GNSS Atmospheric and Multipath Delays - GNSS Remote Sensing: Theory, Methods and Applications - page 27

The ionospheric delay is related to TEC and proportional to the square of signal
frequency when the first order of the refractive index is just considered.

2.3.3 Empirical Ionospheric Models

2.3.3.1 Bent Model

The Bent model is directed towards world-wide empirical ionospheric model using
bottom side ionospheric sounders' data and topside sounding measurements (Bent
et al. 1972 ), which can provide the parameters of the ionosphere from the electron
density profiles, such as TEC and ionospheric delay. In this model, topside of
ionosphere is expressed as three layers and one parabola layer, and the bottom of
the ionosphere is expressed as double parabolic layer. Through inputting the date,
time, station's location, solar flux and sunspot number into Bent model, TEC can be
computed at any time and locations.

2.3.3.2 IRI Model

The first version of International Reference Ionospheric (IRI) model was released
by International Union of Radio Science (URSI) and Committee on Space Research
(COSPAR) in 1978, which is called IRI 1978. With the development of techniques
and the accumulation of measurement data, IRI model has been updated frequently.
Nowadays, the latest version is IRI 2007 (Bilitza and Reinisch 2008 ). Data sources
of IRI model include the world-wide ionosondes, the powerful incoherent scatter
radars, the ISIS and Alouette topside sounders, in situ instruments and several
satellites and rockets. In this model, parameters, such as electron density, ion
density, electron temperature, ion temperature, main components of positive ions
et al., are expressed with mathematic formula. Ionospheric electron density profile
can be derived easily after inputting date, time, location, sunspot number and some
other parameters. Then the TEC and ionospheric delay can be calculated. This
model has been used widely in the ionosphere research and radio science. However,
the International Reference Ionospheric (IRI) model as an empirical model, which
just gives the mean variation of the ionosphere and cannot give more details.

2.3.3.3 Klobuchar Model

Klobuchar model was designed by Klobuchar ( 1987 ) for single-frequency GPS
users. In this model ionospheric time delay is considered as a constant in the night,
which is equal to 5 ns, and a cosine function on the daytime. Time delay T g can be
mathematically expressed as follow.

D D c C A cos 2
B

T g .t T P / (2.23)