Beidou one, Beidou two and Beidou three are often heard. They are not separate satellites, but are made up of satellites. The Beidou generation has basically been abandoned, the Beidou two generation has formed regional navigation, and the Beidou three generation has built the official constellation. To understand the similarities and differences of the three generation "Beidou" navigation satellite in China, we should first understand the following satellite navigation systems, and see which navigation is Beidou 1, two and three respectively.

1. Difference of navigation system principle

Satellite navigation technology has developed two major categories:

A kind of satellite navigation system adopts the principle of Doppler velocity measurement, that is, using the Doppler frequency shift to measure the navigation signal to get the rate of distance change to navigate and locate. However, because of this way, there are some shortcomings such as continuous real-time navigation and so on, so the satellite navigation using this principle is basically gone.

Another kind of satellite navigation system adopts the principle of time distance, that is, finding the distance by using the time of measuring the navigation signal to navigate and locate. At present, most of the satellite navigation systems adopt this method, including the three generation of "Big Dipper" navigation satellite, but even so, they are still different.

Because satellite navigation system can be divided into active (active) and passive (passive) two. China's Beidou one satellite navigation system uses active mode, that is, when users navigate and locate, they should send signals to the satellite voluntarily. Beidou two and Beidou three satellite navigation system adopt passive and active combination. When conducting passive satellite navigation, users only need to receive navigation satellite signals. The global positioning system (GPS), Russia's "gal", "Galileo" and other satellite navigation systems all adopt passive mode.

1) Beidou 1 positioning principle

The Beidou satellite navigation system uses satellite radio to determine the location of users. At ordinary times, the ground central station continuously asks the users whether they need to locate signals through 2 Beidou 1 geostationary orbit satellites, and the user terminals are usually only in the state of light listening without sending messages. When positioning is needed, the user terminal sends the location signal to the central station via 2 Beidou 1 geostationary satellite. At this time, the ground center station calculates the distance between the user terminals and each satellite by measuring the round-trip time of the signal. Since the position of the satellite is known, we can use these 2 distances to measure the data and further calculate the user position. Finally, the ground station sends the location signal to the user terminal via 1 satellites.

The advantages of this method are less satellite, as long as 2 satellites are running; navigation and positioning, sending short messages and precise time service and other functions. But its positioning accuracy is not high, and the user capacity of the system is limited.

2) Beidou two and Beidou three positioning principle

Beidou two and Beidou three satellite navigation system have first implemented the integrated navigation system of satellite radio and satellite radio navigation services. The so-called satellite radio navigation service is that the users passively measure the radio navigation signals from at least 4 consecutive navigation satellites under the satellite clock control, and simultaneous interpreting the different distances of the users to these satellites according to the different transmission time of these satellite signals, and then get the three-dimensional coordinates and speed of the users through the mathematical calculation of the user terminals.

Beidou navigation system map

Two, navigation coverage difference

According to the coverage of the navigation satellite, the satellite navigation system can also be divided into the regional satellite navigation system and the global satellite navigation system. China's Beidou 1, two satellite navigation system and Japan, India's satellite navigation system belong to the former. The Beidou three satellite navigation system in the United States, Russia, Europe and China belongs to the latter.

Three. Different navigation satellites orbit

Navigation satellites can also be classified into earth orbit, earth orbit, inclined earth synchronous orbit and geostationary orbit satellite according to the orbit height of satellites. China's Beidou 1 is the geostationary orbit navigation satellite, while Beidou two and three are composed of the earth's circular orbit navigation satellite, the geostationary orbit navigation satellite and the geostationary orbit navigation satellite. At present, the navigation satellites in the US, Russia and Europe operate in the earth's circular orbit. The navigation satellite constellations in India and Japan are composed of oblique geostationary orbit navigation satellites and geostationary orbit navigation satellites.

Four, the unique characteristics of the three generation of the Beidou.

The "Beidou" global satellite navigation system with high precision, high reliability, high reliability and high security has some features and features that the US, Russia and Europe do not possess. The space segment uses three constellations composed of orbiting satellites, and more satellites than other satellite navigation systems. Therefore, it has strong anti blocking ability, especially in low latitude. The performance characteristics of Dou area are more obvious; Beidou three can provide navigation signals with multiple frequency points, and can improve the service accuracy through the combination of multi frequency signals. The satellite system innovating integrates navigation and communication capabilities, and has five functions: real-time navigation, fast positioning, precise timing, location reporting and short message communication services.

Since Beidou three navigation satellite needs many new technologies, China has launched 5 Beidou three test satellites from 2015 to 2016, and verified the key technology of satellite load represented by high-precision spaceborne atomic clocks and autonomous operation of constellations. The key technology of satellite platform based on lightweight, long life and high reliability is based on satellite earth links and inter satellite chains. The key technologies of the navigation satellite operation control, the 98% localization device and the key components are "made in China".

Since the launch of the first Beidou three (2) earth orbit navigation satellite in November 5, 2017, China has officially started to build the "Beidou" global navigation satellite system. Compared with Beidou two, Beidou three has improved greatly in terms of accuracy and reliability in addition to the service area extended from regional coverage to global coverage. Its single star design life has increased from 8 years to 10 to 12 years, and for the first time, the "guaranteed service" index has been put forward.

In addition, there are the following major technological innovations or improvements. The Beidou three medium earth orbit satellite adopts a new special platform for navigation satellite. It has the technical characteristics of high power density, high load bearing capacity, flexible layout of equipment, and strong adaptability of functional expansion. It can provide greater adaptability for the follow-up function and demand expansion of the system, and will realize the positioning, timing and navigation services of the satellite navigation system. The application of Yung based data transmission and new business load can be used as the broadcast node of space-based data transmission network. As the world's only navigation system composed of 3 kinds of orbiting satellites, Beidou three will also expand global search and rescue, global position reporting and satellite based enhancement services in the future according to international standards.

Beidou three geostationary orbit satellite and tilted earth synchronous orbit satellite adopt large satellite platform. It can integrate various loads, and is compatible with space enhanced system, mobile point beam power enhancement, short message communication and location reporting system. It becomes the central node of space-based data transmission network.

The Beidou three constellation is equipped with a phased array intersatellite link (the communication measurement link between satellites) for the first time, which solves the problem of monitoring satellites abroad and becomes a major feature. In this way, we can realize the monitoring and injection function of the satellites running abroad, and achieve two-way precision ranging and communication between satellites, so that we can carry out multi satellite measurement, autonomously calculate and modify the orbit position and clock system of the satellites, greatly reduce the dependence on the ground stations, and improve the positioning and service accuracy of the whole system. The inter satellite link is the key to realizing the autonomous navigation of the "Beidou". It not only realizes the communication and data transmission between the Beidou satellites, but also can distance each other, automatically maintain the formation and reduce the pressure of ground management and maintenance. The so-called autonomous navigation means that even if all the ground stations fail, the more than 30 Beidou navigation satellites can provide precise positioning and timing through the inter satellite links. The ground users can receive the navigation satellite signals through the terminals such as mobile phones, and can still locate and navigate.

As we all know, the performance of the atomic clock on the navigation satellite has an important influence on the performance of the entire satellite navigation system. The Beidou three satellite uses our new high precision rubidium atomic clock and hydrogen atomic clock. Compared with the atomic clock used by Beidou two, the atomic clock on Beidou three has greatly reduced in volume and weight, and the frequency stability has increased by 10 times a day, and the comprehensive index has reached the international leading level. Its hydrogen atomic clock will be increased by an order of magnitude more than that of the Beidou two Ruby atomic clock. The stability of rubidium atomic clock is 10-14 seconds per 10 thousand seconds, and the stability of hydrogen atomic clock is 10-15 seconds per 10 thousand seconds.The progress of the atomic clock technology has directly promoted the positioning accuracy of the Beidou system from 10 meters to meters, and the speed and timing accuracy have increased by one magnitude simultaneously.

Beidou three has further improved the index of continuity, stability and availability. With many new technologies, the anti-jamming capability of the satellite has been enhanced. The unplanned interruption index is 0.4 times a year, reaching the international advanced level. It adopts multiple reliability "reinforcement" measures, which can maximize the insurance coefficient of the system. For example, after the completion of the system, the number of satellites running is greater than the number of satellites necessary for service, that is, the satellite has backup, equipped with multiple rubidium atomic clocks, forming "double insurance" to provide services together, and adopting a number of measures, such as software redundancy, fault self diagnosis and fault self repair, to ensure the reliability of the system.

The satellite is also the first orbit autonomous integrity monitoring function of the satellite in the world. This function is of great practical value to users in civil aviation, automatic driving and other life safety areas. It will add better interoperability signals, B1C and B2a signals that are better compatible with other satellite navigation systems in the world. Its new navigation signal system and powerful on orbit reconstruction function will also greatly enhance the user experience, because by compatible Interoperability Technology, it can lay the foundation for users to receive multiple signals on the terminal, and provide users with a variety of options.

Extended reading

Development history of Beidou navigation system

At present, satellite navigation has become an important spatial information infrastructure in today's society. Its application is restricted by imagination, and has wide social, economic, technological and national defense significance.Therefore, our country attaches great importance to the construction of the satellite navigation system, and launches in three stages, that is, building an active regional satellite navigation system first, then building a regional satellite navigation system with passive and active combination, and ultimately building a global satellite navigation system with passive and active combination.

The first phase is from October 2000 to December. Our country has launched 4 Beidou navigation satellite (the last two are backup). They operate in the geostationary orbit with longitude 60 degrees apart, thus building the world's first active regional satellite navigation system. The system not only provides regional navigation and positioning, but also provides two-way digital message communication and precise time service. It is especially suitable for users who need navigation and mobile data communication. Its service range is domestic; the positioning accuracy is 20 meters; the timing accuracy is 100 ns; the short message word is 120 words per time.

The period from 2007 to 2012 is the second stage. 16 Beidou two navigation satellites have been launched in China, and 14 regional Beidou two (5 geostationary orbit navigation satellites + 5 geostationary orbit navigation satellites + 4 middle earth orbit navigation satellites) have been built, and a regional satellite navigation system combining passive and active satellite navigation methods has been built. If GPS can only tell the user where and where it is, "Beidou" can also send out the user's location information, so that others can know the situation of the users, and better solve the problems of who, what and where. Its service scope is the Asia Pacific region; the positioning accuracy is 10 meters; the speed accuracy is 0.2 M / sec; the timing accuracy is 50 ns; the short message word is 120 words per time.

From 2017 to 2020, the third phase, China will launch 35 Beidou three navigation satellite (5 geostationary orbit satellites + three geostationary orbit satellites + 27 medium orbit satellites), and build a global navigation satellite system with passive and active navigation methods. Its service range is global; the positioning accuracy is 2.5 to 5 meters; the speed accuracy is 0.2 M / sec; the timing accuracy is 20 ns, and the number of SMS words increases each time. It will provide free positioning service and 0.2 M / s speed measurement service for civilian users free of charge, with a precision of 10 meters. With the initial service provided by the Beidou ground-based augmentation system, it can also provide meters, sub meters, decimeter and even centimeter level services. At that time, the positioning accuracy of Beidou will be comparable to that of the US GPS.

According to the plan, the launch of the 18 Beidou three satellite in 2018 has already completed the basic services for the "one belt and one road" country and region. By the year 2020, Beidou three system has been providing the whole world with all-weather and all-weather, providing high precision and reliable positioning, navigation and time service for all kinds of users. The "Beidou" development blueprint is to build a comprehensive national positioning, navigation and timing system. With the "Beidou" system as the core, we will build a comprehensive national positioning, navigation and timing system that integrates space, ground, underwater, indoor, seamless, safe, credible, efficient and convenient.

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