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range of 800-1,336km altitude. The ionospheric products deduced from the Doppler
measurements are recorded at each count interval of about 10 s, and are used to derive
the ionospheric TEC. The ionospheric corrections are available at the CDDISwebsite
(CDDIS 2011 ). For more details on DORIS mission refer to Fleury et al. ( 1991 )or
Yin and Mitchell ( 2011 ).
4.3.6 Combination of Different Techniques
Although each of the above mentioned techniques is capable of providing infor-
mation about the ionosphere, each technique has its pros and cons depending on
its characteristics. The classical input data for development of GIM are obtained
from dual-frequency observations carried out at GNSS stations. However, GNSS
stations are in-homogeneously distributed around the world, with large gaps partic-
ularly over the oceans; this fact reduces the precision of the GIM over these areas.
On the other hand, dual-frequency satellite altimetry missions such as Jason-1 (see
Sect. 4.3.2 ) provide information about the ionosphere precisely above the oceans; and
furthermore LEO satellites, such as F3C (see Sect. 4.3.3 ) provide well-distributed
information of ionosphere on globe. Combining different techniques for developing
the ionospheric maps would significantly improve the accuracy and reliability of
the developed model, as the combined model uses the advantages of each particular
method and provides a more accurate result than from each single techniques alone.
Several studies have investigated the development of combined models of the
ionosphere. Todorova et al. ( 2007 ) developed combinedmodels of VTEC fromGNSS
and satellite altimetry data. Alizadeh et al. ( 2011 ) developed models using combi-
nation of GNSS, satellite altimetry and F3C measurements. Both studies aimed at
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VTEC [TECU]
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Fig. 4 GNSS, satellite altimetry, and F3C combined GIM, 17UT, day 202, 2007 (Alizadeh et al.
2011 )
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