Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 340

− 150 − 120 − 90 − 60 − 30 0 30 60 90 120 150

90

90

60 60

30

30

0 0

− 30

− 30

− 60

− 60

− 90

− 90

− 150 − 120 − 90 − 60 − 30 0 30 60 90 120 150

1- to 5-minute outages
6- to 10-minute outages

11- to 15-minute outages
16- to 20-minute outages

21- to 25-minute outages

Figure 7.14 Availability of the GPS constellation with a 5º mask angle with two satellites
removed from the constellation.

With three satellites out of service, the overall availability of the GPS constella-
tion drops to 99.197%. The number of outage occurrences increases dramatically
and outages now last up to 65 minutes. The locations and corresponding durations
of these outages are shown in Figure 7.15.
The scenario of having three satellites out of service at the same time should be
a very rare occurrence. However, if it were to happen, the user could examine
the predicted availability over the course of the day and plan the use of GPS accord-
ingly.
As mentioned previously, the determination of satellite positions and the result-
ing GPS availability for any location and point in time is not intuitive and requires
software to perform the calculations. GPS prediction software that allows a user to
determine GPS coverage for a single location or for multiple locations is commer-
cially available. Some GPS receiver manufacturers also include prediction software
with the purchase of a receiver. The typical input parameters used to perform GPS
availability predictions are as follows:

GPS almanac data: The position of the satellites at a reference epoch may be
obtained from several different sources: the USCG NAVCEN Web site or a
GPS receiver that outputs almanac data.

•

Location: Latitude, longitude, and altitude of the location(s) for which the
prediction is to be performed.

•