manner to build a comprehensive PNT system. Through the evaluation of the system, we constantly improve and modify it until a comprehensive PNT system can meet future needs.Currently, the construction and application of the BeiDou system in China has developed rapidly. The system market has grown from less than 1 billion RMB in 2000 to about 12 billion RMB in 2005. By 2016, it exceeded 80 billion RMB in value, while the market capacity has expanded nearly 8 times. It shows that the construction and application of the BeiDou system has gradually shifted from system-oriented to service-oriented. The space-based PNT system based on the BeiDou system has gradually developed through market promotion, and it is duty-bound to assume the breakthrough of China's PNT system construction.

A space-based PNT system based on the BeiDou system is a huge industrial system involving variousfields and industries,with a long industrial chain, comprehensive and wide penetration. It has just started in China, although the BeiDou system is steadily being constructed, the front end of the industrial chain has been clear, but the follow on industrial division and industrial chain in the middle and late stages are still unclear. In the process of continuous evolution and development, there are deficiencies such as a vague concept, system patchwork,narrow vision, superficial theory and lack of effective functions. In recent years, a series of new concepts such as Internet of Things, Cloud Computing and Smart Earth have emerged in thefield of information technology. Countries have raised these up to a strategic level and invested considerable manpower, material resources andfinancial resources to build and promote applications. Through the launch of new concepts,new models and new technologies, CAST has comprehensively demonstrated new developments in the construction of the space-based PNT system based on the BeiDou system with forward-looking theory and keenness.

The space-based PNT system using the BeiDou system is a comprehensive concept. With the BeiDou system and its space-based augmentation system as the core, it combines space remote sensing information, space communications networks, and interplanetary navigation satellite networks and other space-based systems, and assists in integration of digital broadcasting networks, the Internet, and the Internet of Things. Information infrastructure is related to location,direction, speed, time information, and time synchronization.With the support of a large-capacity space database, various spatial information can be processed and fused accordingly,and high-availability and integrity are provided to the users on the Earth's surface, in near-Earth space and in deep space detection providing different positioning accuracy and different amounts of information. Strict, robust and maintainable high-precision space application services, with extremely important military and civilian values, will become a major national space and information infrastructure, and an important part of strategic emerging industries[6-8].

2.1 Key Connotations

2.1.1 Core

The BeiDou system is the cornerstone of the space-based PNT system, including BeiDou 1, BeiDou 2, BeiDou 3 systems, and future navigation satellite systems.

2.1.2 Augmentation

The satellite navigation augmentation system was developed under the US GPS system implementing the Selective Availability (SA) policy. Although the United States canceled the SA policy in 2000, the accuracy of navigation and positioning has been significantly improved. However, with the continuous promotion and deepening of the application of GNSS,the existing systems are still unable to meet some of the positioning accuracy, usability and integrity. For the requirements of high-end users, various satellite navigation augmentation systems have emerged for this purpose, and the system is basically divided into two types.

1) Satellite-based augmentation system (SBAS)

The SBAS is equipped with a satellite navigation enhanced signal transponder via a geostationary orbit (GEO) satellite,which can broadcast various correction information, such as ephemeris error, satellite clock error and ionospheric delay,to the users, thereby realizing the positioning accuracy of the original navigation satellite system.

2) Ground-based augmentation system (GBAS)

The main principle of the GBAS is to perform differential positioning for the ground benchmark station, and send the differential correction corresponding to each satellite to the ground main control station. The main control station via a VHF (Very High Frequency) network, broadcasts high-precision differential correction information and integrity information to provide users with round-the-clock precision approach navigation services in their area of operation.

2.1.3 Comprehensive

Based on the BeiDou system, the space-based PNT system uses various space-based systems as the main nodes, including deep space detectors, space-based remote sensing systems, and space-based communications networks. These nodes are all under a unified high-precision space-time benchmark framework.

2.1.4 Interplanetary satellite navigation network

The construction of a Lagrange space navigation satellite or a planetary surface navigation satellite system can complete the deep space and near-Earth integrated large space-based navigation system, which can break through the navigation gap in remote detection and also support the satellite in the near-Earth space. The autonomous operation of the navigation system is achieved. Navigation satellites can be expanded into man-made spaceports (further expanded into man-made planets), used as transit bases or research bases for planetary exploration in addition to navigation.

2.1.5 Emerging industries

New industries based on major technological breakthroughs and major demands have a major tie-making effect on the overall economic and social development. Long-term knowledge and technology, low material consumption, large growth potential, and comprehensive benefits will emerge. The space-based PNT industry will be a unique new industry.

2.2 Basic Characteristics

2.2.1 Strategic

The construction of the BeiDou system has promoted the rapid development of satellite navigation basic components and key components in manufacturing, electronic map and application software development, terminal equipment manufacturing, mobile location service industry and navigation information service industry, and basically formed a complete new industrial chain. Based on the above, the space-based PNT system will also cover space-based remote sensing information, provide various types of users through the spacebased communications network, and the types of information services will be more comprehensive. The new industry will truly become an important economic growth point in China,which is not only an important component of the country's strategic emerging industries, but will also have the ability to drive the development of other related high-tech industries,and is a strategic driving force for industrial upgrade and leapfrog development.

2.2.2 Publicity

The space-based PNT system based on the BeiDou system will be part of the national infrastructure, one of the largest aerospace projects in the country under construction, and one of the largest intelligent information industry infrastructures.It will also become the main market of China's intelligent information industry. The driving force, not belonging to the department, has the characteristics of public benefit, and is a space-based information platform that provides services for the whole society.

2.2.3 Stability

With the BeiDou system as an important component, it has the ability to operate over a long-term and stably, providing uninterrupted services including all-weather, high-precision navigation satellite and constellation services for the users on the Earth's surface and near-Earth space. Ground facilities will participate in the contribution.

2.2.4 Dynamic scalability

The concept is not closed. With the advancement of technology and the growth of demand, the connotation and extension of the space-based PNT system can be dynamically expanded. For example, with the steadily advancing and maturing of manned landing or deep space exploration, it can be used for deep space. The demand for interplanetary satellite navigation systems, which becomes an important part of the space-based PNT system, will be more and more.

2.2.5 Compatibility and interoperability

Space-based PNT will not only be compatible and interoperable with multiple navigation satellite systems, but will provide comprehensive interoperability of space-based information systems such as space-based navigation, satellite remote sensing and space-based remote sensing.

3 DESlGN OF PNT SYSTEM BASED ON BElDOU SYSTEM

Figure 1 The space-based PNT system

The PNT system should be designed with the BeiDou system as its core, integrating all space-based information sources that can meet the needs of users' applications[9-11],including nine programs as shown in Figure 1.

3.1 Satellite Strengthening Program

As far as the long-term development of the BeiDou system is concerned, in the satellite navigationfield, the gap between the BeiDou and foreign advanced systems continues to shrink;the strategy of following development will no longer apply to the BeiDou system. How will the BeiDou system should develop after 2020 is now quite urgent. To answer this question,you don't have to trace the cutting-edge systems or technologies, but to the essence of the navigation satellite system.As defined in The National Defense Science and Technology Dictionary: “A receiver that is carried by a moving object on the Earth receives a radio coded signal broadcast by a navigation satellite system, and obtains the position and velocity of the moving object to achieve navigation.” High-precision spatio-temporal data measurement capability is the essence of satellite navigation systems.

Focusing on the pseudo range measurement error in the BeiDou system, the strengthening satellite program comprehensively studies the measurement principle of the system,and proposes the measurement error indicators of the BeiDou system from the aspects of the construction process of system error index system, the original core index screening, and the system error structure design. The program will conduct in-depth study and analysis of various error sources in the BeiDou system through high-precision spatio-temporal data measurement, looking for technical ways to weaken the impact of these errors to improve system service accuracy and capability. This is the top priority in the future development of the BeiDou system.

3.2 Satellite Re-recognization Program

Once a certain scale of navigation satellite systems has been established with the perception, decision-making and inter-satellite link capabilities, it becomes possible to realize intelligent operation and application of navigation satellite systems through artificial intelligence to improve the efficiency of the whole system. It needs to re-recognize and redefine the appli-cation and operational management of the navigation satellite system. The program focuses on the following aspects:

1) Redefinition. Stemming from the construction mode,working methods, indicator systems, development or evolution of navigation satellite systems;

2) Intelligent model. Based upon the means, deep neural network, reinforcement learning, group intelligence,game, distributed intelligence, etc., complete on-board tasks such as perception, decision-making, coordination, and planning;

3) Intelligent application. For different user experiences,top-level task requirements are proposed, the system completes task itself, and outputs the results.

4) Intelligent operation. For the complete operational process of the system, an intelligent method is used to complete the tasks and operational management.

3.3 Satellite Augmentation Program

Civil aviation, surveying and mapping and other application areas propose higher requirements for navigational service performance, and the need for the navigation system is further enhanced in a complex terrain environment and complex electromagnetic environment. The mission for the satellite navigation augmentation system is to improve the performance of navigation satellite main system while expanding the main system function to ensure the perfect implementation of navigation, positioning, timing and attitude measurement. The program focuses on three main areas:

1) Accuracy enhancement. Accuracy is a measure of the consistency of an estimated or measured value of a terminal location and velocity with the true location or velocity at a given time. Accuracy enhancement refers to the ability to increase the accuracy of measurements and calculations or to reduce errors;

2) Integrity enhancement. The navigation satellite system itself has the ability to monitor integrity, but it takes several hours to give warning and only has a single source of information, that the integrity is urgent to be enhanced;

3) Usability enhancement. Completeness, reliability,time availability and precision should be enhanced. It should be noted that the failure of the system hardware or warning information that causes the interruption of operation will lead to a decline in this service quality assessment indicator.

3.4 Cloud Satellite Program

With the acceleration of the modernization process for the BeiDou system, the research on the re flected signal based on the BeiDou navigation satellites and its application in thefield of remote sensing detection is on the rise. The BeiDou navigation satellite re flected signal does not need a separate signal source transmitter. Many on-orbit satellites are available with rich signal sources. The re flected signal receiving devices can be configured on the ground, in an aircraft or a low-orbit satellite to realize real-time observation of the ground. The application of BeiDou re flected signal based on a low-orbit satellite configuration has become a development trend because of its high altitude, large detection range, continuous and long-term observation ability. The program puts emphasis on three main areas:

1) Sea surface height measurement, sea surface windfield inversion, sea ice and soil moisture measurement;

2) Detection and application of the BeiDou re flected signal of marine ships;

Detection and application of the BeiDou reflected signal based on low-orbit satellites.

3.5 Inter-satellite Link Program

Due to the long revisit period of low-orbit remote sensing satellites, the application opportunity is limited. A modernized,high-performance remote sensing system, if it can't deliver the right information to the right person or the platform it needs, its true performance is zero. Therefore, real-time communications and intelligence distribution systems are the most critical. The program mainly aims at enhancing the mission response, the timeliness of detection data and multi-method coordination of low-orbit remote sensing satellites with the help of the inter-satellite and satellite-ground links constructed by the BeiDou system.

1) BeiDou satellites will provide higher bandwidth in the future and will be suitable for transmitting large amounts of data over its operational range;

2) Using the global short message transmission and reception capability of BeiDou navigation satellites,it can quickly inject remote control commands into low-orbit remote sensing satellites;

3) Based on the communications link of the global Bei-Dou system, it effectively reduces the limits of the measurement and control and data transmission window for low-orbit satellites, and improves the utilization of the low-orbit satellites.

3.6 Pivot Satellite Program

At present the current development and research within China on the integration of satellite navigation and communications (network) is focused on how to use the communications (network) to provide navigation information services to make up for the gaps and shortcomings in satellite navigation capabilities, and to provide seamless PNT capabilities. The focus is not yet on a fusion of PNT and communications (network).

The program will realize the complete integration of navigation and communications, focusing on satellite navigation enhancement by adding navigation messages, precision correction parameters and integrity in the broadcasted information with the space-based communications system signal modulation mode, multiple access and coding unchanged. Design and measure communications integrated signal, in addition to the auxiliary enhancement function, supporting ranging, so that the communications mode has the dual function of communications and navigation. In addition, to realize global mobile communications functions on the selected MEO navigation satellites that carry mobile communications payload are also one of the research directions.

3.7 Satellite Security Program

The security of the space-based PNT system is to ensure the security of the PNT system as a whole as well as the effective use of PNT information by its own or friendly land,sea and air platforms, especially in the case of interference caused with the space-based PNT system. This means ensuring continuous PNT services. The security of the space-based PNT system is divided into physical security and information security. The physical security of the PNT system refers to the security of the PNT infrastructure. When the enemy initiates a hard PNT destruction approach, effective measures are taken to ensure the security of the infrastructure of the PNT signal source and the ground control segment.

The program refers to ensuring the correctness and integrity of the PNT information. When the enemy initiates a soft PNT strike approach, it adopts enhancement, backup,and integration of other undisturbed information to ensure an uninterrupted PNT service. According to the technical approach of the integrated PNT system defense technology, the corresponding space-based PNT system security protection measures are adopted.

3.8 Varible Satellite Program

The BeiDou system can provide all-weather, all-day,high-precision navigation information services for users on the Earth's surface and in near-Earth space. However, for military applications, during wartime, the BeiDou system is likely to be disturbed or destroyed that will not be able to provide stable and reliable navigation information for the combat platform. On the other hand, navigation satellite systems usually can only meet the navigation requirements of ground-based,space-based and near-Earth space platforms. For high-orbit or deep-space missions, navigation satellite systems will no longer be applicable.

The program uses periodic variable satellites or pulsars as a space-time reference. It does not rely on artificial beacons at all and does not interact with the surrounding environment. It is a completely autonomous navigation method. Moreover, a periodic variable satellite satisfies the requirement for high-precision navigation during the whole process of ground-based,space-based, low-Earth orbit, medium- and high-orbit, deep space and interstellar space missions.

3.9 Micius Satellite Program

Quantum mechanics is a dynamic theory that unifies the volatility and particle properties of matter. Quantum theory is one of the two greatest advances in physics in the 20th century; the other one is the relativity theory. The establishment of quantum theory has opened up the road for human beings to understand the laws of material movement in the world and created a new era of physics. Man's understanding has evolved from traditional science and technology theory from classical to quantum theory, as quantum states have many different classical physical states; they are based on quantum mechanics such as quantum computer theory, quantum information, quantum signals. Processing has evolved and a large number of emerging interdisciplinary disciplines such as quantum cryptography.

The essential difference between the Micius satellite program and the traditional positioning system is the difference in the signals used. The traditional positioning system uses electromagnetic wave pulses, while the Micius satellite program will use a pulse signal with quantum characteristics, that is,photons without charge and mass. In the development process of the BeiDou system, we have adopted more strategies to follow GPS. At a relatively delayed technical level, we should take more leaps and bounds. The Micius satellite program will be the main research direction of our leapfrog development strategy.

4 CONSTRUCTlNG A WORLD-CLASS DEVELOPMENT MODEL OF SPACE-BASED PNT SYSTEM BASED ON BElDOU SYSTEM

The most important feature of the information resources provided by the space-based PNT system with the BeiDou system as its core is its sharing, which is different from human resources, material resources and capital resources.Integrated spatio-temporal benchmarks and services that provide high-precision PNT capabilities with higher availability,integrity, robustness and maintainability is the goal for the space-based PNT system. Effective configuration of the Bei-Dou-based PNT system is the premise for realizing the PNT information service. In order to have a reasonable and effective configuration, the construction of the PNT system based on the BeiDou system should follow a development model of government-led, enterprise implementation, multi-participation, market promotion, and coordinated promotion.

4.1 Government Leading Role

Since the information industry is a high-input, high-risk industry, major projects like the space-based PNT system based on the BeiDou system cannot be completed by the strength of individual enterprises and research institutions, coordinated operations and joint researches are need, Large-scale researches and development activities are more appealing for government-led organizations.

From the experience of other projects such as the BeiDou system project and China High-resolution Earth Observation System (CHEOS) Major Special Project we can see that those projects had small scale and low output in the early stage,which were behind those of the developed countries. By implementing a government-led development model, it is not only conducive to taking China's late-comer advantages,but also avoiding repeating the old mistakes that developed countries have had to deal with, thus achieving a leap-forward development, rapidly narrowing the gap between and greatly enhancing China's international competitiveness in PNTfield.

4.2 The Role of CAST

To develop a space-based PNT system, the key is to cultivate leading enterprises to take advantage of the backbone implementation role of leading enterprises, taking a market-oriented approach and take advantage of joint interests in all aspects to form an industrial community.

CAST undertook the construction of the BeiDou 1 and BeiDou 2 navigation satellite systems, and is also the main player of the BeiDou 3 system. It plays a good backbone role and shoulders the historical mission of leading enterprises in the industry. It also provides a comprehensive information network through the use of mobile communications networks,digital broadcasting networks, the Internet, and the Internet of Things, to provide users with a world-wide PNT capability through the development of China's PNT industry.

It is important to integrate multi-source information of the space-based system and other sources of PNT with the goal of developing a fast integrated and reconfigurable architecture,abstract methodology and filtering algorithm related to any navigation sensor and sensor combination scheme, reducing system integration cost, and providing users with high-precision under full-source and global service conditions, thus providing positioning, navigation and timing capabilities to meet changing mission requirements and environmental changes.

4.3 The Role of Multi-participation

The space-based PNT system based on the BeiDou system will create a huge industrial chain, involving positioning, navigation, timing, geographic image information, communications and weather, traffic, yellow pages, hotels, restaurants and shopping guides, tour guides and many other aspects, which drives a huge market. On the other hand, it also indicates that there will be a series of problems in this industry chain, such as serious resource duplication, so the interfaces of the components will not be uniform.

To promote and accelerate the development of the system,we must deal with the relationship of various links in the industrial chain as soon as possible, and develop each member group in the industrial chain into a multi-participating industrial alliance, with scientific management, earnestly safeguard the interests of all members, while promoting the members'collaboration and integration. Establishing an industrial alliance is a win-win for all members and will definitely enhance the competitiveness of the industry. The establishment of an industrial alliance should be coordinated by the government and all members should work together to complete it.

4.4 The Role of the Market

The optimal configuration of location resources through an open market system not only reduces the cost to the government organization, but also ensures the power of optimal resource allocation. By having the market open it is conducive to the promotion of the best in the industry. In the space-based PNT system, a large quantity of information will be generated in the form of service information through mutual transactions, which will require the promotion within the market.

The development of the space-based PNT system using the BeiDou system requires the mobilization of various resources to bring together all forces and funds. The operation of an environmental infrastructure that relies solely on government actions and lacks social enthusiasm is unsustainable and does not meet the needs for operating capital, increased manpower and for equipment evaluation. The management and operation of infrastructure should be transformed from a government-run enterprise to a market-oriented one. Under the base of an open market, social capital, social public, enterprises and institutions should collaborate. Diversified investment options such as those based upon government, enterprises and individuals are important to accelerate the development of the system industry.

5 PROMOTlNG THE ADVANTAGES OF THESPACE-BASED PNT SYSTEM BASED ON THEBElDOU SYSTEM

5.1 The Theory and Method of Aerospace Systems Engineering is the Foundation for Building a World-class PNT System Based on the BeiDou System

With the trend of information society development, the space-based PNT system using the BeiDou system will no longer be limited to a single PNT information service, but is an integrated system with the BeiDou navigation and positioning function as its core. The system will be combined with geographical location information and remote sensing information, and integrated into infrastructures such as mobile communications network, digital broadcasting network, Internet, Internet of Things to realize integrated service functions such as navigation and positioning, tracking and monitoring,emergency rescue and security services. The system has the characteristics of complexity, technology-intensive, high risk and a long development period, which require the establishment of a new organizational management mechanism to be in charge of the research, development and application of the space-based PNT system based on the BeiDou system using new ideas, theories and methods. The construction of the space-based PNT system faces not only technical problems,but also organizational issues. There is a need to establish a systematic management model including planning, research,design, manufacturing, testing and operation to organize and coordinate thousands of management technicians, a large number of collaborative units and countless terminals. The systems engineering theory and method applied in space projects have ensured the smooth construction of the BeiDou system and will be the premise and basis for the smooth implementation of the space-based PNT system based on the BeiDou system.

5.2 CAST's Expertise in Satellite Development Offers the Best Opportunity for Building a World-class Space-based PNT System

Characteristics of an industrial enterprise are the basis for its survival and future development. The core for maintaining its advantage is to use industrial characteristics and continuously optimize the industrial structure.

In the process of developing the BeiDou system based PNT system, CAST has the advantages of paying attention to the development trend of China's satellites, and always takes into account the development of space-based PNT system in accordance with satellite development. As a starting point, the development of the space-based PNT system based on the BeiDou system will mutually benefit from satellite development. Combining with the requirements of the PNT system based on the BeiDou system, it will highlight its own advantages and characteristics, improve the overall level and competitiveness of the system, consolidate the benefits from the incumbent international PNT, and also ensure the spacebased PNT system realizes its own development in the process of promoting satellite development.

5.3 Space Technology is an Important Element for Construction of a Space-based PNT System Based on the BeiDou System

The space-based PNT system with the BeiDou System as its core, combines satellite navigation technology, geographic information technology (GIS), satellite remote sensing technology, satellite communications and sensor technology, computer technology and network communications technology to collect, process, manage, analyze, disseminate and use data information, which is typically a complex project. The project featured as large scale, complex systems, technology intensive and comprehensive requires integrating the expertise of many disciplines to ensure success.

CAST covers the entire industrial chain from chip, module, terminal integration to satellite systems, including payload design and manufacturing. It has national first-class satellite platform design experience and has been in the design and operation of the BeiDou system since the beginning. In short,CAST can provide key technical support for the establishment of a space-based PNT system based on the BeiDou system.

6 CONCLUSlONS

The research on the PNT system in China is currently only in its infancy. After the United States issued the “National Positioning Navigation Timing System Research Final Research Report”, China realized the inherent rationality and technical close correlation of PNT integration. Gradually it was realized that the development of China's autonomous satellite positioning system should be integrated into the construction of the country's unified planning on the PNT platform. From the top down, the development of the national PNT system focuses on providing a safe, reliable, robust,and compatible PNT capability that is constantly improving in terms of performance, complementing each discipline and backing up each other. Naturally, the role of the BeiDou system in the national PNT system is irreplaceable. Based on its own advantages, opportunities, it gives play to the active implementation and promotion of the system. CAST will build a model of space-based PNT system with the BeiDou system as its core that meets the requirements of the country's emerging science and technology industries, and will promote space transformation and its sustainable development, thus promoting the process of the new industrial revolution.