Impact of the low latitude ionosphere
disturbances on GNSS studied with athree-dimensional ionosphere model
Susumu Saito and Naoki Fujii
Communication, Navigation, and Surveillance Department,
Electronic Navigation Research Institute, JapanEIWAC2010, Akihabara, Tokyo, 20101

Contents
1. GNSS and the ionosphere
2. Anomalies in the low latitude ionosphere
3. 3-D ionosphere delay model
4. Studies with the model
4.1.GBAS
4.2.SBAS
5. External monitors
5.1. Backscatter radar as a plasma bubble monitor
5.2. IS radar
6. Summary
EIWAC2010, Akihabara, Tokyo, 20102

Satellite positioning and the ionosphereGroup refractive index2ene�
µ =1+ 28π me �0 f 2

IonosphereGroup delay
Delay

Position
error

40.3
Iρ = 2f� rec

ne (l)dl

sat

Radio wave propagation is delayed by ionospheric plasma thatchange the refractive index
At 1.57542 GHz (GPS L1), 16 cm delay per 1 TECU (1016m-2).
EIWAC2010, Akihabara, Tokyo, 2010
3

GBAS and ionospheric inhomogetuity

Delay

Measured
True range (known)δρg=ρg-ρg0
True delay
True range (unknown)(unknown)

ρu0=ρu-δρuρu0’=ρu-δρg
Estimated

Reference stations
BroadcastδρgMeasured
Broadcasted

Ionospheric delay is one of the largest error source.
Inhomogeneous ionosphere (δρg ≠ δρu) results indifferential correction errors.
EIWAC2010, Akihabara, Tokyo, 2010
4

SBAS (Satellite-based augmentation system)
MSAS (MTSAT Satellite Based Augmentation System)
MTSAT

Master control station (MCS)
Ground monitor station (GMS)
Monitor and ranging station (MRS)GPS

SapporoAugmentation
information

Hitachi-ota
Kobe

Tokyo

HawaiiFukuokaCorrection information (every 5º in latitude
and longitude) based on MCS, GMS, and MRS
observations.
Correction information is sent from MCS to
MTSAT
MTSAT broadcasts correction information

Australia

Naha

EIWAC2010, Akihabara, Tokyo, 20105

Ionospheric Anomaly and SBAS/GBAS

GBAS- Local sharp ionospheric delay gradients resultsin error.
SBAS- Small-scale Ionospheric irregularities may be
miss-detected.
- Ionospheric irregularities smaller than the gridsize (5ºx5º) cannot be well corrected.
Both- Scintillation associated with ionosphericirregularities may degrade availability of GNSS.
EIWAC2010, Akihabara, Tokyo, 20106

Ionospheric anomalies in low latitudes
Storm Enhanced Density (SED)

Equatorial Anomaly
Vertical delay (m)4020Equatorial
anomaly

0
-20
-40
0

120

Longitude

240

360

20

10

0

Plasma Bubble
Vertical delay over Japan on 7 April 2002

50

8.0

40

6.4

30

4.8

[Foster et al., 2002]

Delay (m)TEC (1016m-2)

Latitude

Solar max., March, 11 UT

Equatorial anomaly (Always)- Enhanced ionospheric plasma density
around ±15º magnetic latitude
Plasma bubble (low latitude, frequent)- Local plasma density depletion
SED (mid-latitude, rare)- Increased ionospheric density
associated with severe magnetic
storms EIWAC2010, Akihabara, Tokyo, 20107

Figure 3. Plasma densities measured by ROCSAT-1 along
the trajectory shown in Figure 2. The UT and geographic
longitude at which the four DMSP trajectories crossed thatof ROCSAT-1 are indicated. DMSP F12 and F15 detected
EPBs at approximately the same longitude (115!E) as thedensity depletion crossed by ROCSAT-1.

of MLat are given below.
[25] Figure 8 shows EPB occurrence rates plotted as afunction of MLT. The figure also indicates the MLT
meridians where the four DMSP satellites crossed themagnetic equator. Three empirical points should be noted.
First, occurrence rates were similar in the 2 years. This isconsistent with measurements from the DMSP F14 and F15
satellites that detected EPBs in 17.7% and 18.5% of their
2000 and 2002 orbits, respectively. Second, between 1930
and 2000 MLT the occurrence rates rose to sustained highlevels in both years. Consistent with this result, while it was
still operating in 2000, F11 seldom detected EPBs near
1915 MLT. The orbital planes of DMSP F12, F14, and F15

Plasma bubbles and ionospheric density
variation
ROCSAT-1 orbital plane precesses !7! per day, a similar
distribution of points, advanced/delayed by 2 days, represents MLat locations of the satellite near the 2200/2000 MLT
meridians. The 2100 MLT meridian was chosen for display

Plasma density (ROCSAT-1)

Unique phenomenon in low
latitude.
Extreme depletion in plasma
density inside of a bubble.

plasma buble

Very sharp edges (15-30 km).

6 of 12

Delay (m)Frequent occurrence aftersunset in solar maximum
period.

Vertical delay over Japan on 7 April 2002
TEC (1016m-2)

Sharp spatial gradient inionospheric delay.

[Burke et al., 2004a]

Figure 4. Plasma densities measured on 2 March 2000 by DMSP F12 between 1246 and 1256 UT (left)
and by DMSP F15 between 1326 and 1338 UT (right). In both cases, the deep depletions were observed
at approximately the same longitude (115!E) as the EPB that ROCSAT-1 encountered (Figure 3).

50

8.0

40

6.4

30

4.8

EIWAC2010, Akihabara, Tokyo, 20108

Need of modeling

General characteristics of the plasma bubble is rather
well known unlike storm-enhanced density (SED).
Number of observations of ionospheric gradients withshort baselines are limited: “worst case” may not havebeen recorded yet.
Modeling study based on the large amount of paststudies on plasma bubble should be effective.

EIWAC2010, Akihabara, Tokyo, 20109

3-D plasma bubble shape and ionosphericdelay
plasmasphere
upper ionosphereplasma bubble
virtual thin layer(2D model)
magnetic field

meridional cut

zonal cutPlasma bubble impacts on satellites at west/east low
elevation angles or high elevation were not significant.
Plasma bubble develops along the magnetic field line.
Total delays may be different for the same ionosphericpierce points, which cannot be described by 2-D models.
EIWAC2010, Akihabara, Tokyo, 201010

3-D ionospheric delay model with plasma
bubbles
Background plasma distribution * plasma bubble
Background plasma distribution:-

NeQuick [Giovannni and Radicella, 1990; Radicellaand Zhang, 1995]

Plasma bubble:-

defined on the equatorial vertical plane withequivalent longitude and altitude-

represented as depletion normalized by background(no plasma bubble) density

Written in FORTRAN (Platform independent).
EIWAC2010, Akihabara, Tokyo, 201011

Plasma bubble model

400
200

1

10
0

Vertical delay (m)600

20Latitude

Altitude (km)800

Electron density (/1012 m-3)

High solar activity, March, 11 UT
Vertical delay
Vertical crosssection1000
2
30

100 m/s0-100-100 0 100
zonal distance (km)-20
130135 140
Longitude

16
12
8
4
0

Plasma bubble
parameters- Width: 100 km- Top altitude 800 km- Depletion: 100 %- Scale of wall: 20 km- Eastward velocity:
100 m/s- Rectangular crosssection- Tilted-dipole
magnetic field
EIWAC2010, Akihabara, Tokyo, 2010
12

Test : Slant ionospheric delay variation
Plasma bubble drifts at aconstant zonal velocity.
Receiver is fixed on a
ground.

Plasma bubble

A satellite is picked up
from standard 24 satellite
constellation.
Delay changes as local timegoes by (backgroundchanges), as a satellites
moves, and as a plasma
bubble passes over.
EIWAC2010, Akihabara, Tokyo, 201013

Test: Slant ionospheric delay variation
15
Delay at L1 (m)Delay at L1 (m)Model (Medium solar activity, March)
15
10

5
0
00:00

00:30

01:00

01:30

Example observed at
Okinawa on 24 March 2004

10

5

0
23:30

00:00

00:30

01:00

Slant ionospheric delay at (135ºE, 25ºN) with a plasma
bubble in March with medium solar activity is modeled.
Delay depletion due to a plasma bubble is reproduced.
The result looks similar to observed delay variation.
EIWAC2010, Akihabara, Tokyo, 2010
14

GBAS simulation
Plasma bubble drifts at a constantzonal velocity.
Airborne receiver moves toward
the reference station at a constantvelocity.
A satellite is picked up fromstandard 24 satellite constellation.
Plasma bubble

Positioning errors calculated withthe delays of the reference andairborne receivers.
No monitor neither on the
ground nor airborne.
EIWAC2010, Akihabara, Tokyo, 201015

Plasma bubble model

400
200

1

10
0

Vertical delay (m)600

20Latitude

Altitude (km)800

Electron density (/1012 m-3)

High solar activity, March, 11 UT
Vertical delay
Vertical crosssection1000
2
30

100 m/s0-100-100 0 100
zonal distance (km)-20
130135 140
Longitude

16
12
8
4
0

Plasma bubble
parameters- Width: 100 km- Top altitude 800 km- Depletion: 100 %- Scale of wall: 20 km- Eastward velocity:
100 m/s- Rectangular crosssection- Tilted-dipole
magnetic field
EIWAC2010, Akihabara, Tokyo, 201016

Simulation: Satellite geometry and plasma
bubble location (2)Background parameters- season: March- solar activity: high- UT = 11 at t = 0
Receiver- ground: 135ºE, 25ºN
- air: 134.6ºE, 25ºN, 80 m/s. (Approach to RW09)
Satellite geometry- The worst case geometry of (1)Run simulations by changing the plasma bubble
initial location from 130 to 140ºE.
EIWAC2010, Akihabara, Tokyo, 201017

Satellite geometry and plasma bubble location:
Result with the worst vertical error
N

40
20

Distance (km)
30
20
10

0
W

E

Error/VPL [m]

10
S

0

-10

-20
11:00

Total
VerticalHorizontal
North
East
VPL
11:04
11:08
UT

Seven visible satellites.
10.2 m vertical error at 6 km
from the reference station.
Two satellites were impacted.
The southward low elevation
satellite was mainly impacted.
The impact on the highelevation satellite was not
significant.
EIWAC2010, Akihabara, Tokyo, 201018

Satellite geometry and plasma bubble location:
Result with the worst vertical error
N
40
20

Distance (km)
30
20
10

0
W

E

Error/VPL [m]

10
S
0

-10

-20
11:00

Total
VerticalHorizontal
North
East
VPL
11:04
11:08
UT

Seven visible satellites.
1.2 m vertical error at 6 km
from the reference station.
One satellites was impacted.
Small error mainly due tosmall delay difference.
EIWAC2010, Akihabara, Tokyo, 201019

Low latitude ionosphere effects on SBAS(ENAC-ENRI Internship project)

Impacts of the low latitude ionosphere on SBAS has
been studied with simulations using the 3-D
ionosphere model.
- Strong ionospheric gradient associated with theequatorial anomaly makes it difficult to derive
ionospheric correction term.
- Plasma bubbles are hardly detected by SBAS groundmonitor stations and result in large user error.
Further studies are planned to be conducted.
- More simulations with different conditions- Optimal distribution of ground reference staitons- Backscatter radar monitoring of plasma bubbles for
SBAS
EIWAC2010, Akihabara, Tokyo, 201020

External monitors

GNSS measurements are “point measurements”.
- There are a lot of “blank” area.
- GNSS measurement are used both fornavigation and monitoring: not independent
A technique to monitor ionopsheric anomalies
effectively in a wide area would be useful.
- It should be independent of GNSS signals:
external monitor
There are a number of techniques that have beenused to study the ionosphere.
EIWAC2010, Akihabara, Tokyo, 201021

Plasma bubble detection by radar

Plasma bubble accompany
plasma irregularities ofvarious scale sizes fromkilometer down to meter.
Irregularities can bedetected effectively by abackscatter radarusing VHF band.
Backscatter radar can
detect plasma bubbles.

Plasmabubble(a) Radar backscatter(b) Airglow(ionospheric density)(c) Backscatter map
on airglow[Otsuka et al., 2004]
EIWAC2010, Akihabara, Tokyo, 201022

Backscatter radar
plasmairregularity
B (magnetic
field)

λradarλirregularity

2 λirregularity = λradar=> scattered waves
are in-phase

Detects echoes scattered byplasma irregularities
Intensified echo when radar
wave vector is twice theirregularity wavelength(Bragg scattering)• 2 kradar = kirregularity
Irregularities aligned withmagnetic field => radar beam
perpendicular to magnetic
field for strong echo
• kradar • B = 0

Radar

VHF band (typically 30-50
MHz) is often used.
EIWAC2010, Akihabara, Tokyo, 201023

Backscatter radars

103 m

560-Yagi circular array
Nagoya Univ. VHF radar (30.8 MHz,
Indonesia)

Southward distance
From EAR (km)Equatorial Atmosphere Radar (47 MHz, Indonesia)

Echo power on 28 March 2006
South/High~1600 km above the300
magnetic equator
150

0
East

-250
0
250
West
Zonal distance From EAR (km)[Saito et al., 2008]
Echo power in one of the beams on 31 March 2006

18-Yagi linear array[Courtesy of Y. Otsuka]

2-D image of plasma bubbles by electronic beam swinging
Wide coverage area
Cost effective (Nagoya Univ. Radar: ~ 200,000 USD)EIWAC2010, Akihabara, Tokyo, 201024

Backscatter radar coverage•20ºN, 135ºE•8 beams (azimuth

Radar coverage is determinedby the geometry of radar
beams and the Earth’s magnetic
field.

angles -50, -35, -20,
-5 10, 25, 40, 55º)•range 300-1500 km

350 km
450
550
650
750
850
950
Radar

Plasma bubble develops along
the magnetic field- Radar monitors “magnetic
field line”.
- Different covered area fordifferent altitudes.
Magnetic conjugate area in theother hemisphere is also
covered.
EIWAC2010, Akihabara, Tokyo, 201025

Simulation Study of Backscatter Radarfor GBAS
Three major blocks:
1. Ionosphere delay model- 3-D model with plasma bubbles [Saito et al., ION
GNSS 2009]
2. Backscatter radar observation model- Multi-beam radar- Reject satellites of which ray-paths seen from theground reference pass through plasma bubbles
3. GBAS simulation- Range correction and positioning error estimation
for an approaching airplane- Based on the information from the blocks 1 and 2
EIWAC2010, Akihabara, Tokyo, 201026

Backscatter radar observation model

Radar beam

B
Plasma bubble
expected volume

Magnetic
equatorecho

OK

To conjugate
points in theotherhemisphereNG

plasma bubble
magnetic field
North

South

Backscatter radar detects plasma bubbles in the radar beams
Satellite ray-paths crossing the same magnetic field lines as
the detected plasma bubbles are rejected.
EIWAC2010, Akihabara, Tokyo, 201027

Radar location and effects

Error/VPL [m]No monitor
Distance (km)40 30 20 10 0
40

Radar at 10ºN

20

Total
VerticalHorizontal
North
East
VPL

Radar at 20ºN

Radar at 30ºN

detecteddetected

misseddetected

0

-20
11:00

11:04
UT

11:00

11:04
UT

11:00

11:04
UT

11:00

11:04
UT

11:08

Radar at too high latitudes may miss-detect plasma
bubbles and error may remain.
Closer to the magnetic equator, the more effective.
EIWAC2010, Akihabara, Tokyo, 201028

Incoherent scatter (IS) radar
Plasma density

Plasma bubbles[Hysell et al., 2006]

Frequency(MHz)

Peak Power
(MW)

MU

46.5

1

Jicamarca

50

3

ALTAIR

422/160

2.5

Arecibo430

2.5

Poker Flat

450

1.3

EISCAT Svalbard

500

1

EISCAT

933/224

1.5/5

Millstone Hill

1290/440

2.5/5

Sondrestrom

1290

3.5

ARSR

1300

2

ALTAIR radar

Electron density (and basic plasma parameters) can be measured directly.
The most powerful tool to monitor the ionosphere.
ARSRs that are being decommissioned can be converted to IS radar.
EIWAC2010, Akihabara, Tokyo, 201029

Summary (1)A 3-D ionospheric delay model that account for the low
latitude ionosphere including the equatorial anomaly andthe plasma bubble has been developed.
The model is a very useful tool to examine the impacts of
the low latitude ionospheric anomalies on GNSS
applications.
GBAS-

3-D ionosphere delay model can be used to study theeffect of ionospheric anomalies in more realistic manner.

-

The model was used to validate the baseline SARPs of
GAST-D (single-frequency CAT-III GBAS).
EIWAC2010, Akihabara, Tokyo, 201030

Summary (2)SBAS- Strong ionospheric gradient associated with theequatorial anomaly makes it difficult to derive
ionospheric correction term.
- Plasma bubbles are hardly detected by SBAS groundmonitor stations and result in large user error.
An external plasma bubble monitor by a backscatterradar is proposed and its effects are investigated withthe 3-D ionosphere model.
- Backscatter radar monitor can significantly reduce
the potential error caused by the plasma bubble.
ARSRs that are going to be decommissioned could be
converted to IS radars to monitor the ionosphericdensity directly, because the frequency and power issuitable for IS measurements.

EIWAC2010, Akihabara, Tokyo, 201031