GPS stands for Global Positioning System, and that name captures well what it does: indicate your position on the globe. How it does that is a fascinating tale that doesn’t require advanced mathematics to understand.

Before delving into the details of how that system operates, it’s helpful to have a basic knowledge of how positions on Earth are specified. The classic view provides one that’s easy to visualize.

Imagine a simple grid made of lines that wrap the globe ‘vertically’ originating at the Earth’s North Pole and fanning out then coming together at the South Pole. Then add a set of ‘horizontal’ lines that wrap the globe at right angles to the first set. That grid is the familiar longitude and latitude ‘mesh’ that is painted on every school room globe and map. Each line of lat and long are marked off in degrees that specify how far along one of them you are located.

Now, how is that ‘mesh’ used in connection with the GPS system?

The U.S. Air Force maintains a set of 24 operational GPS satellites (with 3 spares). Each satellite has the electronics and software needed to measure its own location, most importantly the distance from the Earth. It does that by sending a radio beam out and measuring the time needed to hit the Earth where the signal is picked up by ground stations.

Since distance = velocity * time, that measurement is pretty straightforward. Radio waves travel at the speed of light (~186,00 mph or ~300,000 kph) and the electronics measure the delay from when the beam is sent to when it’s picked up.

Satellites (and ground stations) can measure that delay accurately because they have clocks synchronized by atomic clocks that measure time to incredible accuracy. GPS Receivers don’t have atomic clocks inside, but perform some tricks to compensate.

Also because of variations in the atmosphere, the motion of the satellites, reflections off buildings, and other imperfections from this neat description, GPS systems have to correct for small errors in order to get the precision needed to locate your receiver to within a few meters.

Distance provides only one (albeit important) piece of the puzzle. Imagine you’re told you are 600 miles west of Kansas City. That puts you at the center of a circle of radius 600 miles, with KC somewhere on the circumference (the rim). But you don’t know exactly where. Now you’re told you are also 400 miles east of Denver. Another circle. Those two circles intersect at two points. A third intersecting circle will place you at exactly one unique point (to within measurement accuracy).

Since the GPS system is formed by satellites in space in cooperation with ground stations, it operates in three dimensions, not just the two provided by the surface of the Earth. So, the Global Positioning System uses spheres rather than circles. The calculations are more complex, but the idea is the same. Where the surfaces of four spheres intersect they determine a point, the point at which your GPS receiver is located.

Your GPS receiver is designed to ‘listen’ for the signals from four of those satellites, and uses the info provided to calculate your latitude and longitude. It overlays that unique point onto a map that resembles your surroundings, allowing you to navigate your way to anywhere you wish.