Open a map on your phone and a blue dot can often place you on a street within seconds. Behind that simple display is a global timing and ranging system involving satellites about 20,200 kilometres above Earth, ground-control stations and a receiver inside your device.
GPS satellites continuously broadcast one-way radio signals containing their position and very precise timing information. Your receiver measures how long those signals took to arrive. Because radio waves travel at the speed of light, the receiver can estimate its distance from each satellite.
One distance is not enough to locate you. By combining measurements from several satellites, the receiver uses a process known as trilateration to solve for position. In practice, four or more satellite measurements are commonly used so the receiver can determine three-dimensional position while also correcting its own clock error.
GPS.gov says the system is designed around a nominal constellation of 24 operational satellites, while additional satellites are normally flown to improve availability. The satellites are arranged so users can generally see multiple spacecraft from most places on Earth.
A useful misconception to clear up is that basic GPS does not normally require your phone to transmit its location to a satellite. The satellites broadcast; your phone listens and calculates. Mapping apps then take the coordinates and match them against their own road, address and business databases.
Accuracy depends heavily on conditions. GPS.gov says GPS-enabled smartphones are typically accurate to within about a 4.9-metre radius under open sky. Tall buildings, bridges, trees, indoor use and reflected signals can make the result worse. A signal bouncing off a wall travels farther before reaching the receiver, producing what is known as a multipath error.
Modern phones also combine GPS with Wi-Fi, cellular networks, Bluetooth and motion sensors. That is why a device may estimate your position before it has a strong satellite fix.
GPS is also a timing system. Its atomic-clock-based signals help synchronize telecommunications, financial systems, scientific instruments and parts of critical infrastructure. So the technology behind the blue dot is doing much more than giving driving directions: it is continuously turning precise measurements of time into a position on Earth.
Source: GPS.gov — GPS Overview, Trilateration, Space Segment and GPS Accuracy.