Recent Military Use of GPS (1990–present).

The 1990–1991 crisis in the Persian
Gulf, the first major test24 of GPS in a combat situation, proved beyond a doubt
the importance and utility of the NAVSTAR. Some say that GPS revolutionized
combat operations on the ground and in the air during Operation Desert Storm
and was—as one Allied commander noted—one of two particular pieces of
equipment that were potential war winners (the other was night-vision devices).
Among the many uses of GPS in Operation Desert Storm, navigation proved to
be a crucial technique for desert warfare.26 GPS satellites enabled coalition
forces to navigate, maneuver, and fire with unprecedented accuracy in the vast
desert terrain almost 24 hours a day27 despite difficult conditions—frequent
sandstorms, few paved roads, no vegetative cover, and few natural landmarks.
Although on average, each U.S. Army maneuver company (e.g., tank, mechanized
infantry, or armored cavalry) had at least one GPS receiver, the demand
for receivers was so great that more than 10,000 commercial units were hastily
ordered during the crisis so that more coalition forces could benefit from the

Other operations made possible or greatly enhanced by GPS include precisionbombing,
artillery fire support, the precise positioning of maneuvering troop
formations, and certain special forces operations such as combat search-andrescue
missions. As well as being carried by foot soldiers, GPS receivers were
attached, in some cases with tape, to vehicles and helicopter instrument panels
and were also used in F-16 fighters, KC-135 tankers, and B-52 bombers.

Since the Persian Gulf War, the United States has employed GPS in several
peacekeeping and military operations. During Operation Restore Hope in 1993,
GPS was used to air drop food and supplies to remote areas of Somalia because
of lack of accurate maps and ground-based navigation facilities. U.S. forces entering
Haiti in 1994 also relied on GPS. During the present Balkan crisis, GPS port planes at night to their drop zones where food and medicine is then
parachuted close to towns and villages.
has assisted in delivery of aid to the Bosnians by guiding U.S. Air Force transport planes at night to their drop zones where food and medicine is then
parachuted close to towns and villages.

GPS Grows Up (1980–1989).

Efforts to expand the fledgling GPS program suffered
some growing pains during the development phase.
The first setback was brought on by a 1979 decision by the Office of the
Secretary of Defense (OSD) to cut $500 million (approximately 30 percent) from
the budget over the period FY81–FY86.19 As a result, the GPS program was restructured
and the scope of the program reduced. The final satellite constellation
was cut from 24 to 18 satellites (plus three satellites serving as on-orbit
spares); Block II development satellites were dropped; and the design was
scaled down in terms of weight, power, and nuclear and laser hardening.20
Plans for attainment of an early limited two-dimensional capability in 1981
were also dropped.

Funding for GPS was somewhat unstable during the early stages of the program
even though it received support from many elements of the services. Because
GPS is a support system and not a standard weapon system with a clear mission
and a history of well-defined operational concepts, early understanding of the
value of the system was less straightforward than with tanks or aircraft. This increased
the need to sell the program, particularly to potential users. The JPO
addressed this problem, especially during Phase I, by emphasizing one of the
more tangible capabilities of the system: increased bombing accuracy. The fact
that GPS was a joint program also increased the need to sell the program to
multiple services. No one service was anxious to bear the entire financial load
for a support system that was to be used by all services. As a result, GPS had
service support difficulties. For example, the program was zeroed out in 1980
through 1982, but was reinstated by OSD.21 It appears that OSD support contributed
to the survival of the program.

GPS suffered another setback as a result of the Space Shuttle Challenger accident
in 1986. As the only planned launch vehicle for GPS satellites at that time,
the loss of the shuttle caused a 24-month delay in the scheduled launch of the
second generation of GPS satellites, the Block IIs. Originally, the JPO planned to
launch the first 12 satellites (Phase I) on refurbished Atlas F boosters and to use
the McDonnell-Douglas Delta for the next series of launches (Phase II). Around 1979, the JPO had responded to DoD decisions which designated the Space
Shuttle as the principal launch vehicle for Air Force missions. Although the
Block IIs were built to be compatible with shuttle deployment, the JPO decided
to switch back to the Delta II as the GPS launch vehicle following the Challenger

The first Block II satellite was eventually launched in February 1989 from Cape
Canaveral AFS, and became operational for global use in April 1989. Since then,
there have been 23 more Block II satellite launches. Like the Block I satellites,
the Block IIs were produced by Rockwell International. The Block II satellites
differ from the Block Is in shape and weight and incorporate design differences
that affect security and integrity.22 Significant Block II satellite enhancements

• Radiation-hardened electronics to improve reliability and survivability
• Full selective availability (SA) and anti-spoofing (AS) capabilities to provide
system security
• Automatic detection of certain error conditions and switching to nonstandard

code transmission or default navigation message data to protect users
from tracking a faulty satellite and to maximize system integrity.
Block II satellites launched after 1989 have the additional capability of operating
for up to 180 days without contact from the control segment. They are
called Block IIAs. This represents a significant improvement over the earlier
Block I and II satellites, which required updating from the control segment after
only 3.5 days.

Further progress was made on the control and user equipment segments of GPS
during this period. As part of the transition to an operational and sustainable
system, the control segment was transferred to a new master control station located
at Falcon AFB, CO. System testing was completed, and successful interoperability
was demonstrated between the ground control stations, the satellites,
and the “user” navigation equipment. Rockwell-Collins was chosen as the
contractor for the production GPS user equipment. By the turn of the century,
an estimated 17,000 U.S. military aircraft will be equipped with GPS, and 60,000
portable receivers will be in use by U.S. ground forces and on military

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.

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.

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

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.


Throughout time people have developed a variety of ways to figure out their
position on earth and to navigate from one place to another. Early mariners relied
on angular measurements to celestial bodies like the sun and stars to calculate
their location. The 1920s witnessed the introduction of a more advanced
technique—radionavigation—based at first on radios that allowed navigators to
locate the direction of shore-based transmitters when in range.1 Later, the development
of artificial satellites made possible the transmission of more-precise,
line-of-sight radionavigation signals and sparked a new era in navigation
technology. Satellites were first used in position-finding in a simple but reliable
two-dimensional Navy system called Transit. This laid the groundwork for a
system that would later revolutionize navigation forever—the Global
Positioning System.

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