Technical data

D
D
Appendix D - The Global Positioning
System (GPS)
The Global Positioning System (GPS) is a military satellite based system
developed by the U.S. Department of Defence (DoD) to provide global,
all-weather, precise navigation and timing capability to users 24 hours
a day.
Civilian use of GPS is made freely available at the users own risk, but is
subject to the prevailing DoD policy or limitations, and to an individuals
understanding of how to use the GPS.
However, current practice of the U.S. Department of Defence (DoD) is to
deliberately degrade signals from the constellation of GPS satellites. In
practice, this is achieved by applying errors in the form of Selective
Availability (SA). This intervention reduces the accuracy obtainable by
civilian users of the Standard Positioning Service (SPS).
1.5. DoD policy is to set the level of SA degradation to give a horizontal
accuracy of 100 meters (95% of the time). The DoD has discontinued
the application of SA.
In today’s satellite constellation there are a minimum of 24 operational
satellites (plus several operational spares) in 6 orbital planes, at an
altitude of about 22,000 km. Each satellite has an orbital period of
approximately 12 hours, with the position of the constellation repeating
every 24 hours. This geometry is such that it furnishes direct line-of-
sight timing signals from at least four satellites (usually more).
Accordingly, the GPS system can deliver to a receiver station, accurate
3-D position, velocity, and time, anywhere on or near the earth’s
surface.