The system segment is the core of the GNSS augmentation system with the mission of monitoring GNSS satellites and generating differential corrections and integrity information over a wide area. System segment technologies are the key to the construction of GNSS augmentation systems and determine the service capability of GNSS augmentation systems. GNSS augmentation technologies used in system segments can be divided into two types: technology used in the SBAS system segment and technology used in the GBAS system segment.
Technologies of SBAS system segment
Preliminary information
SBAS system segments are mainly composed of monitor stations and master stations. Monitor stations receive and process signals from the GNSS and SBAS satellites (Parkinson et al. 1996). More specifically, monitor stations collect dual frequency code carriers and the pseudorange. The detection of cycle slip errors and repair can be implemented with dual frequency code carriers and the dual frequency carrier is used to smooth the dual frequency pseudorange, subsequently removing any ionospheric delay. Meteorological parameters are utilized to compute the tropospheric delay (SC-159 2013). The geometric range can be computed using the navigation message and the position of the monitor station. Common view time transfer is used to estimate the clock offset of the monitor station and the synchronized pseudorange (Chen et al. 2017; Tsai 1999). Finally, the monitor stations forward the resultant data to the master stations. Master stations generate differential corrections and integrity information with regard to each monitored satellite and each monitored ionosphere grid point (IGP). Differential corrections include long-term corrections, fast corrections, and ionospheric corrections. Long-term corrections and fast corrections are used to mitigate the slowly changing errors of the satellite clock-ephemeris and the rapidly changing errors of the satellite clock, respectively. Ionospheric corrections are used to revise the atmospheric delay and IGPs are used to deduce the pseudorange errors in the ionospheric pierce point. The accuracy of position provided by GNSS can be improved with differential corrections. Integrity Information includes the user differential range error (UDRE), clock-ephemeris covariance matrix, and the grid ionospheric vertical error (GIVE). UDRE is used to compute the integrity error bound for satellite clock-ephemeris errors and GIVE is used to compute the integrity error bound for ionospheric errors. Using GIVE and UDRE, the integrity protection levels of SBAS can be determined to analyze the probability of integrity. After generation by the master stations, the differential corrections and integrity information are quantified. All these data are packaged into SBAS messages and sent to navigation earth stations, as illustrated in Fig. 2. These stations then upload this augmentation information to SBAS satellites in space, and it is broadcast to users. The user receives the basic navigation signal from the GNSS satellite and the augmentation signal from the SBAS satellites, and thus determines location and information concerning safety.
Research progress
Satellite navigation augmentation systems, especially SBAS, have been a popular topic for many years. Existing SBAS systems include the Wide Area augmentation System (WAAS) in the United States, the European Geostationary Navigation Overlay System (EGNOS) in Europe, the System of Differential Correction and Monitoring (SDCM) in Russia, BDSBAS in China, the Mtsat Satellite-based Augmentation System (MSAS) in Japan, and Gps-Aided Geo-Augmented Navigation (GAGAN) in India. WAAS has been in full use for 16 years, providing the LPV200 service. China successfully launched the GEO-1 satellite (SBAS PRN 130) on November 1, 2018. BDSBAS is still under construction, with three GEOs yet to be launched (Liu 2019). The BDSBAS signal has been broadcast since November 9, 2018 (Liu 2019). BDSBAS is now broadcasting augmentation information to test the performance of SBAS. The augmentation information broadcast by BDSBAS does not conform to the service requirements for the Minimum Operational Performance Standards 229 of Radio Technical Commission for Aeronautics (RTCA MOPS 229); hence BDSBAS cannot yet provide services for civil aviation.
The goals surrounding the construction of BDSBAS are to perform wide area differential processing and integrity monitoring for BDS/GNSS satellites in China and its surrounding areas, broadcast augmentation messages via the B1C and B2a signals to users through the GEO satellites, and to improve the accuracy, integrity, continuity, and availability of the service. BDSBAS will initially suit the requirements of civil aviation users for APVI precision approach, ultimately reaching the level required for CAT-I precision approach. BDSBAS interoperates with WAAS, EGNOS, and other SBAS to provide information for the RTCA L1CA and RTCA DFMC interfaces. The BDSBAS satellites are located at 80°E, 110°E, and 140°E (Chen 2019; Guo et al. 2019; Liu 2019).
Few papers have been produced concerning BDSBAS. Chen Jinping, chief engineer of the Beijing Satellite Navigation Center, both designed and carried out preliminary testing on BDSBAS, producing a general design for BDSBAS including the system work mode, information processing mode, and analysis (Chen 2019), as shown in Fig. 3. The pseudorange and code phase of the monitor stations are returned to the information processing center at the master station in order to calculate the corrections necessary for the satellite, the ionospheric grid delay, and the corresponding information concerning integrity for use with multi-GNSS. Each monitor station is equipped with three independent receivers which are used for the calculation of differential corrections, integrity checks, and the backup of data. The master stations simultaneously process augmentation information for RTCA L1CA with the B1C signal and DFMC interfaces with the B2a signal. This information is then broadcasted by the GEO satellites.
BDSBAS uses the kinematics model and the dynamics model for orbit and clock correction, respectively. According to analysis of the regional monitoring network data, the accuracy of the corrections for orbit offset is similar to that of the corrections of the clock offset. Results show that the UDRE of BDS-3 is near to 0.4 m which is conservative compared with that of WAAS and EGNOS. The parameters UDRE and GIVE can provide the integrity confidences for the corresponding corrections. Degradation parameters are useful for guaranteeing service integrity on the rare occasions when SBAS users fail to receive differential corrections. BDSBAS monitor stations across China have been selected as users to analyze the performance of BDSBAS in terms of the position domain and the single point position accuracy of BDSBAS, which is similar to that of WAAS and EGNOS. However, the probability of integrity is lower for BDSBAS than it is for WAAS and EGNOS. BDSBAS will soon be able to provide RTCA L1CA, RTCA DFMC, and other protocol augmentation information (Chen 2019).
Dual frequency range error (DFRE) is a critical integrity parameter in DFMC SBAS, and the method used to calculate DFRE has not yet been introduced in the relevant literature abroad. Shao Bo, an engineer at the 20th Research Institute of China Electronics Technology Group Corporation developed an integrity method for use with DFRE, using a projection method. The satellite clock-ephemeris covariance matrix is used to find the maximal projection direction, and the projection of the covariance matrix in this direction is defined as a DFRE which can form a bound for satellite correction error (Shao 2019; Shao et al. 2011).
The DFRE of BDS and GPS were solved and compared with the maximal corrected error using the observations made by the 24 monitor stations in China shown in Fig. 4. The DFRE, calculated using the projection method, can form an envelope for the maximal corrected error, which is suitable for monitoring the integrity of different constellations or types of satellites (Shao 2019; Shao et al. 2012). Results show that the DFRE solved with the projection method can bound the satellite correction error with a probability of 99.9%, and is suitable for use with different constellations or different kinds of satellites. After further validation, the method can be applied to the DFRE calculation of BDSBAS (Shao 2019; Shao et al. 2011).
In 2013, the China Aerospace Science and Technology Corporation Ltd signed a contract with the Algerian space agency regarding the Algerian communication satellite (Alcomsat-1). Alcomsat-1 utilizes a DFH-4 satellite and is equipped with 33 transponders, including the L1 and L5 navigation augmentation transponder payloads. The China Aerospace Science and Technology Corporation Ltd constructed a satellite augmentation system based on the Alcomsat-1, named Al-SBAS, in order to provide services for Algeria and the surrounding area. The Alcomsat-1 communications satellite was launched on December 11, 2017, and is located at 24.8° W in a geostationary orbit (Li 2019).
Compatible with ICAO standards and based on ALCOMSAT-1, the SBAS aims to improve the positioning accuracy and integrity in Algeria and the surrounding area, providing services for users in many fields such as surveying, transportation, aviation, railways, and the ocean (Li 2019), as seen in Fig. 5. The system collects GPS observations and solves GPS satellite ephemeris errors, clock errors, and ionospheric errors together with the corresponding integrity parameters in real-time, and broadcasts differential corrections and integrity information through GEO satellites with a high accuracy and a significant capability for integrity augmentation (Li 2019).
The ground segment of Al-SBAS is composed of 18 monitor stations in Algeria, a data processing center in Algiers, and an uplink station in Algiers. Signals that are supported include GPS, GLONASS, BDS, and GALILEO. The data processing center is used to perform local system redundancy, automatic switch-over, autonomous state monitoring, and fast seamless recovery. The system collects data from GPS monitor stations to generate satellite corrections, ionospheric corrections, and integrity parameters, and then broadcasts these augmentation data through the GEO satellite Alcomsat-1 which is equipped with two way L1 and L5 navigation augmentation payloads (Li 2019).
The positioning accuracy of the single frequency and dual frequency services is at a sub-meter level. Follow-up plans include continuous operation, monitoring and evaluation of the L1 augmentation service, updating the L5 augmentation signal, BDS augmentation services, the issue of a signal-in-space ICD, and public services. AL-SBAS improves the positioning accuracy and integrity of GPS in the Algerian region. The construction and final testing of the ground monitor stations, data processing center, and the uplink station for AL-SBAS have been completed (Li 2019). The future prospects of Al-SBAS in Algeria and its surroundings are therefore promising.
The Wide Area Precise Positioning System (WAPPS) is a GNSS differential system for real-time high accuracy positioning and navigation as shown in Fig. 6. By referring to the work mode of SBAS, WAPPS broadcasts corrections by broadcasting from satellites and via the internet to ensure that users perform precise point position-ing (PPP) (Shen et al. 2019). With the advantages of the low density of stations, wide range of services, variation in services, and the simple user terminal, WAPPS is applied to marine transportation, surveying, and accuracy agriculture, among others. WAPPS provides services that are related to the safety, for which integrity with high accuracy in real-time can be obtained from the WAPPS signal produced by broadcasting satellites and the WAPPS service on the internet (Wang 2019e).
Similar to other differential systems such as SBAS and GBAS, integrity monitoring also needs to be performed for WAPPS. The performance of both the pseudorange and carrier correction is monitored to guarantee the service performance of PPP. The percentage of missed alerts is 10−3, the percentage of false alerts is 10−5, and the time taken to produce an alert is less than 10 s. WAPPS provides services using dual frequency ionosphere-free combination. Dual frequency ionosphere free combination can remove any ionospheric delay sufficiently, even when abnormal phenomena occur in the ionosphere. The carrier cycle slip and other abnormalities in the user segment need to be guaranteed by the receiver, which are not considered in the integrity monitoring of the system. WAPPS integrity fault modes can be divided into step faults and slow drift faults. All faults will affect the pseudorange, the carrier observation, and the positioning performance. Results reveal that in normal status, integrity monitoring can ensure the requirements for missed alerts and false alerts. When step faults or slow drift faults occur in the corrections, the alert has to be sent to a user within a short period of time. A slow drift rate of 0.1 m/s can be detected within 5 s with carrier phase monitoring (Wang 2019e).
An engineer at Space Star technology CO., Ltd recently predicted the service performance of single frequency BDSBAS and dual-frequency Multi-constellation (DFMC) SBAS in China without consideration of the broadcast of clock-ephemeris covariance matrix and augmentation information quantification (Chen et al. 2019). The results of this research are better than the BDSBAS performance derived from the general engineering community, which has led to many questions. Considerable work is needed to forecast the service performance of BDSBAS.
Summary
BDSBAS is still under construction. China successfully launched the first SBAS satellite and began broadcasting its augmentation signal last year. Two SBAS satellites still remain to be launched. Although BDSBAS has been broadcasting an augmentation signal for approximately 1 year, BDSBAS is still being tested and therefore cannot yet provide services for civil aviation.
Several studies have been carried out concerning the system segment of BDSBAS. The methods used for augmentation information are being developed from institute to university. Some scholars have given the design and preliminary analysis of BDSBAS the principle of differential correction and integrity parameters, and the service performance of BDSBAS. The algorithms used in the system segment of SBAS are significant for the construction of BDSBAS and are hence popular targets for research.
Technologies of GBAS system segment
Preliminary information
GBAS is a kind of differential GNSS (DGNSS) that is applied to aircraft for a precision approach. GBAS is able to provide CAT-I and higher level precision approach and landing guidance services for aircrafts equipped with the corresponding airborne equipment within the airspace of the terminal area of an airport (Geng 2019).
Aviation GBAS consists of a ground segment and an airborne segment. The ground segment consists of a reference receiving subsystem, a ground processing subsystem, a maintenance management subsystem, and a VDB (very high frequency Data Broadcasting) subsystem. The airborne segment mainly refers to the multi-mode receiver (MMR). The reference receiving subsystem receives the ranging signal of the GNSS satellites and forwards it to the ground processing subsystem. The ground processing subsystem then generates the augmentation information for GBAS users. The maintenance management subsystem is used to ensure the normal work of the ground processing subsystem can detect any faults in the ground processing subsystem and send control commands to the ground processing subsystem. After receiving the augmentation information, the VDB subsystem transmits it to GBAS users, receiving the augmentation information from the MMR and calculating the location and the integrity information for the pilot. In detail, the ground processing subsystem generates the differential corrections for the visible satellite by combining observations from each reference receiver. The integrity information of the visible satellite or the navigation system is formed at the same time through the real-time monitoring of the navigation signal or the abnormality at the ground stations. The final approach segment (FAS) data, calibration, and integrity information are then transmitted through VDB to the airborne users, as illustrated in Fig. 7. The airborne user receives the augmentation information from the VDB subsystem and generates flight information which is displayed in the flight control instruments and display. As GBAS normally uses specialized reference stations that are located close to each other, which are always distributed around an airport, and the distance between the airborne users and the GBAS stations is close (less than 50 km), the errors between them are strongly correlated and GBAS can thus improve the positioning accuracy and integrity of the airborne users.
Research progress
GBAS is currently under development worldwide. The GBAS sls-3000 ground stations by Honeywell were installed at Malaga airport in Spain early in 2007. France conducted a signal-in-space verification of the CAT-I ground station for GBAS that was installed at Toulouse in 2006, and continuously monitors GBAS performance. As of 2009, the FAA has placed multiple CAT-I GBAS installations into service using the Honeywell SLS-4000 ground station. In 2017, the FAA began to carry out system design approval for the GAST-D system. However, the FAA had to suspend this project due to a lack of funding. In 2018, Japan finished the development and deployment of ground and airborne subsystem software for GBAS, and began to perform data collection and analysis. Japan plans to conduct air-ground experiments in 2019. In China, the China Electronics Technology Group Corporation is performing research and development on the GBAS Approach Service Type D (GAST-D), using its CAT-I GBAS products as a base.
China is also conducting research on NBGAS, using the BDS. NBGAS is a pivotal part of BDS. NBGAS uses an advanced system architecture, data processing system, and software to realize multiple mode positioning accuracy augmentation, utilizes various broadcasting means to broadcast augmented data products, and provides positioning with accuracy at the meter, decimeter, and centimeter level, or on the millimeter level after post-processing.
Geng Yongchao, a senior engineer at the CETC Northwest Group Co., LTD, developed GBAS for civil aviation, including the concept of using GBAS for civil aviation, the civil aviation GBAS architecture, and progress in GBAS. The CETC Northwest Group Co., LTD has performed flight testing with inspection aircrafts. After performing a static check, a taxi check, GBAS reception, and a flight test, the group concluded that lgf-1a GBAS equipment can provide aircraft precision approach, automatic landing, and taxi guidance services under the current regulatory specifications (Geng 2019).
Cai Yi, a chief designer at the China Research & Development Academy of Machinery Equipment, developed NBGAS, with test equipment and a user terminal as shown in Fig. 8. NBGAS refers to the national BeiDou ground-based augmentation system which can provide service for ships, trains, and cars, etc. Unlike GBAS, which is designed to serve civil aviation, NBGAS is designed to provide a high precision positioning service for many areas. NBGAS focuses on the services of satellite broadcasting, digital radio, and mobile communication. NBGAS uses different wide reference stations (RSs) than GBAS, and pays significant amounts of attention to position accuracy. Obviously, NBGAS cannot support the current ICAO GBAS standards or provide service for civil aircrafts as does GBAS. NBGAS are composed of BeiDou augmentation RSs which are distributed all over China, data processing systems, data broadcasting systems, and user terminals. BeiDou wide area RSs and regional RSs collect the GNSS signals and forward these data to the corresponding data processing systems. NBGAS data processing systems include industrial data processing systems and national data processing systems, both of which output high accuracy real time (RT) products from the meter level to the millimeter level, which is sent to users through data broadcasting systems. Finally, users such as the cars drivers and pilots can obtain NBGAS services.
The real-time positioning accuracy for NBGAS was tested at the meter level, decimeter level, centimeter level, and post-processing millimeter level augmentation by Cai Yi. Results showed that the positioning accuracy of NBGAS meets or exceeds the design performance. According to the service capability tests for real-time positioning accuracy at the meter level to the post-processing millimeter level, the service capability of NBGAS’s positioning accuracy meets and is superior to that indicated by the system design (Cai 2019).
Summary
With the rapid development and improvement of BDS, the system is being widely applied in many areas. As an important navigation system, the ground-based augmentation system has begun to provide services for all kinds of users by combination with BDS. Three of the typical ground-based augmentation systems are under discussion: the civil aviation ground-based augmentation system, the Chinese ground-based augmentation system, and the national BeiDou ground-based augmentation system. The civil aviation ground-based augmentation system represents the traditional type of GBAS that is focused on civil aviation. The Chinese ground-based augmentation system represents the GBAS that is focused on aviation which was developed in China. The national BeiDou ground-based augmentation system refers to NBGAS, which provides service for trains and cars, etc. The Chinese ground-based augmentation system has passed technical reviews and system tests so far, and has reached the stage of verification flight. To implement the CAT II/III research, the first verification flight of the Chinese GBAS was performed in April 2019, and further GAST-D technical tests are being carried out during the second half of 2019 and into 2020. NBGAS officially started work in 2014 and provided real-time accuracy at the centimeter and the post-processing millimeter level in 2016. In 2017, NBGAS service performance specification version 1.0 was released and NBGAS began to undergo thorough testing in 2018. The augmentation for the real-time positioning accuracy at the meter level, decimeter level, centimeter level, and post-processing millimeter level of NBGAS was tested, with results indicating that the positioning accuracy of NBGAS either meets or exceeds the design performance. NBGAS can provide real time accuracy at the meter and decimeter level as of 2019.
In summary, the development of GBAS in China has reached an early stage. The research of GBAS is considered significant, and has practical value in terms of aerospace missions in the future. The research into NBGAS with civil aviation has caught the attention of many researchers and is increasingly popular.