Global Positioning System Reference
In-Depth Information
12:10 UTC
Azimuth-elevation sky map
North
12/23/94Z
42.35N
71.08W
0.00km
7 deg 3D
19
17
27
16
West
East
6
23
24
26
PDOP 2.8
HDOP 1.6
GPS
South
Figure 7.17
Azimuth/elevation plot.
Another feature of mission planning software involves determining the number
of satellites that will be visible at a location over the course of a day. This informa-
tion is useful for applications that may require the maximum number of visible sat-
ellites. Figure 7.20 displays the number of visible satellites over a 24-hour period
with all of the satellites operational. As shown in this figure, the minimum number
of satellites available is 6 and the maximum is 10 for this location. At lower lati-
tudes (near the equator), it is possible to have 12 or more GPS satellites visible at a
time.
Availability prediction software generally also plots DOP over the course of the
day.
7.5
GPS Integrity
In addition to providing a position, navigation, and timing function, a system such
as GPS must have the ability to provide timely warnings to users when the system
should not be used. This capability is known as the integrity of the system. Integrity
is a measure of the trust that can be placed in the correctness of the information sup-
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