Global Positioning System Reference
In-Depth Information
TABLE 5.12 Difference Between GPS and UTC Times
Date
GPS-UTC Time (sec)
6 Jan 1980
0 (Start of GPS system time)
1 Jul 1981
1
1 Jul 1982
2
1 Jul 1983
3
1 Jul 1985
4
1 Jan 1988
5
1 Jan 1990
6
1 Jan 1991
7
1 Jul 1992
8
1 Jul 1993
9
1 Jul 1994
10
1 Jan 1996
11
1 Jul 1997
12
1 Jan 1999
13
(121 - 128),
β
2
(129 - 136),
β
3
(137 - 144). These data can be used to correct
the time received from the satellite for ionospheric effect. The applications
of these data are discussed in the next section.
5.15 IONOSPHERIC MODEL
(
3,7-10
)
The atmosphere around the earth will affect the traveling speed of the GPS
signal and cause measurement errors. These errors should be corrected. For GPS
application, the atmosphere is usually divided into two portions: the ionosphere
and the troposphere. Troposphere is the closer of the two to the surface of the
earth while ionosphere is above the troposphere. The troposphere contains neutral
particles and ionosphere contains free ions. The ionosphere will cause a code
delay but a carrier phase advance.
(
10
)
This section presents a correction model
for the ionospheric error.
Besides the selectivity availability (SA), which will be discussed in the next
section, the ionospheric effect can cause one of the most significant position errors
in a GPS receiver. If a receiver operates on both the L1 and L2 frequencies, such
as in a military receiver, the time delay
t
1
at frequency L1 caused by the
ionospheric effect can be calculated as
(
9
)
f
2
t
1
=
δ(t)
(
5
.
8
)
f
1
f
2
−
where
f
1
and
f
2
are the frequencies at L1 and L2 respectively,
δ(t)
is the
measured time difference between frequencies
f
1
and
f
2
from the same satellite.
This
t
1
can be considered as the measured value.
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