Testing of GPS user

Testing of GPS user equipment began in March 1977 before any satellites were
in place. A system of solar-powered ground transmitters was set up on the
desert floor at the Army’s Yuma Proving Ground in Arizona to simulate GPS
satellites. These transmitters, known as pseudolites (taken from the term pseudosatellites),
broadcast a signal that has a structure similar to that of a GPS
satellite.18 Although the signals were coming from the ground rather than from
space, they provided a geometry that approximated that of the satellites. By the
time four Block I satellites were in orbit (1978), the JPO was running tests on
several types of user equipment carried on aircraft, helicopter, ships, trucks,
jeeps, and even by men using 25-pound backpacks.

The final segment of GPS—a prototype ground control system—was located at
Vandenberg AFB, CA, during this period. With all the basic components of the
system in place, the JPO was given the go-ahead to proceed with full-scale development
of GPS in August 1979.