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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
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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)
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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?
