Global Positioning System Reference
In-Depth Information
The terms used in computation of ionospheric delay are as follows:
Satellite-Transmitted Terms
α
n
: The coefficients of a cubic equation representing the amplitude of the
vertical delay (4 coefficients - 8 bits each).
β
n
: the coefficients of a cubic equation representing the period of the model
(4 coefficients - 8 bits each).
Receiver-Generated Terms
ξ
: elevation angle between the user and satellite (semicircles).
A
: azimuth angle between the user and satellite, measured clockwise positive
from true North (semicircles).
L
u
: user geodetic latitude (semicircle).
l
u
: user geodetic longitude (semicircle).
GPS time: receiver-computed system time.
Computed Terms
x
: phase (radians).
T: obliquity factor (dimensionless).
t
: local time (sec).
φ
m
: geomagnetic latitude of the earth projection of the ionospheric intersection
point (mean ionospheric height assumed 350 km) (semicircle).
λ
i
: geomagnetic latitude of the earth projection of the ionospheric intersection
point (semicircle).
φ
i
: geomagnetic latitude of the earth projection of the ionospheric intersection
point (semicircle).
ψ
: earth's central angle between user position and earth projection of iono-
spheric intersection point (semicircles).
5.16 TROPOSPHERIC MODEL
(
11
)
Compared with the ionospheric effect, the tropospheric effect is about an order
of magnitude less. The satellites do not transmit any data to correct for the
tropospheric effect. There are many models to correct the error. Here only a
simple model will be presented. The delay in meters is given by:
(
11
)
2
.
47
sin
ξ
+
0
.
0121
meters
=
(
5
.
19
)
where
ξ
is the elevation angle between the user and satellite.
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