Ionospheric Integrity Lessons from the WIPP Todd Walter Stanford University Todd Walter Stanford University

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  • Ionospheric Integrity Lessons from the WIPP

    Todd Walter Stanford University

  • HistoryIonospheric Storms and Disturbances Originally Tested Via ScenariosSimulated disturbances added to simulated ionosphereGenerally, large geographic features were placed near center of networkIonospheric Algorithm Originally Based on JPL GIM CodeTuned to work on scenariosLive data from WRSs not yet available

  • Ionospheric ModelsProvides TruthGood for initial algorithm validationVery SmoothAverage TECLoses small-scale variationsSpatial gradients smoothed as wellUseful Tool Before Data Was AvailableHowever Does Not Faithfully Represent Real-World Instantaneous Ionosphere

  • Example ScenarioFrom Ionospheric Specification for the Wide Area Augmentation System (WAAS) Simulation Studies by Steve Chavin, ION GPS-96dTEC/dt = 0.74 TECU/minGradient = 0.085 TECU/kmShell Height = 360 km

  • ProblemAlgorithm Tuned to Work on ScenariosAll passed easilyDid not Work as Well on Real DataRequired extensive retuningSimulated Ionosphere Did Not Faithfully Reproduce Real IonosphereReal disturbances worse than predictedReal slant-to-vertical errors better than predictedFailing Scenario Could Prove Loss of IntegrityHowever, Passing All Scenarios Would Not Demonstrate Positive IntegrityWorst-case scenario is algorithm dependentDoes not demonstrate probability of missed detection requirement is met

  • National Satellite Test-BedPrototyping Occurred During Solar MinimumNo significant ionospheric disturbances observedCaused Us to Become OverconfidentPerformance Dominated by Receiver ArtifactsReasonability checks instituted to mitigate these errorsToo aggressive, would remove much of solar max observed behaviorEarly PrototypingDual-Frequency Survey ReceiversSingle ThreadedInitiated in 1993Full Deployment Started in 1996

  • 11 Year Solar CycleSolar activity changes dramatically over an 11 year solar cycleIonosphere at the peak is much worse than at minimumMost disturbances at peak and declining phaseNSTBWIPP

  • WIPPAt the End of 1999 FAA Certification Required a Change in the Safety AnalysisLevel D code not considered reliableThreat models required for all monitorsRigorous accounting for monitor observabilityCertification of Ionospheric Algorithms Left to Ionospheric ExpertsExperts Created Threat Models From DataReliable threat not hypotheticalMust protect against worst observed conditionsMust overbound historical observationsMust have a demonstratable probability of missed detection

  • Supertruth Data25 WRS - 3 Threads Each - Carrier Leveled - Biases Removed - Voting to Remove ArtifactsClean Reliable Data Collected at the Peak of the Solar Cycle Contained Worst Observed Gradients (Temporal and Spatial over CONUS)Most Severely Disturbed Days Formed the Basis for Threat ModelIonospheric Disturbances Are Deterministic, but Sampled RandomlyWorst cases are sampled over timeWill appear in the data as they move w.r.t IPPsApply Data Deprivation to Model Effects of Poor Observability

  • Ionospheric Measurements

  • Storm Example

  • Differences in Vertical DelayDifference in Vertical Delay vs. IPP Separation Distance for Two Days:Quiet Day July 2nd 2000Disturbed Day July 15th 2000

  • CONUS Ionosphere ThreatNot Well-Modeled by Local Planar FitIonosphere well-sampled1Ionosphere poorly sampled2Ionosphere Changes Over the Lifetime of the CorrectionUser Interpolation Introduces Error

    1. Robust Detection of Ionospheric Irregularities, Walter et al. ION GPS 20002. The WAAS Ionospheric Threat Model, Sparks et al. Beacon Symposium 2001

  • Well-Sampled IonosphereChi-Square Metric Acts as Storm-DetectorTest using small decorrelation valueNominally ~ 35 cm one-sigmaPassing test accepts larger valueTypically ~ 85 cm one-sigmaAnalytic ApproachDoes not require data except as validationFully specified before the first storm data of April 2000

  • Under-Sampled IonospherePurely an Empirical Threat Model Worst Storm Data UsedIPPs Removed From Estimation to Simulate Poor SamplingThree quadrant removal schemesStorm detector used on remaining dataThreat Based on Worst Deviation for Given Set of MetricsThreat Region Is 5x5 Degree Cell Centered on IGP

  • CORS Data

  • Goal of Sigma UndersampledTo protect against an unfortunate sampling of the ionosphere such that we fail to detect an existing disturbance

    Presumes the ionosphere is non-uniform near the IGP, i.e. it is divided into at least two states: a quiet one that is sampled, and a disturbed one that is not

  • Goal of Data DeprivationTo divide the IPPs into two groups: one that samples a relatively quiet ionosphere and does not trip the chi-square, the other that samples ionosphere not well-modeled from the quiet pointsData deprivation is used to simulate conditions that were not actually experienced but may reasonably be experienced in the futureIt allows us to investigate threats that occurred in well observed regions as though they had occurred in poorly observed regionsWant to have the quiet IPP distribution match those that may occur on operational system

  • Storm DaysOver the last 5 years, ~100 active days have been identified~50 affected or would have affected WAAS performance~45 supertruth files generatedWorking on files for the lesser days16 days affect our empirical threat model (serious effect)All supertruth files available to international SBAS community

  • Temporal Threat ModelAlso an Empirical Threat Model Worst Storm Data UsedStorm detector usedLook to See Largest Change With Respect to Planar EstimateOverbound of Worst Rate of Change Ever ObservedCorrelation with Spatial GradientErrors are currently double counted

  • No Storm Detector> 3 m / min> 6 m /2 min> 7 m over 5 minWith Storm Detector< 0.5 m / min< 1.25 m /2 min< 2.5 m over 10 minTemporal Threat Model

  • Post IOC StormsOctober 29-31 and November 20, 2003 Were Some of the Worst Storms Ever ObservedConservatism in GIVE Calculation Protected UsersNo HMI observed at any locationNone even close

    However Worse Than PredictedStill much uncertainty in ionosphere

  • ConclusionsFAA Certification Required All Users Bounded Under All ConditionsIonospheric deviations are deterministicIonospheric deviations are observableThreat Models Essential for Limiting Ionospheric BehaviorEach Monitor Must Account for the Limits of Its ObservabilityApproach is Very ConservativeWe are still learning!


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