Global Positioning System Reference
In-Depth Information
The vertical TEC may vary between 1 TECU and 300 TECU depending on a number of
factors such as local time, geographic location, season, solar activity level etc. The frequency
dependence of the first order ionospheric term has been plotted for elevations 5° and 30° in
Fig. 2 at different levels of ionospheric ionization characterized by vertical TECs such as i)
250 and 150 TECU correspond to TEC during extreme space weather conditions, ii) 50 TECU
corresponds to mid latitude day time and iii) 5 TECU corresponds to mid latitude night time
TEC.
Fig. 2. Frequency dependence of the first order term at different levels of ionospheric
ionization and elevation angles.
As Fig. 2 demonstrates, the first order ionospheric term can be more than 100 m at GNSS
L-band frequencies (1 - 2 GHz) during times of high TEC at low elevation angles.
3.1.1 Second order term
Higher order ionospheric terms include the second and third order ionospheric terms, and
the excess path length. Equations (5, 6, 8) indicate that the second order term depends on the
electron density
n
e
and as well as on the geomagnetic induction
B
. The electron gyro
frequency
f
g
=
eB
/(2
πm
) is usually less than 1.4 MHz. The value of
B
can be derived as ~5x10
-5
Tesla for
f
g
= 1.4 MHz and considered constant throughout the propagation. Thus, for the
worst case condition with
f
g
= 1.4 MHz and Θ = 0, the second order term can be simplified as
(using Eqs. 5 and 8)
7
11.28
×
10
()
2
d
=
TEC
(30)
Igr
3
f
()
2
Ig
d
is measured in meters, TEC in electrons/m
2
and
f
in Hz. Using the above
approximation, the frequency dependence of the second order term at different levels of
ionospheric ionization and elevation angles has been plotted in Fig. 3.
where
Figure 3 shows that during the worst case conditions the second order ionospheric term on
code observables can be as big as about 500 millimeters at GNSS L-band frequencies. Due to
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