The Wide Area Augmentation System (WAAS)

Todd Walter

Stanford University

http://waas.stanford.edu

Todd Walter

Stanford University

http://waas.stanford.edu

2

Conclusions

WAAS is used to provide aircraft navigation from enroute through vertically guided approach

Integrity was and is the key challengeImportant to understand what can go wrong

and how to protect usersCareful analysis of feasible threats

New civil frequencies and additional constellations will further improve performance

3

Outline

The Wide-Area Augmentation SystemIntegrity AnalysesComparison with Terrestrial

Navigational AidsFuture Directions

4

WAAS

Wide Area Augmentation System (WAAS) – Program Status5Federal Aviation

Administration

WAAS Architecture

38 Reference

Stations

3 Master

Stations

4 Ground

Earth Stations

2 Geostationary

Satellite Links

2 Operational

Control Centers

Courtesy:

Wide Area Augmentation System (WAAS) – Program Status6Federal Aviation

Administration

Geostationary Satellites (GEO)

• Provides Dual Coverage Over United States

Telesat107W

PanAmSat133W

Courtesy:

Wide Area Augmentation System (WAAS) – Program Status7Federal Aviation

Administration

8

Error Sources

Satellite errorsEphemerisClockSignal

Propagation errorsIonosphereTroposphere

Local ErrorsMultipathReceiver Noise

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Master Station Schematic

10

Complicated Schematic

Correction Processor

UDRE

UDRE, GIVE UDRE Alarms Out

GIVE, Delays UDRE

GIVE, Delays

UDRE

Internal (Verification)

External (Validation)

Safety Processor

UDRE

UDRE

Delays

Reset

WRE Bias Fault

SCP

PID - Thread Select - Operator Alert

Larger UDRE

CCC Monitor

User Position Monitor (Internal Only)

Old But Active Data (OBAD)

UDRE

UDRE

Broadcast - UDRE

ICC - L1/L2 Bias Estimator - Iono Delays

CNMP Monitor,

WRE Bias Monitor

GIVE Monitor (Internal Only)

WNT Monitor UDRE Monitor

Range Domain Monitor (Internal Only)

Broadcast (Internal) Alarms (External)

Signal Quality Monitor

L1L2

L1L2 Relative Bias Monitor

UDRE 2irr eg, /Tirreg, trip

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GPS Performance (Usually)

On a good day, thered circle encloses 95%of the GPS position fixes.

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Major GPS Faults About Twice a YearExample: Ephemeris Failure on April 10, 2007

On a bad day, theGPS errors can bemuch worse.

WAAS & GBASeliminate theselarge errors.

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Integrity Approach

Aviation integrity operates on a guilty until proven innocent principleError bound is the maximum possible

value given the measurements

This is unlike conventional systems that describe the most likely errors

Protection level is a 99.99999% bound on worst reasonable conditionsVery different from 95% achieved

accuracy

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Failure of Thin Shell Model

Quiet Day Disturbed Day

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Undersampled Condition

Courtesy:Seebany Datta-Barua

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11/20/200321:00:00 GMT

Wide Area Augmentation System (WAAS) – Program Status17Federal Aviation

Administration

Localizer Performance Vertical (LPV) CoverageLocalizer Performance Vertical (LPV) Coverage

Courtesy:

Wide Area Augmentation System (WAAS) – Program Status18Federal Aviation

Administration

WAAS RNP 0.3 Current Coverage

Courtesy:

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WAAS LPV and LPV-200 Vertical Position Error Distributions July 2003 to June 2006

Courtesy:

FAA Technical Center

3 years

20 WRSs

1 Hz data

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Navigational Aids

Instrument Landing System (ILS)Glideslope antenna for verticalLocalizer for horizontal

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ILS Installations: Each Runway EndRequires At Least Two Transmitters

1318 ILS’s nationwide

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Airports with WAAS LPVNo GPS Equipment Required at Airport

50 Pieces of WAAS Equipment Serve the Continent

As of November, 2009• 1820 WAAS-based LPV’s• ~1000 for non ILS runways

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Localizer Approaches at Moffett Field

Courtesy:Sharon Houck

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Utility of Protected Accuracy from WAAS

WAAS (& GBAS) tunnels:• Do not flare like ILS• Do not have beam bends• Are programmable• Are adaptable

• Localizer performance with vertical guidance (LPV)

• Safer than lateral nav.(non-precision approach)

• Same decision ht. as Cat I• GBAS for Cat. II & III

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Current WAAS Performance

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Future L1/L5 Performance

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L1-only Threats Reference station multipath, noise, cycle slips, and

other receiver errors – WRE bias, CNMP Satellite clock/ephemeris – UDRE/MT28 Erroneous ionospheric delay estimates - GIVE Erroneous WRE clock estimates, interfrequency

bias estimates – RDM L1 code/carrier incoherence – CCC L1 signal deformation – SQM Multiple convolved threats – UPM, convolution

analysis Antenna biases, GEO biases – specific analyses

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L1/L5 Threats Reference station multipath, noise, cycle slips, and

other receiver errors – WRE bias, CNMP – unchanged

Satellite clock/ephemeris – UDRE/MT28 – need to remove L1/L2 bias from Fast Correction, could remove uncertainty from UDRE

Erroneous ionospheric delay estimates - GIVE – uneeded

Erroneous WRE clock estimates, interfrequency bias estimates – RDM – IFBs uneeded, WRE clocks may be handled by UDRE/UPM

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L1/L5 Threats cont. L1/L5 code/carrier incoherence – CCC –

new/updated monitor, MERR reduced without GIVE while threat is potentially increased

L1/L5 signal deformation – SQM – new/updated monitor, MERR reduced while threat is increased. User space will need to be greatly restricted

Multiple convolved threats – UPM, convolution analysis – new/updated monitor specific to L1/L5 user

Antenna biases, GEO biases – specific analyses – new/updated analysis, effects are greater

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CCC Threats

Iono-free ccc metric expected to have 2.6 times as much noise as L1-only versionTrip thresholds may need to be increasedCould lead to UDRE bumps or trips

MERR substantially reduced without GIVE Need much reduced thresholds and test

L1 only monitor has margin thanks to GIVENeed to collect data to see real test

Need L5 data, but L2C might be OK

High risk factor for not being able to match L1 UDREs

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Frequency Dependencies

L1 error L5 error RSS Sum of absolutes

Iono error

Clock / ephemeris or tropo

L1 Only

1 0 1 1 1 1

Iono-free

2.6 3.5 0 1f12

f12 − f5

2

=2.26

f52

f12 − f5

2

=1.26

PRL1 ji =ΔR igl j

i + ΔBi + I L1 ji +Tj

i −bj + ML1 ji +νL1 j

i

φL1 ji =ΔR igl j

i + ΔBi −I L1 ji +Tj

i −bj + NL1 ji λ1 + mL1 j

i + nL1 ji

PRL1 ji =ΔR igl j

i + ΔBi +γ ×I L1 ji +Tj

i −bj + ML5 ji +νL5 j

i

φL5 ji =ΔR igl j

i + ΔBi −γ ×I L1 ji +Tj

i −bj + NL5 ji λ1 + mL5 j

i + nL5 ji

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SDM Threats

Bias threats on L1 have 2.26 times greater influence on iono-free users

L5 biases also must be added MERR substantially reduced without GIVE Nominal and threat biases potentially 3.5 times

those on L1 onlyBias terms in VPL potentially very beneficial to iono-

free user

Need to further restrict user space L1 and L5 may need to be a very tight box around

monitor choiceDifficult to get international buy-in

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Conclusions

WAAS is used to provide aircraft navigation from enroute through vertically guided approach

Integrity was and is the key challengeImportant to understand what can go wrong

and how to protect usersCareful analysis of feasible threats

New civil frequencies and additional constellations will further improve performance

34

Potential of L1/L5 GPS/Galileo Performance