Vanguard I orbits more than 400 miles above the Earth and will orbit for hundreds of years. Every satellite carries some kind of radio transmitter. One kind is called a radio beacon which sends signals that enable engineers to track the satellite. Another kind of transmitter sends scientific information gathered by the satellite's instruments. This sending of information is called telemetry.
Some satellites carry equipment to receive signals.
Engineers beam signals to these satellites to turn the instruments on and off. Most satellites stop functioning long before they fall to Earth. Their batteries go dead, or their electronic equipment breaks down. Many silent satellites are circling the Earth. Page 1 Weather, or meteorological, satellites help scientists to forecast and study weather.