The Navstar Global Positioning System (GPS) is a satellite-based, global radio-navigation system and one of several Global Navigation Satellite Systems (GNSS). Summarizing its history or workings in a blog post is a bit like giving a three-minute lecture on quantum theory or French literature. Therefore, this post, on the history of GPS, and the next one, on GPS basics, are intended only as an introduction to a series of posts on different aspects of the system, related technologies, and their applications. The next post will also include a brief reading list for those who wish to explore the subject further.
Satellite navigation began with the very first artificial satellite, Sputnik I, which the Soviet Union launched in 1957. Two U.S. physicists were able to calculate the satellite’s orbital parameters by analyzing the Doppler shift in its signals as measured from a single ground station at a known position and applying Kepler’s laws of orbital motion. Then their boss, Frank T. McClure, realized that the inverse process could be used to determine the position of a ground station by measuring the Doppler shifts in a signal from a satellite in a known orbit.
The first system based on this principle was Transit, which became operational in 1964 and was used primarily by the U.S. Navy. Ships and submarines had to wait up to 100 minutes for one of its half-dozen satellites to come into view, then record the Doppler shift of the received signal and the navigation message it carried for 10 to 20 minutes and process these measurements in digital computers that were huge and primitive by today’s standards. This yielded the latitude and longitude of a slow-moving ship or submarine with the accuracy of a couple of hundred meters.
A comprehensive study of satellite-based radio-navigation commissioned by the U.S. Air Force, called USAF621B, produced a final classified report in 1966 that detailed 12 ways it could be done. The choice of which approach to take was made at a meeting at the Pentagon over Labor Day weekend in 1973, called and chaired by Dr. Brad Parkinson, then a U.S. Air Force colonel. The fundamentals were revolutionary at the time. By measuring their range (distance) from at least four satellites and using a simple crystal clock, GPS receivers would be able to calculate their 3D position with about 10 m of accuracy and their time within 30 ns. The system would be passive — that is, the receivers would not have to communicate with the satellites — therefore it would allow an infinite number of users. Another innovation was the decision to broadcast the signal using CDMA spread spectrum, which is now standard for cell phones. Finally, the system would require space-hardened atomic clocks, which did not yet exist.
Parkinson won Pentagon approval for GPS in December 1973 and the first satellite was launched just 44 months later. A series of tests in 1978 proved that the program had met all its goals. In September 1983, following the shoot down by the Soviets of a Korean Airlines plane over the Kamchatka Peninsula allegedly due to a navigation error, President Reagan announced that GPS would be guaranteed to the world. In June 1993, the constellation was completed with the launch of the 24th satellite and GPS achieved full operational capability (FOC) in July 1995.
In addition to the basic architecture of GPS, many scientific and technical advances were required to make it possible and as ubiquitous as it is today. Let us cite a few. On the scientific side, we learned the laws of orbital motion thanks to the observations and calculations of Nicolaus Copernicus, Johann Kepler, Galileo Galilei, Isaac Newton, and many others. On the engineering side, we developed rockets, satellites and ways to precisely monitor their orbits, and radiation-resistant atomic clocks. We also refined the ancient concept of latitude and longitude and developed a sophisticated global coordinate frame, the World Geodetic System 1984, which GPS uses. Finally, the evolution from the first GPS receiver, which was six feet tall and had two seats, to the GNSS chips in millions of cell phones today, required the miniaturization revolution in electronics.
GPS is a dual-use system, meaning that it is for use by both the military and civilians. Early on, Parkinson published the system’s specs so that civilians could use it, too, and the first civil GPS receiver was built by students at the University of Leeds, in England. However, for the first two decades of GPS, the Air Force, citing security concerns, introduced errors in its civilian signal. This intentional degradation of the civilian signal was called Selective Availability (SA). Yet, anyone using a differential system could receive corrections that would cancel it out. In fact, the United States Coast Guard was broadcasting the corrections to the errors that the United States Air Force was putting into the system. Finally, on May 1, 2000, on orders from President Clinton, the Air Force turned off SA, instantly making the accuracy of civilian GPS receivers about six times better.
Today, GPS is accurate to at least about 3 m 95 percent of the time, but real-time kinematic (RTK) corrections can improve this accuracy to about 2 cm and geodetic techniques can achieve accuracies of a fraction of a millimeter.
Following the development of GPS by the United States, other countries began to develop analogous global and regional systems. Today, Russia operates the Globalnaya Navigazionnaya Sputnikovaya Sistema or GLONASS, Europe is building Galileo, China operates the BeiDou Navigation Satellite System (BDS, previously known as Compass), India the Navigation Indian Constellation (NavIC, previously known as the Indian Regional Navigation Satellite System or IRNSS), and Japan the Quasi-Zenith Satellite System (QZSS). The total number of GNSS satellites in now close to 130.