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

Todd Walter

Stanford University

http://waas.stanford.edu

Todd Walter

Stanford University

http://waas.stanford.edu

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Purpose

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

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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 continuityMay also cause increased error due to

interrupted carrier smoothing

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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

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Critical Properties for Single Frequency Use

GBASShort-baseline gradients

Rate of change, velocity, and width of gradientDepletions

SBASDecorrelation on thin shell

How similar are nearby measurements?

Undersampled errorsHow large are features that are undetected?

Temporal ChangesHow fast will a vertical delay change?

Nominal vs. DisturbedHow does performance vary over time?

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Critical Properties for Dual Frequency Use

Fade depth vs. durationTime between fadesRegions of sky that can be

simultaneously affectedCorrelation between L1 and L5

frequenciesEffect on phase tracking loopTimes, locations, and severityEffect on SBAS messages

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GBAS/LAAS Concept

Courtesy: FAA

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Contributors to Local Differential Ionosphere Error

Diff. Iono Range Error  =  gradient slope × min{ (x + 2 vair), gradient width}

70 m/s

5 kmLGF

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

For 5 km ground-to-air separation at CAT I DH: x = 5 km; = 100 sec; vair = 70 m/s

“virtual baseline” at DH = x + 2 vair = 5 + 14 = 19 km

Simplified Ionosphere Wave Front Model:

a ramp defined by constant slope and width

Courtesy:Sam Pullen

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20 November 2003 20:30 UT

Courtesy:SeebanyDatta-Barua

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Ionosphere Delay Gradients 20 Nov. 2003

0 50 100 150 200 250 300 3500

5

10

15

20

25

30

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WAAS Time (minutes from 5:00 PM to 11:59 PM UT)

Sla

nt Io

no D

elay

(m

)

Sharp falling edge; slant

gradients 250 – 400 mm/km

Initial upward growth; slant

gradients 60 – 120 mm/km

“Valleys” with smaller (but anomalous) gradients

Courtesy:Sam Pullen

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•Geostationary Satellites

•Geo Uplink Stations

•Network of Reference Stations

•Master Stations

WAAS Concept

Courtesy: FAA

Courtesy: FAA

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Thin-Shell Model

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Correlation Estimation Process

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Ionospheric Decorrelation About a Planar Fit (1st Order)

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Ionospheric Decorrelation Function (1st Order)

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Equatorial Ionosphere1st Order

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Equatorial Sigma Estimate1st Order

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Sigma Estimate 1st Order (Sliced by Time)

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Failure of Thin Shell Model

Quiet Day Disturbed Day

Courtesy:SeebanyDatta-Barua

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Undersampled Condition

Courtesy:Seebany Datta-Barua

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WAAS Measurements

Courtesy:Seebany Datta-Barua

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Temporal Gradients

200 s

Slide Courtesy Seebany Datta-Barua

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Nominal C/N0 without Scintillation

Ionosphere

Nominal

Carrier to Noise density Ratio (C/N0) C/N0

(dB-Hz)

100 s

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Ionospheric Scintillation

Electron density irregularities

Ionospheric scintillation

25 dB fading

100 s

Carrier to Noise density Ratio (C/N0) C/N0

(dB-Hz)

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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.

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Strong Ionospheric Scintillation

C/N0

(dB-Hz)

18 March 2001Ascension Island

Data from Theodore Beach,AFRL

100 s

7 SVs out of 8(worst 45 min)

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Benefit from a back-up channel

60 s (zoomed-in plot)

Lost L2C, but tracked L1

Loss of L1 & L2CLoss of L2C alone

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Summary

LISN provides an excellent opportunity to better understand important extreme characteristics of the equatorial ionosphereDelay

Gradients, thin-shell decorrelation, small scale features, frequency of occurrence

ScintillationFade depth, duration, time between fades, spatial

correlation, frequency correlation, phase effects, message loss, and patterns of occurrence

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Sigma Estimate 1st Order (Sliced by Time)

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Solar Max Quiet DayJuly 2nd, 2000

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CASE I: Moderate scintillation on 5 March 2011 (UT)

Less than 10 dB fluctuations

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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.

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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.

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CASE II: Strong scintillation on 15 March 2011 (UT)

Our way to indicate no C/N0 output (loss of lock)

More than 15 dB fluctuations

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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 isbetter 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.

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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.

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CASE III: Strong scintillation on 16 March 2011 (UT)

More than 15 dB fluctuations

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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%)

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Previous Studies

- El-Arini et al. (Radio Sci, 2009) observed highly-correlated fadingsbetween L1 and L2. (L1 and L2 military receiver and 20 Hz outputs)

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Previous Studies

- Klobuchar (GPS Blue Book) showed signal intensities of L1 and L2 during scintillation.

- Deep fadings are not highly correlated in this example.

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Ionospheric Decorrelation(0th Order)

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Ionospheric Decorrelation Function (0th Order)

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Estimation of Ionospheric Gradients

Station Pair Method

Mixed Pair Method

Time Step Method

• Long baselines• Free from satellite

IFB calibration error

• Long and short baselines• IFB calibration error on both SV and RR

• Short baselines• Free from IFB calibration error• Corrupted by iono.

temporal gradients

T1 T2

S2S1 S2S1 S1

IPP

Slide Courtesy Jiyun Li

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GBAS: Gradient Threat

Ionosphere

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SBAS: Undersampled Threat

IonosphereEstimated

Ionosphere

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Obliquity Factor

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Ionospheric Threat

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Nominal Day Spatial Gradients Between WAAS Stations

Slide Courtesy Seebany Datta-Barua

Typical Solar Max Value:Below 5 mm/km

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Spatial Gradients Between WAAS Stations During Anomaly

Slide Courtesy Seebany Datta-Barua

Storm Values:> 40 mm/kmup to 360 mm/km

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Disturbed Ionosphere Decorrelation

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Simultaneous Loss of Satellites

Chance of simultaneous loss is strongly dependent on reacquisition time of receiver

20 sec Loss

18 sec

Max of 4 SV Loss

Slide Courtesy Jiwon Seo

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Simultaneous Loss of Satellites

Chance of simultaneous loss is strongly dependent on reacquisition time of receiver

Max of 2 SV Loss

2 sec LossSlide Courtesy Jiwon Seo

18 sec

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Number of Tracked Satellites Simulating 20 sec reacquisition time (WAAS MOPS limit)

Using 45 minutes of severe scintillation data

4 or more: 97.9 %, 5 or more: 92.3 %, 6 or more: 68.1 %

4 or more tracked SVs

5 or more

6 or more

20 secReacquisition Time

65 %

100 %

Time

Percentage

2 sec

Slide Courtesy Jiwon Seo

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Number of Tracked Satellites Simulating 2 sec reacquisition time

4 or more: 100 %, 5 or more: 100 %, 6 or more: 98.3 %

WAAS MOPS limit (20 sec) should be reduced

4 or more tracked SVs

5 or more

6 or more

20 secReacquisition Time

65 %

100 %

Time

Percentage

2 sec

Slide Courtesy Jiwon Seo

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Correlation of Fades between Satellites

45 min

8 SVsin view

* Worst 45 min data from the 9 day campaign at Ascension Island in 2001

300 s

PRN 11

PRN 4

Instance of loss of lock (each blue dot)

15% correlation

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Availability of LPV-200 (parametric study)

Assuming max temporal range error (0.5 m/s)

- High availability for short reacquisition time (< 2 s)

L1/L5 Correlation Coefficient

0 1

Reacquisition Time (s)

0

10< 50%

99.5%

> 50%

> 75%

> 90%

> 95%> 99.9%

Availability of a single user at Ascension Island

Availability Level