Satellites orbiting the Earth communicate with stations on the ground using radio waves. The only difference between the radio waves picked up by the satellite radio on your office desk and those used for traditional FM and AM broadcasts is the wavelength and frequency. The U.S. government allocates specific frequencies for commercial, scientific and military satellites so they don't interfere with television, FM and other kinds of radio communications.
Radio Frequency and Wavelength
All kinds of radio waves are a form of electromagnetic radiation, which also includes visible and ultraviolet light, X-rays and gamma rays. Each type of electromagnetic radiation has a characteristic range of wavelengths determined by the speed of light, 186,000 miles per second. Radio frequencies range from about 100 kHz to 300 GHz and have corresponding wavelengths of 3,000 meters to 1 millimeter. Satellite communications range from about 20 meters to the shortest radio wavelengths.
The Federal Communications Commission organizes satellite radio frequencies into several groups, including amateur, meteorological, scientific and maritime. For example, some weather satellites use frequencies between 460 and 470 MHz. Each group occupies several blocks of frequencies; the FCC intersperses satellite with other kinds of radio users, such as aircraft, radio astronomy, ham radio operators and cellular phones.
How Satellites Work
A satellite is a package of radio equipment powered by solar cells. An antenna on the satellite receives an uplink signal from a transmitter on the ground. The satellite amplifies the signal and broadcasts same information on a different frequency, called the downlink, so the two signals don't interfere with one another. A second station on the ground, located up to thousands of miles from the transmitter, receives the satellite downlink.
Need For Satellites
Most commercial radio signals do not travel more than a few hundred miles at most, and the curvature of the Earth limits high-frequency radio to about 60 miles. Radio signals follow a straight line from the transmission antenna, called the line of sight; any position that cannot see the antenna tower cannot receive the signal. Some radio signals bounce off the upper layers of the atmosphere back to Earth, but these happen unpredictably; most of the waves go straight into space. Satellites allow radio communications to travel reliably around the Earth.
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