Global Positioning System Reference
In-Depth Information
The ionospheric reconstruction algorithm MART can integrate the STEC from all available
GPS receivers to all GPS satellites visible from each site of the KGN network above a user-
specified elevation cut-off angle (usually 15°). The unknown electron density profile is
expressed in 4-D (longitude-latitude-height and time) voxel basis functions over the
following grid: longitude 124E°-130E° in 1° increments, latitude 33N°-39N° in 0.5°
increments, altitude 100 km - 1000 km in 25 km increments and time: 1 h increments of
linear change in the electron density per voxel. As there are a sparse number of ions in the
ionosphere above 1000 km, the effect on the inversion for ionospheric electron density
profiles is very small and the ionosphere is only considered up to an altitude of 1000 km.
Furthermore, it is faster to invert the unknown ionospheric density parameters because of
the reduced number of unknown variables. In addition, the fewer leaving rays from the
ionospheric space of above defined latitude and longitude range are not used, but are
useful to further obtain ionospheric profiles outside the latitudinal/longitudinal
boundaries space. Using the STEC of all ray paths passing the ionospheric grid cells from
the Korean GPS network, the 4-D ionospheric electron density profiles can be derived
through the tomography reconstruction algorithm. To verify the reliability of GPS
ionospheric tomography reconstruction results, the available ionosonde station (Anyang)
in South Korea provides an independent comparison with the GPS tomographically
reconstructed electron density profiles. The electron density profiles at 25 km height steps
from GPS reconstruction and ionosonde data match well in October and November with a
root-mean-square (RMS) of
11
3
el
× . For example, Figure 11 shows a comparison of
the GPS reconstructed electron density profile at 9:00 UT on 1 October 2003 with the
available ionosonde data at Anyang station (37.39°N, 126.95°E) and the profiles from the
IRI-2007 model. It can be seen that the GPS tomographically reconstructed density profile
is in a good agreement with ionosonde data and the IRI-2007 model, but is closer to the
ionosonde, which confirms the validity of our GPS ionospheric reconstruction approach
(Jin et al., 2006 and 2008).
0.3
10
/
3.3 F-2 layer ionospheric response to storm
It is well known that geomagnetic storms may profoundly affect the global ionosphere and
upper atmosphere, inducing great variations in such parameters as the Total Electron
Content (TEC), the F2-layer peak density (NmF2) and its height (hmF2). These influences
vary with location, season, local time and solar activity. The responses of the ionosphere to
geomagnetic storms have been studied for several decades using moderately priced to
expensive instrumentation, such as ionosondes and Incoherent Scatter Radar (ISR) (Lei et al.
2004). However, it is well known that ionospheric storms have a global impact on ionization,
and under very disturbed conditions the ionospheric response to severe storms often
presents significant changes in the distribution of ionization with latitude and altitude.
Furthermore, ionosondes cannot measure the topside ionosphere and sometimes suffer from
absorption during storms, whereas Incoherent Scatter Radars have geographical limitations.
Nowadays the GPS satellites, being in high altitude orbits (~20,200 km), are very useful for
studying the structure of the entire ionosphere, even the plasmasphere. Moreover, GPS is a
low-cost, all-weather, near real time, and high-resolution atmospheric sounding technique.
Therefore, GPS has been widely used to monitor the ionosphere (e.g. Jin et al. 2004-2007; Yin
et al., 2004).
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