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LAAS Study of Slow-Moving Ionosphere Anomalies and Their Potential Impacts

This presentation discusses the study of slow-moving ionosphere anomalies and their potential impacts on the LAAS system. It includes an analysis of data from various locations and the assessment of worst-case threat models.

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LAAS Study of Slow-Moving Ionosphere Anomalies and Their Potential Impacts

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  1. LAAS Study of Slow-Moving Ionosphere Anomalies and Their Potential Impacts Ming Luo, Sam Pullen, Seebany Datta-Barua, Godwin Zhang, Todd Walter, and Per Enge Stanford University (with funding from FAA SatNav LAAS Program Office, AND-710) ION GNSS 2005 Long Beach, CA. Session E5 16 September 2005

  2. Presentation Outline • LAAS Ionosphere Anomaly Threat Model • Ionosphere Anomaly Data Analysis • 20 Nov. 2003 data in MI/OH (summary) • 31 Oct. 2003 data in Florida • Potential Impact on LAAS • “Worst-case” threat model assessment • “End-around check” data-replay assessment • Conclusions and Ongoing Work

  3. An illustration of the impact on LAAS users Front Speed Airplane Speed 70 m/s LGF 45 km LAAS Model of Iono. Spatial Anomaly Iono Front Slope Front Speed Max Iono delay Nominal Iono Width Simplified model: a wave front ramp defined by the “slope” and the “width”. • Moving wave frontscenario: • Iono wave front moves in the same direction as the airplane does and “catches” the airplane from behind before reaching the LGF • Stationary front scenario: • Ionospheric wave front stops moving before reaching the LGF

  4. Iono. Anomaly from JPL IGS/CORS Data (20 Nov. 2003; 20:15 – 21:00 UT)

  5. 35 30 25 20 15 10 5 0 0 50 100 150 200 250 300 350 Subset of OH/MI Stations that Saw Similar Ionosphere Behavior on 11/20/2003 Stations from Groups B and D Initial upward growth  analysis continues… Sharp falling edge; slant gradients  300 mm/km from previous work Slant Iono Delay (m) Slant Iono Delay (m) Weaker “valley” with smaller (but still anomalous) gradients WAAS Time (minutes from 5:00 PM to 11:59 PM)

  6. Iono. Anomaly from JPL IGS/CORS Data: 10/31/03 01:00 ─ 02:40 UT

  7. Iono. Anomaly from JPL IGS/CORS Data: 10/31/03 03:00 ─ 04:40 UT

  8. Iono. Anomaly from JPL IGS/CORS Data: 10/31/03 05:00 ─ 06:40 UT

  9. CORS Stations in Florida and SE Region

  10. Slant Delay Observed at GNVL (Gainesville) and PLTK (Palatka), FL: PRN 29, 31 Oct. 2003 L1 Code-minus-Carrier Data 25 GNVL PLTK 20 Difference • GNVL and PLTK are ~ 60 km apart. • PRN 29 is at 15-20 • Estimated Slant Slope: 210 mm/km • Speed between the two stations appears ~ 200 m/s 15 10 5 Slant Iono. Delay (m) 0 -5 -10 -15 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 Hours Past Midnight UT on 31 Oct. 2003

  11. Slant Delay Observed at GNVL and PLTK, FL: PRN 10 (SVN 40), 31 Oct. 2003 L1 Code-minus-Carrier Data 14 GNVL 12 PLTK • GNVL (Gainesville) and PLTK (Palatka) are ~ 60 km apart. • PRN 10 is at 70-80 • Estimated Slope: • 100 mm/km • Appears to be slow-moving: • ~ 60 m/s between these two stations. Difference 10 8 6 Slant Iono. Delay (m) 4 2 0 -2 -4 2 3 4 5 6 7 8 9 Hours Past Midnight UT on 31 Oct. 2003

  12. Slant Delay Observed at GNVL and PLTK, FL: PRN 10 (SVN 40), 31 Oct. 2003 Post-Processed L1 – L2 Data PLTK satellite 40 11 GNVL satellite 40 difference between two IPPs 10 • Plot of same event using L1– L2 data is very similar to L1 code-minus- carrier result • Data gaps are due to (semi- codeless) L2 loss-of-lock 9 8 7 Slant Iono. Delay (m) 6 5 4 3 2 1 3 4 5 6 7 8 Hours Past Midnight UT on 31 Oct. 2003

  13. Slant Delay Observed at PLTK and JXVL (Jacksonville), FL: PRN 10, 31 Oct. 2003 L1 Code-minus-Carrier Data 20 JXVL PLTK Difference Using JXVL instead of GNVL shows very similar “slow-moving” event on PRN 10 (PLTK and JXVL are ~ 75 km apart) 15 10 Slant Iono. Delay (m) 5 0 -5 2 3 4 5 6 7 8 9 Hours Past Midnight UT on 31 Oct. 2003

  14. Slant Delay Observed at NAPL (Naples) and MTNT (West of Miami), FL: PRN 10, 31 Oct. 2003 L1 Code-minus-Carrier Data 12 NAPL 10 MTNT Difference 8 For PRN 10, a slow-moving pattern similar to that seen from NE Florida is also observed in SW Florida (~ 400 km away) 6 4 Slant Iono. Delay (m) 2 0 -2 -4 -6 2 3 4 5 6 7 8 Hours Past Midnight UT on 31 Oct. 2003

  15. LAAS Threat Model Parameter Bounds approved in Sept. 2004 • Max error and slope are in the vertical (zenith) direction • Two changes proposed based on more-recent data analysis: • Interpret numbers in slant direction (change max. slope const. to ~ 50 m) -> still bounds all verifiable observed events • Restrict max. slope of slow-moving events to 200 mm/km or less

  16. Availability Assessment for Stationary Fronts at Memphis PSP Site (7/18/05 almanac, all SV healthy) • No geometry (among 145) has vertical error greater than 10 m. • The maximum VPLH0 among these geometries is about 5 m. • Note that max. error exceeds VPLH0 for all geometries VPLH0

  17. Availability Assessment for Stationary Fronts at Memphis PSP Site (7/18/05 alm, all one-SV-out cases) • With one satellite out: • Maximum vertical error is 22 m • 46 geometries (out of 4060) have errors > 10 m • (46/4060 = 0.0113)

  18. Availability Assessment for Stationary Fronts at Memphis with Slant Slope Limit ≤ 200 mm/km (7/18/05 alm, all one-SV-out cases) Even with one SV out, no geometry (among 4060) has vertical error greater than 10 m.

  19. PRN 4 PRN 5 PRN 6 PRN 10 PRN 17 PRN 24 PRN 28 PRN 29 Differential Slant Delay Observed between PLTK and GNVL, FL: All Satellites, 31 Oct. 2003 15 PRN 29 (low-elevation; fast-moving iono. @ 210 mm/km) 10 PRN 10 (high-elevation; slow-moving iono. @ 100 mm/km) 5 0 Differential Slant Iono. Delay (m) -5 Recall that GNVL and PLTK are ~ 60 km apart -10 -15 0 2 4 6 8 10 Hours Past Midnight UT on 31 Oct. 2003

  20. 15 PRN 4 PRN 5 PRN 6 10 PRN 10 PRN 17 PRN 24 5 PRN 28 PRN 29 0 Differential Error (m) -5 Vertical Position Error -10 -15 0 2 4 6 8 10 12 Hours Past Midnight UT on 31 Oct. 2003 Range and Position Error between PLTK (“user”) and GNVL (“LGF”), FL: 31 Oct. 2003 • Range errors from all satellites are included • Based on actual GPS constellation on Oct 31 of 03 • Max vertical error is about 6 m at about 04:30 UT • If scaled down to typical ≤ 5-km phys. separation between user and LGF, diff. error would be significantly smaller

  21. Conclusions and Ongoing Work • Iono. anomaly data analysis has turned up at least one verifiable slow-speed event in Florida • ~ 60 m/s event on high-elevation PRN 10 is confirmed by multiple reference stations spread around Florida • OH/MI gradients are more severe, but all verified points analyzed to date are moving faster than 140 m/s • Data analysis continues to support “finalized” threat model • Slow-speed events should remain in GBAS threat model, but reduction of max. gradient is advisable • Impact on LAAS availability (of integrity) is not severe if maximum slow-speed slant slope is ≤ 200 mm/km • “End-around-check” replay of Florida data shows that worst-case position error is well below 10-meter VAL (and may be “boundable” by inflated VPLH0)

  22. Backup Slides follow…

  23. Iono. Anomaly from JPL IGS/CORS Data (29 Oct. 2003; 20:00 – 20:45 UT)

  24. Iono. Anomaly: Threats to WAAS • WAAS corrections are based on planar fits to measured iono. delays • Thus, threats include: • deviations from linearity (mitigated by chi-square “storm detector”) • bubbles of enhanced or depleted iono. delay that fall inside WAAS iono. pierce points (mitigated by “undersampled” threat model)

  25. 11/20/2003 Ionosphere Storm as Seen From OH/MI CORS Cluster (for SVN 38) Slant Iono Delay (m) Time (hours, UTC)

  26. 119 83 SUP3 PTIR 149 PCK1 84 SUP2 40 PARY NOR3 NOR2 MIO1 NOR1 SAG1 HBCH 79 134 MPLE CASS PWEL 132 BAYR 74 FRTG AVCA YOU2 124 GRAR YOU1 150 OKEE 27 57 METR BFNY LANS CLRE 91 89 BRIG 128 37 UNIV SOWR DET2 HRUF ADRI SIBY 70 UPTC 95 GUST GARF TLDO DEFI 62 TIFF 151 88 WLCI WOOS LSBN 122 HRN1 KNTN 24 106 PIT1 16 MTVR 175 SIDN FREO PAPT 97 COLB MCON 171 LEBA GALB STKR 105 IUCO PKTN 92 GRTN GALP ERLA VAST LOU1 120 UVFM CORS Stations in Ohio/Michigan Region 46 301 302 45 356 Group A 303 44 186 Group C 196 Group D 193 192 285 292 43 345 177 Group B 316 42 217 236 337 330 Group E 261 307 41 248 265 40 249 234 340 275 39 213 178 Group F 347 375 38 -87 -86 -85 -84 -83 -82 -81 -80 -79

  27. 25 8 7 20 6 No. of Occurrences 5 15 4 10 3 2 5 1 0 150 200 250 300 350 400 450 0 -100 0 100 200 300 400 500 600 Histograms of Velocity Normal to Front (Vn) Based on Three-Station Trigonometric Fit At Sharp Falloff In “Valley” Section Further analysis placed doubt on low-speed results Normal Velocity Vn (m/s) Normal Velocity Vn (m/s)

  28. Satellites In View on 31 October 2003 for GNVL (Gainesville), Florida

  29. Slant Delay Observed at PLTK and JXVL, FL: PRN 24, 31 Oct. 2003 L1 Code-minus-Carrier Data 18 JXVL 16 PLTK Difference 14 While gradient is smaller (~ 30 mm/km), note persistence of gradient over 2.5-hour period 12 10 8 Slant Iono. Delay (m) 6 4 2 0 -2 -4 0 1 2 3 4 5 6 7 8 9 Hours Past Midnight UT on 31 Oct. 2003

  30. Slant Delay Observed at NAPL and MTNT, FL:PRN 29, 31 October 31 2003 For PRN 29, a faster-moving pattern similar to that seen from NE Florida is also observed in SW Florida (~ 450 km away), but MTNT data jump makes precise analysis difficult

  31. Slant Delay Observed at PNCY and MRKB, FL: PRN 10, 31 Oct. 2003

  32. Slant Delay Observed at PNCY and MRKB, FL: PRN 29, 31 Oct. 2003

  33. Slant Delay Observed at DFNK and TALH, FL: PRN 5, 31 Oct. 2003

  34. Slant Delay Observed at DFNK and TALH, FL: PRN 10, 31 Oct. 2003

  35. Slant Delay Observed at DFNK and TALH, FL: PRN 24, 31 Oct. 2003

  36. Slant Delay Observed at DFNK and TALH, FL: PRN 29, 31 Oct. 2003

  37. Slant Delay Observed at DFNK and TALH, FL: PRN 28, 31 Oct. 2003

  38. Florida Data Analysis Summary • CORS data from Florida region on 10/31/03 (UT) provides the clearest example of slow-moving iono. fronts seen thus far • Event on high-elevation PRN 10 is confirmed by multiple reference stations spread around the state • L1-L2 results look very similar to L1 code-minus-carrier • Slopes of slow-moving events studied to date are as large as ~ 100 mm/km (slant) • Fast-moving events show possible larger gradients • Fortunately, gradients of this size are unlikely to be hazardous to LAAS • LAAS threat simulation results to come…

  39. Impact of Florida Anomaly on WAAS • To be filled in if needed…

  40. Updated Candidate Threat Model (proposed by FAA in Aug. 2005) Max error and slope are in the slant direction Slow-speed possibility is removed

  41. Sam’s Proposed Threat Model as of Today… Max error and slope are in the slant direction

  42. Time-to-detect vs. Ionospheric Rate (From Stanford IMT) • Airborne Monitoring: • Assume only GMA Code Carrier Divergence with time constant of 200s • Iono rate ≥ 0.035 m/s: 5 s • Iono rate = 0.01 - 0.035 m/s: 200 – (rate – 0.01) × 8 ×103 s • Iono rate < 0.01 m/s: No detection • LGF Monitoring: • When Iono rate ≥ 0.02 m/s: MQM Ramp detects <= 5 seconds • When 0.01 ≤ Iono rate < 0.02 m/s: CUSUM detects first. • When Iono rate < 0.01 m/s: No LGF detection

  43. Max Error at Memphis, Stationary Iono. Front (All SVs Healthy, LGF Monitoring, 7/18/05) • Sensitive to slope but not to width. • The maximum error is 9.9 m.

  44. Geometry Screening via Reduced VAL for Memphis PSP Site (7/18/05 almanac, all one-SV-out cases) • VPLH0 limit (to eliminate all errors > 10 m)  3.13 • Resulting availability loss: 945/4060 = 0.2328 • For all-SV-healthy, same VPLH0 limit gives availability loss of 24/145 = 0.1665

  45. Geometry Screening via Reduced VAL for Memphis PSP Site (7/18/05 almanac, all-SV-healthy case) Although no error exceeds 10 m for all-SV-healthy case; since VPLH0 must be  3.13 to protect all 1-SV-out scenarios, the resulting availability loss for all-SV-healthy case becomes 24/145 = 0.1665

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