GPS finds your place on Earth by listening to satellites. Each satellite sends out its own position and the time. Your receiver then works out where you are.
Three parts, one system
GPS.gov says GPS is owned by the United States. It gives people three services: position, navigation and time. The Space Force builds and runs the satellites and the ground network.
The system has three parts. The first is the satellites. The second is a set of stations around the world. They track the satellites, keep them in the right orbits and adjust their clocks. The third is your receiver.
Satellites that fly 12,550 miles up
NASA says at least 24 satellites circle Earth in six orbital planes. They fly about 20,200 kilometers (12,550 miles) up. Each one goes around the planet every 12 hours. Today more than 30 are working, and each carries backup atomic clocks.
The signals travel one way, from satellite to receiver. Your receiver does the listening.
A signal becomes a position
At regular times, each satellite sends out where it is and what time it is. Your receiver picks up these messages. It then measures how long they took to arrive.
From that, the receiver calculates your position in 3D. It also works out the time.
Timing, the quiet half of GPS
NASA says GPS can give a position to meter-level accuracy. It can give the time to the 10-nanosecond level. GPS.gov lists timing as one of its three services, next to position and navigation.
Open to all, with a military side
The civilian service is free to all users. It works day and night, in every part of the world. Armed forces of the United States and its allies use a different service. So do approved government agencies.
From Sputnik to a third civil signal
The idea goes back to the Sputnik era. Scientists could follow that early satellite by a shift in its radio signal, called the Doppler Effect. The United States Department of Defense launched its first satellite for the system in 1978. The full set of 24 satellites was working in 1993.
The United States plans to add a second and a third civil signal. GPS.gov says the second will make civilian use more precise. The third is mainly meant for safety uses, such as aviation.
