It wasn’t all that long ago when the person riding in a car’s front passenger seat was almost as important as the driver: with a map on her lap, she gave the driver directions and — depending on how good she was at reading a map — got the driver to the destination. This skill has been on the decline for some time now—thanks to navigation devices. Their intelligence isn’t inside the car, but rather circling the Earth.
To ensure that the system functions all the time and from almost every location on the planet, at least 24 satellites are in orbit around the globe. To find the precise location of a car, at least three of the satellites are needed. They send their current position and exact time to the device’s GPS receiver. It measures the time that passes until the signals arrive, and this signal propagation delay allows the car’s exact distance from the three satellites to be calculated, pinpointing its position on Earth.
For a more accurate positioning, a fourth satellite is involved . The reason: in actuality, the clock of the navigation device isn’t precise enough to perfectly calculate the signal delay. The fourth signal is needed to factor out the inaccuracies.
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