Analytical results of ionospheric delay
gradient based on GPS monitoring stations
near Suvarnabhumi airport in Thailand
Sarawoot Rungraengwajiake1, Assoc. Dr. Pornchai Supnithi1
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

The 3rd ENRI International Workshop on ATM/CNS (EIWAC) 201319-22 February 2013 Tokyo, Japan

Outline• Introduction- Ionospheric effects on GNSS signals- Ionospheric delay gradient effects on GBAS

• Experimental Setup• Results and Discussions• Conclusions

Ionospheric effects on GNSS signals
Time delay

Scintillation

n=1

Ionosphere

nion

ts

Ionospherets*

n1n2n3n4Vn5

Amplitude andPhase fluctuation•The extra time delay produced by theionosphere is given by, t  t s*  t s Ionosphericdelay

40.3
Ne ds
cf 2 s*40.3
I  c   t  2 TEC
fts* : Traveling time with an ionospherets : Traveling time without an ionosphere•The ionospheric irregularities cause thefluctuations in the refractive index (n).

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

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

(second)(meter)Ionospheric effects to GBAS
GBAS (Ground-Based Augmentation System)

GNSS Satellites• The reference stations provide thedifferential corrections to the receiverthat are equipped in the aircraft in thenearby area.
• However, the ionospheric irregularities
cause the error of the differentialcorrection information that broadcast tothe aircraft.

Ionosphere
AirplaneDifferential correction
Reference Station• For the GAST-D (GBAS Approach
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).

Ionospheric effects to GBAS
20 November 2003

413 mm/km

The extreme ionospheric delay gradientobserved in US.
S. Datta-Barua, et al., 2010.

Ionospheric effects to GBAS
Simplified ionosphere wave front model• Here, we focus on the Front Slope or“Ionospheric delay gradient”.
• Ionospheric delay gradient problems
1. Due to the physical ionospheric separation between
aircraft and reference station.
2. Due to the ionospheric irregularities (plasma bubbles,SED).

GNSS Satellites

Front Slope

Ionosphere• Q : How large of the ionospheric delay
gradient in the low latitude regions can be?

Front Speed
Viono_front

Front Width

AirplaneDifferential correction
Vairplane

hd

Reference Station

In this study, we focus on theionospheric delay gradientassociated with plasma bubbles
in Thailand, which is located inlow- latitude region

How to derive TEC from GPS data?
GPS Satellites•For dual-frequencies GPS receiver, the Slant
TEC can be derived by both psedorange andcarrier phase linear combinations.

1 m2IonosphereSlant TECPseudorange :STECP  k (P2  P1 )Carrier phase :STECL  k (L1  L2 )where k=9.5196 for TEC expressed in TECU.

Elevation Angle
150

TECP
TECL100•

Since the STECP is nosier than STECL but the
STECL still has an initial ambiguity which
frequently causes the STECL to have negativevalues.
Generally, STECL is adjusted to STECP level.

STECadj  STEC  BS  BRSatelliteIFBReceiverIFB

50
STEC (TECU)•STECP
0STECadj  STECL  (STECP  STECL )arc

-50-100

STECL-150
99.5

1010.5Time (UTC)1111.5

12Receiver bias calibration
1 September 2011

Single difference method

STEC

STEC (TECU)

40
KMITL
STFD

30
20
100-10-20
1010.51111.51212.51313.51414.515dSTEC

kk
KSTECadj_1  STEC1  BS  BR _1
kadj _ 2

STEC

 STEC  B  BR _ 2
k2dSTEC (TECU)

10Ionosphericdisturbance

Quiet time

8BrSTFD-BrKMITL

6421010.51111.5

12KSkkdSTEC k  STECadj_1  STECadj _ 2

 ( STEC1k  STEC2k )  ( BR1  BR 2 )12.5Time (UTC)1313.51414.5

15(STECSTFD-STECKMITL) +(BrKMITL-BrSTFD)

Short baseline experiments• Short baseline experiment needs to be carried out
to 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.

Short baseline experiments

dSTEC k  (STEC1k  STEC2k )  ( BR1  BR 2 )40.3  STEC1k (t )  STEC2k (t ) I (t )  2 df Ionospheric delay gradient (mm/km)

ROTI (Rate of TEC change index)• In order to check the ionosphericirregularities, we use the rate of TECchange index or ROTI to indicate
whether the plasma bubbles occur ornot.

STFDKMITROT (i)  STEC(i  1)  STEC(i)ROTI AERO

1st September 2011

1 N2(ROT(i)ROT)N i1• The ROTI is defined by Standard
deviation of rate of TEC change with
5-minute window.

ROTI (Rate of TEC change index)

ROTI (5 minute window) of18 days of September 2011

ROTI (Rate of TEC change index)

15 days have high ROTI values

Results and Discussions
1st September 2011
dSTEC
STFD-KMITdSTEC (TECU)

15105• In the results, we selected the data on
1 September 2011 to analyze.

0-58101214

1618202224• The constant level can be consideredas the differential receiver biases.

AERO-KMITdSTEC (TECU)

151050-581012141618Time (UTC)202224

PRN2, 9, 14, 21, 29

Results and Discussions
1st September 2011
dSTEC
STFD-KMITdSTEC (TECU)

15105

BrSTFD-BrKMIT
4.28 –5.12 TECU

0-581012141618

202224

AERO-KMITdSTEC (TECU)

15
10

uncertainty offset
BrAERO-BrKMIT

55.18 –6.78 TECU

101214• Therefore, we consider the differentialreceiver biases for each satellite.
L. Ciraolo, et al., 2007.

0-58• However, we found a slight offset
exist in AERO-KMIT direction due to theuncertainty offset in the adjustment
step (L. Ciraolo, et al., 2007).

1618Time (UTC)202224

PRN2, 9, 14, 21, 29

Results and Discussions
1st September 2011
dSTEC

I (t )

STFD-KMITSTFD-KMIT

100105I (mm/km)

dSTEC (TECU)

15

BrSTFD-BrKMIT
4.28 –5.12 TECU

0-95.23 mm/km-100-581012141618

AERO-KMIT

1510202224

8101214

uncertainty offset inthe adjustment step

BrAERO-BrKMIT

55.18 –6.78 TECU

01618202224202224

AERO-KMIT

107.7 mm/km

100I (mm/km)

dSTEC (TECU)

00-100-581012141618Time (UTC)202224810

PRN2, 9, 14, 21, 29

12141618
Time (UTC)Results and Discussions
22nd September 2011I (t )PMF

STFD-KMIT200

I  0.58-310PMF

I (mm/km)

100

0-100-200
10

STFD-KMIT-210  17.61-410-510-612141618202210-250

24-200-150-100-50

AERO-KMIT

50100250I  7.46-310010PMF

I (mm/km)

20010192.36 mm/km0  19.75-4

10-5-100-200
10150

AERO-KMIT-2200010-612141618Time (UTC)20222410-250-200-150-100-500I (mm/km)

50100

150200250

Conclusions• From this study, we show the ionospheric delay gradientsnear Suvarnabhumi airport, Thailand, in September equinox
2011.

• Based on 15-day observation, we found that the maximum
ionospheric delay gradient can reach 192.36 mm/km on 22ndSeptember 2011.
• For the GBAS implementation in this area, the localionospheric threat model should consider the maximum
ionospheric delay gradient.

Thank you for your attention!
Question?