Ming Luo, Sam Pullen, Seebany Datta-Barua,

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

Presentation Outline LAAS Ionosphere Anomaly Threat Model
Ionosphere Anomaly Data Analysis 20 Nov data in MI/OH (summary) 31 Oct data in Florida Potential Impact on LAAS “Worst-case” threat model assessment “End-around check” data-replay assessment Conclusions and Ongoing Work 16 September 2005

LAAS Model of Iono. Spatial Anomaly
Iono Front An illustration of the impact on LAAS users Front Speed Airplane Speed 70 m/s 45 km LGF Slope Front Speed Max Iono delay Width Nominal Iono Simplified model: a wave front ramp defined by the “slope” and the “width”. This slide shows a simplified model of an ionosphere spatial gradient anomaly. The gradient itself is modeled as a linear change in vertical ionosphere delay between “high” and “low” delay zones. The upper left shows the baseline model identified from the worst-case (sharpest gradient) point in the WAAS data shown previously, where the amplitude of the wave (high-to-low vertical delay difference) is 6 m, the width of the gradient is 19 km, and the wave moves forward at 110 km/s. Given that this wave sweeps over a LAAS-equipped airport, the worst case from the aircraft’s point of view is a wave front that approaches from directly behind an aircraft on approach, overtakes the ionospheric pierce point of an aircraft before the aircraft reaches its decision height (note that a typical jet aircraft final approach speed is about 70 m/s, which is slower than the wave front). After the wave front overtakes the aircraft, a differential range error builds up as a function of the rate of overtaking (in this case, 110 – 70 = 40 m/s) and the slope of the gradient (316 mm/km). Before the wave front reaches the corresponding LGF pierce point, there is no way for the LGF to observe (and detect and exclude) the onset of the anomaly . The worst-case timing is that which leads to the maximum differential error (often this means the time immediately before LGF detection and exclusion) at the moment when the aircraft reaches the decision height (where the tightest VAL applies). Note that this worst-case event and timing, if it ever were to occur, would only affect one aircraft. Other aircraft on the same approach would be spread out such that the wave front passage would create no hazard for them (VAL far from the decision height is much higher than the error that could result from this anomaly). Moving wave front scenario: 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 16 September 2005

Iono. Anomaly from JPL IGS/CORS Data (20 Nov. 2003; 20:15 – 21:00 UT)
16 September 2005

Subset of OH/MI Stations that Saw Similar Ionosphere Behavior on 11/20/2003
5 10 15 20 25 30 35 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 50 100 150 200 250 300 350 16 September 2005 WAAS Time (minutes from 5:00 PM to 11:59 PM)

Iono. Anomaly from JPL IGS/CORS Data: 10/31/03 01:00 ─ 02:40 UT
16 September 2005

16 September 2005

CORS Stations in Florida and SE Region
16 September 2005

L1 Code-minus-Carrier Data
Slant Delay Observed at GNVL (Gainesville) and PLTK (Palatka), FL: PRN 29, 31 Oct. 2003 L1 Code-minus-Carrier Data 25 GNVL 20 PLTK 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 Slant Iono. Delay (m) 5 -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 16 September 2005

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 -2 -4 2 3 4 5 6 7 8 9 Hours Past Midnight UT on 31 Oct. 2003 16 September 2005

Post-Processed L1 – L2 Data
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 16 September 2005

L1 Code-minus-Carrier Data (PLTK and JXVL are ~ 75 km apart)
Slant Delay Observed at PLTK and JXVL (Jacksonville), FL: PRN 10, 31 Oct. 2003 L1 Code-minus-Carrier Data 20 JXVL PLTK Difference 15 Using JXVL instead of GNVL shows very similar “slow-moving” event on PRN 10 (PLTK and JXVL are ~ 75 km apart) 10 Slant Iono. Delay (m) 5 -5 2 3 4 5 6 7 8 9 Hours Past Midnight UT on 31 Oct. 2003 16 September 2005

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 -2 -4 -6 2 3 4 5 6 7 8 Hours Past Midnight UT on 31 Oct. 2003 16 September 2005

LAAS Threat Model Parameter Bounds approved in Sept. 2004
Elevation Speed Width Slope Max Error Low elevation < 12 0 – 1000 m/s 25 – 200 km 30 – 150 mm/km 25 m High elevation ≥ 12 70 – 1000 30 – 500 0 – 70 30 – 250 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 September 2005

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 16 September 2005

With one satellite out:
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 = ) 16 September 2005

(7/18/05 alm, all one-SV-out cases)
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. 16 September 2005

Differential Slant Delay Observed between PLTK and GNVL, FL: All Satellites, 31 Oct. 2003
15 PRN 29 (low-elevation; fast-moving 210 mm/km) PRN 4 PRN 5 PRN 6 PRN 10 PRN 17 PRN 24 PRN 28 PRN 29 10 PRN 10 (high-elevation; slow-moving 100 mm/km) 5 Differential Slant Iono. Delay (m) -5 Recall that GNVL and PLTK are ~ 60 km apart -10 -15 2 4 6 8 10 Hours Past Midnight UT on 31 Oct. 2003 16 September 2005

Range and Position Error between PLTK (“user”) and GNVL (“LGF”), FL: 31 Oct. 2003
2 4 6 8 10 12 -15 -10 -5 5 15 Differential Error (m) PRN 4 PRN 5 PRN 6 PRN 10 PRN 17 PRN 24 PRN 28 PRN 29 Vertical Position Error Hours Past Midnight UT on 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 16 September 2005

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) 16 September 2005

Backup Slides follow…

Iono. Anomaly from JPL IGS/CORS Data (29 Oct. 2003; 20:00 – 20:45 UT)
16 September 2005

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) 16 September 2005

11/20/2003 Ionosphere Storm as Seen From OH/MI CORS Cluster (for SVN 38)
Slant Iono Delay (m) Time (hours, UTC) 16 September 2005

CORS Stations in Ohio/Michigan Region
MTVR PIT1 METR UNIV PARY LANS TIFF UPTC PWEL MPLE SUP3 SUP2 WOOS CLRE BRIG GALP GUST COLB GALB PAPT PTIR UVFM KNTN AVCA HRUF BAYR SAG1 PCK1 BFNY WLCI LEBA SIDN SOWR ERLA HBCH YOU2 YOU1 CASS GRTN SIBY STKR ADRI LSBN MCON DEFI FREO PKTN FRTG GRAR NOR3 NOR2 NOR1 HRN1 DET2 GARF TLDO IUCO OKEE VAST MIO1 LOU1 16 24 27 37 40 57 62 70 74 79 83 84 88 89 91 92 95 97 105 106 119 120 122 124 128 132 134 149 150 151 171 175 46 45 302 301 356 Group A 303 44 Group C 186 196 Group D 192 193 43 292 285 345 177 316 Group B 42 236 217 337 330 Group E 261 41 307 248 265 40 249 234 340 39 275 178 213 Group F 347 38 375 -87 -86 -85 -84 -83 -82 -81 -80 -79 16 September 2005

Further analysis placed doubt on low-speed results
Histograms of Velocity Normal to Front (Vn) Based on Three-Station Trigonometric Fit At Sharp Falloff In “Valley” Section -100 100 200 300 400 500 600 5 10 15 20 25 No. of Occurrences 150 200 250 300 350 400 450 1 2 3 4 5 6 7 8 Further analysis placed doubt on low-speed results Normal Velocity Vn (m/s) Normal Velocity Vn (m/s) 16 September 2005

Satellites In View on 31 October 2003 for GNVL (Gainesville), Florida
16 September 2005

Slant Delay Observed at PLTK and JXVL, FL: PRN 24, 31 Oct. 2003
L1 Code-minus-Carrier Data 18 JXVL 16 PLTK 14 Difference 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 -2 -4 1 2 3 4 5 6 7 8 9 Hours Past Midnight UT on 31 Oct. 2003 16 September 2005

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 16 September 2005

Slant Delay Observed at PNCY and MRKB, FL: PRN 10, 31 Oct. 2003
16 September 2005

Slant Delay Observed at PNCY and MRKB, FL: PRN 29, 31 Oct. 2003
16 September 2005

Slant Delay Observed at DFNK and TALH, FL: PRN 5, 31 Oct. 2003
16 September 2005

16 September 2005

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… 16 September 2005

Impact of Florida Anomaly on WAAS
To be filled in if needed… 16 September 2005

Updated Candidate Threat Model (proposed by FAA in Aug. 2005)
Elevation Speed Width Slope Max Error Low elevation < 12 100 – 1000 m/s 25 – 200 km 30 – 150 mm/km 50 m High elevation ≥ 12 30 – 500 Max error and slope are in the slant direction Slow-speed possibility is removed 16 September 2005

Sam’s Proposed Threat Model as of Today…
Elevation Speed Width Slope Max Error Low elevation < 12 0 – 1000 m/s 25 – 200 km 30 – 150 mm/km 50 m High elevation ≥ 12 100 – 1000 30 – 500 0 – 100 25 – 200 km Max error and slope are in the slant direction 16 September 2005

Time-to-detect vs. Ionospheric Rate (From Stanford IMT)
Airborne Monitoring: Assume only GMA Code Carrier Divergence with time constant of 200s Iono rate ≥ m/s: 5 s Iono rate = m/s: 200 – (rate – 0.01) × 8 × 103 s Iono rate < 0.01 m/s: No detection There are multiple monitors in the current Stanford Integrity Monitor Testbed (IMT) that can detect abnormal ionosphere event. These include the MQM (carrier phase) Step test, carrier-smoothing innovation test, and code-carrier divergence test. (Please see the recent AIAA paper by Gang Xie, et.al., for more details.) Since each monitor was designed to target different potential failure modes in LGF measurements, their times-to-detect vary with apparent iono. delay rate-of-change as well as elevation angle. Based on recent failure testing conducted at Stanford University, we have found that MQM is the fastest when the iono. rate is above a certain level (> 0.02 m/s for a high-elevation-angle satellite), and the CUSUM code-carrier divergence method is the best when the iono. rate is lower than this but still anomalous (< 0.02 m/s and > 0.01 m/s). For this analysis, it is assumed that no monitor detects iono. events with apparent iono. delay rates-of-change at the LGF lower than 0.01 m/s (this is likely required to meet the LGF continuity sub-allocation during ionosphere storms). The overall time-to-detect by the LGF is shown as the blue line in this plot. Note that test results may strongly associated with factors unique to the Stanford IMT such as siting, antenna type, etc. The value used here may need to be adjusted to suit a more generic LAAS site. The goal of on-aircraft ionosphere divergence monitoring is to detect anomalous ionosphere gradients soon after the aircraft itself is affected (and before the anomaly can be observed by the LGF). However, for this option, only the code-carrier divergence Geometric Moving Averaging (GMA) Method (the more traditional code-carrier divergence monitor) is considered (we expect that the CUSUM approach is too cumbersome to be implemented in avionics unless absolutely necessary). As shown as the red line in the this plot, it typically it takes much longer time for the GMA monitor in the Stanford IMT to detect the same iono. event compared to the MQM or divergence CUSUM methods. Note that the time constant for the GMA monitor in the IMT is 200 seconds. Because airborne multipath generally has shorter time correlations, we expect to be able to reduce this time constant in the airborne application in order to speed up the detection. Investigation of this is underway, but for now, we believe that the red line is conservative. References: details of the IMT monitor algorithms represented in this plot are in: [1] G. Xie, et.al., "Integrity Design and Updated Test Results for the Stanford LAAS Integrity Monitor Testbed (IMT)," Proceedings of ION 2001 Annual Meeting. Albuquerque, NM, June 11-13, 2001, pp Internet URL: < [2] M. Luo, S. Pullen, et.al., “Assessment of Ionospheric Impact on LAAS Using WAAS Supertruth Data,” Proceedings of the ION 58th Annual Meeting, Albuquerque, NM, June 24-26, 2002, pp Internet URL: < [3] S. Pullen, M. Luo, et.al., “LAAS Ionosphere Spatial Gradient Threat and Role of Airborne Monitoring,” RTCA SC-159 WG-4 Meeting, Washington, D.C., January 16, 2003. [4] G. Xie, S. Pullen, et.al., “Detecting Ionosphere Gradients with the Cumulative Sum (CUSUM) Method,” Proceedings of the 21st International Communications Satellite Systems Conference (ICSSC). Yokohama, Japan, April 15-19, 2003, Paper AIAA Internet URL: < (requires subscription access to AIAA online electronic publications database) LGF Monitoring: When Iono rate ≥ 0.02 m/s: MQM Ramp detects <= 5 seconds When ≤ Iono rate < 0.02 m/s: CUSUM detects first. When Iono rate < 0.01 m/s: No LGF detection 16 September 2005

Max Error at Memphis, Stationary Iono
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. 16 September 2005

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 = For all-SV-healthy, same VPLH0 limit gives availability loss of 24/145 = 16 September 2005

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 = 16 September 2005

Ming Luo, Sam Pullen, Seebany Datta-Barua,

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