The GPS

GPS, the Global Positioning System, has become an integral part of our lives, so much so that imagining a world without it seems nearly impossible. This space-based radio-navigation system, initially dubbed Navstar GPS, is the brainchild of the United States government and is operated by the United States Air Force. From the simplicity of setting time on your phone to guiding self-driving cars, GPS fundamentally changed the way we navigate.

At its core, GPS is a constellation of 24 satellites orbiting Earth, about 12,500 miles (~20,000 kilometers) above us. Each satellite completes an orbit every 12 hours, constantly broadcasting navigation signals. Ground receivers catch these signals, enabling them to pinpoint time, location, and velocity with remarkable precision—some receivers can even pinpoint their location to within 0.4 inches (one centimeter).

Here’s how it works: GPS receivers are programmed with the whereabouts of each satellite at any given moment. They determine their own location by measuring how long it takes for signals from at least four satellites to reach them. Since radio waves travel at a consistent speed, the receiver can calculate its distance from each satellite.

Imagine a satellite as a flashlight shining on the ground, creating a circle of light. With one satellite, the GPS receiver could be anywhere within that circle. Add two more satellites, and their circles intersect at just one point—where the receiver is located. This process, known as trilateration, is fundamental to GPS technology.

GPS receivers are widely used in cars, guiding drivers with constantly updated locations plotted on electronic maps. Beyond automobiles, GPS is crucial for navigation in various modes of transportation including aircraft, ships, submarines, trains, and even the Space Shuttle.

The origins of GPS date back to World War II, with the United States and British navies using ground-based radio signals for navigation. The concept evolved significantly with the launch of Sputnik, the world’s first artificial satellite, by the Soviet Union in 1957. American scientists discovered that they could determine Sputnik’s location based on the Doppler shift of its radio signals, leading to the idea of a space-based navigation system. 

This led to the development of satellite navigation technology in the late 1950s, with the US Navy deploying the Transit satellite system, also known as NAVSAT, in the 1960s. In 1973, Pentagon officials conceptualized a “defense navigation satellite system,” laying the foundation for Navstar GPS. By 1995, the system achieved full operational capacity with the positioning of the 24th satellite.

GPS satellites broadcast two types of signals: one for military and the other for civilian use. Initially, the civilian signal was intentionally degraded for national security reasons. However, in 2000, this practice was discontinued by order of President Bill Clinton, propelling GPS to become a global standard for navigation.

Several other countries have developed their own space-based navigation systems. For instance, the European Union operates the Galileo system, while China has developed Beidou. These systems, along with others, are collectively known as Global Navigation Satellite Systems (GNSS).