You open a map on your phone, and a little blue dot shows exactly where you are standing. It feels like magic, but it is actually the result of clever science and a lot of satellites.
GPS stands for Global Positioning System. It was developed by the United States, at first for military use, and later it was opened to everyone. The system uses a group of satellites that travel around the Earth, high above us. Each satellite constantly sends out a signal that says, in effect, "This is where I am, and this is the exact time."
Your phone does not send anything to the satellites; it only listens. Because the signals travel at a known speed, the phone can calculate how far away each satellite is from the time it takes the signal to arrive. With signals from at least four satellites, the phone can work out its position on the Earth.
For this to work, timing must be incredibly accurate. That is why every satellite carries very precise atomic clocks. A tiny mistake in time would lead to a large mistake in distance.
GPS is not perfect. Tall buildings, tunnels and thick forests can block or reflect the signals, so the blue dot sometimes jumps around. Still, the system has changed our lives: it guides ships and planes, helps rescue teams find people, and makes sure the pizza delivery reaches the right door.