Development and Innovation of BeiDou Navigation Satellite System

XIE Jun,ZHANG Jianjun

China Academy of Space Technology,Beijing 100094

Abstract:The completion of the BeiDou global navigation satellite system has made China becoming the third country owning a global navigation satellite system independently.It has embarked on a development path with Chinese characteristics,enriched the development route of the world navigation satellite system,and enhanced China's core competitiveness. The article describes the development and construction of Beidou Navigation Satellite System (BDS) and summaries the main achievements of BDS-1,BDS-2 and BDS-3 systems from the three aspects of independent innovation,system research and technological breakthrough.The article also analyzes the technical innovation characteristics of the BDS satellites comprehensively,providing a reference for subsequent research and planning of the next generation of the BDS satellites that are more ubiquitous,more integrated,and more intelligent.

Key words:BeiDou,navigation,innovation,next generation

1 OVERVIEW

China started to plan the BeiDou Navigation Satellite System (BDS) optimized for its national conditions in the 1980s,laying down a three-step strategy.The first step is to construct the BDS-1 system,providing services for China.The second step is to construct the BDS-2 system,providing services to the Asia-Pacific region.The third step is to construct the BDS-3 system to complete its own global navigation satellite system,which was designated as a major national project in December 2009.The BDS-3 system is to provide a global basic navigation,positioning and timing services compatible with the existing global navigation satellite systems (GNSS) with the ability of providing high-precision positioning and short message communication services.The completion of the system is of great significance in promoting the transformation and upgrading of China's space industry,developing China from a major space player to a space power,ensuring national security and peaceful development,while enhancing China's international status.It is also a major contribution for China's space industry to the construction of a global public service infrastructure.

Facing the technical difficulties in developing the BDS-3 space segment and products,with a short development cycle,and requirement for high product consistency,the BDS team insisted on independent innovation,focusing on technological research,and extensive testing and verification to promote reliability,thus successfully completed the construction of BDS-3 system.

At present,30 satellites of the BDS-3 system are operating normally in orbit,with service functions and performance fully meeting the technical requirements.The accuracy,integrity,continuity,and availability of the system operation have reached an internationally advanced level,realizing the goal of developing a world-class navigation satellite system under the principles developed by China and dedicated to the world.

This article comprehensively analyzes the technical innovation characteristics of the BDS satellites,summarizes the main achievements of BDS-1,BDS-2 and BDS-3 satellites,and provides reference for the follow-up research and planning of the next-generation of the BDS satellites which are“more ubiquitous,more integrated and more intelligent”.

2 INDEPENDENT INNOVATION OF BDS SYSTEM

2.1 “Gradually and Orderly”Innovation Strategy

BDS is a global navigation satellite system independently developed and operated by China but compatible with other navigation satellite systems in the world.China gradually formulated a three-step strategy of development considering the country's economic and technological level using the innovated QIAN Xuesen systems engineering theory and method.At the same time,the technological level of BDS satellites has made a sustained development.

2.1.1 BDS-1 satellites

The first two BDS-1 navigation satellites were launched on October 31 and December 21,2000 respectively.The BDS-1 satellites adopted key technologies such as large satellite solar wings,sailboard driving mechanisms,high-power signal micro discharge for a navigation satellite,high-gain and high-power signal transmission,large fixed surface deployable antennas,satellite multi-band signal and electromagnetic compatibility.It successfully enabled the on-orbit verification of China's three-axis stabilized satellite platform,solved the problem of continuous operation with long life,high-safety power transmission,high-reliability drive and payload capacity,and realized the localization of high-power solar cell arrays and drive mechanisms.

Tests were carried out to verify the satellite-ground integrated active positioning technology system after the BDS-1 satellites entered their preset orbits.The BDS-1 system,also known as BeiDou Navigation Satellite Demonstration System,had provided an integrated service combining satellite navigation,positioning with a location service for the first time in the world.With the short message communication service capability,the BDS-1 system could provide users in China services with the positioning accuracy of 20 m and the timing accuracy of 20 ns.The positioning service was provided to China.The service capability was successfully brought about by resolving the key problem of developing China's navigation satellites.

2.1.2 BDS-2 satellites

14 BDS-2 satellites were launched between 2010 and 2012,promoting a series of key technologies by using GEO and IGSO satellites for high-precision navigation,and enriching the international satellite navigation technology base.

BDS-2 GEO satellites were designed based on the hybrid power supply systems,which increased the antenna unit and expanded the capacity of the satellite platform.China adopted a flexible and efficient spatial data management system for the IGSO/MEO satellites for the first time,integrating information of multiple subsystems to improve the level of satellite information management and for autonomous operation.The satellite payloads adopted key technologies including spacebased time-frequency reference establishment and maintenance technology,active and passive location integrated design technology,and high stability signal broadcast channels.The design of a compatible multi-signal Radio Navigation Satellite System(RNSS) and Radio Determination Satellite System (RDSS) payload was adopted for the first time in the world,successfully solved the problem of power compatibility between L-,S-and C-band multi-frequency points for high-sensitivity reception and high-power transmission on one satellite,with the functions of active location,passive location and short message communication.

The BDS-2 satellites also used key domestic technologies for satellite-borne rubidium atomic clocks.The stability of the atomic clocks has reached an international advanced level and met the engineering technical requirements.The on-board thermal control system improved with new high-precision and high-stability thermal control technology,provided a high-precision and high-stability temperature environment for the onboard atomic clocks,and high-power amplifiers,and achieved the required temperature control accuracy of ±0.3℃/orbital period for the on-board atomic clocks .

The BDS-2 regional navigation satellite system steadily guarantees unique short message communication and location service report functions of the BDS-1 system.It provided users in China and surrounding areas services with the horizontal positioning accuracy of better than 5 m,the vertical positioning accuracy of better than 8 m,and a timing accuracy of better than 50 ns.It realized China's space-based space-time reference for the first time,laying a good foundation for the design and construction of the BeiDou global navigation satellite system.

2.1.3 BDS-3 satellites

China launched 30 BDS-3 satellites including 24 MEO satellites,3 IGSO satellites and 3 GEO satellites between 2009 and 2020,further developing BDS into a hybrid constellation system.It provides basic satellite navigation,positioning and timing services,global short message communication,and international search and rescue services to global users,as well as satellite-based augmentation,high-precision point positioning and regional short message communication services for users in China and surrounding areas.

Aiming at supplying world-class service,the BDS-3 satellites were developed on a truss navigation satellite platform with advanced performance,long life and high reliability to meet the requirement for launching multiple satellites on one rocket.The platform has reached an international first-class level in terms of satellite carrying efficiency,payload platform weight ratio and power mass ratio,thus realizing the rapid deployment of the BDS constellation.

Aiming to improve the precise orbit determination and time synchronization accuracy of the constellation,and support all-weather global satellite in-orbit management,BDS-3 system broke through the multi-orbit hybrid constellation inter-satellite link technology,solved the core technology bottlenecks of BDS enhancing its capability from providing services from regional to global.The navigation constellation has achieved a one-stop management capability.

The BDS-3 system has realized a unique and innovative global navigation signal solution with high reliability.Through adding pilot channels,user acquisition sensitivity and weak signal reception robustness have been enhanced,and the ranging accuracy has been increased by 4 times.At the same time,the signal utilization efficiency,compatibility,and interoperability have been improved,and the quantity and quality of navigation signals have been greatly increased,realizing the smooth transition of multiple signals within the BDS-3 system and bringing compatibility with other GNSS.

The completion of the BeiDou global navigation system signifies China as the third country in the world which has its own global navigation satellite system.

Tests with more than 50 GNSS monitoring stations distributed around the world show that the BDS-3 system can provide worldwide positioning services with the horizontal positioning accuracy of 1.6 m and the vertical positioning accuracy of 3.5 m.The BDS-3 system becomes China's first space system that serves the world at an internationally advanced level.The service performance of the BDS has been greatly contributed to enhance China's initiative in the field of international satellite navigation.

2.2 “Priority and Order”Innovation Plan

The development goals of BDS-3 are to independently construct a global satellite navigation system with key technologies featuring constellation deployment,high-precision space-time reference,and autonomous operation.The development principles of BDS-3 are openness,compatibility,independence,and gradualness.

Core tasks such as BeiDou positioning,navigation and timing (PNT) were carried out through navigation satellites.At the same time,more incremental tasks and experimental verification were conducted according to the technical development,national needs and future construction trend.

1) Core tasks:to meet the needs of providing worldwide basic PNT services,as well as the compatibility with other GNSS in the world.Centralized resource allocation of the satellite system in the constellation to ensure that the service performance of the system is better than that of other satellite navigation systems for users in China and surrounding areas.

2) Incremental tasks:with the continuous improvement of the BDS in PNT services,incremental tasks such as global position reports and short messages,Ka-band high-speed ground links,and L-band data links were further implemented on the satellites.These incremental tasks can provide standard and unified spacetime reference services for all kinds of users,providing a means for realizing integrated application services of navigation,communications,and remote sensing,as well as conducting on-orbit test and verification.

As a space reference based on precise radio measurement and positioning,the BDS satellites are equipped with highly stable on-board atomic clocks,high-precision ranging receivers and spherical beam array antennae in order to ensure the high stability of the navigation signal broadcast delay and the accuracy of the satellite phase.The satellites are featured with high-precision measurement,high-continuity information processing,high-stability channel transmission,high-power transmission,high-reliability availability and integrity technologies.

3 SYSTEMATIC BDS

3.1 Hybrid Navigation Constellation

BDS is composed of a space segment (a hybrid navigation constellation),ground segment (including master control stations,monitoring stations,time synchronization/uplink stations)and a user segment (including chips,modules and antennae,as well as terminals,application systems and application services).

The navigation constellation in the space segment is the core component of BDS.According to the requirements of providing basic PNT services globally and providing characteristic services in China and surrounding areas,the space segment of BDS created the first GEO/IGSO/MEO hybrid constellation in the world,overcoming key technological issues in constructing navigation constellation with GEO/IGSO satellites.Among the issues,the optimized design of heterogeneous constellations,the analysis of availability and robustness of heterogeneous constellations,and the key technologies of multi-orbit satellites with compatible backups have reached an international leading level.

Figure 1 Configuration of the BDS-3 constellation

3.2 Inclined Geosynchronous Satellite Orbit Satellite

Geosynchronous satellites are the most in-demand in the BDS system and the basis of feasibility estimation for developing China's navigation satellite system.In order to meet the navigation signal coverage requirements for China and its surrounding areas,plus to take into account the impact of orbital maneuvers of satellites in orbit on the availability of the constellation,synchronous orbit satellites were adopted to the innovative design of BDS.The world's first IGSO satellite was successfully launched in August 2010.The BDS navigation signal phase consistency is better than 1ns (high precision),the satellite's on-orbit availability is better than 0.99 (high availability),and the loadto-weight ratio was increased by 30% (high functional density ratio).

3.3 Full-truss Satellite Platform

China domestically developed its first full-truss satellite platform enabling the launch of multiple navigation satellites into orbit on one rocket with direct injection,thus achieved a quick launch and the fast deployment of the BeiDou navigation constellation.

In order to launch multiple satellites on one rocket,the satellite platform has enriched functions and its technical level improved,reaching a world-class level in satellite carrying efficiency,payload platform weight ratio,power-quality ratio,etc.

3.4 Batch Development

BDS-3 system is China's largest satellite constellation at present.The batch development,quick launch,and rapid deployment of the satellites demanded higher requirements for the current satellite development model,manufacturing efficiency and product quality.

To meet the target batch development ability of 4 satellites per batch,delivering one batch every 4 months,the satellites were developed based on the mode of completed satellite batches,streamlined tests,thus establishing an efficient and specialized test environment.The satellites were batch produced and assembled,realizing the transformation of the production mode from single satellite assembly to batch manufacturing thus improving the efficiency of satellite assemble,integration and testing (AIT).

At the same time,a digital AIT status management system was established to realize a real-time control of the AIT for the entire satellite,thus successfully solving the problems of multi-satellite parallelism,multi-line coexistence,and multi-constraint coexistence during the batch production of BDS-3 satellites,enabling the production of 8-12 BDS-3 satellites annually,shorten the manufacturing cycle by 1/3.The batch production capacity of navigation satellites has therefore been doubled,which strongly enabled the BDS-3 system to be constructed within three years.

Figure 2 BDS-3 MEO satellite

3.5 Fully Realized Localization of On-board Products

The BDS-3 navigation satellite adheres to the concept of original innovation and independent development,and unswervingly promotes the localization and independent control of satellite components.

The BDS-1 satellite took the lead in realizing the localization of basic satellite products such as solar wing,windsurfing drive mechanism,momentum wheel.The products work stably in orbit,laying a good foundation for the development of subsequent satellites.The BDS-2 satellite broke through the localization of products represented by the on-board atomic clock at the core of the navigation satellite,ensuring the smooth progress of the BDS engineering construction.Its successful design,quality control,test verification and other processes and experience are also comprehensive for the BDS-3 satellite.The implementation of localization has laid the foundation and set an example.

In accordance with the goal of ‘100% localization of spaceborne products',the BDS-3 satellite has been established an integrated system from design,development,verification to application,giving full play to the domestic superior scientific research strength,unswerving,realizing hard work,and scientific research.The application of more than 110 domestic components such as on-board CPU,high-performance DSP,large-capacity FLASH and SRAM memory was completed,a complete satellite product and application guide was formed,and finally the aim of 100% localization of the BDS system onboard products was realized.

4 TECHNOLOGICAL BREAKTHROUGHS OF BDS

4.1 High-precision Satellite Clock Technology

The miniaturized and high-precision rubidium atomic clocks for the BDS satellite were developed and used in the BDS satellite for the first time.Chinese universities and research institutes worked together and solved the bottlenecks of the rubidium lamp energy level transition frequency control technology,high stability frequency signal locking technology,and product long life index evaluation under vacuum.The stability of satellite-borne atomic clocks has increased by 2 orders of magnitude,and its performance is equivalent to the latest international ones.

Figure 3 Atomic clocks on BDS-3

4.2 Multi-service Co-satellite Joint

It was the first time in the world to propose and develop a multi-service co-satellite joint broadcast technology,to realize the technology integration of navigation positioning and short message position report service,and location sharing and control.The two-way timing service index was used in BDS-3 system with the accuracy increased from 50 ns to 10 ns.The designed three-frequency navigation signal effectively improves the success rate and speed of navigation message resolution for high-precision users (centimeter level).

4.3 Inter-satellite Link Technology

The BDS team proposed and realized a high-speed communications link and precision measurement between MEO and IGSO successfully for the first time in the world,with a domestic leading and international advanced comprehensive capabilities and performance.

Figure 4 Topology diagram of BDS-3 inter-satellite backbone network

To solve the technical problems caused by the continuous yaw maneuver of the navigation satellite attitude and the satellite dual-end motion,the BDS team proposed a high-precision pointing control method for satellite attitude flight prediction.The BDS team has achieved inter-satellite high-precision and high-stability signal acquisition,utilizing tracking and signal optical coherence tracking technology,and achieved the acquisition,tracking and stable chain establishment for high-speed narrow-beam signals without a beacon.

4.4 Global Short Message Communication Technology

Carrying forward the characteristics of the BDS short message communication service,the BDS team proposed and realized global short message communication service for the first time,breaking through the key technology barriers for global short message transmission.

The regional message communication capacity has been greatly improved,the signal system has been smoothly transitioned.It has the short message service capacity of more than 10 million times per hour,and the short message communication capacity has increased to 1000 Chinese characters (the capacity is 10 times,and the user transmission power has reduced by 1/10).

Realizing global short message communication reception.Each MEO satellite allows simultaneous access to 12 users,doubling coverage globally.The transmission signal information rate is 400 bps,with the ability of short message service of 300,000 times per hour,and the maximum message length can be 40 Chinese characters.The services provided by the system including short message transmission,emergency collection,location report and has reached an international leading level.

5 CONCLUSIONS

The BDS was developed and constructed under the principle of constructing a first-class system developed by China and dedicated to the world.The BDS focuses on innovative development,and emphasizes the needs to break through technical bottlenecks and strengthening core capabilities,taking the mission of leading development to an international advanced level.We will continue to strengthen the research and development of key and basic technologies,and constantly improve the level of domestic technological innovation and product control,strongly supporting the construction of a space power and to better serve the national economic construction and social development.

The next-generation BDS satellites aim at providing a spaceground integrated space-time reference,autonomous operation and maintenance,and on-demand navigation information service,realizing global high-precision positioning,high continuity and high reliability.This will enable meeting users' requirements such as automatic driving,dynamic mapping and unmanned system navigation,while meeting the needs of safety and realize is an improvement in GNSS service and performance.