High-resolution ionospheric total
electron content observations using
dense GNSS receiver networks
T. Tsugawa1, M. Nishioka1, A. Saito2, Y. Otsuka3, S. Saito4,T. Maruyama1, T. Nagatsuma1, K. T. Murata1, and M. Ishii1
1 NICT,

2 Kyoto University, 3 Nagoya University, 4 ENRI

• Brief introduction of NICTʼs space weatheractivities• High-resolution GNSS-TEC observations• DRAWING-TEC projectAbout NICT• Headquarter: Koganei, Tokyo• Staff: permanent researchers: 300,
temporal researchers: 400,
administrative: 200(approximately).
• The “ONLY National Institute” ofInformation and Communications
technology in Japan• Our originality was in ionosphericobservations for monitoring shortwave propagations• Our study fields expand not only
narrow meaning of ICT, but also
wide areas.

Forecast centers of International Space
Environment Service (ISES)Space Research Center(Warsaw, Poland)
Lund Space Weather Center

Hydro-meteorological Service(Lund , Sweden)(Moscow, Russia)

Royal Observatory of Belgium(Brussels, Belgium)
Institute of Atmospheric Physics(Plague, Czech)

Natural Resource Canada(Ottawa, Canada)

RRA (Jeju, Korea)

NICT (Tokyo, Japan)

NOAA/SWPC(Boulder , U.S.A.)

Beijing Astronomical Observatory(Beijing China)
National Physical Laboratory(New Deli, India)

INPE(Sao Jose dos Campos, Brazil)

Hermanus Geomagnetic Observatory(Herumanus, South Africa)

IPS Radio and Space Services(Sydney, Australia)

NICT Space Weather Forecast Center
Real-time space weather monitoring
Exchanging forecast and data
among ISES forecast centers

Result of simulation• Flare prediction• Geomagnetic activity prediction• High energy particle prediction

NICT Space Weather Forecast Center
Forecasts and current conditions of flare activities,
geomagnetic activities, high-energy particle flux, HF
propagation condition by web, e-mail, RSS, fax

Domestic users: satellite operators, operators of power company,museum, operators of HF radio communication, etc.

Space Weather Forecast Meeting• We (forecasters and researchers) have a meeting to make aforecast every afternoon.

Space weather: cause and effect< Time range of forecast >[X-ray, EUV, light, radio wave]• Approximately 8 min. (light speed)[ solar energetic particles ]• tens minutes to several hours[Interplanetary shocks]• approximately 1 hour (from ACE)
• two or three days (from the sun)• twenty-seven day (solar rotation)[solar cycle]• approximately 11 years

Ionospheric effects on radio applications

GPS navigation and positioning• Pseudorange includes ionospheric propagation delay which is
the largest error of GPS positioning/navigation for general
single-frequency GPS receivers.

Differential GPS positioning• Steep spatial gradient of ionospheric electron density causes
differential GPS positioning errors.

GPS scintillation• Several 100m scale ionospheric irregularity causes GPS
scintillation which results in loss-of-lock on GPS signalsin the worst case.

Derivation of TEC using GPS• Total electron content (TEC) can be derived by comparing
the pseudorange/phase delays of the two GPS signals.
P1 P2: Pseudorange
L1 L2: Carrier phase
I: Total electron content
f1, f2: Frequency
ρ: True range between the GPS satellite
and receiver• TEC is a measure ofintegrated electron density
in 1m2 column.
• 1 TECU(=1016electrons/m2)
is frequently used as ameasuring unit of TEC.

High-resolution TEC observation using adense GPS receiver network

GEONET consisting of more than1,200 GPS stations

Detrended TEC with 60-min window revealed
medium-scale traveling ionospheric disturbances(MSTID) [Saito et al., GRL, 1998].

• A dense GPS receiver network makes it possible toobserve high-resolution two-dimensional TEC variations.

GPS-TEC maps in Japan
http://seg-web.nict.go.jp/GPS/GEONETHigh resolution GPS-TEC maps in Japan
Absolute TEC

12:20 UT
(21:20 JST)

12:40 UT
(21:40 JST)

13:00 UT
(22:00 JST)ROTI (~10km scaleirregularity)

Loss-of-Lock (~100mscale irregularity)Equatorial plasma bubble (EPB)

135.6nm airglow images observed by
TIMED/GUVI [Christensen et al., 2003]

Schematic picture of plasma bubbles• Plasma bubbles generally can develop and create instability inthe low-latitude ionosphere after the sunset.
• Plasma bubbles generally move eastward and have thestructure extending along the magnetic field line.
• A prompt penetrating magnetospheric electric field during themagnetic storm helped to trigger the super plasma bubble
observed at mid-latitudes.

ROTI and LOL at Okinawa, Japansunrise at 100 km alt.

sunrise at 300 km alt.

• Sat. zenith angle: < 45 deg.

sunset at 300 km alt.

• Slant TEC, ROTI, and Rate of GPS‐LOL (5‐min window) on Apr 30, 2001.

sunset at 100 km alt.

Okinawa（0100）• Rate of LOL of 2 or more GPS satellites during
2002‐2006.
• During Mar‐Apr in 2002, the RLOL in thenighttime (21‐24 JST) exceeds 30% (once perthree days on the average).

Global GNSS Receiver Networks

We have collected all the available GPS receiver data (more than
6,000 receivers as of Jan. 2012) and made the database of TEC.

Earthquake- and Tsunami-induced TEC variationsJapan (GEONET): ~1,200 receivers
Korea (KMA): ~80 receivers(KMA collects Korean GPS receiver data andprovides GTEX data)[Tsugawa et al., EPS, 2011].

Summary of earthquake/tsunami-inducedionospheric variations• Concentric waves propagating away from the ionospheric epicenter:the 1st fast wave  acoustic waves generated from the propagating Rayleigh wave.
the 2nd and 3rd waves  atmospheric gravity waves generated in the lower ionosphere.
the 4th and following waves  atmospheric gravity waves generated by tsunami wavefronts.
• Plasma depletion near the epicenter  plasma displacement due to neutral winds.
• Short-period TEC oscillation  acoustic wave resonance.

High resolution GPS-TEC mapsRegion
JAPAN# of GPS Rec. ~1,200 receivers

N. America~2,700 receivers

Europe~1,200 receivers

Detrended
TEC Map(60-minWindow)[Tsugawa et al., 2007].

[Otsuka et al., 2012].

Dense Regional And Worldwide INternational
GNSS-TEC observation （DRAWING-TEC)
http://seg-web.nict.go.jp/GPS/DRAWING-TEC

Southeast Asian GNSS Networks
Available for Ionospheric Researches
PresentNear Future Image

TID?
EIA?

TID
EIA

Plasma Bubble?

EIA?
TID?

Plasma Bubble

EIATID• Dense and wide‐coverage GPS receiver network can reveal their spatialstructures, propagation directions, and temporal evolutions.
• The GPS‐TEC maps greatly contribute to the ionospheric researches and thenowcast/forecast of space weather.
• However, it is difficult to collect or share the GNSS data in some countries dueto government or institute data policy.

GPS Observation Data （RINEX format）

Filename: ssssdddh.yyo
ssss: marker nameddd: day of the year
h: file sequence number
yy: 2-digit year

Header Partyear, month, day, hour,min, sec, flag, # ofPRNs, PRNs
1 epochGNSS‐TEC exchange (GTEX) format (v0.3)
Filename: ssssdddh.yy_TEC
ssss: marker nameddd: day of the year
h: file sequence number
yy: 2-digit year
Header PartRINEX files used toderive slant TEC
Rec. Position in Lat, Lon, Alt
Types of obs. in RINEX
Types of data productInterval according to RINEX
year, month, day, hour,min, sec, flag, # ofPRNs, PRNs
1 epochGNSS-TEC data sharing based on GTEX• NICT have developed the database of“GTEX” data for more than 6,000 GNSS
receivers in the world. These data are
available via the NICT science cloud,OneSpaceNet (OSN).

• Since the 1st AOSWA workshop held inChiang Mai, Thailand in February 2012,
we are now developing the GTEX data
Detrended TEC overof Thailand, Indonesia, South Korea,Thailand. [Courtesy ofWatthanasangmechai
and China collaborated with KMA, KMITL, K.
(KMITL)]LAPAN, and CMA, respectively.

Detrended TEC overIndonesia by SUGARnetwork.

• We can provide software products
to convert RINEX data to GTEX
data (Fortran 77) , and to makehigh-resolution TEC grid data
(Fortran 77) and map images (IDL).
• NICT recently released a Windows
software “RNX2GTEX” which areavailable via the NICT website.
http://seg-web.nict.go.jp/e-sw/download/index_e.html

Asia‐Oceania Space Weather Alliance: AOSWA
http://aoswa.nict.go.jp• Objective: make a regional linkage of information of space
weather for operations and researches• GTEX data sharing is one of important topics.

The 1st AOSWA workshop at Chiang Mai, Thailand during 2224 February 2012.
- 10 countries, 30 organizations, 76 participants- 41 oral presentations, 21 poster presentations, 1 tutorial lecture- an excursion- business meeting

ICAO Asia and Pacific
Ionospheric studies task force (ISTF)• ICAO plans to use aviation navigations based on GNSS, such as
GBAS and SBAS. ICAO recognizes a necessity to evaluate theionospheric effects on such navigations.
• ICAO Asia and Pacific have discussed about the effect of low-latitude
ionospheric disturbances such as plasma bubble since 2009 andestablished the ionospheric studies task force (ISTF) in July 2011.
• In the 2nd meeting of ISTF
held at Bangkok in Oct. 2012,
the ionospheric data format
for data sharing among
countries were discussed.
• The GTEX format proposed
by Japan (ENRI, NICT) wereadopted as the sharing
format in ISTF.
• GTEX format will be fixed by
the next meeting (Jul. 2013).

Summary• High-resolution TEC observations using dense GNSS receiver
networks can be a powerful tool to monitor and research medium-scale(~100-1,000 km) ionospheric disturbances such as plasma bubble.
• NICT started “DRAWING-TEC” project to expand the high-resolution
TEC observation area with collaboration of ionosphere and GNSS
researchers in the world.
• This project consists of (1) standardizing GNSS-TEC data (GTEXformat), (2) developing dense TEC mapping technique, and (3) sharing
the standardized TEC or GNSS data. The TEC-DRAWING project
would promote studies of medium-scale ionospheric variations and theireffect on GNSS.

Acknowledgement
GNSS receiver data or GTEX-TEC data are provided by GSI, UNAVCO, IGS,SOPAC, CORS, WCDA, CHAIN, PANGA, KASI, KMA, EPN, BKGE, OLG, IGNE, DUT,ASI, ITACyL, ESEAS, SWEPOS, SATREF, BIGF, TrigNet, Geoscience Australia, IPS,RBMC, SUGAR, DPT, and KMITL.