The Forerunners of GPS

DoD’s primary purposes in developing GPS were to
use it in precision weapon delivery and to provide a capability that would reverse
the proliferation of navigation systems in the military.2 Beginning in the
early 1960s, the U.S. Department of Defense began pursuing the idea of developing
a global, all-weather, continuously available, highly accurate positioning
and navigation system that could address the needs of a broad spectrum of
users and at the same time save the DoD money by limiting the proliferation of
specialized equipment that supported only particular mission requirements. As
a result, the U.S. Navy and Air Force began studying the concept of using radio
signals transmitted from satellites for positioning and navigation purposes.
These studies developed concepts and experimental satellite programs, which
became the building blocks for the Global Positioning System.

The Navy sponsored two programs which were predecessors to GPS: Transit
and Timation. Transit was the first operational satellite-based navigation system.
3 Developed by the Johns Hopkins Applied Physics Laboratory under Dr.
Richard Kirschner in the 1960s, Transit consists of 7 low-altitude polar-orbiting
satellites that broadcast very stable radio signals; several ground-based monitor
stations to track the satellites; and facilities to update satellite orbital parameters.
Transit users determine their position on earth by measuring the Doppler
shift of signals transmitted by the satellites.

Originally designed to meet the Navy’s requirement for locating ballistic missile
submarines and other ships at the ocean’s surface, Transit was made available
to civilian users in 1967. It was quickly adopted by a large number of commercial
marine navigators and owners of small pleasure craft and is still operated
by the Navy today.4 Although it has proved its utility for most ship navigation,the system has a number of drawbacks. It is slow, requiring a long observation
time, provides only two-dimensional positioning capability, has limited coverage
due to the intermittent access/availability of its signals (with periods of unavailability
measured in hours), and requires users to correct for their velocities—
all of which make Transit impractical for use on aircraft or other rapidly
moving platforms. Nonetheless, Transit was important to GPS because it resulted
in a number of technologies5 that were extremely useful to GPS and
demonstrated that a space system could offer excellent reliability.

Timation, a second forerunner of GPS, was a space-based navigation system
technology program the Navy had worked on since 1964.6 This program incorporated
two experimental satellites that were used to advance the development
of high-stability clocks, time-transfer, two-dimensional navigation, and demonstrate
technology for three-dimensional navigation. The first Timation satellite
launched in 1967 carried very stable quartz-crystal oscillators; later models
orbited the first atomic frequency standards (rubidium and cesium). The
atomic clocks had better frequency stability than earlier clocks, which greatly
improved the prediction of satellite orbits (ephemerides) and would eventually
extend the time required between control segment updates to GPS satellites.
This pioneering work on space-qualified time standards was an important contribution
to GPS.7 In fact, the last two Timation satellites were used as prototype
GPS satellites.

In the meantime, the Air Force was working on a similar technology program
that resulted in a design concept called System 621B; it provided threedimensional
(latitude, longitude, and altitude) navigation with continuous
service.8 By 1972, the system had already demonstrated the operation of a new
type of satellite ranging signal based on pseudorandom noise (PRN).9 To verify
the PRN technique, the Air Force ran a series of aircraft tests at White Sands

Proving Ground in New Mexico using ground- and balloon-carried transmitters to simulate satellites. The technique pinpointed the positions of aircraft to
within a hundredth of a mile.

At that time, the Air Force concept envisioned a global system consisting of 16
satellites in geosynchronous orbits whose ground tracks formed four ovalshaped
clusters extending 30 degrees north and south of the equator. This particular
geometry allowed for the gradual evolution of the system because it required
only four satellites to demonstrate its operation capabilities. That is, one
cluster could provide 24-hour coverage of a particular geographic region (for
example, North and South America).

However, no real progress was made toward full-scale development of System
621B until 1973. Part of the reason for this was that the Air Force work had
stimulated additional work on satellite navigation, giving rise to a number of
competing initiatives from the other services. By the late 1960s, the U.S. Navy,
Air Force, and Army were each working independently on radionavigation systems
that would provide all-weather, 24-hour coverage and accuracies that
would enhance the military capabilities of their respective forces.10 The APL
had made technical improvements to Transit and wanted to upgrade the system,
while the Naval Research Laboratory was pushing an expanded Timation
system and the Army had proposed using its own system, SECOR (Sequential
Correlation of Range). To coordinate the effort of the various satellite navigation
groups, DoD established a joint tri-service steering committee in 1968
called the NAVSEG (Navigation Satellite Executive Group). The NAVSEG spent
the next several years deciding what the specifics of a satellite navigation
system should be—how many satellites, at what altitude, signal codes, and
modulation techniques—and what they would cost.

Finally, in April 1973, the Deputy Secretary of Defense designated the Air Force
as the lead agency to consolidate the various satellite navigation concepts into a
single comprehensive DoD system to be known as the Defense Navigation
Satellite System (DNSS). The new system was to be developed by a Joint
Program Office (JPO) located at the Air Force’s Space and Missile Organization,
with participation by all military services. Colonel Brad Parkinson, program director
of the JPO, was directed to negotiate between the services to develop a
DNSS concept that embraced the views and needs of all services.By September 1973, a compromise system was evolving which combined the
best features of earlier Navy and Air Force programs. The signal structure and
frequencies were taken from the Air Force’s 621B. Satellite orbits were based on
those proposed for the Navy’s Timation system, but higher in altitude, giving twelve-hour instead of eight-hour periods. While both systems had proposed
the use of atomic clocks in satellites, only the Navy had tested this idea. The
system concept that emerged is what is known today as the NAVSTAR Global
Positioning System. In December 1973, DoD granted the JPO approval to
proceed with the first phase of a three-phase development of the NAVSTAR
GPS.