AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

International collaboration on space weather forecast

Paper: SCT1.9
Slant TEC gradient analysis during 12-13
September, 2014
Sarawoot Rungraengwajiake1, Assoc. Dr. Pornchai Supnithi1
Prasert Kenpankho1, Dr. Susumu Saito2, Nattapong Siansawasdi3,
Dr. Apithep Saekow4
1Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Thailand
2Electronic Navigation Research Institute (ENRI), Japan
3Aeronautical Radio of Thailand (AEROTHAI), Thailand
4Stamford International University, Thailand

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015Outline• Ionospheric delay gradient effects on Ground-Based
Augmentation System (GBAS)• Experimental Setup• Results and Discussions• Conclusions

2AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Total Electron Content (TEC)
The ionosphere is an ionized part of theatmosphere consisting of electrons andions••

It is ionized due to the solar radiation anddepends on (Locations: latitude, longitude,Local time and seasons, and Solar activities(sunspot number, solar storm, etc.)

Solar Radiation

Total Electron Content (TEC)on the Slant path (STEC)
Height1,000 km

350 km
50 km+1 m2-- + - +- - ++ - + - +- ++-+Unit (TECU)STEC = ∫ Ne ds (10TEC electrons/m)
16s

Ne : Electron density (electrons/m3)
S : Distance along the propagation path

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

GPS-Derived TEC
Slant TEC (STEC) is derived by differencing the pseudo ranges(P1 and P2) or the phases (L1 and L2) of the two frequencies.
STEC =()

2

2 f1 f 222k f1 - f 2(())( L1λ1 - L2λ2 )2

2 f1 f 2
STEC =22k f1 - f 2()

f1 = 1575.42 MHz,f2 = 1227.60 MHz
λ1 = 0.1904 m
λ2 = 0.2444 m
k = 80.62 (m3/s2)( P2 - P1 )[Blewitt, 1990]zVertical TEC (VTEC) can becomputed from the STEC value andthe zenith angle.

VTEC = STEC.cos( z )[Tsugawa, T., AGU, 2010 ]5

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Instrumental bias estimation
 The satellite bias ordifferential code bias
can be use data from theInternationalGNSSService(IGS).

Satellitebias

VTE=C ( STEC− bs − br ) ⋅ cosχ

(ftp://ftp.unibe.ch/aiub/CODE/)

STEC

VTEC

Instrumental biases

VTEC-bsbr

Receiverbias

Absolute
TEC

VTE=C ( STEC− bs − br ) ⋅ cosχ

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

STEC (removed biases )STFD
1005000510152015201520KMITLSTEC (TECU)

1005000510AERO

1005000510Time (UTC)AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

VTEC (removed biases)
STFD
1005000510152015201520KMITLVTEC (TECU)

1005000510AERO1005000510Time (UTC)AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ground-Based Augmentation System (GBAS)GPS Satellites•

e = c1 - dIonosphereρTroposphereAirplane

The GBAS processor calculates thepseudorange error (e) of each GPSsatellite.
True distanceObserve distanceC1•

The augmentation information(error e) is then broadcast to theapproaching airplanes•

The pseudorange errors are assumed
to be the same in the nearby area.

•The coverage area is about 40-60 km
around the airport.

Broadcast AugmentationInformation

c1-dReference Stations

VHF Antenna
GBAS Processor

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric Delay Gradients with GBASGPS Satellites• The reference stations provide thedifferential corrections and integrity
information to the receiver that are
equipped in the aircraft in the nearby
area.

IonosphereρTroposphere

Airplane

Incorrect Augmentation
Information

c1-d
VHF Antenna
GBAS Processor

C1• However, the ionospheric irregularities
can cause the error of the differential
correction information that is broadcast
to the aircraft.

• For the GAST-D (GBAS Approach
Reference Stations Service Type D), the error of differential
corrections shall be less than 1.5 m
within 5 km of the runway threshold(300 mm/km).

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric effects to GBAS
Simplified ionosphere wave front model• Front Slope or “Ionospheric delay gradient”• Causes of Ionospheric delay gradient,

1. Due to the physical ionospheric separation between
aircraft and reference station.
2. Due to the ionospheric irregularities (plasma bubbles,SED).

GNSS Satellites• Q : How large of the ionospheric delaygradient in the low latitude regions can be?
Front Slope

Ionosphere

Front Speed
Viono_front

Front Width

Airplane
Vairplane

Differential correction

hd

Reference Station

In this study, we investigate on
the ionospheric delay gradientassociated with plasma bubbles
in Thailand, which is located inlow- latitude region.
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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric effects to GBAS
20 November 2003

413 mm/km

This extreme event stimulate the ionosphericdelay gradient research in various regions.

S. Datta-Barua, et al., 2010.

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric effects to GBASGNSS Satellites

Plasma bubble
S. Saito 2011.

Ionosphere
AirplaneDifferential correction???

Reference Station

The ICAO has recently realized theimpact of this issue and recommended
each country to investigate ionosphericdelay gradient in that region.

• Plasma bubble frequently occurs in low-latitude
region after sunset , and more occurrence duringhigh solar activity period..
• This phenomena can cause ionospheric delay
gradient and also scintillation, which degrades
the GBAS performance.
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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Short baseline experiments• Short baseline experiment needs to be carried outto monitor the ionospheric delay gradients near
Suvarnabhumi international airport.
• Three dual-frequency GPS receivers have been
installed as part of a cooperation project of
1. King Mongkut’s Institute ofTechnology Ladkrabang (KMITL)
2. Electronic Navigation ResearchInstitute (ENRI), Japan
3. Aeronautical Radio of Thailand
Ltd. (AEROTHAI)
4. Stamford International University• This project started July 2011.

16

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Experimental Setup
Antenna•
GPS Receiver

PCSerialCoaxial cable•

The dual-frequency GPS receiverscollect the pseudorange and thecarrier phase at 1-Hz frequency.
The raw GPS data (binary) are sent
to the data server at every 1 hour.

ProPak®-V3

STFDInternetAntenna

GPS Receiver
SerialCoaxial cable

DL-V3™

KMITLPC•

The raw GPS data are converted into
RINEX (Receiver Independent Exchange)
format (ASCII).

17

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric delay gradient calculation
1 September 2011

Single difference method

STEC

STEC (TECU)

40
KMITL
STFD

30
2010

PRN20-10-20
1010.51111.51212.51313.51414.515dSTECkSTECadj=
STEC1k + BSK + BR _1_1

STEC = STEC + B + BR _ 2kadj _ 2k2dSTEC (TECU)

10Ionosphericdisturbance

8Quiet time

6

brSTFD-brKMITL

421010.5

1111.512KS

12.5

Time (UTC)1313.51414.5(STECSTFD-STECKMITL) +(BrKMITL-BrSTFD)

kk=dSTEC k STECadj_1 − STECadj _ 2= ( STEC1k − STEC2k ) + (bR1 − bR 2 )1518

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric delay gradient calculation

kdSTEC=( STEC1k − STEC2k ) + (bR1 − bR 2 )40.3  STEC1k (t ) − STEC2k (t ) ∇I (t ) =2 df 
Ionospheric delay gradient (mm/km)
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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Dst Index

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

ROTI (Rate of TEC change index)
SUNSETMIDNIGHT• In order to detect the ionosphericirregularities, we use the rate of TECchange index or ROTI.

ROT=(i ) STEC (i + 1) − STEC (i )Irregularities=ROTI

1 N
2(())
ROTi−ROT∑
N i =1

Quiet Condition

Example ROTI• The ROTI is defined by Standard
deviation of rate of TEC change with
5-minute window.
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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

KMITL station
KMITL station

150100STEC (TECU)STEC (TECU)

150

500-500510152000526 AUG, 2015
0.52520252025202527 AUG, 2015
00510152025051510Time (UTC)KMITL station

KMITL station

150200STEC (TECU)STEC (TECU)

200.5Time (UTC)100
500-5015101ROTIROTI50-5025101001000-1000510152025051015

128 AUG, 2015

ROTIROTI

10.500051015Time (UTC)2025

29 AUG, 2015

0.5051015Time (UTC)22

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Dst Index

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Dst

111213

14

11

12

13

14

ROTI

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

ROTI – Sept. 2014
0102

03

0405

06

07

08

09

10

111213141525AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

ROTI – Sept. 2014
161718

1920

2223242526272829302126

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

PRN21, 12 Sept, 2014
STFD

KMIT

STEC KMITL station on 12th Sep. 2014

200

150

150

100100STEC (TECU)STEC (TECU)STEC KMITL station on 12th Sep. 2014

200

50

50

00-50

-50

-100

05

1015

20

25-10005Time (UTC)10152025152025

Time (UTC)

11
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.5

ROTI

ROTI

0.6

0.4

0.5
0.4

0.3
0.3
0.2
0.2
0.1
0.1
005

1015Time (UTC)202500510Time (UTC)27

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

PRN 31

PRN 21
IPPs masking30o PRN21

IPPs masking30o PRN313030252511.6042UT20

Latitude (Deg)

Latitude (Deg)20151051023.6892UT
518.1781UT

0-580

1585

90

10595
100
Longitude (Deg)110

14.9903UT

0115

120-580

8590

105
100
95
Longitude (Deg)11011512028

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

12 Sept, 2014
PRN 21
80
KMITL
STFD

STEC (TECU)60

PRN 21
1004080200

PRN2160

010000500015000

10Gradient (mm/km)-20dSTEC (TECU)

94 mm/km40

2005-200-5-40-60
12.5

0

10000

5000

15000

1313.51414.5

15.5
15Time (UTC)1616.51717.5Time (samples)

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

12 Sept, 2014
Ionospheric delay gradient,KMIT/STFD station allPRN
120

PRN 31102 mm/km

100

All PRNs

Iono Gradient (mm/km)

8060

PRN

Speed (m/s)

40

21124

20259003191-20-40-60

0510152025

Time (UTC)30

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

13 Sept, 2014
KMIT

STFD

KMITL station
150100STEC (TECU)STEC (TECU)

150

500-50051015200051005101520251520251ROTIROTI

50-5025

10.501000.50051015Time (UTC)

2025Time (UTC)31

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

13 Sept, 2014
Ionospheric delay gradient,KMIT/STFD station allPRN
150

100Iono Gradient (mm/km)

500

PRN

Speed (m/s)

24160

29

187.5-50-100

PRN 29-150

162 mm/km-2000510152025Time (UTC)32

AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Previous processing-step proposed•In order to process a lot of GPS data forionospheric delay gradient monitoring,
the previous studies proposed the dataprocessing-step, focus on the SED eventon 20 November 2003.

•However, they did not consider theplasma bubble occurrence in equatorial
and low-latitude regions.

S. Datta-Barua, et al., 2010.

J. Lee, et al., 2011.

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Ionospheric delay gradient for equatorial
an d low-latitude regions
Raw RINEX data

Pre-processing

Pseudorange and carrier
phase measurements of dualfrequencies GPS receiver

Processed by GPStk- Cycle slip detection- Short-arc removal

Update IonosphericThreat Model

Update a new boundManualValidation

Validate the true ionosphericphenomena

Using carrier (L1 , L2) andcode measurement (C1 , P2)

Compute Slant TEC(STEC)* Biases are not calibrated

Find Plasma Bubble
Occurrence(using ROTI)

ROTI > threshold

Find Maximum
Ionospheric gradient(Station Pair)* Receiver biases are calibratedusing station pair method

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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Conclusions• The geomagnetic storm during 26-27 Aug, 2014 nor 12-13
Sept, 2014 did not cause the ionospheric disturbance as seenfrom the ROTI plot around low-latitude KMITL station, but
ROTI plots show disturbance around sunset time during thewhole of September

12-13 Sept, 2014
• We see the disturbance around the sunset time during 12-13
Sept, 2014, likely caused by the plasma bubbles (commonduring the equinoctial months).
• The maximum slant TEC gradient on the East-West direction
is measured to be 162 mm/km (PRN29)
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AOSWA 2015, Fukuoka, Japan, 2-5 March, 2015

Thank you for your attention!
Question?

36AOSWA 2015, Fukuoka, Japan, 2-5 March, 201537