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Differences between BeiDou-3 and the previous two generations of BeiDou satellites

On November 5th, the first 2 satellites of BeiDou-3 network (the third-generation Chinese navigation satellite) were sent into space via a single carrier rocket, marking the official start of the construction of BeiDou navigation satellite system. Then, what is the difference between BeiDou-3 and the first two generations of BeiDou navigation satellites (BeiDou-1 and BeiDou-2)? What new technologies are used in BeiDou-3?

Different kinds of navigation satellite systems

To figure out the differences and similarities between the three generations of BeiDou navigation satellites, we should first understand what kind of navigation satellite systems they belong to.

So far, the satellite navigation technology has developed into two main types. The first type adopts the principle of Doppler Velocity Measurement. Namely, it gets the range rate by measuring the Doppler shift of navigation signals for navigation and positioning. However, this type of navigation system has some disadvantages. It cannot ensure continuous real-time navigation. Therefore, few navigation satellite systems adopt this method now. The other type adopts the principle of time ranging. Namely, it measures the travel time of navigation signals to get the distance for navigation and positioning. Most navigation satellite systems currently adopt this method including the three generations of BeiDou navigation satellites. Even so, they are still different.

Navigation satellite systems can also be divided into these two kinds, active and passive. BeiDou-1 navigation satellite system adopts the active mode, that is, users have to send signals to satellites for navigation and positioning. However, BeiDou-2 and BeiDou-3 combine active and passive modes. Users only need to receive signals sent out by navigation satellites. Navigation satellite systems such as GPS in the USA, GLONASS in Russian, and GALILEO in EU are all in passive mode.

Navigation satellite systems can also be divided into two types according to their coverage, global navigation satellite systems (GNSS) and regional navigation satellite systems (RNSS). BeiDou-2 and BeiDou-3 of China and navigation satellite systems of Japan and India belong to the former, while that of the USA, Russia, and Europe, and BeiDou-3 of China belong to the latter.

Moreover, according to the height of orbits, navigation satellites can be divided into near-earth, MEO, IGSO, and GEO satellites. BeiDou-1 satellites are in GEO, while BeiDou-2 and BeiDou-3 navigation constellations consist of MEO, IGSO, and GEO satellites. At present, navigation satellites of the USA, Russia, and Europe are operating in MEO, while that of India and Japan are in IGSO and GEO.

Unique functions of BeiDou navigation satellite system

Satellite navigation has already become a significant infrastructure for spatial information now. As its application can only be limited by imagination, it has profound significance in society, economy, science and technology, and national defense. Therefore, China attaches great importance to navigation satellite system and implements the launches in three stages, a navigation satellite system for active region first, then a regional navigation satellite system of both active and passive services, and finally a global navigation satellite system of both active and passive services.

The first phase ranges from October to December, 2000. China launched 4 BeiDou-1 navigation satellites (the later two as backups) successively operating in GEOs 60 degrees of longitude apart. Accordingly, the first navigation satellite system for active region in the world was completed. This system can provide regional navigation and positioning, two-way short message communication, and precise timing, specially useful for users in need of combining navigation and mobile data communication. It covers all China and provides positioning accuracy within 20 meters, timing accuracy within 100ns, and short message within 120 Chinese characters.

The second phase ranges from 2007-2012. China launched 16 BeiDou-2 satellites and constructed a RNSS with 14 BeiDou-2 satellites (5 in GEO + 4 in MEO) by integrating active and negative modes. GPS can only tell users the time and location. However, BeiDou-2 can also send users' information to others and tell who, what, and where. Serving the Asia-Pacific region, BeiDou-2 provides positioning accuracy within 10 meters, velocity accuracy within 0.2 meter per second, timing accuracy within 50ns, and short message within 120 Chinese characters.

In the third phase (2017-2020), China will successively launch 35 BeiDou-3 navigation satellites (5 in GEO + 3 in IGSO + 27 in MEO) and construct a GNSS by integrating active and passive modes. Serving the whole world, BeiDou-3 raises the positioning accuracy to 2.5-5 meters, velocity accuracy within 0.2 meter per second, timing accuracy within 20ns. It also allows more Chinese characters in short message. It provides civil users with the positioning accuracy within 10 meters, velocity accuracy within 0.2 meters per second, and more precise services for paying users. As the ground-based augmentation systems (GBAS) of BeiDou starts to provide initial services, BeiDou-3 will be able to provide meter, sub-meter, decimeter, and even centimeter level services. Then, it will be comparable with GPS in positioning accuracy.

When completed, the GNSS of BeiDou will provide services with the positioning accuracy within 10 meters for civil users.

The BeiDou navigation satellite system featured with high precision, reliability, and multiple functions has some unique functions and features distinct from global navigation satellites systems of the USA, Russia, and Europe. For example, its space segment is a hybrid constellation consisting of satellites in three kinds of orbits. In comparison with other navigation satellite systems, the BeiDou system operates more satellites in high orbits to achieve better anti-shielding capability, which is particularly observable in terms of performance in the low-latitude areas. BeiDou-3 also provides navigation signals of multiple frequencies, and can improve service accuracy by using combined multi-frequency signals. Moreover, with creatively integrated navigation and communication capabilities, it has five major function: real-time navigation, rapid positioning, precise timing, location reporting, and short message communication services.

Highlights of BeiDou-3

Due to BeiDou-3 navigation satellites' need of many new technologies, China launched 5 BeiDou-3 test satellites successively from 2015 to 2016 and proved the effect of the key technologies of satellite payload represented by high-precision space-borne atomic clocks and autonomous operation of satellites, satellite platform featured with light weight, long service life, and high reliability, navigation satellite operation control based on satellite-ground links, phased-array inter-satellite-link (ISL), and brand-new navigation satellite system, as well as 98% domestic-made devices. All these key devices were made in China.

China started to construct its global navigation satellite system officially from the first two BeiDou-3 MEO satellites launched on November 5th, 2017. Comparing to BeiDou-2, BeiDou-3 not only expands its service coverage from region to globe, but also improves greatly on accuracy and reliability. BeiDou-3 satellites are designed to work 10-12 years instead of 8 in the past and are expected to provide non-stop services for the first time.

Moreover, BeiDou-3 marks the following technological innovations and improvements. BeiDou-3 MEO satellites adopt the new specialized navigation satellite platform. With high power density, payload, and adaptive capacity in function expansion, as well as flexible equipment layout, it can better adapt to follow-up system functions and requirement expansion, provide positioning, timing, and navigation services of the navigation satellite system, and achieve in-orbit functions including space-based data transmission and new business payload as a broadcast node for the space-based data transmission network. BeiDou-3 is the only navigation system consisting of satellites in three kinds of orbits. It will expand other services including global search and rescue, location reporting, and Satellite-Based Augmentation System (SBAS) in line with international standards in the future.

BeiDou-3 GEO and IGSO satellites use the large-scale satellite platform which integrates multiple payloads, achieves space-based augmentation, movable spot beam power augmentation, short message communication, and location reporting systems at the same time, and works as the central node for the space-based data transmission network.

The BeiDou-3 constellation is featured with ISL for the first time which helps to track domestic satellites. In this way, it can monitor domestic satellites operating all over the world and give them orders. It also achieves precise two-way distance measurement and communication between satellites to enable autonomous computing and correction of satellites' orbital positions and service accuracy. Accordingly, the system's dependence on ground stations reduced and the accuracy of positioning and service enhanced. ISL is essential to the autonomous navigation of BeiDou. It enables the communication, data transmission, mutual distance measurement, array keeping between BeiDou satellites, thus reducing the burden on ground management and maintenance. Autonomous navigation refers to that even if the ground station breaks down, more than 30 BeiDou navigation satellites can still provide precise position and time through ISL and users' cell phones can still receive signals from navigation satellites for positioning and navigation.

It is widely known that the function of the atomic clocks on the navigation satellites play an important role in the overall performance of the navigation satellite system. BeiDou-3 satellites are equipped with new and high-performance rubidium and hydrogen atomic clocks of China. Atomic clocks of BeiDou-3 satellites reduced sharply in weight and volume in comparison to that of BeiDou-2 satellites. Moreover, their frequency stability at one day is ten times better and overall index reaches the advanced level in the world. The accuracy of hydrogen atomic clocks on BeiDou-3 is enhanced by an order of magnitude than that of rubidium clocks on BeiDou-2. The stability of rubidium clocks is 10-14 seconds in every ten thousand seconds per day, and hydrogen atomic clock 10-15 seconds. The improvement of atomic clocks promoted the positioning accuracy of BeiDou system to 1 meter from 10 meters and the accuracy of velocity measurement and timing were also raised by one order of magnitude.

BeiDou-3 further improves continuity, stability, and usability. It adopts several new technologies to enhance satellites' anti-interference ability and unintended interrupt 0.4 time per year, reaching advanced international standards. It also adopts multiple measures to produce the greatest degree of reliability of the system. For example, the number of satellites in service must be more than the necessary quantity, that is, the system must has backup satellites. This system is equipped with several rubidium atomic clocks to provide service with "double insurance". Moreover, it adopts other measures including software redundancy and fault self-diagnosis and repair to enhance reliability.

BeiDou-3 also achieves autonomous in-orbit monitor of its capabilities and integrity for the first time. This function is quite practical for users in fields related to life safety such as civil aviation and automatic transmission. BeiDou-3 will be equipped with B1C and B2a interoperable signals with optimized performance to achieve compatibility with other navigation satellite systems in the world. Its brand-new navigation signal system and in-orbit reconstruction function greatly improves user experience. The compatibility and interoperability of BeiDou enable users to receive more signals on their ends and provide them with more options.

According to the plan, 18 BeiDou-3 satellites will be launched around 2018 to serve nations and regions along the Belt and Road. BeiDou-3 will provides all-time, all-weather, and high-accuracy positioning, navigation, and timing services for global users around 2020. The plan for BeiDou is about to construct an integrated system of positioning, navigation, and timing service. This reliable, secure, efficient, and convenient system centered on BeiDou will cover all places including outer space, ground, underwater, and indoors.

Extended Reading: How do BeiDou satellites navigation and positioning work?

BeiDou-1 differs distinctly from BeiDou-2 and BeiDou-3 in how they work.

BeiDou-1 navigation satellite system adopts radio determination satellite service to determine users' location. Usually, the ground station keeps asking users if they need signals for positioning by two GEO satellites of BeiDou-1. The user terminals only receive but never send messages. The user terminal in need of positioning applies to the ground station for signals through these two satellites. Then, the ground station calculates the distance from the user terminal to every satellites by measuring the round-trip time of signals. Since locations of satellites are already known, these two resulting data can be used to further calculate the user's position. Finally, the ground station send the positioning signal to the user terminal through one satellite.

This method has some advantages such as it needs two satellites only. It also has many functions including navigation, positioning, short message communication, and precise timing. However, the positioning accuracy of the system is low and the user capacity is limited.

BeiDou-2 and BeiDou-3 navigation satellite systems integrate radio navigation satellite service and radio determination satellite service for navigation and positioning for the first time in the world. Through the radio navigation satellite service, users measure radio navigation signals continuously send out by at least four navigation satellites controlled by satellite clocks. Then, users can calculate their distances to these satellites based on the different transmission time of these signals and get their three dimensional coordinates and speed through the mathematical operation of the user terminal.