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Ionospheric Studies Required to Support GNSS Use by Aviation in Equatorial Areas. Todd Walter Stanford University http://waas.stanford.edu. Purpose. To identify important ionospheric properties that must be better understood for GNSS use by aviation in equatorial areas. Ionospheric Issues.

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Ionospheric studies required to support gnss use by aviation in equatorial areas

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

Todd Walter

Stanford University

http://waas.stanford.edu


Purpose
Purpose in Equatorial Areas

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


Ionospheric issues
Ionospheric Issues in Equatorial Areas

  • 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


Sbas ionospheric working group siwg
SBAS Ionospheric Working Group (SIWG) in Equatorial Areas

  • 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


Critical properties for single frequency use
Critical Properties for Single Frequency Use in Equatorial Areas

  • 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?


Critical properties for dual frequency use
Critical Properties for Dual Frequency Use in Equatorial Areas

  • 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


Gbas laas concept
GBAS/LAAS Concept in Equatorial Areas

Courtesy: FAA


Contributors to local differential ionosphere error
Contributors to Local Differential Ionosphere Error in Equatorial Areas

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


20 november 2003 20 30 ut
20 November 2003 in Equatorial Areas20:30 UT

Courtesy:

Seebany

Datta-Barua


Ionosphere delay gradients 20 nov 2003

35 in Equatorial Areas

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


Waas concept
WAAS Concept in Equatorial Areas

Courtesy: FAA

Courtesy: FAA

  • Network of Reference Stations

  • Master Stations

  • Geostationary Satellites

  • Geo Uplink Stations


Thin shell model
Thin-Shell Model in Equatorial Areas


Correlation estimation process
Correlation Estimation Process in Equatorial Areas



Ionospheric decorrelation function 1 st order
Ionospheric Decorrelation Function (1 in Equatorial Areasst Order)


Equatorial ionosphere 1 st order
Equatorial Ionosphere in Equatorial Areas1st Order


Equatorial sigma estimate 1 st order
Equatorial Sigma Estimate in Equatorial Areas1st Order


Sigma estimate 1 st order sliced by time
Sigma Estimate 1 in Equatorial Areasst Order (Sliced by Time)


Failure of thin shell model
Failure of Thin Shell Model in Equatorial Areas

Courtesy:

Seebany

Datta-Barua

Quiet Day

Disturbed Day


Undersampled condition
Undersampled Condition in Equatorial Areas

Courtesy:

Seebany

Datta-Barua


Waas measurements
WAAS Measurements in Equatorial Areas

Courtesy:

Seebany

Datta-Barua


Temporal gradients

200 s in Equatorial Areas

Temporal Gradients

Slide Courtesy Seebany Datta-Barua


Nominal c n 0 without scintillation
Nominal C/N in Equatorial Areas0 without Scintillation

Ionosphere

Carrier to Noise density Ratio (C/N0)

C/N0

(dB-Hz)

Nominal

100 s


Ionospheric scintillation
Ionospheric Scintillation in Equatorial Areas

Electron density irregularities

Ionospheric

scintillation

Carrier to Noise density Ratio (C/N0)

C/N0

(dB-Hz)

25 dB fading

100 s


Challenge to worldwide lpv 200
Challenge to Worldwide LPV-200 in Equatorial Areas

Challenge to expand LPV-200 service to equatorial area

- Strong ionospheric scintillation is frequently observed

in the equatorial area during solar maxima.


Strong ionospheric scintillation
Strong Ionospheric Scintillation in Equatorial Areas

7 SVs out of 8

(worst 45 min)

18 March 2001

Ascension Island

Data from

Theodore Beach,

AFRL

C/N0

(dB-Hz)

100 s


Benefit from a back up channel
Benefit from a back-up channel in Equatorial Areas

Lost L2C, but tracked L1

Loss of L2C alone

Loss of L1 & L2C

60 s (zoomed-in plot)


Summary
Summary in Equatorial Areas

  • 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


Sigma estimate 1 st order sliced by time1
Sigma Estimate 1 in Equatorial Areasst Order (Sliced by Time)


Solar max quiet day
Solar Max Quiet Day in Equatorial Areas

July 2nd, 2000


Case i moderate scintillation on 5 march 2011 ut
CASE I: in Equatorial Areas Moderate scintillation on 5 March 2011 (UT)

Less than 10 dB fluctuations


Histogram of c n 0 difference during scintillation
Histogram of C/N in Equatorial Areas0 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.


Percentage of c n 0 difference during scintillation
Percentage of C/N in Equatorial Areas0 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.


Case ii strong scintillation on 15 march 2011 ut
CASE II: in Equatorial Areas Strong scintillation on 15 March 2011 (UT)

More than 15 dB fluctuations

Our way to indicate

no C/N0 output (loss of lock)


Percentage of c n 0 difference during scintillation1
Percentage of C/N in Equatorial Areas0 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.


Receiver response during the 800 s of strong scintillation
Receiver response during the 800 s of strong scintillation in Equatorial Areas

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.


Case iii strong scintillation on 16 march 2011 ut
CASE III: in Equatorial Areas Strong scintillation on 16 March 2011 (UT)

More than 15 dB fluctuations


Percentage of c n 0 difference during scintillation2
Percentage of C/N in Equatorial Areas0 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%)


Previous studies
Previous Studies in Equatorial Areas

- El-Arini et al. (Radio Sci, 2009) observed highly-correlated fadings

between L1 and L2. (L1 and L2 military receiver and 20 Hz outputs)


Previous studies1
Previous Studies in Equatorial Areas

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

during scintillation.

- Deep fadings are not highly correlated in this example.


Ionospheric decorrelation 0 th order
Ionospheric Decorrelation in Equatorial Areas(0th Order)


Ionospheric decorrelation function 0 th order
Ionospheric Decorrelation Function (0 in Equatorial Areasth Order)


Estimation of ionospheric gradients
Estimation of Ionospheric Gradients in Equatorial Areas

T1

T2

IPP

S1

S2

S1

S1

S2

Slide Courtesy Jiyun Li


Gbas gradient threat
GBAS: Gradient Threat in Equatorial Areas

Ionosphere


Sbas undersampled threat
SBAS: Undersampled Threat in Equatorial Areas

Estimated

Ionosphere

Ionosphere


Obliquity factor
Obliquity Factor in Equatorial Areas


Ionospheric threat
Ionospheric Threat in Equatorial Areas


Nominal Day Spatial Gradients Between WAAS Stations in Equatorial Areas

Typical Solar Max Value:

Below 5 mm/km

Slide Courtesy Seebany Datta-Barua


Spatial Gradients Between WAAS Stations During Anomaly in Equatorial Areas

Storm Values:

> 40 mm/km

up to 360 mm/km

Slide Courtesy Seebany Datta-Barua



Simultaneous loss of satellites
Simultaneous Loss of Satellites in Equatorial Areas

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

Slide Courtesy Jiwon Seo

20 sec Loss

18 sec

Max of 4 SV Loss


Simultaneous loss of satellites1
Simultaneous Loss of Satellites in Equatorial Areas

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

Slide Courtesy Jiwon Seo

2 sec Loss

18 sec

Max of 2 SV Loss


Number of tracked satellites
Number of Tracked Satellites in Equatorial Areas

  • 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 %

100 %

4 or more tracked SVs

5 or more

Time

Percentage

Slide Courtesy Jiwon Seo

6 or more

65 %

2 sec

20 sec

Reacquisition Time


Number of tracked satellites1
Number of Tracked Satellites in Equatorial Areas

  • 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

100 %

4 or more tracked SVs

5 or more

Time

Percentage

Slide Courtesy Jiwon Seo

6 or more

65 %

2 sec

20 sec

Reacquisition Time


Correlation of fades between satellites
Correlation of Fades between Satellites in Equatorial Areas

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

300 s

PRN 11

8 SVs

in view

15%

correlation

Instance of loss of lock

(each blue dot)

PRN 4

45 min


Availability of lpv 200 parametric study
Availability of LPV-200 (parametric study) in Equatorial Areas

99.5%

Assuming max temporal range error (0.5 m/s)

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

10

< 50%

Availability of a single user at Ascension Island

Reacquisition

Time (s)

Availability

Level

> 50%

> 75%

> 90%

> 95%

> 99.9%

0

L1/L5 Correlation

Coefficient

0

1


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