Listening practice: The clocks that tell your phone where it is

When you open a map on your phone, the little blue dot may look simple. It seems to say, here you are. But behind that dot is a story about clocks, satellites, radio signals, and even Einstein. According to GPS.gov, the Global Positioning System is a United States owned utility that provides positioning, navigation, and timing services. It has three main parts: satellites in space, control stations on Earth, and the receiver in your hand. The receiver does not ask the satellites, where am I? Instead, it listens for signals that carry two kinds of information: where a satellite was, and what time the signal left that satellite.