What Is GPS and How Does It Work?

What GPS is

GPS stands for Global Positioning System. It is a satellite navigation system that allows a receiver, such as the chip inside your phone or car, to determine its precise location anywhere on or near the surface of the Earth. Alongside location, it can also provide your speed, your direction of travel, and a highly accurate time reading.

GPS was originally developed and is operated by the United States government, specifically its military, but it has long been available for anyone in the world to use free of charge. It is one of several similar systems now in operation globally, and the general term for this category of technology is a global navigation satellite system. In everyday speech, though, GPS is often used loosely to mean satellite positioning in general.

The system is designed to work in all weather, day or night, almost anywhere outdoors. It does not require you to transmit anything or pay a subscription; your device simply listens to signals that the satellites are constantly broadcasting.

GPS is one member of a wider family. Several countries and regions now operate their own satellite navigation systems, and many modern devices quietly combine signals from more than one of them at the same time. Doing so gives a receiver more satellites to work with, which generally means a faster and more reliable fix. Even so, because GPS was the first such system to reach widespread public use, its name has become an everyday shorthand for the whole idea of satellite positioning.

The three parts of the system

GPS is usually described as having three main segments that work together.

• The space segment is the constellation of satellites orbiting the Earth. The system is designed so that enough satellites are always available for a receiver to get a fix from most places on the planet.
• The control segment is a network of ground stations run by the operators. These stations track the satellites, monitor their health, and send up corrections to keep their clocks and orbital information accurate.
• The user segment is everyone’s receivers: the phones, car navigation units, fitness watches, and countless other devices that pick up the signals and calculate a position.

The satellites orbit in what is called medium Earth orbit, at an altitude of roughly 20,200 kilometres, which is about 12,550 miles. The United States commits to keeping at least 24 operational satellites available the vast majority of the time, and in practice the Space Force has flown around 31 operational satellites for well over a decade to provide extra coverage and reliability. Each satellite circles the Earth about twice a day.

How a receiver finds its location

The heart of GPS is a clever use of timing. Each satellite carries an extremely accurate atomic clock and continuously broadcasts a radio signal. That signal contains two crucial pieces of information: the exact time the signal was sent, and details of where the satellite is in its orbit.

Your receiver knows the speed at which radio signals travel, which is the speed of light. By comparing the time stamped in the signal with the time it actually arrives, the receiver can work out how long the signal spent travelling. Multiplying that travel time by the speed of light gives the distance between the receiver and that particular satellite.

Knowing the distance to a single satellite is not enough. It only tells your device that it lies somewhere on the surface of an imaginary sphere around that satellite. But by measuring the distance to several satellites at once, the receiver can find the single point where all those spheres intersect. That point is your location. This technique of fixing a position by combining several distance measurements is often called trilateration.

Why four satellites are needed

In principle three distances would locate a point in three-dimensional space. In practice GPS needs signals from at least four satellites. The reason is the clock inside your receiver. Satellites carry atomic clocks of extraordinary precision, but your phone cannot. The signal from a fourth satellite gives the receiver the extra information it needs to correct its own clock error, which is essential because even a tiny timing mistake would translate into a large positioning error. With four or more satellites in view, the device can solve for its position and the exact time together.

Why the signals must be so precise

Because radio signals travel at the speed of light, the timing involved is staggeringly fine. Light covers roughly 300,000 kilometres every second, so an error of even a millionth of a second in the clocks would throw the calculated distance off by hundreds of metres. This is why the satellites rely on atomic clocks and why the ground control stations constantly monitor and correct them.

The satellites’ positions must also be known with great accuracy, since the whole calculation depends on knowing exactly where each signal came from. The control segment regularly updates this orbital data and uploads it to the satellites so that the information they broadcast stays correct.

What affects GPS accuracy

Under good conditions a typical consumer device can place you within a few metres. Several factors can reduce that accuracy.

• Obstructions. Tall buildings, dense tree cover, tunnels, and being indoors can block or weaken the signals, which is why positioning often struggles in city centres or underground.
• Signal bounce. In built-up areas signals can reflect off surfaces before reaching the receiver, making the measured distance longer than it really is.
• The atmosphere. The signals slow slightly as they pass through layers of the atmosphere, introducing small errors that systems try to correct for.
• Number of satellites in view. The more satellites a device can see, and the better they are spread across the sky, the more accurate the fix.

Modern phones improve on raw GPS in clever ways. They often combine signals from several satellite systems at once, and they use assisting data from mobile networks and other sensors to find a location faster and more reliably, especially in difficult environments.

Everyday uses of GPS

The most familiar use of GPS is navigation: the turn-by-turn directions in a car or on a phone, and the moving dot that shows where you are on a map. But its reach extends far beyond getting from place to place.

GPS guides delivery fleets and ride-hailing services, tracks shipping containers and aircraft, and helps farmers steer machinery with precision across fields. Fitness trackers use it to measure the distance and pace of a run or ride. Emergency services use it to locate people who call for help. Perhaps less obviously, the system’s extremely accurate time signal is used to synchronise power grids, financial transactions, and communication networks, making GPS a quiet backbone of modern infrastructure as well as a mapping tool.

A free, shared utility

One of the remarkable things about GPS is that this global capability is open to everyone at no cost. You never send a request to the satellites or reveal your position to them; your device only listens. That is why a navigation app can work even when you have no mobile signal, as long as it has downloaded the map in advance and can see the sky.

From a constellation of satellites broadcasting the time thousands of kilometres overhead, a small chip in your pocket can work out where you stand to within a few steps. Once you understand that it all comes down to measuring how long signals take to arrive, the everyday magic of the blue dot on the map starts to make sense.