Global Positioning System Reference
In-Depth Information
satellites and these angles range 92.38 - 130.98 degrees.
(
9
)
The spacing has been
optimized to minimize the effects of a single satellite failure on system degra-
dation. At any time and any location on the earth, neglecting obstacles such as
mountains and tall buildings, a GPS receiver should have a direct line of sight
and be receiving signals from 4 to 11 satellites. A majority of the time a GPS
receiver can receive signals from more than four satellites. Since four satellites
are the minimum required number to find the user position, this arrangement can
provide user position at any time and any location. For this 24-satellite constel-
lation with a 5-degree elevation mask angle, more than 80% of the time seven
or more satellites are in view.
(
9
)
A user at 35 degrees latitude corresponds to the
approximate worst latitude where momentarily there are only four satellites in
view (approximately .4% of the time).
The radius of the earth is 6,378 km around the equator and 6,357 km passing
through the poles, and the average radius can be considered as 6,368 km. The
radius of the satellite orbit is 26,560 km, which is about 20,192 km
(
26
,
560
−
6
,
368
)
above the earth's surface. This height agrees well with references 6 and 9.
This height is approximately the shortest distance between a user on the surface
of the earth and the satellite, which occurs at around zenith or an elevation angle
of approximately 90 degrees. Most GPS receivers are designed to receive signals
from satellites above 5 degrees. For simplicity, let us assume that the receiver
can receive signals from satellites at the zero-degree point. The distance from
a satellite on the horizon to the user is 25,785 km
(
26
,
560
2
−
6
,
368
2
)
.These
distances are shown in Figure 3.1.
FIGURE 3.1
Earth and circular satellite orbit.
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