laas study of slow-moving ionosphere anomalies and their potential impacts

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

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

16 September 2005 3

Max Iono delay

Front Speed

Nominal IonoWidth

LAAS Model of Iono. Spatial Anomaly

Iono Front

• 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

Simplified model: a wave front ramp defined by the “slope” and the “width”.

Slope

An illustration of the impact on LAAS users

Front Speed

Airplane Speed 70 m/s

45 km

LGF

16 September 2005 4

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

16 September 2005 5

Subset of OH/MI Stations that Saw Similar Ionosphere Behavior on 11/20/2003

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)

Sla

nt Io

no D

elay

(m

)S

lant

Ion

o D

elay

(m

)

Sharp falling edge; slant gradients 300 mm/km from

previous work

Weaker “valley” with smaller (but still anomalous)

gradients

Initial upward growth analysis

continues…

Stations from Groups B and D

16 September 2005 6

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

16 September 2005 7


16 September 2005 8


16 September 2005 9

CORS Stations in Florida and SE Region

16 September 2005 10

Slant Delay Observed at GNVL (Gainesville) and PLTK (Palatka), FL: PRN 29, 31 Oct. 2003

• 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

4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6-15

-10

-5

0

5

10

15

20

25GNVL

PLTKDifference

Hours Past Midnight UT on 31 Oct. 2003

Sla

nt I

ono.

Del

ay (

m)

L1 Code-minus-Carrier Data


Slant Delay Observed at GNVL and PLTK, FL: PRN 10 (SVN 40), 31 Oct. 2003

• 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.

2 3 4 5 6 7 8 9-4

-2

0

2

4

6

8

10

12

14

GNVL

PLTKDifference


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m)



Slant Delay Observed at GNVL and PLTK, FL: PRN 10 (SVN 40), 31 Oct. 2003

• 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


Sla

nt I

ono.

Del

ay (

m)

Post-Processed L1 – L2 Data

3 4 5 6 7 8

1

2

3

4

5

6

7

8

9

10

11PLTK satellite 40

GNVL satellite 40difference between two IPPs


Slant Delay Observed at PLTK and JXVL (Jacksonville), FL: PRN 10, 31 Oct. 2003

Using JXVL instead of GNVL

shows very similar “slow-moving” event

on PRN 10

(PLTK and JXVL are ~ 75 km

apart)

2 3 4 5 6 7 8 9

-5

0

5

10

15

20


Sla

nt I

ono.

Del

ay (

m)

JXVL

PLTK

Difference



Slant Delay Observed at NAPL (Naples) and MTNT (West of Miami), FL: PRN 10, 31 Oct. 2003

For PRN 10, a slow-moving pattern

similar to that seen from NE Florida is also observed in SW Florida (~ 400

km away)

2 3 4 5 6 7 8-6

-4

-2

0

2

4

6

8

10

12

NAPL

MTNT

Difference


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ay (

m)



LAAS Threat Model Parameter Bounds approved in Sept. 2004

Elevation Speed Width Slope Max Error

Low elevation < 12

0 – 1000m/s

25 – 200km

30 – 150mm/km

25 m

High elevation ≥ 12

70 – 1000 m/s

25 – 200km

30 – 500mm/km

25 m

0 – 70m/s

25 – 200km

30 – 250mm/km

25 m

• 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


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 geometriesVPLH0


With one satellite out:

• Maximum vertical error is 22 m

• 46 geometries (out of 4060) have errors > 10 m

• (46/4060 = 0.0113)

Availability Assessment for Stationary Fronts at Memphis PSP Site (7/18/05 alm, all one-SV-out cases)


Availability Assessment for Stationary Fronts at Memphis with Slant Slope Limit ≤ 200 mm/km

Even with one SV out, no geometry (among 4060) has

vertical error greater than 10 m.

(7/18/05 alm, all one-SV-out cases)


Differential Slant Delay Observed between PLTK and GNVL, FL: All Satellites, 31 Oct. 2003

0 2 4 6 8 10-15

-10

-5

0

5

10

15D

iffe

rent

ial S

lant

Ion

o. D

elay

(m

)

PRN 4

PRN 5

PRN 6

PRN 10

PRN 17

PRN 24

PRN 28

PRN 29


Recall that GNVL and PLTK are ~ 60 km apart

PRN 29 (low-elevation; fast-moving iono. @

210 mm/km)

PRN 10 (high-elevation; slow-moving iono. @

100 mm/km)


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 smaller0 2 4 6 8 10 12

-15

-10

-5

0

5

10

15

Dif

fere

ntia

l Err

or (

m)

PRN 4

PRN 5

PRN 6

PRN 10

PRN 17

PRN 24

PRN 28

PRN 29

Vertical Position Error



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)

Backup Slides follow…


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


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)


11/20/2003 Ionosphere Storm as Seen From OH/MI CORS Cluster (for SVN 38)

17:40 18:30 19:20 20:10 21:00 21:50 22:30 23:300

5

10

15

20

25

30

35All Stations in OH-MI area, SV 38, 11/20/03, 17:00 - 24:00; Figure filename: sv38_17-24.fig

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(m)

Time (hours, UTC)


CORS Stations in Ohio/Michigan Region

Group A

Group B

Group D

Group C

Group E

Group F

-87 -86 -85 -84 -83 -82 -81 -80 -79

38

39

40

41

42

43

44

45

46

MTVRPIT1

METR

UNIV

PARY

LANS

TIFF

UPTC

PWELMPLE

SUP3

SUP2

WOOS

CLREBRIG

GALP

GUST

COLB

GALB

PAPT

PTIR

UVFM

KNTN

AVCA

HRUF

BAYR

SAG1

PCK1

BFNY

WLCI

LEBA

SIDN

SOWR

ERLA

HBCH

YOU2YOU1

CASS

GRTN

SIBY

STKR

ADRI

LSBN

MCON

DEFI

FREO

PKTN

FRTGGRAR

NOR3NOR2

NOR1

HRN1

DET2

GARFTLDO

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

177

178

186

192193

196

213

217

234

236

248

249

261

265

275

285292

301302

303

307

316

330337

340

345

347

356

375


Histograms of Velocity Normal to Front (Vn) Based on Three-Station Trigonometric Fit

Normal Velocity Vn (m/s)

-100 0 100 200 300 400 500 6000

5

10

15

20

25

No.

of

Occ

urre

nces

At Sharp Falloff In “Valley” Section

Further analysis placed doubt on low-speed results

150 200 250 300 350 400 4500

1

2

3

4

5

6

7

8

Normal Velocity Vn (m/s)


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

x

4 60

30

0

30

210

60

240 120

300

150

330

N

E

S

W

'x' denotes last point in timeelevation is 90 degrees at center

6.0 Hours of Coverage from 10/31/03 0:00:00 for GPS Visibility for GNVL, Florida

x

5

x

6

x

7

x

8

x

9

x

10

x11

x

13

x

17

x

20x21x

24

x

26

x27 x

28x

29

x

30


Slant Delay Observed at PLTK and JXVL, FL: PRN 24, 31 Oct. 2003

While gradient is smaller (~ 30 mm/km), note persistence of gradient over

2.5-hour period

0 1 2 3 4 5 6 7 8 9

-4

-2

0

2

4

6

8

10

12

14

16

18

JXVL

PLTKDifference


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m)



Slant Delay Observed at NAPL and MTNT, FL:PRN 29, 31 October 31 2003

4 5 6 7 8 9 10-10

-5

0

5

10

15

20

Hours in Oct 31, 2003 (hr)

Sla

nt

De

lay

(m

)

Slant Delay Observed at NAPL and MTNT, FL, Oct 31, 03, PRN 29

NAPL

MTNTDifference

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


Slant Delay Observed at PNCY and MRKB, FL:

PRN 10, 31 Oct. 2003

2 3 4 5 6 7 8 9-4

-2

0

2

4

6

8

10


Sla

nt

De

lay

(m

)Slant Delay Observed at PNCY and MRKB, FL, Oct 31, 03, PRN 10

PNCY

MRKBDifference


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

4 5 6 7 8 9 10-4

-2

0

2

4

6

8

10


Sla

nt

De

lay

(m

)

Slant Delay Observed at PNCY and MRKB, FL, Oct 31, 03, PRN 29

PNCY

MRKBDifference


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

1 2 3 4 5 6 7-4

-2

0

2

4

6

8


Sla

nt

De

lay

(m

)

Slant Delay Observed at DFNK and TALH, FL, Oct 31, 03, PRN 5

DFNK

TALHDifference



2 3 4 5 6 7 8 9 10-4

-2

0

2

4

6

8

10

12


Sla

nt

De

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(m

)Slant Delay Observed at DFNK and TALH, FL, Oct 31, 03, PRN 10

DFNK

TALHDifference


Slant Delay Observed at DFNK and TALH, FL:

PRN 24, 31 Oct. 2003

0 1 2 3 4 5 6 7 8 9-4

-2

0

2

4

6

8

10

12


Sla

nt

De

lay

(m

)


DFNK

TALHDifference



4 5 6 7 8 9 10-6

-4

-2

0

2

4

6

8

10

12


Sla

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De

lay

(m

)


DFNK

TALHDifference



0 0.5 1 1.5 2 2.5 3 3.5 4

0

5

10

15

20


Sla

nt

De

lay

(m

)


DFNK

TALHDifference


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…


Impact of Florida Anomaly on WAAS

• To be filled in if needed…


Updated Candidate Threat Model (proposed by FAA in Aug. 2005)

Max error and slope are in the slant direction

Slow-speed possibility is removed


Low elevation < 12

100 – 1000m/s

25 – 200km

30 – 150mm/km

50 m


100 – 1000 m/s

25 – 200km

30 – 500mm/km

50 m


Sam’s Proposed Threat Model as of Today…

Max error and slope are in the slant direction


Low elevation < 12

0 – 1000m/s

25 – 200km

30 – 150mm/km

50 m


100 – 1000 m/s

25 – 200km

30 – 500mm/km

50 m

0 – 100m/s

25 – 200 km

30 – 150mm/km

50 m


0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050

20

40

60

80

100

120

140

160

180

200

Ionospheric Rate (m/s)

Tim

e-to

-det

ect

(s)

Simplified Time-to-detect vs Ionospheric Rate

by LGFby Airborne

GMA Only

MQM

CUSUM

Time-to-detect vs. Ionospheric Rate (From Stanford IMT)

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

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


Max Error at Memphis, Stationary Iono. Front (All SVs Healthy, LGF Monitoring, 7/18/05)

050

100150

200250

0

50

100

150

2000

2

4

6

8

10

Iono Slope (mm/km)

Max Error at Memphis, Stationary Front, LGF Monitoring, July 18,05

Iono Width (km)

Ma

x E

rro

r (m

)

•Sensitive to slope but not to width.

•The maximum error is 9.9 m.


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


2 2.5 3 3.5 4 4.5 52

3

4

5

6

7

8

9

10

VPL-H0 (m)

Ma

x V

ert

ica

l E

rro

rs (

m)

Vertical Error vs VPL-H0, Memphis, 071805, All SVs, Stationary Front, LGF

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

Geometry Screening via Reduced VAL for Memphis PSP Site (7/18/05 almanac, all-SV-healthy case)

laas study of slow-moving ionosphere anomalies and their potential impacts

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