Final answer:
GPS relies on an array of 24 satellites orbiting the Earth and calculates a device's locational coordinates based on the time it takes for satellite signals to reach the device.
Step-by-step explanation:
GPS relies on an array of 24 satellites orbiting the Earth, and at least 4 of them are visible from any spot on Earth. Each satellite carries a precise atomic clock. Your GPS receiver detects the signals from those satellites that are overhead and calculates your position based on the time that it has taken those signals to reach you. Suppose you want to know where you are within 50 feet (GPS devices can actually do much better than this). Since it takes only 50 billionths of a second for light to travel 50 feet, the clocks on the satellites must be synchronized to at least this accuracy-and relativistic effects must therefore be taken into account.