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– If GPS autonomous integrity is not available, FMS cross
compares GPS with other navigation aids or inertial
– FANS I implementation
Two major form factors are defined by AEEC.
–ARINC 755 – “Digital MMR”
• GPS, ILS, GLS and MLS
• Replaces ARINC 710 ILS receiver
–ARINC 756 – “Analog MMR”
• GPS, ILS, VOR, Marker Beacon, MLS and GPS
• Optional FMS functionality
• Replaces ARINC 547 ILS/VOR receiver
Commercial Air Transport Airplane
manufacturers have adopted the
multimode receiver as the
standard architecture for GNSS
integration
Multi-Mode Receiver
Courtesy of:
The Boeing Company W100.23
Basic GPS System Limitations
• Integrity
– Notification Time: 15 Minutes or Greater WAAS (CAT-I) - 6 sec
– Not Sufficient for Civil Aviation LAAS (CAT-II/III) - 2 sec
• Availability
– 24 Satellites - 70% WAAS (CAT I) - 99.9%
– 21 Satellites - 98% LAAS (CAT II/III) - 99.999%
– Not Sufficient for Primary Means Navigation
• Accuracy
– Enroute - OK WAAS (CAT I) - 7.6 m
– Not Sufficient for Precision Approaches LAAS (CAT II/III) - 1 m
• Not Sufficient for Safety of Flight for All Operations
}
}
}
The Boeing Company W100.24
GPS Performance Issues
• Integrity
– GPS alone does not meet civil aviation requirements for integrity
– Satellite signal monitoring insufficient. (large periods of time where
satellites are not monitored by ground network).
– Time to correct anomalies too long. (On the order of hours)
– Augmentation is required
– Receiver Autonomous Integrity Monitoring (RAIM)
– Checks signal integrity by doing a consistency check using
redundant data
– Requires at least 5 satellites to perform RAIM
– Other augmentation system (GBAS, SBAS etc)
• Availability of GPS service depends on availability of augmentation
• GPS performance characteristics are time-varying
The Boeing Company W100.25
GPS Augmentations
• RAIM is the principle augmentation to GPS today
– Results in some limitation on availability of service
• Several other augmentations to GPS are under development
– Satellite Based Augmentation System (SBAS)
–Wide Area Augmentation System (WAAS) - US
–European Geostationary Overlay System (EGNOS) - Europe
–Multifunction Satellite Augmentation System (MSAS) - Japan
– Ground Based Augmentation Systems (GBAS)
–Local Area Augmentation System (LAAS) – US
–GBAS program in Australia
– Ground Based Regional Augmentation Systems (GRAS)
–Emerging concept. - Australia
The Boeing Company W100.26
ABAS Status
• ABAS – GNSS integrity assured by integration of information
available the airplane
– RAIM is considered ABAS
– Some form of RAIM is included in all aviation receivers
– ABAS also includes
– GNSS + baro measurements
– GNSS + inertial measurements: Hybrid GNSS/INS
– “Loosely coupled” systems
– “Tightly coupled” systems
» AIME (Northrup Grumman)
»MSS (Honeywell HIGHTM System)
– Could potentially include other combinations
RAIM Uses Redundancy to Detect Faults
Good Position Fix
Bad/Misleading Position Fix
Satellite range measurements
Ranging Bias
Normal
Conditions
Error
Conditions
Present
Residual inconsistencies reveal misleading information
Good Position Fix:
• Measurements agree with one another
• Residual is small
Bad/Misleading Position Fix:
• Measurements contradict one another
• Residual is large
Good Position Fix:
• Measurements agree with one another
• Residual is small
Bad/Misleading Position Fix:
• Measurements contradict one another
• Residual is large
Ability to Detect Faults Depends on Geometry
i n
S
A A
ii
i i for 1, 2, 3, . . . ,
Magnitude of detection Statistic Caused by Bias
Position Error Caused by Bias 2
2
2
1 =
+
=
Each satellite will have a
different slope
Alert Limit
Magnitude of the Detection Function
Magnitude of the
Position Error
D FA T − Set by P
( ) ( ( T ) T )
n
A HTH HT S I H H H H 1 1 Where: and − − = = −
Fundamentals of RAIM
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