Post on 09-Jan-2016
description
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