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.
It Started with Basic Research.
For centuries, the only way to navigate was to look at the position of the sun and stars and use dead reckoning. Even after modern clocks were developed, making it possible to find one's longitude, the most accurate instruments could yield a position that was accurate only to within a few miles. However, when the Soviet Union launched Sputnik on October 4, 1957, it was immediately recognized that this "artificial star" could be used as a navigational tool. The very next evening, researchers at the Lincoln Laboratory of the Massachusetts Institute of Technology (MIT) were able to determine the satellite's orbit precisely by observing how the apparent frequency of its radio signal increased as it approached and decreased as it departed--an effect known as the Doppler shift. The proof that a satellite's orbit could be precisely determined from the ground was the first step in establishing that positions on the ground could be determined by homing in on the signals broadcast by satellites.
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. 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.
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. 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. 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. 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 technologies 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. This program incorporated two experimental satellites that were used to advance the development of high-stability clocks, time-transfer, and two-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 three dimensional (latitude, longitude, and altitude) navigation with continuous service. By 1972, the system had already demonstrated the operation of a new type of satellite ranging signal based on pseudorandom noise (PRN). 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 oval shaped 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.
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. 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 time based 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 I’s 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. The system would also contribute to monitoring. An earlier attempt to gain approval for the system was made in August 1973, but failed because the program presented to DoD at that time was not representative of a joint program, but rather a repackaged version of the Air Force’s System 621B.
The second phase of GPS was devoted to full-scale engineering development, and the third to production and deployment of the GPS segments. This funding was apparently just enough to cover the satellites but not enough for the other elements of the first phase of the program.
The complete GPS space system includes 24 satellites, 11,000 nautical miles above the Earth, which take 12 hours each to go around the Earth once (one orbit). They are positioned so that we can receive signals from six of them nearly 100 percent of the time at any point on Earth. You need that many signals to get the best position information. Satellites are equipped with very precise clocks that keep accurate time to within three nanoseconds - that's 0.000000003, or three billionths, of a second. This precision timing is important because the receiver must determine exactly how long it takes for signals to travel from each GPS satellite. The receiver uses this information to calculate its position.
The first GPS satellite was launched in 1978. The first 10 satellites were developmental satellites, called Block I. In 1981 it was decided that the US Space Shuttle program would be the primary carrier for the deployment of the GPS satellites. The completion of the Block I deployment was delayed when the Space Shuttle Challenger was destroyed just after launch. The Challenger accident grounded the shuttle fleet thus delaying the deployment of the final Block I satellites. From 1989 to 1993, 23 production satellites, called Block II, were launched. The launch of the 24th satellite in 1994 completed the system.
The Global Positioning System 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. 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. 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. According to DoD plans, future GPS satellites will continue to serve the nuclear detection mission.
GPS and the Future.
It is often forgotten that GPS is still a military device built by the Department of Defense at a cost of $12 billion and intended primarily for military use. That fact has led to one of the few controversies surrounding the remarkably successful system. As with any new technology, progress brings risk, and GPS potentially could be used to aid smugglers, terrorists, or hostile forces. The Pentagon made the GPS system available for commercial use only after being pressured by the companies that built the equipment and saw the enormous potential market for it. As a compromise, however, the Pentagon initiated a policy known as selective availability, whereby the most accurate signals broadcast by GPS satellites would be reserved strictly for military and other authorized users. GPS satellites now broadcast two signals: a civilian signal that is accurate to within 100 feet and a second signal that only the military can decode that is accurate to within 60 feet. The Pentagon has also reserved the ability to introduce errors at any time into the civilian signal to reduce its accuracy to about 300 feet.
In March 1996, the White House announced that a higher level of GPS accuracy will be made available to everyone, and the practice of degrading civil GPS signals will be phased out within a decade. The White House also reaffirmed the federal government's commitment to providing GPS services for peaceful civil, commercial, and scientific use on a worldwide basis and free of charge.
The future of GPS appears to be virtually unlimited; technological fantasies abound. The system provides a novel, unique, and instantly available address for every square yard on the surface of the planet--a new international standard for locations and distances. To the computers of the world, at least, our locations may be defined not by a street address, a city, and a state, but by a longitude and a latitude . With the GPS location of services stored with phone numbers in computerized "yellow pages," the search for a local restaurant or the nearest gas station in any city, town, or suburb will be completed in an instant. With GPS, the world has been given a technology of unbounded promise, born in the laboratories of scientists who were motivated by their own curiosity to probe the nature of the universe and our world, and built on the fruits of publicly supported basic research.
The technology of the Global Positioning System is allowing for huge changes in society. The applications using GPS are constantly growing. The cost of the receivers is dropping while at the same time the accuracy of the system is improving. This affects everyone with things such as faster Internet speed and safer plane landings.
Even though the system was originally developed for military purposes, civil sales now exceed military sales (See Figure 1 below).
On May 1, 2000 President Clinton announced that the government will no longer scramble signals from the GPS satellites. This means that civilians are now able to enjoy the high accuracy that the military has had for years. The DGPS techniques and the equipment needed to use them will no longer be necessary to get the same effects on accuracy.
