Ionospheric Studies Required to Support GNSS Use by Aviation in Equatorial Areas

1. Ionospheric Studies Required to Support GNSS Use by Aviation in Equatorial Areas Todd Walter Stanford University http://waas.stanford.edu

2. Purpose To identify important ionospheric properties that must be better understood for GNSS use by aviation in equatorial areas

3. Ionospheric Issues • Incorrect ionospheric delay values at the aircraft can create integrity problems if improperly bounded, or availability problems when the bounds become too large • Scintillation may cause the loss of tracking of one or more satellites causing a loss continuity • May also cause increased error due to interrupted carrier smoothing

4. SBAS Ionospheric Working Group (SIWG) • SIWG has produced two white papers • “Ionospheric Research Areas for SBAS” • February 2003 • “Effect of Ionospheric Scintillation on GNSS” • November 2010 • Papers identified equatorial region as most challenging • Also identified need to collect data and better characterize effects

5. Critical Properties for Single Frequency Use • GBAS • Short-baseline gradients • Rate of change, velocity, and width of gradient • Depletions • SBAS • Decorrelation on thin shell • How similar are nearby measurements? • Undersampled errors • How large are features that are undetected? • Temporal Changes • How fast will a vertical delay change? • Nominal vs. Disturbed • How does performance vary over time?

6. Critical Properties for Dual Frequency Use • Fade depth vs. duration • Time between fades • Regions of sky that can be simultaneously affected • Correlation between L1 and L5 frequencies • Effect on phase tracking loop • Times, locations, and severity • Effect on SBAS messages

7. GBAS/LAAS Concept Courtesy: FAA

8. Contributors to Local Differential Ionosphere Error Simplified Ionosphere Wave Front Model: a ramp defined by constant slope and width GPS Satellite Error due to code-carrier divergence experienced by 100-second aircraft carrier-smoothing filter Error due to physical separation of ground and aircraft ionosphere pierce points 70 m/s LGF 5 km Courtesy: Sam Pullen Diff. Iono Range Error  =  gradient slope × min{ (x + 2 tvair), gradient width} For 5 km ground-to-air separation at CAT I DH: x = 5 km; t = 100 sec; vair = 70 m/s => “virtual baseline” at DH = x + 2 tvair = 5 + 14 = 19 km

9. 20 November 2003 20:30 UT Courtesy: Seebany Datta-Barua

10. 35 30 Initial upward growth; slant gradients  60 – 120 mm/km Sharp falling edge; slant gradients  250 – 400 mm/km 25 20 Slant Iono Delay (m) 15 “Valleys” with smaller (but anomalous) gradients 10 5 0 0 50 100 150 200 250 300 350 WAAS Time (minutes from 5:00 PM to 11:59 PM UT) Ionosphere Delay Gradients 20 Nov. 2003 Courtesy: Sam Pullen

11. WAAS Concept Courtesy: FAA Courtesy: FAA • Network of Reference Stations • Master Stations • Geostationary Satellites • Geo Uplink Stations

12. Thin-Shell Model

13. Correlation Estimation Process

14. Ionospheric Decorrelation About a Planar Fit (1st Order)

15. Ionospheric Decorrelation Function (1st Order)

16. Equatorial Ionosphere1st Order

17. Equatorial Sigma Estimate1st Order

18. Sigma Estimate 1st Order (Sliced by Time)

19. Failure of Thin Shell Model Courtesy: Seebany Datta-Barua Quiet Day Disturbed Day

20. Undersampled Condition Courtesy: Seebany Datta-Barua

21. WAAS Measurements Courtesy: Seebany Datta-Barua

22. 200 s Temporal Gradients Slide Courtesy Seebany Datta-Barua

23. Nominal C/N0 without Scintillation Ionosphere Carrier to Noise density Ratio (C/N0) C/N0 (dB-Hz) Nominal 100 s

24. Ionospheric Scintillation Electron density irregularities Ionospheric scintillation Carrier to Noise density Ratio (C/N0) C/N0 (dB-Hz) 25 dB fading 100 s

25. Challenge to Worldwide LPV-200 Challenge to expand LPV-200 service to equatorial area - Strong ionospheric scintillation is frequently observed in the equatorial area during solar maxima.

26. Strong Ionospheric Scintillation 7 SVs out of 8 (worst 45 min) 18 March 2001 Ascension Island Data from Theodore Beach, AFRL C/N0 (dB-Hz) 100 s

27. Benefit from a back-up channel Lost L2C, but tracked L1 Loss of L2C alone Loss of L1 & L2C 60 s (zoomed-in plot)

28. Summary • LISN provides an excellent opportunity to better understand important extreme characteristics of the equatorial ionosphere • Delay • Gradients, thin-shell decorrelation, small scale features, frequency of occurrence • Scintillation • Fade depth, duration, time between fades, spatial correlation, frequency correlation, phase effects, message loss, and patterns of occurrence

29. Sigma Estimate 1st Order (Sliced by Time)

30. Solar Max Quiet Day July 2nd, 2000

31. CASE I: Moderate scintillation on 5 March 2011 (UT) Less than 10 dB fluctuations

32. Histogram of C/N0 difference during scintillation C/N0(L2C) minus C/N0(L1) at the same epoch during scintillation. Usually 2-3 dB difference between L1 and L2c.

33. Percentage of C/N0 difference during scintillation Percentage of (C/N0 difference > Threshold of C/N0 difference) e.g., Only 4.4% of samples have C/N0 difference of 3 dB or more between L1 and L2C at the same epoch during scintillation.

34. CASE II: Strong scintillation on 15 March 2011 (UT) More than 15 dB fluctuations Our way to indicate no C/N0 output (loss of lock)

35. Percentage of C/N0 difference during scintillation 17.9% of samples have C/N0 difference of 3 dB or more between L1 and L2C during strong scintillation, which is better than the moderate scintillation case (4.4%). Under higher fluctuations, C/N0 difference between two frequency at the same epoch tends to be also higher.

36. Receiver response during the 800 s of strong scintillation Although tracking both frequencies can provide benefit under strong scintillation, the actual receiver response showed that both frequencies were lost simultaneously in 94.6% cases, and L2C-only loss was observed in 5.4% cases. There was no case of L1-only loss during the 800 s strong scintillation.

37. CASE III: Strong scintillation on 16 March 2011 (UT) More than 15 dB fluctuations

38. Percentage of C/N0 difference during scintillation 18.8% of samples have C/N0 difference of 3 dB or more between L1 and L2C during this period, which is similar to the case of 15 March 2011 (17.9%)

39. Previous Studies - El-Arini et al. (Radio Sci, 2009) observed highly-correlated fadings between L1 and L2. (L1 and L2 military receiver and 20 Hz outputs)

40. Previous Studies - Klobuchar (GPS Blue Book) showed signal intensities of L1 and L2 during scintillation. - Deep fadings are not highly correlated in this example.

41. Ionospheric Decorrelation(0th Order)

42. Ionospheric Decorrelation Function (0th Order)

43. Estimation of Ionospheric Gradients T1 T2 IPP S1 S2 S1 S1 S2 Slide Courtesy Jiyun Li

44. GBAS: Gradient Threat Ionosphere

45. SBAS: Undersampled Threat Estimated Ionosphere Ionosphere

46. Obliquity Factor

47. Ionospheric Threat

48. Nominal Day Spatial Gradients Between WAAS Stations Typical Solar Max Value: Below 5 mm/km Slide Courtesy Seebany Datta-Barua

49. Spatial Gradients Between WAAS Stations During Anomaly Storm Values: > 40 mm/km up to 360 mm/km Slide Courtesy Seebany Datta-Barua

50. Disturbed Ionosphere Decorrelation