01summary

Global navigation satellite system (GNSS) is the general name of global satellite navigation system. Including GPS of the United States, GLONASS of Russia, BDS of China and Galileo system of the European Union. From the current development of various systems, GNSS system can provide meter level positioning services for pseudo users by means of single frequency pseudo range positioning, and can meet the positioning needs of most general users.

However, it should be noted that for users and applications in surveying and mapping, surveying, disaster emergency, land and resources, civil aviation and other industries, Beidou GNSS positioning still has many types of problems, such as insufficient accuracy, reliability and continuity, and lack of navigation integrity guarantee, which is difficult to meet the requirements of the above industries or scenarios for high-performance navigation and positioning. Therefore, Some high-performance navigation and positioning application technologies came into being.

02Foundation reinforcement system

Satellite navigation ground-based augmentation system is an important technical means to provide high-precision differential positioning service in a certain geographical range. Based on the continuous operation reference station (CORS) technology, it carries out long-term continuous observation of satellite navigation signals by setting up a certain number of ground reference stations in a certain area, uses the observation data to model and correct the errors of satellite navigation signals, generates high-precision differential correction numbers and broadcasts them to relevant users in the area, so as to assist users to realize high-precision differential positioning. Because the ground-based enhancement system adopts dual multi frequency carrier phase observation and has a large service coverage (generally provincial or municipal), it can provide more accurate navigation and positioning services.

Generally, the ground-based enhancement system generally includes reference station network, data processing and service center, communication network system and user terminal. Of which:

① The base station network is an important part of the ground-based enhancement system. Its main function is to collect satellite navigation observation data all day, transmit the collected and preprocessed observation data to the data processing and service center, and support the system to generate relevant differential correction numbers. General reference stations can support receiving and collecting signals from Beidou (b1b2b3 frequency point), GPS (l1l2l5 frequency point), GLONASS (L1L2 frequency point) and other systems. The number of reference stations in a foundation enhancement system generally depends on the size of the area served by the system and the terrain in the area. The reference station can output various types of navigation observation data, including signal carrier noise ratio, code pseudo range, carrier phase observation value, signal Doppler frequency shift, navigation message and so on.

In the case of multi system reception, the reference station shall have the function of time autonomous synchronization, which can unify the time datum of observation data of different systems to a general time datum (such as Beidou system time), so as to ensure that the observation data of each system maintain time synchronization. In addition, after the navigation satellite signal received by the reference station is interpreted and parameter extraction is completed, the data needs to be transmitted to the central system in real time. The reference station receiver is generally equipped with relevant network transmission equipment (i.e. the transmission path from the reference station satellite navigation receiver to the public network or special line communication line), which can directly transmit data to the existing communication network.

② The data processing and service center is the core of the foundation enhancement system, which mainly undertakes the functions of high-precision differential data processing, system service and system operation monitoring. Collect and store the observation data transmitted by each reference station, and evaluate the quality of these observation data in real time. Then, it completes the multi-path impact analysis of reference station data, ionospheric and tropospheric change analysis, system integrity monitoring and other functions, and calculates the error correction number in the generated area according to the relevant GNSS error model and differential correction number algorithm, so as to provide high-precision location services for relevant users.

At present, the main error correction number calculation methods include virtual reference station (VRS) technology, regional correction number (FKP) technology and main and auxiliary station (MAC) technology. Taking the most common virtual reference station technology as an example, the data processing and service center completes the information fusion and error source modeling of all reference stations. When the mobile station / user uses it, first send its approximate coordinates to the system data processing center, and the system data processing center generates the observation value of the virtual reference station according to the approximate coordinates and returns it to the mobile station / user. The mobile station / user uses the virtual reference station data and its own observation data for difference, so as to obtain high-precision positioning results.

The advantage of VRS technology is that only one data receiving device needs to be added without increasing the data processing capacity of user equipment, and the compatibility of receiver is good. In addition, VRS technology requires two-way data communication. The mobile station should not only receive data, but also send its own positioning results and status. The data exchanged between each mobile station and the data processing center is unique, which has high requirements for the data processing capacity and data transmission capacity of the system data processing and control center.

At present, China is gradually building and improving domestic ground-based high-precision navigation service facilities, and actively carrying out the construction of GNSS ground-based enhancement network based on Beidou in terms of industry and region. The Beidou foundation enhancement system covering major regions and some industries in China has been preliminarily formed. The Beidou foundation enhancement system consists of Beidou reference station system, communication network system, national comprehensive data processing system and data backup system, industrial data processing system, regional data processing system and location service operation platform, data broadcasting system Beidou / GNSS enhanced user terminal and other subsystems.

The system generally uses ground reference stations with a distance of 50 ~ 300 km to broadcast navigation signal correction and auxiliary positioning signals through the ground communication system to provide users with centimeter to sub meter precision navigation and positioning and public terminal auxiliary enhancement services. By the end of 2018, more than 2200 base stations had been built, making it the foundation enhancement system with the largest number of base stations, the widest coverage and stable operation in the world. The system has the basic service capability of providing real-time meter level, decimeter level, centimeter level and post-processing millimeter level high-precision positioning on land all over the country. It can support surveying and mapping, geology, meteorology, land and resources and other industries to provide professional high-precision location services.

At present, the foundation enhancement system mainly serves ground applications, covering professional fields such as surveying and mapping exploration, monitoring and control, driving test and training, precision agriculture, aviation and navigation, as well as public fields such as traffic navigation, tourism and emergency rescue. By receiving the differential correction signal provided by the ground reference station network to improve the satellite navigation accuracy, the optimized positioning accuracy can range from millimeter level to sub meter level. Although the accuracy of foundation reinforcement is very high, the coverage is limited. The positioning target must be within the coverage of the communication signal, but it may form a service blind area in high altitude, sea, desert and mountainous areas where the communication signal is difficult to cover.

03Satellite based precision differential / enhancement technology

Different from the ground-based enhancement system, satellite based precision difference and enhancement technology takes satellite as the communication means of differential correction data broadcasting transmission. Satellite based augmentation system can broadcast ephemeris error, satellite clock error, ionospheric delay and other correction information to users through the satellite navigation augmentation signal repeater carried by geostationary satellites, so as to improve the positioning accuracy of the original satellite navigation system. At present, the construction of satellite based enhancement system is accelerating all over the world. The United States, the European Union, Russia, China, Australia, South Korea, Japan, India and even African countries are building satellite based enhancement services.

Figure 1 basic situation of main satellite based enhancement systems in the world

Wide area augmentation system (WAAS) is one of the earliest satellite based augmentation systems in the world. Because the performance of early GPS system can not fully meet the actual needs of cat-i approach guidance in civil aviation field. Therefore, the Federal Aviation Administration (FAA) has initiated the construction plan of WAAS since the end of 1990, in order to provide the navigation and positioning performance required by cat-i precision approach. WAAS provides services for various types of aircraft in all stages of the whole flight process of departure, journey and arrival. This also includes providing vertical guidance for the landing process under normal flight meteorological conditions at all appropriate sites within the U.S. national airspace system.

The WAAS system consists of 38 reference stations, 2 master control stations and 4 ground uplink stations, as shown in the figure below. The two master control stations are located in FAA and Stanford University and are responsible for GPS error correction information and integrity information processing of the evaluation system.

Figure 2 basic composition of WAAS system

The system receives the GPS downlink data collected by all reference stations in real time, estimates the orbit, clock error and ionospheric error in the data processing center, and then injects these correction numbers into the geostationary satellite through the injection station. These satellites package the relevant differential correction numbers into frames according to the standardized data format and broadcast them to ground users. Users use their own navigation and positioning terminal to realize precise differential positioning on the basis of receiving the differential correction numbers, so as to improve their positioning accuracy. On July 10, 2003, WAAS signal began to officially serve the civil aviation system, covering 95% of the territory of the United States. In 2008, FAA launched the application of WAAS system in helicopters.

In December 2009, a flight of Seattle horizon airlines from Portland to Seattle used the LPV service of WAAS for the first time. The company will cooperate with FAA to provide long-term data to demonstrate the service of WAAS system in civil aviation system.

WAAS system improves the integrity of basic GPS signal and can detect smaller error information more quickly. WAAS has a WAAS integrity and performance group co chaired by FAA and Stanford University to guide the research and development of WAAS integrity monitoring indicators. When the GPS system is unavailable due to the influence of system error or other factors, wass will send prompt information to the user. In addition, the WAAS system is designed according to the most stringent safety standard, that is, when there is any misleading and harmful information that may cause GPS position estimation error, the user can receive the prompt information issued by the system within 6 seconds.

WAAS system is a typical representative of satellite based enhancement system. Its core processing algorithm and process are the core technology widely used for reference by other satellite based enhancement systems. Generally speaking, its core processing algorithms mainly include:

1) Satellite orbit and clock error estimation algorithm module.

It combines a precise satellite dynamics model and a square root information filter to provide very accurate satellite orbit and clock error. It can realize fully automatic and real-time observation data processing for orbit determination and positioning calculation.

2) Navigation signal ionospheric delay estimation algorithm module.

A grid ionospheric algorithm is used for ionospheric delay estimation in WAAS. The basic principle is based on the ionospheric monolayer assumption, taking the Klobuchar model as the background field, projecting all the actual observations in a certain range around the fixed grid point to the grid point position and taking its weighted average. For the processing of hardware delay, the square root information filtering technology is used to estimate the inter symbol deviation between the station and the satellite in real time.

The European geostationary satellite navigation overlay service system (EGNOS) is a satellite based augmentation system for satellite navigation in Europe, which is jointly established by the European Space Agency and the European aviation navigation safety organization. Similar to WAAS, the system uses some key technologies for reference. Through the monitoring of GPS and GLONASS systems, the integrity and accuracy of user navigation and positioning are improved by means of differential correction number and integrity information service.

EGNOS consists of three geostationary orbit satellites (GEO), ground station network and user equipment. The ground station network includes 34 ranging and integrity monitoring stations (RIMS), 4 main control centers (MCC) and 6 ground navigation information injection stations (NLEs). EGNOS system receives GPS and GLONASS observation data at the same time through the monitoring station (rims station) on the ground. The observation data are sent to the main control center for processing to obtain wide area differential correction information and integrity information, which are injected into GEO satellite and broadcast to users. The user can calculate the integrity alarm information by using these information and the data received by the machine for differential positioning. The basic operation principle and service scope of the system are as follows:

Fig. 3 basic operation principle and service scope of EGNOS system

EGNOS system has improved and upgraded the core difference correction method on the basis of WAAS. The ionospheric delay of its broadcast grid point is estimated by European nequick model, which uses DGR profile formula to describe the electron density in the ionospheric range from 90km to F2 layer, so as to more accurately describe the ionospheric variation law over Europe, The ionospheric correction number of navigation signal suitable for Europe is given.

On April 1, 2009, the ownership of EGNOS was transferred from the European Space Agency (ESA) to the European Commission (EC) of the European Union (EU). On October 1, 2009, the EU announced that EGNOS would resume normal service. Unlike WAAS in the United States, which only provides air navigation, EGNOS provides navigation information for aircraft, ships, vehicles and other forms of transportation in aviation, navigation and land transportation.

GPS assisted low earth orbit augmentation system (Gagan) is a satellite based augmentation system deployed in India. In July 2015, India officially released Gagan system service. It plans to provide accurate navigation services for the bay of Bengal, Southeast Asia, the Indian Ocean, the Middle East and Africa. It is reported that after 15 years, the system costs Indian rupees 7.74 billion (about US $123 million). It is jointly developed by Indian Space Research Organization (ISRO) and Indian Aviation Administration (AAI). It adopts SBAS technology developed by Raytheon of the United States and will provide services for SAARC member states (SAARC).

The infrastructure of India's Gagan system includes the ground segment composed of 15 reference stations (the distribution of stations is shown in the figure below), 3 uplink injection stations and 1 mission control center, 2 space segments of geostationary orbit (GEO) satellites carrying GPS enhanced signal broadcasting payload, as well as relevant software and communication links, which can broadcast C-band and L-band navigation enhanced signals, Enhance satellite navigation systems such as GPS. The system will serve more than 50 airports in India.

Figure 4 distribution of Gagan ground stations

At present, Gagan's enhanced signal has been broadcast through the enhanced payload carried by gsat-8 and gsat-10 geo satellites, covering the entire flight information area of India and beyond. In addition, the upcoming gsat-15 satellite will also carry Gagan payload as a backup of the system's space transponder. The satellite will be fixed in the geostationary orbit at 93.5 degrees east longitude, of which two channels are dedicated to the positioning, navigation and timing services of India's Gagan system.

QZSS is a regional navigation system independently developed by Japan. Initially, Japan hoped to improve Japan's satellite navigation service quality through the development of this system, and gradually adapt to Japan's mountainous terrain, serious signal occlusion and other problems. Therefore, QZSS is not only a satellite based enhancement system, but also includes some autonomous navigation functions, that is, it can still provide Japan with basic satellite navigation capability in case of signal interruption of GPS system. Therefore, the number of satellites used in the system has developed from the initial 3 to 4, and finally to 7. The constellation configuration has gradually evolved from 3 inclined geosynchronous satellite orbit (IGSO) satellites to 3 IGSO satellites, 1 GEO satellite, and finally 5 IGSO satellites and 2 geo satellites.

Since 2017, Japan has gradually accelerated the construction process of the system and completed the basic on orbit test in March 2018.

QZSS system is composed of space segment, ground operation control segment and user segment. The space segment is composed of 2 geo satellites and 5 IGSO satellites. IGSO has a unique figure-8 ground trace over Japan, which is also the source of the name of this system.

Figure 5 ground trace of IGSO satellite

The ground control, tracking and monitoring stations of QZSS system are mainly located in Japan, Bangalore, Bangkok, Canberra, Hawaii, Guam and other places, all over the service coverage area.

QZSS system broadcasts GPS enhanced and autonomous navigation signals, and provides short message service in L1s and S-band. Geo orbiting satellite has the ability of l1sb navigation technology verification signal and S-band short message service. Up to now, QZSS system has transmitted 6 service signals: L1 C / A, L1C, L2C, L5, L1 Saif and lex.

It should be pointed out that in addition to the meter level positioning accuracy improvement service, QZSS also broadcasts L-band experiment signal, namely Lex signal, at the frequency of 1278.75mhz. The signal rate reaches 2000bps, while the information rate of GPS signal is 50bps, and the information rate of L1 Saif signal is 250bps. Lex signal can provide more error correction information, making the positioning accuracy of users reach centimeter level. It can be used in driverless, surveying and mapping, precision agriculture and other industries. At the same time, the lex signal frequency point coincides with the E6 frequency point of the European Galileo system, that is, QZSS can also supplement the Galileo system when the Galileo system officially provides services.

In addition, L1 Saif signal also provides GPS and other satellite health information, and timely notifies the user not to use the abnormal satellite after the GPS satellite is abnormal, so as to avoid getting wrong positioning results

04Auxiliary GNSS system (a-gnss)

Auxiliary GNSS (a-gnss or A-GPS) refers to the process of using mobile communication network to provide users with necessary auxiliary information to help users correctly receive GNSS signals in harsh environments such as high dynamic and low signal-to-noise ratio. The auxiliary information generally includes the almanac, ephemeris, frequency range, standard time and approximate position of the navigation satellite. By providing auxiliary information, a-gnss enables the GNSS receiver used by the user to roughly understand the approximate range of signal number phase and Doppler frequency shift to be captured before capturing the signal, so as to compress the receiver search frequency band, reduce the noise bandwidth, increase the accumulation time of signal energy, increase the sensitivity of the user receiver and shorten the first positioning time of the user.

A-gnss technology is very effective for the urban environment where the satellite navigation signal is seriously blocked. In the urban environment, there are many tall buildings, the signal received by the receiver will have serious multipath effect, and the signal quality can not be guaranteed. A-gnss is to make the GNSS receiver know the frequency range to be received before receiving by providing the almanac, ephemeris, frequency range, standard time, approximate position and other auxiliary information of the navigation satellite, and then assist in calculating the data, and then provide the location of the satellite used to calculate the GNSS user's position, which can reduce the initial positioning time and improve the sensitivity of the receiver Reduce the energy loss of the receiver, speed up the position calculation, improve the positioning accuracy and positioning performance.

A-gnss must rely on high-performance communication network. In recent years, with the development of modern mobile communication network technology, a-gnss has been deeply combined with 4G, 5g and modern Internet of things, forming more application space. The basic principle of a-gnss based on 5g is to deeply integrate 5g mobile communication technology with a-gnss system, so that their positioning and communication functions can learn from each other, so as to obtain the required location information more efficiently.

Firstly, the user's a-gnss receiver terminal sends a positioning request to the satellite server, uses the 5g based c-ran network to quickly query the available satellite information, and quickly transmits the almanac, ephemeris, frequency range, standard time, approximate position and other auxiliary information to the receiver through the 5g network. The receiver then calculates the positioning result according to the auxiliary data and the captured satellite signal. The Qianxun Lijian service provided by Qianxun location company can provide full coverage a-gnss services for GPS, GLONASS and Beidou systems, and support GNSS navigation and positioning chips that meet the "international standard positioning protocol framework for mobile communication networks" (SUPL protocol). It can be said that a-gnss is one of the important growth points of the combination of satellite navigation technology and modern information network technology.

05Summary

In recent years, with the rapid development of satellite navigation, satellite navigation has not only satisfied with the most basic navigation, positioning and timing services, but gradually penetrated into various professional industries and fields, and gradually incubated many types of new high-performance application technologies. From the current situation, these new application technologies are mainly concentrated in the level of high-precision and reliable positioning. With the continuous development of modern information technology and the deepening demand for space-time information infrastructure in various industries of economy and society, satellite navigation technology will accelerate the collision with different categories of economic and social development and form more new applications.

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