ENRI Int. Workshop on ATM/CNS. Tokyo, Japan (EIWAC2010)[EN-107] Impact of the low latitude ionosphere disturbanceson GNSS studied with a three-dimensional ionosphere model+∗

S. Saito ∗ N. FUjii ∗Communication Navigation and Surveillance Department
Electronic Navigation Research Institute (ENRI)
Tokyo, Japan[susaito | fujii ]@enri.go.jp

Abstract The ionosphere is one of the most serious error source in GNSS. Among other error sources, the ionosphereinduced error is the most diﬃcult one to correct it, because of its dynamic variability. Since the behavior of theionosphere is diﬀerent for regions, especially for magnetic latitudes, ionospheric threat model suitable for each regionis needed for advanced use of GNSS in aviation. This paper introduces a three-dimensional ionosphere delay model
for low latitude ionosphere developed in ENRI. Studies with this model on the ionospheric impacts on aviation use of
GNSS are presented.
Keywords GNSS, Ionosphere, CNS

1

INTRODUCTION

must be cared. To evaluate such events, modelingstudies based on the knowledge obtained in the long
history of the ionosphere studies are eﬀective. At
the Electronic Navigation Research Institute (ENRI),a three-dimensional ionosphere model that takes into
account the equatorial anomaly and the plasma bubble has been developed to study the impact of the low
latitude ionosphere on GNSS[1, 2].

Spatial gradient of the ionospheric plasma density
is one of the most important error source of diﬀerential GPS (DGPS) systems, such as a ground-based
augmentation system (GBAS) or a space-based augmentation systems (SBAS). In the low magnetic latitude regions, eﬀects of low latitude ionospheric phenomena, especially the plasma bubble and the equatorial anomaly must be taken into account carefully.
The important parameter for GNSS is the ionospheric delays that is proportional to the total number of electrons, or total electron contents (TECs), between a satellite and a receiver. The ionospheric delay
I is given by
40.3
I = 2 T EC
(1)fwhere f is the frequency of the satellite signal in Herzand T EC is the number of electrons integrated overthe satellite-receiver path and expressed in the unit ofelectrons per square-meter given as∫ sat
T EC =
ne ds
(2)

2

LOW LATITUDE IONOSHERE

There are two major low latitude ionospheric phenomena that accompany signiﬁcant spatial inhomogenuity of the ionospheric delay. One is the equatorial
anomaly, and the other is the plasma bubble.
2.1

EQUATORIAL ANOMALY

Equatorial anomaly is a pair of belts where ionospheric plasma density is high and having peaks around±15◦ in magnetic latitude. Ionospheric delay also has
the maxima at the equatorial anomaly crests. Fig. 1
shows an example of global ionospheric vertical delay
distribution reproduced by an empirical model. Equatorial anomaly always exists, but the location and intensity signiﬁcantly change in time to time. At theequatorward and poleward edges of the equatorial anomaly,
there are large-scale ionospheric delay gradient.
For GBAS, the large-scale gradient increases the
background ionosphere variability and would act toincrease the protection level. For SBAS, the largescale gradient could have more signiﬁcant impact because it assumes smooth variation of the ionospherein space.

recwhere ne is the number density of electrons per cubic
meters.
In the past studies, observed TECs are used to develop ionosphere models. However, since the ionospheric structures are constraint by the Earth’s magnetic ﬁeld, they have characteristic three-dimensional
anisotropic features. To evaluate the impacts of theionosphere appropriately to have more optimized ionosphere threat model, the ionosphere should be treatedthree-dimensionally.
Since the integrity requirement is extremely high,quite rare events that may not have ever been observed271

S. Saito

0120

from the empirical model. NeQuick model [3, 4] isadopted as the background ionosphere model. Using
the NeQuick model, TEC along an arbitrary path canbe calculated. The equatorial anomaly is described inthis model.
Another component is the depletion due to theplasma bubble. It is expressed as relative depletionto the background and deﬁned in the vertical planeover the magnetic equator. Multiplying the relative
depletion to the background delay, the total electron
density at the particular point is calculated. Integrating the electron density along the ray path, the TEC,
and hence the ionospheric delay between the satellite
and the receiver is derived.
Example of modeled depletion in ionospheric delays due to a plasma bubble is shown in Figure 4. Figure 4a shows the zonal-altitudinal cross section of theionospheric electron density over the magnetic equator. The vertical ionospheric delay distribution withthe plasma bubble at 133.7◦ E at the magnetic equatoris shown in Figure 4b.

20

40
20
0
−20
−40

Delay at L1 (m)Latitude (deg.)

Ionospheric delay for high solar activity at 11 UT in March

10

0

240
Longitude (deg.)

Figure 1 Global distribution of ionospheric verticaldelay reproduced by an empirical model for the L1
frequency for high solar activity at 11 UT in March.

Delay at L1 (m)Latitude (deg.)

Vertical delay over Japan on April 7, 2002

8.0

6.4

4.8
Longitude (deg.)

4 STUDIES WITH THE MODEL

Figure 2 Vertical delay over Japan at 21:28 JST on
April 7, 2002.

2.2

4.1

PLASMA BUBBLE

The plasma bubble is a low-latitude and equatorial ionospheric phenomenon where the low density
plasma in the bottom-side ionosphere explosively rises
to the topside.
Inside the plasma bubble, plasma density is extremely lower than that of outside. The boundaryis very sharp, and the local plasma density changes
100 % to less than 10 % within a few tens kilometer.
Therefore, the ionospheric delay gradient at the edgesof the plasma bubble can be extremely steep.
Fig. 2 shows the observed depletions in ionosphericdelay associated with plasma bubbles. Two depletions elongated in the North-South direction around
130 and 135◦ E are caused by plasma bubbles.
The plasma bubble has a very characteristic threedimensional structure. The shape of the plasma bubble is aligned with the earth’s magnetic ﬁeld lines. It
is a cleft of the ionosphere with a fan-like shape extended in north-south and very thin in east-west (Fig. 3).
Therefore, it is tallest over the magnetic equator, andthe latitudinal extent is determined by the top altitude
at the magnetic equator. To account for the characteristic shape of the plasma bubble appropriately, athree-dimensional ionosphere model is developed andused in this study.

3 3-D IONOSPHERE DELAY MODEL
The model consists of two major components. One
is the background ionospheric density distribution taken

272

GBAS

The eﬀects of the plasma bubbles on GBAS wasstudied by Saito et al. (2010) [2] to show the potential
threat that undetected error may exceed the limit prescribed by ICAO [5]. It has been shown that the range
error due to the plasma bubble is largest when the raypath between a satellite and a receiver passes along
the plasma bubble edges in the equatorial anomaly at
low elevation angles. Dependence of the threat on theapproach direction is also studied.
Fig. 5a shows an example of simulated GBAS user
error without monitors. In this simulation, one plasma
bubble is assumed. An airplane is assumed to approach eastward to a GBAS reference station.Parameters
used in the simulation are summarized in Table 1.
In this simulation, two satellites are impacted by theplasma bubble (Fig. 5c). The results shows that the
expected vertical error is larger than 10 m all the time.
This study is further extended to validate the requirements for anomalous ionosphere mitigation deﬁned in the baseline SARPs of GAST-D, which is the
single-frequency CAT-III GBAS [6]. Exhaustive investigation on this issue are being done [7].
4.2

SBAS

For SBAS which aims at wide-area augmentation,
both the plasma bubble and the equatorial anomaly
can be a serious problem. To expand SBAS service
areas to low latitude regions, both the issues must be
cleared. Ionosphere characterization is a key point toassure integrity of the system. However, it requires
huge amount of data, and it takes long time to collectthem. Studies based on realistic models are suitable

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan (EIWAC2010)(a)

Table 1 Parameters of simulation.
Background ionosphere

x 10
2

900

1.8

800

1.6

25

700

1.4

20

600

1.2

500

1

400

0.8

12

High solar activity, March, 11 UT
16
35
30

14

193 (high solar activity)

12

Local time

20 LT (11 UT)
Plasma bubble

Initial longitude134◦ E300

0.6

Zonal width

100 km

200

0.4

15

10

10

8
5

Vertical delay (m)March equinox

Latitude

SeasonAltitude (km)3

(F10.7)

Electron density (m )Solar radio ﬂux index(b)Electron density
1000

6

0
−5

4

−10

Depletion level

100 %

100

0.2

2
−15

Equatorial height1500 km

Scale length of boundary

20 km

Eastward velocity

100 m s−1

0
−100
0
100
Zonal distance (km)25◦ N, 135◦ E

Carrier-smoothing

100 sec

135
140
Longitude

0

Figure 4 (a) Zonal-altitudinal cross section of themodeled electron density over the equator associatedwith a plasma bubble. (b) Vertical ionospheric delay
at the L1 frequency with the plasma bubble at 133.7◦ E
at the magnetic equator.

Reference station
Location−20130

Airplane
Initial location

25◦ N, 134.6◦ E

Velocity

80 m sec−1
Satellites

Constellation

RTCA standard 24 satellites

Elevation mask

5◦

(a)

(b)
Plasmasphere

Plasmasphere

Upper ionosphere

Upper ionosphere>~ 1000 km>~ 1000 km

Plasma bubble

Plasma bubble

Magnetic field

West

Magnetic equator
South

North

East
~ 100 km~ 5000 km

Figure 3 Illustration of the plasma bubble shape in (a)meridonal and (b )zonal planes.

273

for this purpose, because various conditions can becreated and tested.
At ENRI, constructing an ionosphere threat model
for MTSAT Satellite-based Augmentation System (MSAS)
has been tried by using the three-dimensional ionosphere delay model. It has been shown that the equatorial anomaly has a great impact on the threat model
to increase the uncertainty associated with the ionospheric inhomogenuity. It is because the change in
the ionospheric delay associated with the equatorial
anomaly cannot be well described by the ﬁtting algorithm that takes into account ﬁrst order variation in
space to estimate ionospheric delays at the ionospheregrid points. For plasma bubbles, it is found to be diﬃcult to be detected by the ground monitoring stations.
This means that the plasma bubble can threaten users.
Further studies are now being conducted.

S. Saito

(a)
30

40
40

Distance (km)
20

(b)
10

0

40
40

30

Distance (km)
20

(c)
10

0
N

VerticalNorth
East
Horizontal
Total
VPL
20

09
02

20

20

06

Error (m)Error (m)

W

0

15

0

E

08

19
S

−20
11:00

11:02

11:04
UT

11:06

11:08

−20
11:00

11:02

11:04
UT

11:06

11:08

Figure 5 (a) Simulation results without monitors. Parameters used in the simulation are summarized in Table 1.
Red, green, blue, cyan, and black lines show the positioning errors in the east, north, and vertical (upward) directions and the total error, respectively. Brown line shows the vertical protection level. (b) Same as (b), but with abackscatter radar monitor. Parameters used in the simulation are summarized in Table 2. (c) Skyview plot of thesatellite positions. Concentric circles show the elevation angles of 0, 30, and 60◦ . Satellites with its number in red
color are impacted by the plasma bubble.

4.3

PLASMA BUBBLE DETECTION BY A VHF
RADAR

Table 2 Parameters of radar monitor.

The plasma bubble is known to accompany plasma
irregularities of various scale sizes from kilometers
down to meters. Using a VHF radar that transmits
VHF radio waves (usually at 30–50 MHz), intense
backscatter echoes from the irregularities can be detected, when the radar beam is pointed in the direction perpendicular to the wavefronts of the irregularities that are parallel to the magnetic ﬁeld line, because
plasma irregularities develop along them. By swinging radar beams, the ionosphere in a wide area can bescanned to monitor whether any plasma bubbles exist
in the radar coverage.
To investigate how useful such backscatter radars
are for GBAS, a simulation study has been conducted[8]. In the simulation, any satellites of which ray
paths to the ground reference station passes any portions of a plasma bubble that can be detected by aradar are removed from GBAS correction (Fig. 6).
Fig. 5b show the result of a simulation with a backscatter radar monitor with the same parameters as Fig. 5a.
It can be seen that positioning error of a GBAS user isdrastically reduced from more than 10 m without any
ionosphere monitors to almost zero with a radar.
Further simulations can be conducted to investigate how much extent such a backscatter radar can beutilize to monitor plasma bubbles, not only for GBAS
but also for SBAS. Optimal location of a radar canalso be investigated.

Location20◦ N, 135◦ E

Number of beams

8

Beam width5◦

Beam directions-50, -35, -20, -5 10, 25, 40, 55◦

Observation range

300–1500 km

5

SUMMARY

In this paper, the three-dimensional ionosphere delay model that takes the plasma bubble into accountand suitable for low latitude regions. Several studies with the model to evaluate the ionospheric impact
on GNSS for aviation are presented. For GBAS, themodel is utilized to establish the international standards for CAT-III GBAS as well as to study the newly
proposed external ionosphere monitoring system. For
SBAS, impacts of the equatorial anomaly and the plasma
bubble on the ionosphere characterization are estimated.
The model is shown to be very much useful to develop
systems based on GNSS.

274

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan (EIWAC2010)

Satellite (Rejected)

References

Satellite (OK to use)

[1] Saito, S., T. Yoshihara, and N. Fujii, “Development of an ionospheric delay model with plasma
bubbles for GBAS,” Proceedings of ION 2009
International Technical Meeting, 2009.

Radar beam
Plasma bubble
expected volume
Echo[2] Saito, S., T. Yoshihara, and N. Fujii, “Study ofeﬀects of the plasma bubble on GBAS by a threedimensional ionospheric delay model, ” Proceedings of ION GNSS 2009, 2009.

Radar
Reference station[3] Di Giovanni, G. and S. R. Radicella, “An analytical model of the electron density proﬁle in theionosphere,” Adv. Space Res., 10, 27–30, 1990.

Figure 6 Principle of radar monitoring of plasma bubble.

[4] Radicella, S. M. and M. L. Zhang, “The improved DGR analytical model of electron density height proﬁle and total electron content in
the ionosphere,” Annali di Geoﬁsica, 38, 35-41,
1995.

6 ACKNOWLEDGMENTS
The study on the low latitude ionospheric eﬀects
on SBAS using the model is conducted as an internship program between ENRI and Ecole Nationale deL’aviation Civile (ENAC), France by Mr. Pierre Louvé
under supervision by SS.

[5] ICAO, “ International Standards and Recommended Practices, Annex 10 to the Convention
on International Civil Aviation,” Volume I, Radio
Navigation Aids, Amendment 82, Nov 2007.

Copyright[6] Murphy, T., M. Harris, S. Pullen, B. Pervan, S.
Saito, M. Brenner “Validation of IonosphericAnomaly Mitigation for GAST D,” Working Paper 20, ICAO-NSP-CSG, Montreal, 2010.

Copyright Statement
The authors conﬁrm that they, and/or their company or institution, hold copyright of all original material included in their paper. They also conﬁrm they
have obtained permission, from the copyright holder
of any third party material included in their paper, topublish it as part of their paper. The authors grantfull permission for the publication and distribution of
their paper as part of the EIWAC2010 proceedings or
as individual oﬀ-prints from the proceedings.

[7] Harris, M., “Development of GBAS Ionosphere
Anomaly Monitor Standards to Support Category
III Operations” EIWAC2010, Tokyo, 2010.
[8] Saito, S., and N. Fujii, “Eﬀects of external ionosphere anomaly monitors on GNSS augmentation
systems studied with a three-dimensional ionospheric delay model - a study for GBAS,” Proceedings of the ION GNSS 2010, 2010.

275

276