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Analysis of the GNSS Augmentation Technology Architecture CHEN JINPING Beijing Global Information Center of Application and Exploitation BACKGROUND The development process of GNSS is divided into four stages: (1) experi- mental construction stage (GPS I), (2) infrastructure construction stage (GPS II), (2) augmentation construction stage (Wide Area Augmentation System [WAAS]/ Local Area Augmentation System [LAAS]), and (4) architecture construction stage (GPS III). The architecture construction stage is to meet the requirements from different military and civilian users, consider multi-GNSS compatibility and interoperabil - ity, design the basic GNSS and augmentation system in whole, and emphasize the policy, law, standards, and industrialization of GNSS. The current existing augmentation technologies were developed for legacy GPS, and the augmentation systems are independent of each other. There are no uniform definition and standard. The user is a little confused with the application of these augmentation systems. In the architecture construction stage, the redefinition of GNSS augmenta- tion technology architecture is needed. Future augmentation systems should be constructed for the improvement of basic GNSS performance. INTRODUCTION TO THE CURRENT GNSS AUGMENTATION TECHNOLOGY The current GNSS augmentation technologies are shown in Figure 1. The com- parison of different GNSS augmentation technologies is shown in Tables 1 and 2. 137
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138 GLOBAL NAVIGATION SATELLITE SYSTEMS Wide Area Differential Local Area Differential Accuracy Augmentation Technology Wide Area Precise Positioning Local Area Precise Positioning GNSS Basic Integrity Monitoring Satellite Based Augmentation System Wide Area Differential Integrity Monitoring Local Area Differential Integrity Monitoring Integrity Augmentation Technology Receiver Autonomous Integrity Monitoring Ground Based Satellite Autonomous Integrity Monitoring Augmentation System Inter Satellite Link Integrity Monitoring Space Based GEO Satellite Augmentation Continuity/Availability Augmentation Technology Ground Based Pseudolite Augmentation FIGURE 1 The current GNSS augmentation technologies. Chen_Fig1.eps ANALYSIS OF GNSS AUGMENTATION GOALS AND OBJECTS The goal of GNSS augmentation is to meet the improved performance requirements of the high-level user. GNSS navigation performance requirements defined by the International Civil Aviation Organization (ICAO) and the Radio Technical Commission for Aeronautics (RTCA) are shown in Tables 3 and 4. The above tables show that the accuracy, integrity, continuity, and availability requirements are different in different flight phases. The accuracy requirement can be met easily, but the integrity, continuity, and availability requirements are stringent and cannot be provided by the current GNSS. In addition, some high-level users have required decimeter and centimeter positioning accuracy, and these requirements cannot be provided by the current GNSS either. The objects of augmentation are to allow the basic GNSS, including GPSII, modernized GPS, GLONASS, Galileo, COMPASS regional system and GPSIII, modernized GLONASS, Galileo+, and COMPASS global system, to meet posi - tioning, navigation, and timing service requirements. The constellation, user range error (URE) performance, and integrity mon - itoring performance are different for different GNSS and for their different construction stages. So the accuracy, integrity, continuity, and availability per- formance are at different levels (Table 5). The goals of augmentation show the accuracy requirements of augmentation can be divided into two classes: 1 m level and <1 m level (including decimeter
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TABLE 1 Comparison of Current GNSS Differential Technologies Coverage Processing Principle Broadcasting Performance Status and Station State differential method, generation of WAAS Wide Area Thousands of GEO/radio Accuracy 3 m (single ephemeris, clock correction and ionospheric EGNOS Differential kilometers, beacon, frequency), delay parameters and based on pseudorange MSAS Technology Tens of stations RTCA no initialization measurements (assisted with carrier phase) GAGAN Accuracy dm level Wide Area State differential method, generation of GEO/Internet, (dual frequency GDGPS Precise Global, ephemeris, clock correction and mainly based SOC or self- carrier phase), StarFire Positioning Nearly 100 stations on carrier phase measurements defined protocol 20 minutes OmniStar Technology initialization Measurement differential method, generation Local Area VHF, Accuracy <1 m LAAS Tens of kilometers, of integrated pseudorange correction Differential RTCA/RTCM (single frequency), NDGPS Several stations parameters and mainly based on pseudorange Technology protocol no initialization RBN-DGPS measurements (assisted with carrier phase) Tens of kilometers, Measurement differential method, mainly Accuracy cm level Local Area Several stations, based on carrier phase measurements, Local (dual frequency Precise GPRS/CDMA, or extended area users can receive the local area error carrier phase), CORS Positioning RTCM protocol to thousands of correction parameters processed with the 1~2 minutes Technology stations VRS, FKP, MAC modes initialization 139
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140 TABLE 2 Comparison of Current GNSS Integrity Monitoring Technologies Coverage Processing Principle Broadcasting Performance Status and Station GPS, Risk, 1E-4/hr GNSS Basic Global, Generation of URA or SISA/SISMA with Nav TTA, hrs to mins, GPS Integrity Several to tens of processing of ephemeris and clock error Message Galileo, TTA, 6s Galileo Technology stations Risk, 2E-7/150s WAAS WAAS Thousands of Generation of UDRE, GIVE with processing GEO TTA, 6s EGNOS Integrity kilometers, of ephemeris, clock error and ionospheric grid RTCA Risk, 2E-7/150s MSAS Technology Tens of stations parameters GAGAN LAAS Generation of integrity parameters with VHF LAAS Tens of kilometers, TTA, 2s Integrity processing of differential pseudorange RTCA/ NDGPS Several stations Risk, 2E-9/15s Technology integrated correction parameters RTCM RBN-DGPS Detection and elimination of fault satellite RAIM Visible satellites Almost no TTA measurements with multi measurements — Receiver Technology measurements Pmd, 1E-3 redundancy Generation of integrity parameters with Satellite SAIM monitoring of satellite signal power Nav TTA, 2s feedback signal Under research Technology abnormality, pseudo code abnormality, clock Message Risk, 1E-7/hr measurements error overrun and navigation data mistake ISL Inter satellite Generation of integrity parameters with Nav Integrity crosslink monitoring satellite orbit and clock error Unspecific Under research Message Technology measurements abnormality
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TABLE 3 GNSS Navigation Performance Requirements Defined by ICAO Accuracy (95%) Integrity Flight Phase HAL Continuity Risk Availability H V TTA Risk H V Ocean 3.7 km N/A 7.4 km N/A 5 min 0.99~0.99999 1 × 10–7/h 1 × 10–4 ~ 1 × 10–8/h Domestic 3.7 km N/A 3.7 km N/A 5 min 0.99~0.99999 1 × 10–7/h 1 × 10–4 ~ 1 × 10–8/h Terminal 0.74 km N/A 1.85 km N/A 15 s 10–7/h 10–4 10–8/h 0.99~0.99999 1× 1× ~1× NPA 220 m N/A 556 m N/A 10 s 0.99~0.99999 1 × 10–7/h 1 × 10–4 ~ 1 × 10–8/h APV I 16 m 20 m 40 m 50 m 10 s 10–7/P 10–6/15 s 0.99~0.99999 2× 8× APV II 16 m 8m 40 m 20 m 6s 0.99~0.99999 2 × 10–7/P 8 × 10–6/15 s Cat I 16 m 6~4 m 40 m 15~10 m 6s 10–7/P 10–6/15 s 0.99~0.99999 2× 8× TABLE 4 GNSS Navigation Performance Requirements Defined by RTCA Accuracy (95%) Integrity HAL Flight Phase Continuity Risk Availability H V TTA Risk H V Cat I 16 m 4m 40 m 10 m 6s 0.99~0.99999 2 × 10–7/150 s 8 × 10–6/15 s Cat II 5m 2.9 m 17 m 10 m 2s 10–9/15 s 10–6/15 s 0.99~0.99999 1× 4× Cat IIIA 5m 2.9 m 17 m 10 m 2s 0.99~0.99999 1 × 10–9/15 s 4 × 10–6/15 s 10–9/15s(V) 10–6/15s/15 s(V) 1× 2× Cat IIIB 5m 2.9 m 17 m 10 m 2s 0.99~0.99999 1 × 10–9/30 s(H) 2 × 10–6/30 s(H) 141
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142 TABLE 5 Analysis of Different GNSS Performance Constellation Accuracy Integrity Notes URE 8 m Legacy GPSII 24 satellites Weak Position >10 m Modernized URE ~1 m GPS 30 satellites Improved, but unspecific GPS II Position <10 m URE <1 m GPS III 30 satellites Cat I Position ~1 m Legacy 24 satellites Comparative to Legacy GPSII GLONASS GLONASS Modernized 24 satellites Comparative to Modernized GPSII GLONASS Global SOL service (Cat I) URE <1 m MEO broadcast I/Nav Galileo+ More Galileo 30 satellites Position 3~5 m TTA: 6 s improvement Risk: 2E-7/150 s Augmentation system is integrated in URE ~2 m the basic GNSS Regional System 12 satellites Position ~10 m TTA:6 s COMPASS Risk: 2E-7/approach (Cat I) Global System 30 satellites Comparative to Galileo and GPSIII
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143 ANALYSIS OF THE GNSS AUGMENTATION TECHNOLOGY ARCHITECTURE and centimeter). The integrity requirements of augmentation can be divided into two classes: Cat I level (TTA 6 s, Risk 1E-7/approach) and better than Cat I level (TTA 2 s, Risk 1E-9/15 s). The continuity and availability requirements of aug- mentation are corresponding to the integrity requirements. Additionally, the 1 m level users are high in real time, and their integrity, continuity, and availability requirements are also high. The <1 m level users are slow in real time, and their integrity, continuity, and availability requirements are relatively low. The objects of augmentation show: In earlier stages, for legacy GPS and GLONASS, the constellations consist of 24 satellites, the position accuracy is >10 m, and the integrity performance is weak. So the augmentation technologies and augmentation system is needed to improve the performance. In modernization stage, for modernized GPSII, Galileo, modernized GLONASS, etc., the constel - lations consist of 30 satellites, the position accuracy is <10 m and approaching to 1 m level step by step, the integrity performance has been improved and approaches CAT I, but the performance goals of these systems are unspecific. In architecture stage, for GPSIII, Galileo+ and COMPASS global system, the con - stellations consist of 30 satellites, these systems are interoperable, the position accuracy is 1 m level, and the integrity performance reaches CAT I performance. The augmentation systems should be redesigned carefully. DEFINITION OF FUTURE GNSS AUGMENTATION TECHNOLOGY ARCHITECTURE The current existing augmentation technologies are presented for the earlier GNSS. The future augmentation technology architecture can be defined with the following principles: 1. The 1 m accuracy and Cat I integrity performance will be provided by the basic GNSS as the first layer in global coverage and by the wide area augmentation system, which is constructed by some country or organiza- tion as second layer in regional coverage. 2. The <1 m accuracy performance will be provided by the local area precise positioning system constructed by each country or organization. 3. Better than Cat I level integrity performance will be provided by the local area integrity augmentation system constructed by a specific user group. 4. The continuity and availability performance will be assured by the fact that every constellation consists of 30 satellites and constellation interoperability. The definition of future GNSS augmentation technology architecture is shown in Figure 2. GNSS layer: 1 m accuracy performance is provided by the constellation and signal-in-space (SIS) user range of multi-GNSS. Cat I integrity performance
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144 GNSS Basic Integrity Monitoring 〈 Ground Based Integrity Monitoring 〈 Satellite Autonomous Integrity Monitoring GNSS Layer GNSS (GPS/GLONASS/Galileo/COMPASS) 〈 Inter Satellite Link Integrity Monitoring Wide Area Augmentation Wide Area 〈 Wide Area Differential 〈 Wide Area Differential Integrity Monitoring Augmentation Space Based Augmentation System (SBAS) 〈 Wide Area Precise Positioning Layer Local Area Augmentation 〈 Local Area Differential Ground Based Continuously Operating Local Area 〈 Local Area Differential Integrity Monitoring Reference Station Augmentation System Augmentation 〈 Local Area Precise Positioning (CORS) (GBAS) Layer Receiver Augmentation Application 〈 GNSS Basic Integrity Application 〈 SBAS Application GNSS SBAS CORS GBAS Receiver 〈 Local Area Augmentation Application Receiver Receiver Receiver Receiver Layer 〈 Receiver Autonomous Integrity Monitoring FIGURE 2 Definition of future GNSS augmentation technology architecture. Chen_Fig2.eps landscape
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145 ANALYSIS OF THE GNSS AUGMENTATION TECHNOLOGY ARCHITECTURE is provided by integrating the GNSS basic integrity monitoring and satellite autonomous integrity monitoring (SAIM) and inter-satellite link (ISL) integrity monitoring as the first layer in global coverage. Wide area augmentation layer: Based on the basic GNSS, as the second layer in corresponding areas, the performance of 1 m accuracy and Cat I integrity are provided by wide area argumentation systems, which are constructed by various countries or organizations by integrating wide area difference, wide area precise positioning, and integrity monitoring technology. Local area augmentation layer: Based on the basic GNSS, <1 m accuracy performance is provided by local area precise positioning systems, which are constructed by various countries or organizations with local area precise position - ing technology. Better than Cat I integrity performance is provided by local area integrity argumentation systems, which are constructed by various user groups with local area integrity monitoring technology. Receiver layer: Corresponding to the above three level services, there are four types of receiver for navigation, positioning, and integrity monitoring. At the same time, receiver autonomous integrity monitoring technology is used for related integrity analyzing. CONCLUSIONS AND QUESTIONS From the definition of the technology architecture, the GNSS basic system and the augmentation system are independent as well as coupled and reflect the multiplayer architecture. The development of GNSS basic system and augmenta - tion system may refer to this architecture, so as to realize interoperability step by step in the aspects of design, technique realization, application standard, etc. Of course, multi-GNSS basic system and augmentation system have already formed their definition and been constructed. To implement this architecture, the following questions should be answered: 1, For the performance of 1 m accuracy and Cat I integrity, the following questions should be studied: How to define the corresponding perfor- mance standard, such as the constellation geometry and the SIS URE? How to integrate the technologies of system basic integrity monitoring and SAIM and ISL integrity monitoring? 2. Based on the performance of GNSS basic systems, the following ques- tions should be studied: How to classify definition for GNSS basic sys- tems and the Wide Area Augmentation Systems constructed by different countries or organizations? How to construct independently under the same standard? How to integrate Wide Area Augmentation System and Wide Area Precise Positioning System? 3. Today, many countries are developing local precision positioning sys- tems and local integrity augmentation systems for the legacy GPS. The
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146 GLOBAL NAVIGATION SATELLITE SYSTEMS following questions should be studied: How to adapt the development and change of GPS signal? How to design and construct for multi- GNSS? 4. For different service mode and different user receiver, the following questions should be studied: How to define the standards of naviga- tion and integrity application processing? How to define the relation between system integrity processing and receiver autonomous integrity monitoring?