The Military Evolution of GPS

The Global Positioning System is a 24-satellite constellation that can tell you
where you are in three dimensions. GPS navigation and position determination
is based on measuring the distance from the user position to the precise locations
of the GPS satellites as they orbit. By measuring the distance to four GPS
satellites, it is possible to establish three coordinates of a user’s position (latitude, longitude, and altitude) as well as GPS time. (See Appendix A for a
technical explanation of how GPS works.)
Originally developed by the Department of Defense (DoD) to meet military requirements,
GPS was quickly adopted by the civilian world even before the
system was operational. This section describes the evolution of GPS, from its
conceptualization to the present day, tracing its military development and its
emergence in the civilian world.