Global Positioning System Reference
In-Depth Information
The availability of GPS when satellites are removed from the constellation is
very much dependent on which satellites, or combinations of satellites, are taken out
of service. The Aerospace Corporation has performed a study that determined cases
of one, two, and three satellite failures that resulted in the least, average, and great-
est impact on availability [39]. The choices for satellites to be removed in this analy-
sis were based on those satellites that caused an “average” impact on GPS
availability.
The orbital positions of the GPS satellites removed from the constellation are
given in the following list:
•
Average one satellite—SV A3;
•
Average two satellites—SVs A1 and F3;
Average three satellites—SVs A2, E3, and F2.
•
(Refer to Sections 3.2.1 and 3.2.3.1 for satellite identification and orbital location
information.)
Figures 7.11 and 7.12 display the cumulative distribution of HDOP and PDOP
with up to three satellites removed from the constellation and applying a 5º mask
angle. These plots demonstrate the increasing degradation in system performance as
more satellites are removed from the constellation.
The availability of GPS, based on PDOP
6 and a 5º mask angle, is
99.969% with one satellite out of service. The locations and durations of the result-
ing outages are displayed in Figure 7.13. The maximum outage duration that occurs
is 15 minutes.
The effects of two satellites out of service are shown in Figure 7.14. Outages
now last up to 25 minutes in several locations, but there are only a couple of occur-
≤
99.999
99.99
99.9
99
90
24 satellites
70
23 satellites
50
22 satellites
21 satellites
30
10
1
0.1
0
1
2
3
4
5
6
7
8
9
10
HDOP
Figure 7.11
Cumulative distribution of HDOP with 5º mask angle cases of 24, 23, 22, and 21
satellites.
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