A problem navigators long faced
For most of history, finding your position meant reading stars, landmarks or a compass, and ships relied on dead reckoning, which accumulates error over time. Solving the longitude problem took the marine chronometer in the eighteenth century. In the twentieth century, radio navigation systems such as LORAN improved matters but covered limited areas and were not always accurate. A global, all-weather system that could give a position anywhere on Earth required a new approach, and the space age offered one.
Transit and the Sputnik idea
After the Sputnik launch by the Soviet Union in October 1957, scientists at the Johns Hopkins Applied Physics Laboratory, William Guier and George Weiffenbach, found that they could track the satellite’s orbit by measuring the Doppler shift in its radio signal. Another researcher, Frank McClure, suggested the reverse: if a satellite’s orbit was known precisely, a receiver could use the Doppler shift to work out its own position. This led to the US Navy’s Transit system, which became operational in 1964 and was used to fix the position of submarines and ships. Transit was slow, however, taking minutes to provide a fix, and worked in two dimensions.
Competing programs and a merger
In the 1960s, the US Navy and the Air Force each pursued improved concepts. The Naval Research Laboratory’s Timation program, led by Roger Easton, tested satellites carrying precise clocks. The Air Force had a project called 621B that explored using ranging signals. The idea shared by the programs was that precise timing could replace Doppler measurement.
In 1973, the Department of Defense merged these efforts into a Joint Program Office, with Air Force Colonel Bradford Parkinson leading the new Navstar Global Positioning System. The system took ideas from each earlier program, rather than being the invention of one person, and many engineers and scientists contributed. The first experimental satellites launched in 1978, and testing continued through the 1980s.
How GPS works
A GPS satellite broadcasts its position and the exact time its signal was sent. A receiver on the ground picks up signals from several satellites and measures how long each took to arrive. Because radio waves travel at the speed of light, those delays can be converted into distances. Knowing the distance to three satellites would in principle fix a position, but the receiver’s own clock is not nearly as accurate as the clocks on the satellites. Using a fourth satellite lets the receiver solve for time error as well as for latitude, longitude and altitude.
This approach depends on very accurate clocks on board each satellite, based on an atomic clock. It also requires adjustments for effects of relativity, since the satellites move quickly and are farther from Earth’s gravity than receivers on the ground, so their clocks tick at slightly different rates. Engineers built these corrections into the system.
The constellation
The planned system used 24 satellites in six orbital planes, at an altitude of about 20,200 kilometers, with each satellite circling the Earth roughly twice per day. The arrangement ensures that, in principle, at least four satellites are above the horizon almost anywhere. The system was declared fully operational in 1995, with a full constellation and ground control stations. Military users received a precise signal, while a less precise civilian signal was also broadcast.
The first widespread test of GPS in combat came in the Gulf War of 1991, when troops used early receivers in the desert, though the full constellation was not in place yet. Receivers at the time were bulky and expensive.
Opening to civilians
Civilian GPS access was not the first aim, but it was recognized early. After Korean Air Lines Flight 007 strayed into Soviet airspace and was shot down in 1983, US President Ronald Reagan announced that GPS would be made available for civilian use once it was operational, to improve air navigation. At first, the civilian signal was deliberately degraded in accuracy through a policy called Selective Availability, to limit its military value to adversaries. In May 2000, the United States stopped this practice, which improved civilian accuracy considerably.
By then, a fast-growing market in GPS receivers had appeared, serving aviation navigation, shipping and surveying, along with handheld GPS units for hikers. Smaller chips later put receivers into cars and smartphones. Today, GPS timing signals are also used for tasks that have little to do with maps, such as synchronizing cellular networks and financial systems.
Other systems and limits
GPS is operated by the United States, and other countries have built comparable systems, including Russia’s GLONASS, Europe’s Galileo and China’s BeiDou. Many modern receivers can use signals from several of them at once. GPS signals are weak, and they can be blocked indoors, in tunnels and between tall buildings, and they can be jammed or spoofed, which has become a security concern. Even so, satellite navigation has become one of the most widely used technologies of modern life, and it started as a military program built on a handful of earlier ideas.