Have you ever stopped to think about how your phone knows exactly which street you are on, or how an app warns you about the traffic jam just as you reach the roundabout? Behind that small everyday miracle there is a network of satellites orbiting the Earth thousands of kilometres up. It is called GPS and, although we use it daily, almost no one knows how it really works.
A clock in space
The idea behind GPS is, on the surface, simple: if you know how far you are from several known points, you can calculate where you are. Imagine you are in a city and someone tells you you are three kilometres from the main square. That places you on a circle around the square. If another person tells you you are two kilometres from the station, you now have two circles that intersect at two possible points. With a third piece of data, the distance to another known place, the exact point is resolved.
In space, those “known points” are satellites that emit a signal with two essential pieces of data: their own position and the exact time they emitted it. The receiver in your phone calculates how long that signal took to reach it and, multiplying that time by the speed of light, obtains the distance. By combining several signals from different satellites, the device triangulates your position with surprising precision.
The key: time
Here is where the most delicate trick of the whole system comes in. Light travels at about 300,000 kilometres per second, so an error of one millionth of a second translates into about 300 metres of distance. For the calculation to be useful, the clocks of the satellites and the one in the receiver have to be incredibly synchronized.
The satellites carry extremely high-precision atomic clocks. Your phone, obviously, cannot afford one, so it uses a fourth signal from an additional satellite to correct the offset of its own clock. It is a constant mathematical exercise that happens in milliseconds, without you having to do anything.
More than 30 satellites above your head
The original GPS system is from the United States, but it is not alone. Today several constellations coexist: the European Galileo, the Russian GLONASS and the Chinese BeiDou. The more satellites your phone “sees” at once, the more accurate the position. That is why today’s phones use several constellations at once, and that is why GPS works better in open countryside than between skyscrapers, where signals bounce off buildings and get confused.
These satellites orbit about 20,000 kilometres from the Earth and circle the planet twice a day. At any moment, from a point on the surface, between four and twelve satellites can be “seen”, more than enough to make the calculations.
Much more than getting to an address
Although we associate it with maps and satnavs, GPS moves the whole world. Banks use it to synchronize financial operations with millisecond precision. Power grids depend on it to coordinate the supply. Aeroplanes use it to fly safer and more efficient routes, and modern tractors are guided by satellite to sow in perfectly straight lines. Even delivery services know exactly when your order will arrive thanks to it.
And there is a curious detail: Einstein’s theory of relativity also plays a part in the story. Because the satellites travel very fast and are further from the Earth’s gravity, their clocks run slightly faster than those on the surface. Engineers have to correct that small offset so that the positions remain exact. Without that correction, GPS would accumulate an error of several kilometres a day.
A map that guides us every day
Next time your phone tells you to turn right, it is worth remembering what lies behind it: a network of satellites in space, atomic clocks, relativistic physics and some mathematical calculations that are resolved in a fraction of a second. A technology that started as a military system and today accompanies us in our pocket, helping us not to get lost, to arrive on time and to understand the world around us a little better.
