Ground GNSS Ionosphere Sounding - GNSS Remote Sensing: Theory, Methods and Applications - page 76

point, a stochastic process is used to describe VTEC's variation as time. Parameters
which need to be estimated are the VTEC values of grids.

X

VTEC IPP D W .; '; i/VTEC i C b r C b s (4.26)

i D V A ;V B ;V C

b r , b s are instrument biases. W ( ,', i ) is the weight which is related to the distance
between the grid points to the ionosphere pierce points.

4.3.2 Applications of 2-D GNSS TEC

Nowadays, ground-GNSS observations have been widely used to monitor the
Earth's ionosphere, particularly for global and mid-small scales ionospheric varia-
tions and disturbances from continuous GNSS observations. On one hand, the global
ionospheric models (GIM) with more and more accuracy could correct ionospheric
delay for L-band observations, e.g., InSAR and Radar Altimetry. On the other hand,
GNSS-derived TEC can be used for ionospheric research and useful signals or
indicators of some activities from natural sources (e.g., earthquake and volcano)
and anthropogenic sources (such as rocket launching, explosion and so on).

4.3.2.1 TEC Climatology

The total electron content (TEC) is one of important parameters in the Earth's iono-
sphere, related to various space weather and solar activities. However, understanding
of the complex ionospheric environments is still a challenge due to the lack of direct
observations, particularly in the polar areas, e.g., Antarctic. Now more and more
continuous operational GPS stations are set up, GPS has become a powerful tool to
monitor global and region TEC. The international GNSS service (IGS) has routinely
produced global vertical TEC maps every 2 h for more than 10 years, which can
be used to investigate the ionospheric TEC climatology at seasonal and secular
scales. Figure 4.5 (upper) shows obviously annual variation of long GPS TEC time
series over Antarctica at the point (120E ı , 87.5S ı ), and the mean values of vertical
TEC over the Antarctica have a slight decrease (Jin and Jin 2013 ). The 2,880 MHz
solar flux variations during 2002-2012 are also shown in Fig. 4.5 (bottom). The
vertical TEC variation shows agreement with the solar activities. Higher amplitudes
of vertical TEC time series are found in around 2003 and 2011, which are higher
solar activities years. Although, for the most time, the Sun is not visible in areas near
the South Pole in Antarctica, the variation of amplitudes is obvious. In addition, the
mean values and the standard deviations of about 10 years GPS TEC series over
Antarctica increase with the latitude degrease at latitudinal circles. Probably, this
phenomenon can be explained due to the differences of solar radiation in different
latitude. Solar fluxes become smaller and smaller when the location is getting closer