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Absolute Receiver Autonomous Integrity Monitoring (ARAIM) Todd Walter Stanford University http://waas.stanford.edu

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Page 1: Absolute Receiver Autonomous Integrity Monitoring (ARAIM) Todd Walter Stanford University  Todd Walter Stanford University

Absolute Receiver Autonomous Integrity Monitoring (ARAIM)

Todd Walter

Stanford University

http://waas.stanford.edu

Todd Walter

Stanford University

http://waas.stanford.edu

Page 2: Absolute Receiver Autonomous Integrity Monitoring (ARAIM) Todd Walter Stanford University  Todd Walter Stanford University

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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

Page 4: Absolute Receiver Autonomous Integrity Monitoring (ARAIM) Todd Walter Stanford University  Todd Walter Stanford University

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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