Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 341

− 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 20-minute outages
21- to 30-minute outages

31- to 40-minute outages
41- to 65-minute outages

Figure 7.15 Availability of the GPS constellation with a 5º mask angle with three satellites
removed from the constellation.

• Date of prediction: The date for which the prediction is to be performed. The
GPS almanac can be used to accurately predict for approximately 7 days in
the future.

Mask angle: The elevation angle above the horizon at which satellites are con-
sidered visible by the GPS receiver.

•

Terrain mask: The azimuth and elevation of terrain (buildings, mountains,
and so on) that may block the satellite signal can be entered into the program
to ensure an accurate prediction.

•

Satellite outages: If any satellites are currently out of service, their status will
be reflected in the almanac data. However, if satellites are scheduled for main-
tenance for a prediction date in the future, the software allows the user to
mark those satellites unusable. This data can be obtained from the USCG
NAVCEN Web site.

•

Maximum DOP: As discussed previously, in order to determine availability, a
maximum DOP threshold must be set (e.g., PDOP = 6). If the DOP exceeds
that value, the software will declare GPS to be unavailable. Other applications
may use criteria other than DOP as the availability threshold. This will be dis-
cussed further in Section 7.3 for aviation applications.

•

Once these parameters have been input into the software, the prediction can be
performed. A prediction was performed for Boston (42.35ºN, 71.08ºW) on Decem-