The Impact and Mitigation of Ionosphere Anomalies on Ground-Based Augmentation of GNSS
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Transcript of The Impact and Mitigation of Ionosphere Anomalies on Ground-Based Augmentation of GNSS
The Impact and Mitigation of Ionosphere Anomalies on Ground-Based
Augmentation of GNSS
Sam Pullen, Young Shin Park, and Per Enge
Stanford University
12th International Ionospheric Effects Symposium (IES 2008)
Alexandria, Virginia
Session 4A, Paper #6 14 May 2008
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 2
Significance of Ionosphere Spatial Decorrelation in LAAS/GBAS
vig
VPL
VHF Data Broadcast
LAAS Ground Facility
Vertical Protection Level (VPL)
Ionospheric delay
Broadcast Standard Deviation (Sigma) of Vertical Ionosphere Gradient
Vertical Alert Limit (VAL)
VAL
Source: Jiyun Lee, IEEE/ION PLANS 2006
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 3
Severe Ionosphere Gradient Anomaly on 20 November 2003
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 4
Moving Ionosphere Delay “Bubble” in Ohio/Michigan Region on 20 Nov. 2003
0 50 100 150 200 250 300 3500
5
10
15
20
25
30
35
WAAS Time (minutes from 5:00 PM to 11:59 PM UT)
Sla
nt Io
no D
elay
(m
)S
lant
Ion
o D
elay
(m
)
Sharp falling edge; slant gradients 250 – 330 mm/km
Initial upward growth; slant
gradients 60 – 120 mm/km
Data from 7 CORS stations in N. Ohio and S. Michigan
“Valleys” with smaller (but anomalous) gradients
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 5
Validation of High-Elevation Anomaly Reported by FAATC (SVN 38, ZOB1/GARF, 20/11/03)
20.9 20.95 21 21.05 21.1
0
50
100
150
200
250
300
350
400
Time (hour of 11/20/2003)
Iono
Slo
pe (m
m/k
m)
ZOB1 and GARF, Slopes Comparison; SV38
DF Slope
L1 Slope
Maximum slope from L1-only data 412.8 mm/km
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 6
-102 -100 -98 -96 -94 -92 -90 -88 -86 -84 -82 -80
38
40
42
44
46
48
KNTNSIDN
MCON
PKTN
KNTNSIDN
MCON
PKTN
IPP Tracks of (Low-Elevation) GPS SVN 26 (20 Nov. 2003; 20:30 ~ 21:30 UT in OH-MI)
Satellite Direction of Motion
Iono. Frontat 21:00
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 7
20.6 20.7 20.8 20.9 21 21.1 21.2 21.3 21.4 21.50
5
10
15
20
25
30
35
40
45
50Iono Delay for: SV 26; Elevation:10.06890 - 12.0780
Time(Hour of Day); Date:2003 11 20; Filename: sv26
Sla
nt I
ono
Del
ay (
m)
SVN 26 Slant Delays Observed at WOOS, FREO, LSBN, and GARF
FREO
LSBN
WOOS
GARF
• Sufficient similarity between the two sets of ionosphere delays
exists
• Lines-of-Sight from FREO and WOOS are within the bulk of the “enhanced” ionosphere gradient
GUST
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 8
20.8 20.9 21 21.1 21.2 21.3 21.4-50
0
50
100
150
200
250
300
350
400
Hour of 11/23/03
Iono
Slo
pe,
mm
/km
WOOS and GARF Slope comparision
Severe Ionosphere Slope Validated with L1 data WOOS and GARF, SVN 26, 20 Nov. 2003
DF Slope
L1 Slope
• Maximum Validated Slope: ~ 360 mm/km
• This observation window is very close to the time that peak ionosphere gradients were observed on higher-elevation satellites.
Estimated Slope using L1 Code-minus-Carrier Data
L1-L2 slope
L1-only slope
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 9
Ionosphere Anomaly Front Model:Potential Impact on a GBAS User
Simplified Ionosphere Front Model: a ramp defined by constant slope and width
Front Speed 200 m/s
Airplane Speed ~ 70 m/s
(synthetic baseline due to smoothing ~ 14 km)
Front Width 25 km
GBAS Ground Station
Front Slope 425 mm/km
LGF IPP Speed 200 m/s
Stationary Ionosphere Front Scenario: Ionosphere front and IPP of ground station IPP move with same velocity.
Maximum Range Error at DH: 425 mm/km × 20 km = 8.5 meters
Max. ~ 6 km at DH
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 10
Resulting Revised (and Simplified) Ionosphere Anomaly Threat Model for CONUS
(note: plot not precisely to
scale)
5 15 30 45 65 90
SV elevation angle (deg)
Sla
nt
ion
o.
gra
die
nt
bo
un
d (
mm
/km
)
100
200
300
375
425Flat 375 mm/km
Flat 425 mm/kmLinear bound:
ybnd (mm/km) = 375 + 50(el15)/50
Also bounds on:
Front speed wrt. ground: ≤ 750 m/s
Front width: 25 – 200 km
Total differential delay ≤ 50 m
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 11
Semi-random Results for Memphis LGF at 6 km DH
0 5 10 15 20 25 30 35 40 450
0.02
0.04
0.06
0.08
0.1
0.12
0.14
User Vertical Position Error (meters)
PD
F
Worst-case error, or
“MIEV”, is 41 m
Most errors are exactly zero due to, e.g., CCD detection and exclusion before
anomaly affects users, but all zero errors have been removed from the histogram.
RTCA-24 Constellation; All-in-view, all 1-SV-out, and all 2-SV-out subsets included; 2 satellites impacted simultaneously by ionosphere anomaly
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 12
Simplified Flow Chart for Real-Time Inflation
LAAS Ground Facility (LGF) Real-Time Geometry Screening
SV almanac and current
time Subset Geometry Determination
(N2 constraint)
Worst-Case Ionosphere Error
Determination
Ionosphere Anomaly
Threat Model
Airport Approach
Layout and Ops. Limits
Approach Hazard Assessment
Iterative Sigma/P-Value Parameter
Inflation
Do Any Unsafe Subsets Exist?
Yes
Compare MIEV to Ops. Limits for Available
Subset Geometries
No
Inflated pr_gnd, vig, and/or P-
values
Approved Sigmas/P-Values for Broadcast by VDB
References: J. Lee, et al., Proceedings of ION GNSS 2006
S. Ramakrishnan, et al., Proceedings of ION NTM 2008
LGF acts to make potentially unsafe user geometries unavailable.
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MIEV for Memphis at 6 km Prior to Inflation
0 50 100 150 200 250 30010
15
20
25
30
35
40
45
MIE
V (
m)
WC error from histogram on slide 11
OCS Error Limit at DH
Time Index (5-min updates)
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 14
0 50 100 150 200 250 3001
1.5
2
2.5
3
3.5
4
4.5
5
5.5x 10
-4
Infl
ate
d P
-va
lue
s (m
/m)
Time Index (5-min updates)
Inflated P-values for Memphis at 6 km from LGF (PA = 0.17, PB = 0.27 m/km)
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 15
0 50 100 150 200 250 30014
16
18
20
22
24
26
28
30
MIE
V (
m)
Inflated MIEV (m)
OCS Limit (m)
MIEV for Memphis at 6 km after P-Value Inflation
Time Index (5-min updates)
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 16
0 50 100 150 200 250 3002
3
4
5
6
7
8
Time Index (5-min updates)
Pro
tect
ion
Le
vel (
m)
Uninflated VPL H0
Uninflated VPL eph
Inflated VPLeph
Protection Levels for Memphis at 6 km from LGF
Significant margin (> 2.5 meters) relative to 10-meter FASVAL at DH
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 17
Summary
• Ionosphere anomaly threat model for CONUS has been developed based on validated ionosphere storm gradients discovered since April 2000
• Maximum ionosphere-induced errors are caused by worst-case extrapolation of events on 20 Nov. 2003
• Given lack of ground system observability, mitigation strategy is to inflate broadcast parameters to exclude potentially unsafe subset geometries from use– This requires GBAS ground station to vary inflation
parameters in real time.
– Two methods (using inflated pr_gnd and P-values that vary across satellites, as shown here, and using inflated vig values) have been demonstrated that retain near-100% availability for RTCA-24 constellation and typical actual constellations (with no satellite outages).
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Backup Slides
• Backup slides follow…
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 19
Outline
• Nominal Ionosphere Spatial Decorrelation over Short Baselines
• Anomalous ionosphere spatial decorrelation
– Examples from October and November 2003 storms in CONUS
– Worst-case range-domain errors for GBAS users
• Ionosphere Spatial Anomaly “Threat Model”
• Real-Time GBAS/LAAS Threat Mitigation
– Impact of code-carrier divergence (CCD) monitoring
– Simulation of worst-case vertical position errors
– “Geometry Screening” by broadcast parameter inflation
– Resulting impact on CAT I GBAS user availability
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 20
0 20 40 60 80 100 120 140 160 180 2000
1
2
3
4
5
6
IPP Separation Distance (km)
vi
g o
verb
ou
nd
(m
m/k
m)
vig
Estimates using JPL-processed CORS data
07/02/00, Dst: 2
09/11/02, Dst: -78
07/26/04, Dst: -9407/27/04, Dst: -197
11/09/04, Dst: -223
11/10/04, Dst: -289
vig Overbound Results from Station Pair Method
JPL post-processed CORS “truth” data
Insufficient number of samples to obtain reliable statistics
Solid lines
show vig +
|vig|.
Inflation factors are 2.2 ~ 4.1
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 21
Iono. Anomaly Event Verification Methodology
L1 code and carrier
L1 code and carrier, L2 carrier
“Raw” IGS/CORS Data
JPL Ionosphere “Truth” Processing
Ionosphere Delay Estimates
Find Maximum Apparent Gradients
(Station Pair Method)
Screening Process (Automated)
Database of Extreme Gradients Erroneous Receiver
Steps and L1/L2 Biases Removed Investigate
Remaining Points
Remove “Questionable”Observations
Estimate Gradients from L1 code-minus-carrier
Output Database of Maximum Gradients
Compare L1/L2 and L1 CMC Gradient Obs.
“Validated”Max. Gradient
Database
Note: CORS stations in CONUS are typically 30 – 100 km apart.
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 22
LGF
Iono Front
Runway
i (aircraft)
i (LGF)Vi,Proj
j (aircraft)Vj,Proj
j (LGF)
Vfront
Worst-Case Ionosphere Front Scenarios
Generate two sub-cases for each SV pair (i, j) “i worst” and “j worst”
“i worst” => Apply worst range error to SV i and resulting error to SV j
“j worst” => Apply worst range error to SV j and resulting error to SV i
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 23
Proposed Worst-Case Ionosphere Error Limits
Distance from LGF (km)
Wor
st-C
ase
Iono
sphe
re E
rror
Allo
wed
(m
)
6 7 8 9 10 11 12 13 1420
40
60
80
100
120
140
160
99% TSE
95% TSE
99.9% TSE
28-meter constraint
at 6 km
DH location
14 May 2008 Impact and Mitigation of Ionosphere Anomalies on GBAS 24
Notes on Real-Time Ground-System Parameter Inflation
• Real-time parameter inflation is fundamentally providing integrity in the position domain
• The real-time satellite-specific P-value inflation method shown here achieves 100% availability for all-in-view geometry (for RTCA-24 constellation) at 4 of 5 airports tested (MEM, DEN, DFW, MCO)– One exception (DCA) is shown on following slide
– For P-value inflation approach, inflation required for 6-km separation dominates that required for larger separations
• However, since thousands of subset geometries must be checked at every real-time parameter update interval, simpler methods that retain acceptable availability are also of interest