Absolute Receiver Autonomous Integrity Monitoring (ARAIM)
Todd Walter
Stanford University
http://waas.stanford.edu
Todd Walter
Stanford University
http://waas.stanford.edu
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Introduction GPS is an important component of today’s
aviation navigation infrastructure Its role will continue to increase over the coming years
Future GNSS constellations will also become important contributors
However, their incorporation must be done with great care as the integrity requirements for aircraft guidance are very stringent Less than 10-7 probability of misleading information International standards define different types of GNSS
augmentations to achieve this level of integrity
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Integrity Monitoring Satellite-based and ground-based
augmentation systems provide independent monitoring of the GPS signals through calibrated ground monitorsRequires ground monitoring network
communication channel to aircraft
Receiver Autonomous Integrity Monitoring (RAIM) compares redundant satellite range measurements against each other to identify and eliminate significant faultsRequires a greater number of ranging
measurements than SBAS or GBAS
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ARAIM Protection Level
VPL
GPS
Compass
Galileo
GLONASS
VPLVPLVPL
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RAIM vs. ARAIM LNAV requirements are much less stringent
than LPVAlert limit measure in nautical milesOnly real threat is a large clock error
For LPV MI is hazardous (vs. major)Alert limit in tens of metersMany sources of potentially significant errorsTwo or more smaller errors may combine to
cause a large enough error
ARAIM needed to more carefully account for all threats
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Interoperability of Integrity Interoperability should be a goal not only for
GNSS signals, but also for integrity provision Augmentation systems already internationally coordinated
Open service signals should target performance comparable to or better than GPS L1 signals today
Different service providers may make different design choices and different assurances However, it is important to establish a common
understanding of how RAIM depends on GNSS performance and how signals from different services could be combined to improve RAIM
Cooperation and transparency are essential
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Benefits of Multi-Constellation RAIM
Combining signals from multiple constellations can provide significantly greater availability and higher performance levels than can be achieved individuallySupport for vertically guided approaches
Potential to provide a safety of life service without requiring the GNSS service provider to certify each system to 10-7 integrity levels
Creates a truly international solutionAll service providers contributeNot dependent on any single entityCoverage is global and seamless
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Service Commitment Each service provider should provide
documentation of their service commitment Encourage usage by other statesAllows planning of combined service levelSupports development of interface specifications
and user algorithms
Commitment should include details on:Accuracy, continuity, availability, fault modes,
broadcast parameters, and other operating characteristics
Assurances should be provided for minimum commitments
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Specification of Faults Perform a fault modes and effects analysis
Understand and make transparent potential faults and their effects
Assure low fault ratesOf order 10-5/SV/HourAssure low probability of simultaneous or common
mode faults Ideally below 10-8/Constellation/Hour
Assure a short time to alertNot longer than 6 hours
Maintain independence from other service providers
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Monitoring and Assurance Methods for monitoring conformity of signal
properties relative to provided assurances should be agreed upon mutually by service providers and approval authorities Require clear unambiguous evaluations of assurances May be made by any potential approval authority Desirable to have a means to resolve potential observations
of non-conformity
Long-term monitoring by each sovereign state is an important component of establishing reliability Each constellation still cross-checked by others in user
avionics
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Interface Specification Each system may broadcast different
parameters or provide different levels of assurance
However, a common understanding of how each parameter is used must be reached The parameters must be combined into a single upper bound
for the joint position estimate The upper bound must be safe regardless of which
combinations of satellites are used Also able to account for potentially different properties
Requires a more advanced form of RAIM than is used currently for LNAV Good candidates already exist
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Summary RAIM allows for worldwide aviation navigation without
requiring additional ground infrastructure
Additional GNSS constellations can significantly improve performance and availability
At a minimum, new GNSS constellations should assure that their open service signals support existing LNAV RAIM Should work together to specify a means to achieve multi-
constellation RAIM for vertical guidance
International cooperation and coordination will be essential to achieving this goal
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Specification of Accuracy The dominant error sources should be understood
and characterized Satellite clock and ephemeris within constellation tied to
clear, stable, global, reference frames Code and carrier signals coherently derived from a
common source Well designed signals to reduce multipath, ionosphere,
and distortion effects International coordination already well-established
Document how the expected performance level is indicated to the user Should broadcast expected accuracy for a fault-free
ranging source
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Specification of Availability & Continuity
Description of constellation geometryNumber of satellites, planes, spacing, etc.
Assure minimum levels of operating satellitese.g. .99999 probability of at least 20 primary slots
occupied by satellites broadcasting valid signals
Assure minimum levels of continuity e.g. less than .0002 probability of unscheduled
interruption or fault of previously healthy signal
Lesser minimums support multi-const. RAIM even if they cannot support stand-alone
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Fault Tree and Probability of Hazardously Misleading
Information (PHMI)
Any mode causes HMI
No failures/ rare normal create HMI
failure of sat 1 causes HMI
failure of sat k causes HMI
… …
PHMI0 PHMI1PHMIk
• For each branch, a monitor mitigates the probability of HMI given the failure
• In ARAIM, the monitors are formed by comparing subset solutions
Mode prior probability = ~1
Mode prior prob-ability = ~1e-4
Mode prior prob-ability = ~1e-4
Courtesy:Juan Blanch