Global Positioning System Reference
In-Depth Information
ber 23, 1994. (This and the following predictions are still representative of perfor-
mance circa 2006.) Figure 7.16 shows the location of the GPS satellites and the
satellite horizon line for the selected location at a snapshot in time [12:10 UTC
(USNO)]. The satellites with squares next to them (17, 23, 26, and 27) are the four
used to form the navigation solution. The corresponding HDOP and PDOP for that
point in time also are displayed.
Figure 7.17 is an azimuth and elevation plot that gives the position of the satel-
lites from the perspective of looking at the sky directly overhead from the selected
location. The user is at the center of the concentric circles, with the outermost circle
representing 0º elevation, or the horizon. The second circle is at 7º elevation, which
was the selected mask angle for this portion of the analysis. The third is at 10º, and
each circle increases by 10º. The azimuth is 0º at North and increases in the
clockwise direction.
Figure 7.18 displays the rise and set time for the 25 GPS satellites at the selected
location over a 24-hour period. (There generally are more than the nominal 24 GPS
satellites in orbit in order to maintain a constellation of 24 operational satellites.)
This type of graph can be very useful for a researcher who wants to plan an experi-
ment with a particular set of satellites and doesn't want the satellite geometry to
change significantly due to a rising or setting satellite.
The solid line at the bottom of the graph indicates that GPS is available for the
entire day (PDOP
6). Gaps in this line would indicate that GPS is unavailable. This
unavailability is demonstrated in Figure 7.19 when satellites 16, 25, and 26 are
removed (by the author) from the constellation for simulation purposes. As shown
in this figure, removing three satellites from the GPS constellation would result in
two outage periods during the day for Boston.
≤
Satellite ground traces on a Mercator map
12:10 UTC
12/23/94Z
+
80
+
60
16
19
17
28
18
+
30
26
31
29
24
23
0
2
25
21
4
20
20
−
30
14
9
5
15
12
1
7
−
60
PDOP 2.8
HDOP 1.6
GPS
−
80
60
90
120
150
180
210
240
270
300
330
0
30
60
Figure 7.16
Locations of satellites worldwide.
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