Figure 3: Gps Satellites Communicating With Vessel - NorthStar 961 Reference Manual

Chapter 2 - Introducing the 961/962
Absolute and
repeatable accuracy
Selective availability
Page 2-12

Figure 3: GPS satellites communicating with vessel

GPS's major advantage over all other commercially-available
navigation systems, including loran, is its all-weather,
worldwide availability, and its absolute accuracy. (Those
readers interested in loran should see "Understanding
Loran-C" starting on page 5-7.) Absolute accuracy refers to the
ability of a navigation device to determine your actual lat/lon
coordinates on the surface of the earth, as compared to
position data determined from a paper chart or other
reference. Repeatable accuracy, on the other hand, refers to the
ability of a particular navigation device to return to a position
whose lat/lon coordinates were previously recorded by this
same navigation device.
The Department of Defense developed a system of built-in
errors, called Selective Availability (SA), which was introduced
as a national defense strategy to intentionally degrade the GPS
solution. (The Department of Defense wanted to avoid the
possibility that its precision could be used by hostile forces as
961/962 Operations and Reference Manual, Rev. A