The change in satellite to receiver
signal propagation time due to the
ionosphere is directly proportional to
the integrated free-electron density
in a column of one meter-squared
cross-section along the signal path.
Ionosphere consists of a shell of consists of a shell of
ionized gas plasma surrounding the
Earth between altitudes of about 80
and 1000 Km. In order to model the
TEC parameter over the test area GPS
data from our permanent station named
AUT1 was used. The ionospheric
model is a single-layer model at 350
km from the earth's surface. The model
makes the assumption that all the
electrons are concentrated in a spherical
layer at height H above the earth's
surface. The observable is the plain
difference of the L1 and L2 phases
(L1-L2, both expressed in meters), the
parameters are the coef?cients of a two
dimensional Taylor series development
as given in equation 1 (the independent
arguments are geocentric latitude and
hour angle S of the sun) of the electron
density of a single layer model of the
ionosphere. Therefore for each session
data an electron density containing
?ve parameters is written as follows:
where: S is the hour angle of the sun
which de?ned as the angle between the
sun and the local meridian. The local
solar time t is related to the hour angle
In October 2003 the GPS rinex data,
with 30-sec sampling rate cut-off angle
of 150 along with precise satellite orbits
and clock corrections, was selected
in order to generate ionospheric TEC
data. The GPS data was processed with
Bernese software and the parameters
of equation 1 were estimated using
a least squares algorithm (for
more details see Wild, 1994).
Global information for Solar
and Geomagnetic activity
Due to the dependence of the
ionosphere on the solar radiation and the geomagnetic ?eld, ?rst the sun
spot activity was examined using the
relevant information of NOAA National
Geophysical Data Center, which
provides scienti?c products and services
for geophysical studies. It is known that
the solar cycle varies over a period of
typically 11 years from trough to peak
and then back to another low point. The
conventional index of the cycle is known
as the sunspot number (SSN) which is
de?ned from observations of the sun by
optical telescopes and can be used as
a predictor of ionospheric disturbance.
The increment in sunspots induces a
series of solar ?ares and coronal mass
ejections. Solar ?ares are explosions
on the surface of the sun that produce
a quick directed high-speed blast of
charged particles and electromagnetic
energy. The SSN numbers of October
2003 are presented in Figure 1. As it is
clearly shown the days from 14 to 16
are characterized as quiet with SSN less
than 20, while for 27 to 31 the SSN is
over 160, which means storm event.
In order to have the best possible
global information about the
ionospheric activity we have also
investigated the magnetic conditions
using the available information
from the Data Analysis Center for
Geomagnetism and Space Magnetism
of Kyoto University in Japan.
Variations in Earth's magnetic ?eld are
measured by magnetometers. There are
two measurements readily available
from magnetometer data, the daily A
index and the three-hour K index. The
A is an average of the eight 3-hour K
indices, and uses a linear scale and
going from 0 (quiet) to 400 (severe
storm). The K index uses a quasilogarithmic
scale (which essentially is
a compressed version of the A index)
and goes from 0 to 9 with 0 being quiet
and 9 being severe storm. Generally an
A index at or below 15 or a K index at
or below 3 is best for propagation. As
the K values increase, the severity of
the storm increases. Figure 2 illustrates
the state of geomagnetic conditions
(Kp index, p stands for planetary)
for the days of October 2003.
Same results with ?gure 1 are shown
especially for days between 28 to
early of 31 where the geomagnetic
activity was very high and reached at
its maximum value of 9. From many
researchers these days have been
Days of month characterized as one of the most exciting space weather events in recent
history. For comparison purposes and
in order to better understand the event
and its impact on GPS positioning we produced two ionospheric maps for
the broader area of Thessaloniki where
the permanent GPS station is located.
All the maps were generated using
the Generic Mapping Tools. The ?rst
map is for the 16th day of October on
10 UT time which in general is under
quiet geomagnetic and solar activity
with SSN number almost 20 and
Kp index less than 4. We processed
every one hour the GPS data and the
produced results are illustrated on
?gure 3. It is clearly shown that the
ionospheric conditions are at very low
order of magnitude with TEC values
(at 10 UT) between 13.35 to 13.39
TECU (TEC Units). On the contrary,
during intense storm conditions with
SSN 170 and Kp equal to 9 as it was
expected, TEC values for the 29th
day of October at 10 UT are over 5
times bigger and reached 67.6 TECU.
For these geographic latitudes (40-
42N), TEC values greater than 50
TECU re?ect the enhanced storm
phenomena associated with large
gradients and high ion ?ux values.
Figure 4 illustrates the TEC values
in the broader area of the permanent
station where also a different spatial
variability- plume is sparsely shown. |
In this study a rapid method for
monitoring ionospheric variations
using data from only one permanent
GPS station is presented. TEC data
are estimated using the iono-model
of single layer where dual frequency
GPS data are used in a least squares
procedure. The model was found
to be adequate during quiet and
storm conditions, which indicates
that, for surveying applications
in the vicinity (up to 100 Km) of
permanent GPS tracking stations, the
ionosphere models distributed by the
permanent station are suf?cient for
use. TEC maps are produced with
time resolution of one hour, which
is a rapid scheme of knowing space
weather conditions. Therefore a user
can be quickly informed whether
a day is characterized by a high
ionospheric activity for GPS signals,
in order for him to better handle the corresponding errors. Also, knowing
space weather conditions in a short
time interval can be found useful for
telecommunication systems, as well
as navigation systems. Furthermore,
more reliable ionospheric results using
more sophisticated models for bigger
areas or countries can be produced, if
data from several GPS receivers, being
part of permanent networks, combine.
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