Statistical characteristics ofbackground ionospheric total electroncontent (TEC) in Bangkok, Thailand
P. Supnithi1, S. Rungraengwajiake1, S. Saito2,N. Siansawasdi3, A. 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

Outline Introduction
 Ionospheric effects on GNSSand GBAS
 Receiver bias computation
 Results and Discussions
 Conclusions

2

Ionospheric effects on GNSS signals
Time delay•

The refractive index of the radio wave in theionosphere can estimate by using AppletonHartree equation (Kintner, 2005) :n=1Ionospherenion

Plasma frequency

ts2ωp2nion=1− 2ω

ts*••

The extra time delay produced by theionosphere is given by,δ t = ts* − t s =Ionosphericdelay

40.3
Ne ds
cf 2 s∫*(second)40.3
TEC
2f(meter)

I = c ×δ t =ts* : Traveling time with an ionospherets : Traveling time without an ionosphere

Ne e 2= 80.6Ne
ω =ε 0 me2pRadio wave
frequency

Since the plasma frequency is less than the radio
wave frequency (GPS L1 signal is 1.5 GHz).

ω p2
ω p2
Nenion = 1 − 2 ≈ 1 − 2 ≈ 1 − 40.3 22ωfωωp : Plasma frequency
ε0 : Permittivity of free space
Ne : Electron density
me : Electron masse : Electron’s charge
ω : Radio wave frequency (rad/s)
f : Radio wave frequency (Hz)3

Ionospheric effects on GNSS signals
Scintillation

PRN03
50
40C/No30

Ionospheren1n2n320
100n4Vn5-10-20051015202530

Time (min)

KMITL
8060
STEC (TECU)

Amplitude andPhase fluctuation40
200-20•-40

The ionospheric irregularities cause thefluctuations in the refractive index (n).

4681012141618202224161820

2224

STFD
80

•

It causes the rapid fluctuations in amplitude
and phase of GPS signals.

STEC (TECU)

60
40200-20-40•

In the worst case, the receiver loss of lock thesatellite signal.

468101214Time (UTC)4

Scintillations – 22 March 2012
[The raw GPS satellite signal from the software receiver is provided withthe courtesy of JAXA (Dr. Toshiaki Tsujii)]
PRN03
504030C/N0C/No20
100-10-20051015202530

Time (min)S4 index

PRN03

1S4 Index
1S4-corr
S4-total

0.8

PRN03
0.8

S4 Index

PRN07
0.60.4

PRN110.6

PRN130.4

PRN190.20.2

PRN231000510152025303520304050Time (min)607080Time (min)50Ionospheric Effects to GBAS
Simplified ionospherewave front model

Ionospheric delay gradients
1. Delay gradients with ionosphericdisturbance
2. Background delay gradients (quiet time)

GNSS Satellites

Front Slope

Ionosphere

Front Speed
Viono_front

Front Width

Airplane
Vairplane

hd

Reference Station

In this study, we focus on thebackground ionospheric delay
gradient (quiet-time).
The approach is to use the‘minimum variance’ method6

Previous works[Fujita et al., 2010] uses ‘single difference
method’ together with Kalman filter to determine
integer ambiguities and then compute theionospheric delays. The TEC levels at each station
is not computed.
 [J. Lee et al.,2010] suggests using Maruyama’s
method in the long-term ionospheric monitoring
as a simpler way to estmate the receiver bias in“Simple Truth” solutions.
S.Fujita, T. Yoshihara, S. Saito, “Determination of Ionosphere Gradient in Short
Baselines by Using Single Frequency Measurements,” Journal of Aeronautics,
Astronautics and Aviation, Series A, vol. 42, no. 4, pp. 269 – 276 (2010),
J.Lee, S.Jung, E.Bang and S.Pullen, “Long Term Monitoring of Ionospheric Anomalies toSupport the Local Area Augmentation System ,” ION GNSS 2010, Sept. 20-24, Oregon,USA.

7

Objectives
 Preliminary study of the backgrounddelay gradient near Suvarnabhumi
airport, Thailand
 To estimate the receiver bias using

the minimum variance method in[Ma and Maruyama, 2003]

8

Short baseline experiments
Three dual-frequency GPS receivers havebeen 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

9Causes of Delay Gradients
Low and Equatorial Latitudes- Equatorial Ionospheric Anomaly (EIA) : Large-scale ionospheric density
enhancement around ±15º magnetic latitude- Plasma Bubble: area of depletion of TEC at night time, typically generated fromthe equator due to pre-reversal enhancement
Equatorial Ionospheric Anomaly (EIA)

Plasma Bubble[S.Saito, 2011][Tsujii, 2010]10

Slant TEC
GPS Satellites



1 m2

Ionosphere

Slant TEC

For dual-frequencies GPS receiver,the Slant TEC can be derived byboth psedorange and carrier phaselinear combinations.

Pseudorange :STEC P = k (P2 − P1 )Carrier phase :STEC L = k (L1 − L2 )where k=9.5196 for TEC expressed in TECU.

θ Elevation Angle
150

TEC100•Since the STECP is nosier than STECL but the
STECL still has an initial ambiguity which
frequently lead the STECL to negative value.
Generally, STECL is adjusted to STECP level.

STEC adj = STEC + BS + BR
Satellite
IFBReceiver
IFB

50
STEC (TECU)

•P

TECL

STECP
0STECadj = STECL + ( STECP − STECL ) arc-50-100

STECL-150
99.5

1010.5Time (UTC)1111.51211Satellite and receiver biases
ObservedValue

Absolute Satellites
BiasValueReceiver
Bias

STEC1 = STEC + BS + BRThe satellite biases obtained from IGS, determinedby CODE (Center for Orbit Determination in Europe)

STEC2 = STEC + BR
[Ma and T. Maruyama, 2003]- The VTEC computed from low elevation angle data should not be
different from VTEC compute from high elevation angle data.
- The expected receiver bias that gives the minimum σ of VTEC isthe corrected receiver bias
12

Example

Earthradius

VTECadj = STEC ⋅ cos χ + ( BR − BR' ) ⋅ cos χ

3.660

STECVTECx 10

3.5
3.4

40

3.330

3.2

Height ofionosphere

4STFD
KMITLAEROmin(BrSTFD) = -19.13 TECUmin(BrKMIT) = -20.64 TECUmin(BrAERO) = -16.59 TECUΣσTEC (TECU)

50R cos θ)]
Re + hχ = arcsin( eConditions on data used for receiver bias computation:- Elevation > 35°- The ionospheric height h = 350 km
70elevation
angle

20

3.1

10

30

2.9-102.8-200

51015Time (UTC)20

2.7-25-20-15
Receiver IFB (TECU)-1013

Receiver Bias
August, 2011-10
BrSTFDBrKMITBrAERO-15

Receiver IFB (TECU)-20-25-30-35-40
215

217219221223

225

227 229 231 233
DOY (Day of the year)

235

237

239

241

243

DOY
219

Br_stfd Br_kmit Br_aero-31.14 -38.28 -30.25

220
224

-23.17-22.86-28.1-30.37-21.13-22.79225
226-23.69-22.00-28.99-29.24-19.23-20.96227
228-20.92-19.86-27.8-25.50-18.86-18.46229
230-19.94-25.58-27.32-33.17-18.85-24.22231
233-16.55-21.24-21.34-28.01-13.11-19.53235
237-18.71-15.12-21.70-18.83-18.97-14.75240241-18.09-19.13-20.75-20.64-17.28-16.59245A jump in the receiver bias is partially due to omission of some satellite
14data with remaining cycle slips after the cycle slip correction.

STEC and VTEC (Day 224)
STFD

STFD

100

100505000510150200510KMITL
V T E C (T E C U )S T E C (T E C U )15201520100

50051015205000510AERO

AERO

100

100

5050020KMITL

100015051015Time (UTC)

2000510Time (UTC)15

STEC and VTEC (Day 233)
STFD

STFD

100

100505000510150200510KMITL
V T E C (T E C U )S T E C (T E C U )1520152010050051015205000510AERO

AERO

1001005050020KMITL

100015051015Time (UTC)

2000510Time (UTC)16

STEC and VTEC (Day 241)
STFD

STFD

100

100

505000510150200510KMITL
V T E C (T E C U )S T E C (T E C U )15201520100

50051015205000510AERO

AERO

100

1005050020KMITL

100015051015Time (UTC)

2000510Time (UTC)17Delay Gradients

I=40.3STEC
f2

40.3  VTEC1k (t ) − VTEC2k (t ) ∇I vertical (t ) = 2 f dd = distance between the IPPs
18

Results
29th August 2011
Differential STEC (dSTEC)

Vertical ionospheric delay gradient
STFD-KMIT

20100
75
50
250-25-50-75-100
0201007550
250-25-50-75-1000∇ I vertical (mm/km)

dSTEC (TECU)STFD-KMIT

5432
10-1-2-3-4-5051015551015Time (UTC)d ~ 11.5 km

15201015Time (UTC)20

AERO-KMIT∇ I vertical (mm/km)

dSTEC (TECU)

AERO-KMIT

543210-1-2-3-4-50105d ~ 3.5 km

19

Ionospheric Delay Gradient (Day 233)
STFD-KMIT150

STFD-KMIT-210-310500PMF∇ Iv ertic al (m m /k m )100-410-50-510-100-150

05101520-610-150-100-50

0

AERO-KMIT
150

50

100

150

50

100150

AERO-KMIT-210-310500PMF∇ Iv ertic al (m m /k m )100-410-50-510-100-150-6051015Time (UTC)2010-150-100-50∇Ivertical0(mm/km)

20

Ionospheric Delay Gradient (Day 241)
STFD-KMIT
150-3100-4-50PMF∇ Iv e r tic a l (m m /k m )1050-100

10-15010-505101520-610-150

AERO-KMIT-100-50050

100

150

50100

150

AERO-KMIT-215010100-31050-40PM F∇ Iv er tic a l (m m /k m )STFD-KMIT-2

100-501010-5-100-150-610-150051015Time (UTC)20-100-500∇Ivertical (mm/km)

21

[Fujita et al., 2011]

quiettime

During quiet time,
the TEC gradient iscomputed with the meanaround 0 mm/km.

22Discussions
The leveling errors fromthe standard levelingtechnique can drasticallyreduced by the “integerleveling” whereby theinteger ambiguities are
obtained through PPP
processing and introduced
to the geometric-freeequation

S. Banville et al.,”Ionospheric Monitoring Using Integer-Levelled
Observations,” ION GNSS 2012, Tennessee, USA, 17-21 Sept, 2012.
23

Short baseline experiments
Future20 km

Three dual-frequency GPS receivers havebeen installed as part of a cooperation
project ofCHUL- Courtesy of
JAXA receiver- CUSV (IGS)

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

24

Conclusions
 The preliminary results show that using

Minimum variance method, which issuitable for a few single stations, thebackground ionospheric delay gradient iswithin +,- 50 mm/km for STFD-KMIT
baseline, but have higher values for theKMIT-AERO baseline.
 The use of minimum variance method willneed to be further modified to account forthe integer ambiguities.
25

Thank you for your attention
Q&A

26