Testing the GPS Idea(1974–1979)

The first phase of the GPS program was intended
to confirm the concept of a space-based navigation system, demonstrate
its potential for operational utility, and establish the preferred design.12
The original program was funded at about $100 million and was supposed to
cover four satellites, the launch vehicles, three types of user equipment, a
satellite control facility, and an extensive test program.

The very first NAVSTAR satellites were actually two refurbished Timation
satellites built by the NRL. Known as Navigation Technology Satellite (NTS)
numbers 1 and 2, they carried the first atomic clocks ever launched into space.
Although these experimental satellites functioned for only short periods
following their launches in 1974 and 1977, they proved the concept of timebased
ranging using spread-spectrum radio signals and precise time derived
from orbiting atomic clocks.

Soon after, the first developmental GPS satellites, known as Block Is, were
launched and tested. This series of satellites supported most of the system’s
testing program. Between 1978 and 1985, a total of eleven Block I satellites built
by Rockwell International were launched on the Atlas-F booster; one satellite
was lost due to a launch failure. Others eventually failed due to deterioration of
their atomic clocks or failures of their attitude control system. However, many
of the Block I satellites continued to operate much longer than their design life
of three years—in several cases more than 10 years longer.

Even before the first Block Is were launched, the military had begun planning a
dual role for the GPS satellites. In addition to carrying the navigation and timing
payload, GPS satellites would carry nuclear detonation (NUDET) sensors
designed to detect nuclear weapon explosions, assess nuclear attack, and help
in evaluating strike damage.14 The system would also contribute to monitoring
compliance with the nuclear test ban treaty. The first GPS satellite to carry a
nuclear explosion detection sensor was the sixth Block I satellite, launched on
April 26, 1980.15 The use of satellites for detecting nuclear explosions dates
back to the 1963 Limited Test Ban Treaty between the United States and the
Soviet Union, which prohibited nuclear testing in the atmosphere, underwater,
and in space. To monitor the ban, the U.S. Air Force and the Atomic Energy
Commission (predecessor to the Department of Energy) jointly developed a
series of nuclear detection satellites known as Vela. Since then, nuclear
detection sensors have been orbited on a number of other DoD satellites, including
the NAVSTAR satellites, in an effort to increase the number of detection
satellites in space and to improve the existing detection network.16 The sensors
flown on GPS satellites are similar to those initially used on the Vela satellites.
The satellites which currently make up the GPS constellation all have the
capability to detect nuclear detonations and are presently an important component
in the United States’ capability to monitor compliance with the Nuclear
Non-Proliferation Treaty of 1968.17 According to DoD plans, future GPS satellites
will continue to serve the nuclear detection mission.