The Wide Area Augmentation System (WAAS) Todd Walter Stanford University http://waas.stanford.edu Todd Walter Stanford University http://waas.stanford.edu.

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  • The Wide Area Augmentation System (WAAS)

    Todd WalterStanford University

    http://waas.stanford.edu

  • ConclusionsWAAS is used to provide aircraft navigation from enroute through vertically guided approachIntegrity was and is the key challengeImportant to understand what can go wrong and how to protect usersCareful analysis of feasible threatsNew civil frequencies and additional constellations will further improve performance

  • OutlineThe Wide-Area Augmentation SystemIntegrity AnalysesComparison with Terrestrial Navigational AidsFuture Directions

  • WAAS

    Wide Area Augmentation System (WAAS) Program Status*

    WAAS Architecture38 Reference Stations3 Master Stations4 Ground Earth Stations2 Geostationary Satellite Links2 Operational Control CentersCourtesy:

    Wide Area Augmentation System (WAAS) Program Status*

    Geostationary Satellites (GEO)Provides Dual Coverage Over United StatesTelesat107WPanAmSat133WCourtesy:

    Wide Area Augmentation System (WAAS) Program Status*

  • Error SourcesSatellite errorsEphemerisClockSignalPropagation errorsIonosphereTroposphereLocal ErrorsMultipathReceiver Noise

  • Master Station Schematic

  • Complicated Schematic

    Signal Quality Monitor

    GIVE Monitor

    (Internal Only)

    2irreg, /Tirreg, trip

    Range Domain Monitor

    (Internal Only)

    Broadcast - UDRE

    Correction Processor

    UDRE

    PID - Thread Select - Operator Alert

    Old But Active Data

    (OBAD)

    UDRE

    Alarms (External)

    L1L2 Relative Bias Monitor

    L1L2

    Broadcast (Internal)

    User Position Monitor

    (Internal Only)

    UDRE

    UDRE, GIVE

    UDRE Alarms Out

    GIVE, Delays

    UDRE

    GIVE, Delays

    UDRE

    Internal (Verification)

    External (Validation)

    Safety Processor

    ICC - L1/L2 Bias Estimator

    - Iono Delays

    SCP

    UDRE

    UDRE

    UDRE

    WNT Monitor

    UDRE Monitor

    CCCMonitor

    Delays

    UDRE

    Reset

    CNMPMonitor,WRE Bias Monitor

    WRE Bias Fault

    Larger

  • GPS Performance (Usually)On a good day, thered circle encloses 95%of the GPS position fixes.

  • 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.

  • Integrity ApproachAviation integrity operates on a guilty until proven innocent principleError bound is the maximum possible value given the measurementsThis is unlike conventional systems that describe the most likely errorsProtection level is a 99.99999% bound on worst reasonable conditionsVery different from 95% achieved accuracy

  • Failure of Thin Shell ModelQuiet DayDisturbed Day

    *

    Undersampled ConditionCourtesy:Seebany Datta-Barua

  • 11/20/200321:00:00 GMT

    Wide Area Augmentation System (WAAS) Program Status*

    Localizer Performance Vertical (LPV) CoverageCourtesy:

    Wide Area Augmentation System (WAAS) Program Status*

    WAAS RNP 0.3 Current CoverageCourtesy:

  • WAAS LPV and LPV-200 Vertical Position Error Distributions July 2003 to June 2006Courtesy:FAA Technical Center

    3 years20 WRSs1 Hz data

  • Navigational AidsInstrument Landing System (ILS)Glideslope antenna for verticalLocalizer for horizontal

    *

    ILS Installations: Each Runway EndRequires At Least Two Transmitters1318 ILSs nationwide

    *

    Airports with WAAS LPVNo GPS Equipment Required at Airport50 Pieces of WAAS Equipment Serve the ContinentAs of November, 2009 1820 WAAS-based LPVs ~1000 for non ILS runways

  • Localizer Approaches at Moffett FieldCourtesy:Sharon Houck

    *

    Utility of Protected Accuracy from WAASWAAS (& 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

  • Current WAAS Performance

  • Future L1/L5 Performance

  • L1-only ThreatsReference station multipath, noise, cycle slips, and other receiver errors WRE bias, CNMPSatellite clock/ephemeris UDRE/MT28Erroneous ionospheric delay estimates - GIVEErroneous WRE clock estimates, interfrequency bias estimates RDML1 code/carrier incoherence CCCL1 signal deformation SQMMultiple convolved threats UPM, convolution analysisAntenna biases, GEO biases specific analyses

  • L1/L5 ThreatsReference station multipath, noise, cycle slips, and other receiver errors WRE bias, CNMP unchangedSatellite clock/ephemeris UDRE/MT28 need to remove L1/L2 bias from Fast Correction, could remove uncertainty from UDREErroneous ionospheric delay estimates - GIVE uneededErroneous WRE clock estimates, interfrequency bias estimates RDM IFBs uneeded, WRE clocks may be handled by UDRE/UPM

  • L1/L5 Threats cont.L1/L5 code/carrier incoherence CCC new/updated monitor, MERR reduced without GIVE while threat is potentially increasedL1/L5 signal deformation SQM new/updated monitor, MERR reduced while threat is increased. User space will need to be greatly restrictedMultiple convolved threats UPM, convolution analysis new/updated monitor specific to L1/L5 userAntenna biases, GEO biases specific analyses new/updated analysis, effects are greater

  • CCC ThreatsIono-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 tripsMERR 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 OKHigh risk factor for not being able to match L1 UDREs

  • Frequency Dependencies

    L1 errorL5 errorRSSSum of absolutesIono errorClock / ephemeris or tropoL1 Only101111Iono-free2.63.501

  • SDM ThreatsBias threats on L1 have 2.26 times greater influence on iono-free usersL5 biases also must be addedMERR substantially reduced without GIVENominal and threat biases potentially 3.5 times those on L1 onlyBias terms in VPL potentially very beneficial to iono-free userNeed to further restrict user space L1 and L5 may need to be a very tight box around monitor choiceDifficult to get international buy-in

  • ConclusionsWAAS is used to provide aircraft navigation from enroute through vertically guided approachIntegrity was and is the key challengeImportant to understand what can go wrong and how to protect usersCareful analysis of feasible threatsNew civil frequencies and additional constellations will further improve performance

  • Potential of L1/L5 GPS/Galileo Performance

    WAAS improves the GPS signal to meet the necessary integrity, availability, accuracy, and continuity for use in all phases of flight at all runway endsThe Wide Area Augmentation System (WAAS) is one of 4 Space Based Augmentation Systems (SBAS) in operation or in development around the globe. The Japanese MSAS, and Indian GAGAN systems are derivatives of the U.S. WAAS, and the EU has the EGNOS system.WAAS is in its final configuration with 38 Wide Area Reference Stations (WRS) feeding data to 3 Wide Area Master Stations (WMS), that compute GPS corrections and integrity information which is forwarded to four geographically dispersed Ground Earth Stations (GES), and uplinked the messages to two geostationary earth orbiting (GEO) satellites and broadcast to the users. The system is controlled from two Operational Control Centers (OCC), one located at the National Operational Command Center (NOCC) in Herndon, VA and the second located at the Pacific Operational Control Center (POCC) in San Diego, CA.

    WAAS currently relies on the services of 2 leased geostationary satellites, located at 133W and 107W to provide dual coverage over nearly all of the U.S. and Alaska. Within the GEO footprint, users receive basic corrections and ranging signals to augment accuracy and availability for lateral navigation.

    *This chart shows the area, inside the yellow contour line, where WAAS provides precision approach service, referred to as Localizer Precision Vertical (LPV). LPV enables an aircraft to descend to 200 feet above a runway, equivalent to the legacy instrument landing system (ILS). The LPV service is available at any qualifying runway or landing area without the need to install ground-based navigation equipment. WAAS also provides LPV service to significant portions of Canada and Mexico.Ask: so what if pilot is 1 dot off? Keep in mind that the separation between runways at SFO is 700 feet.------------------------Roll activities not significantly differentJock: mention ILS can go unstable

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