Data Assimilation and Adjusted Spherical Harmonic Model of

Somjai Klinngam1, Pornchai Supnithi1, Athiwat Chiablaem1,

Takuya Tsugawa2, Kornyanat Watthanasangmechai2

1Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Thailand (KMITL)

2Space Environment Laboratory, National Institute of Information and Communications Technology, Japan (NICT)

Data Assimilation and Adjusted Spherical Harmonic Model

of VTEC Map over Thailand

1

Objectives

2The 4th AOSWA Workshop, 24 - 27 October 2016, Jeju, Republic of Korea

PART ❶

To create the adjusted spherical harmonic model (ASHM) of the vertical TEC (VTEC) over Thailand

PART ❷

To assimilate the observed VTEC with the VTEC data of

the IRI-2012 model and the IGS using

the data assimilation technique

with the latitude-dependent factor (LDF)

Vertical Total Electron Content : VTEC

The important ionospheric parameter for studying the ionosphere characteristics and the irregularities.

Satellite

Receiver

Earth

STECVTEC

RE

0

IPP

H Z

Z’

Ionosphere

Courtesy, Sarawoot

RE = Earth’s radius (6,371 km)H = Height of the ionospheric layerIPP = Ionospheric pierce pointZ = Satellite zenith angle

1F(z) =

cosz'

(2) ( ) s rVTEC F z STEC b b

= Receiver biasrb

The 4th AOSWA Workshop, 24 - 27 October 2016, Jeju, Republic of Korea

Slant TEC (STEC)

es

STEC = N ds

= Electron density (electrons/m3)

s = Distance between a GPS receiver

and a satellite (m)

eN

the density of free electrons found in a square meter area along the slant path between a GPS receiver and a satellite.

(1)

We convert the STEC to the VTEC using the single-layer mapping function : F(z)

sb = Satellite bias

3

in total electron content unit (TECU) , 1 TECU = 1016 electrons/m2

Removed

IGS-VTEC

International GNSS Service (IGS)

Global Ionospheric Maps (GIM)

4

Although we can construct the VTEC map over Thailand from the IONEX files of the IGS, which are observed from many stations around the world. The regional VTEC map from the IGS has

• The temporal resolution of 2 hours• The spatial resolution of 2.5◦ x 5◦ in latitude and longitude

http://aiuws.unibe.ch/ionosphere/gim.gif


The IONosphere Map Exchange (IONEX) Thailand LocationLatitude = 0◦ – 25◦N

Longitude = 95◦E – 110◦E

44 grids

The Center for Orbit Determination (CODE) produces daily maps of the Earth’s ionosphere since 1996.

IGS Reference Frame

The ~400 GNSS stations around the world are managed by IGS for the ionospheric observation.

International Reference Ionosphere (IRI-Model)

5

• The empirical model for ionospheric parameter estimation

• The IRI model is an international project sponsored by the Committee on Space Research (COSPAR)and the International Union of Radio Science (URSI) since 1978.

• The current version is IRI-2012 Model

RUN ONLINE :http://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html


The IRI-2012 model can also estimate the VTEC at anytime and anywhere around the world.

The GPS stations

6

Geographic Equator

The observation sites of SEALION project

KMITL


No. Stations

1 CHAN

2 CHMA

3 NKRM

4 NKSW

5 PJRK

6 SISK

7 SOKA

8 SRTN

9 UDON

10 UTTD

11 DPT9

12 KMIT

Thailand

GPS Observation in Thailand

Low-latitudeNear the magnetic equator

At the moment, the GPS station network is being expanded to over 100 stations.

These GPS stations are managed by1. Department of Public Works and Town & Country Planning (DPT)2. King Mongkut’s Institute of Technology Ladkrabang (KMITL)3. South East Asia Low-latitude ionospheric Network (SEALION)

LTTE

CU

VTEC

Experimental Setup

Receiver Independent Exchange Format (RINEX)

7

JAVAD-GPS receiver ComputerGPS antenna

LT

Deg

ree

IPP Latitude

LT

Deg

ree

IPP Longitude

July 8th, 2012Quiet day and High solar activity

Elevation cut-off angle = 30 degrees, SLM height = 350 m, Cycle slip, satellite and receiver biases are removed

A dual-frequency GPS receiver at KMITL station (SEALION project)



PART ❶

To create the adjusted spherical harmonic model (ASHM)

of the vertical TEC (VTEC) over Thailand

s

nm

nm nm

IPP sun fixed longitude

' Scaled IPP co latitude

P Normalized associated Legendre functions

a and b Unknown coefficients

n,m Degree and order

N Maximum degree

N n

nms nm s nm s

n 0 m 0

VTEC( , ') P [cos( ')]{a sin(m ) b cos(m )}

9

Adjusted Spherical Harmonic Model (ASHM)

(3)

0

θ = Spherical angel (or latitude span)

= IPP co - latitude

= Minimum co - latitude of the IPPs

LT = Local time

UT = Universal time

λ = Geographic longitude

Scaled IPP co-latitudeIPP sun-fixed longitude

0

90' ( )


s LT UT

PART ❶

2p Number of observation, q Number of unknown coefficient : q (N 1)

Least Square Method

qx1 qxp pxq qxp px1

T -1 TX = (J J ) (J T ) (4)

Unknown coefficients matrix (anm and bnm)

Geometry matrix

Observed VTEC matrix

10

ASHMASHM

Coefficients

matrix: X


OBS-VTECObserved VTEC from 12 stations

For example, 08 JULY 2016 at 15.00 LT

VTEC Map Construction

Using the coefficient of ASHM

We calculate the VTEC at each location using the coefficients of ASHM.

ASHM-VTEC

N = 3

ASHM-VTEC

IGS-VTEC IRI-VTEC

Thailand

11

Thailand

VTEC Map over Thailand


Results & Discussion

• The IRI-VTEC map gives the close ionospheric characteristic of Thailand than the IGS-VTEC map.

• The ASHM-VTEC map shows the lower VTEC values at lower latitudes.

• The ASHM-VTEC map does not cover the ionosphere over near countries : Myanmar, Laos, Vietnam, Cambodia and Malaysia

Global Ionospheric Maps (GIM)

ASHM-VTEC


PART ❷

To assimilate the observed VTEC with the VTEC data of

the IRI-2012 model and the IGS using

Data Assimilation Technique

with the Latitude-Dependent Factor (LDF)

13

VTEC Data AssimilationPART ❷

Observation Data Model Data

Median VTEC

calculation

Observed VTEC from 12 Stations on 08 JULY 2012 at 15.00 LT

OBS-VTEC THG-VTEC

The spatial resolution of 2.5°x 5°in latitude and longitude, respectively

1. IRI-VTEC

2. IGS-VTEC


Assimilation

Missing grids


Latitude-Dependent Factor : LDF

• The LDFIRI and IDFIGS are used to weight the IRI-VTEC and the IGS-VTEC, respectively before assimilation.

• For the same latitude levels, the IRI-VTEC and the IGS-VTEC follow the same trend.

• The IGS-VTEC is more than the observation (THG-VTEC) and the IRI-VTEC at all grids.

• The LDFIGS is higher than the LDFIRI at all times.

IGS-VTEC on 08 JULY 2012 at 15.00 LTIRI-VTEC on 08 JULY 2012 at 15.00 LT

VTEC from each grid

on 08 JULY 2012 at 15.00 LTLatitude-Dependent Factor :

LDFIRI and LDFIGS

𝐋𝐃𝐅𝐈𝐑𝐈,𝐭 = 𝐦𝐞𝐝𝐢𝐚𝐧 𝐋𝐃𝐈𝐑𝐈,𝐢 𝐭

𝐋𝐃𝐅𝐈𝐆𝐒,𝐭 = 𝐦𝐞𝐝𝐢𝐚𝐧 𝐋𝐃𝐈𝐆𝐒,𝐢 𝐭Grid Number

15.00 LT

The weight may not be constant at all time.

Latitude (Degree)

TEC

U

Latitude (Degree)TE

CU

The 4th AOSWA Workshop, 24 - 27 October 2016, Jeju, Republic of Korea15


THG-VTEC IRI-VTEC

Assimilation

with LDFIRI

Results : VTEC Data Assimilation

Assimilation

THG-VTEC IGS-VTEC

Assimilation

with LDFIGS

Assimilation

IRI-VTECTHG-VTEC Assimilation with LDFIRI

IGS-VTEC Assimilation with LDFIGSTHG-VTEC

Ass

imila

tio

nA

ssim

ilati

on

17

Conclusions

We have constructed the VTEC map over Thailand based on

• the coefficients of the adjusted spherical harmonic model (ASHM).

• the data assimilation (IRI-2012 Model, IGS)

with the latitude-dependent factor (LDFIRI and LDFIGS).

Future works :

Increase the number of GPS stations and observation periods (season, period of solar activity)

Other assimilation methods ( Kalman Filter, etc.)


1. National Institute of Information and Communications Technology (NICT), Japan

and Department of Public Works and Town & Country Planning (DPT), Thailand

for data support.

2. King Mongkut's Institute of Technology Ladkrabang (KMITL)

and Ministry of Science and Technology of Thailand (MOST)

for financial support.

Thank you for your attention.

Acknowledgements


Documents

Data Assimilation and Adjusted Spherical Harmonic Model of