The basic idea: distance from time
The Global Positioning System, usually called GPS, is a U.S.-operated network of satellites that lets a receiver work out where it is and what time it is. The core idea is simple even if the engineering is not. Each satellite continuously broadcasts a radio signal that includes its own position and the exact time the signal was sent. A receiver compares that time with its own clock, calculates how long the signal took to arrive, and multiplies by the speed of light to get a distance.
One distance puts the receiver somewhere on a sphere around that satellite. Distances from three satellites narrow the position to a point. In practice a fourth satellite is needed, because the receiver’s own clock is cheap and inaccurate. The extra measurement lets the receiver solve for its clock error as well as for its position. This is why the system is as much about time as it is about location.
The scale of the timing problem is striking. A GPS satellite orbits at roughly 20,200 kilometers (about 12,550 miles) above the Earth. A signal from a satellite directly overhead takes about 0.067 seconds to arrive. Because radio waves travel about 300,000 kilometers per second, a clock error of one microsecond is roughly 300 meters of distance error, and an error of ten nanoseconds is about three meters. That is why each satellite carries an atomic clock, a technology that matured through the twentieth century.
Where it came from
Satellite positioning grew out of Cold War military needs. Navies had long relied on celestial observation, the marine chronometer and later radio beacons, but missile submarines needed a fix on their position that worked anywhere, in any weather. The U.S. Navy’s Transit system, which began in the 1960s, answered part of that need using the Doppler shift of satellite signals. Other early work included Navy research on satellite-borne clocks and an Air Force concept for a more precise three-dimensional system. In 1973 the Defense Department combined these efforts into one program, which became Navstar GPS. The detailed story of that program is covered in our article on the Navstar constellation.
The first prototype satellite of the program was launched in 1978. The system was declared to have initial operational capability in December 1993 and full operational capability in 1995, when the planned constellation of 24 satellites was in place. Today the constellation is generally somewhat larger than 24, which improves coverage and gives spares.
How the system is organized
Engineers usually describe GPS as having three parts.
- The space segment: the satellites, arranged in six orbital planes so that a user on Earth can typically see several of them at once. Each orbit takes a little under 12 hours.
- The control segment: ground stations that track the satellites, update their orbit data and keep their clocks in step.
- The user segment: the receivers, from military sets to the chips in smartphones.
The signal also has to account for physics that sounds odd at first. Satellite clocks run slightly differently from clocks on the ground because of the satellites’ speed and their distance from Earth’s gravity. The net effect is often quoted as a gain of roughly 38 microseconds per day. If it were ignored, position errors would grow by kilometers within a day. The satellite clock rates are therefore adjusted in advance.
From military tool to public utility
Early GPS was designed for the military, and a deliberate degradation called selective availability limited the accuracy of the public signal. Over time the system opened to everyone, as described in our look at civilian GPS access. Early receivers were large and expensive, and the story of GPS receivers shows how hardware shrank into handheld units and eventually into a single chip. Pilots adopted satellite positioning for flight, and the effect on aviation navigation is its own chapter.
By the 2000s, positioning was built into phones and cars, and navigation by screen replaced paper maps for many people. That shift also depended on advances in integrated circuits and in wireless data.
Beyond maps
Navigation is the most visible use, but it is not the largest. A few examples show the range.
- Logistics: shipping companies, railways and delivery fleets track vehicles and cargo.
- Surveying and farming: high-precision techniques that combine signals with correction data allow very accurate land measurements and guided machinery.
- Timing: because GPS delivers precise time, many systems use it as a reference clock. Cellular networks, power grids and financial systems often rely on it for synchronization, usually without most users being aware.
- Aviation and shipping: satellite positioning supports air traffic control and port operations alongside older systems.
Why the change felt so large
Before satellite positioning, finding an exact location usually required either local knowledge, printed maps and landmarks, or specialized equipment operated by trained people. Satellite signals made the same information available anywhere outdoors, at any hour, to anyone with a small receiver. That widened who could navigate precisely and let businesses track vehicles, cargo and equipment in ways that had been impractical. It also created new dependencies, since many everyday services now assume a position or time signal is available.
Other systems and limitations
GPS is not the only satellite navigation system. Russia’s GLONASS, the European Union’s Galileo and China’s BeiDou provide similar services, and modern receivers often use several at once. The generic term for all of them is satellite navigation, and “GPS” is strictly the U.S. system.
The signals are weak by the time they reach the ground, so they can be blocked indoors, in tunnels and in dense city canyons, and they can be jammed or spoofed. Many planners therefore treat satellite positioning as a powerful tool that works best alongside backup methods rather than as a single point of failure. Even for the self-driving cars that are tested today, satellite positioning is generally combined with other sensors rather than used alone.
A note on credit
No single person invented GPS. Engineers and officers in the Navy, the Air Force and several laboratories contributed pieces, and the program office assembled them. Names such as Bradford Parkinson, who led the joint program office, and Roger Easton, who worked on Navy satellite clocks and timing, are often mentioned. Historians tend to describe the result as a product of many overlapping efforts rather than a single invention.