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Communication Satellites
by Light-Science.com
The satellites in the GEO provide communication of telephones, television broadcasting and the Internet. Antennas send and receive the radio waves for transmitting and communicating.
If you were in Europe and sending a call to the United States, the signals could pass from a public telephone network to the nearest Earth station, which then sends the radio waves to a GEO satellite. The satellite's job is to amplify before transmitting the radio waves to the antenna in the United States. The US signal is sent over the telephone network allowing the radio waves to reach their destination.
The ground equipment needed to transmit and receive signals to and from the satellites is called an Earth station. Many of these Earth stations are located in large buildings with the antenna as the gateway. Through this gateway, many thousands of telephone calls are sent and received from a satellite. There are also Earth stations that have been designed to fit into small spaces, such as on ships and planes. But the key to the smooth operations of these Earth stations, is an antenna's ability to send and receive signals.
The GEO satellites above the equator seem to stay stationary over the same spot on Earth. However, they are not in the same spot, appearances can be deceiving. The satellites, which are approximately 36,000 km above Earth, requires the same amount of time to complete one orbit, as the Earth itself uses to spin on its axis. From an Earth station they seem to remain in the same place.
Antennas, Transponders, and Footprints
The radio waves sent to communication satellites are a part of the electromagnetic spectrum. Radio waves have frequencies. The satellites send radio waves at frequencies that allow passage through Earth's atmosphere without being absorbed by water vapor.
The earliest antennas allowed a spilling of signals in every direction. This was a waste of the satellite's limited power. Scientists began creating a sophisticated type of antenna that would transmit a high-powered, but narrow beam at a specific Earth area. Since these antennas are too large to be placed inside the nose cone of a launch rocket, they have been developed in such a way as to unfurl, or deploy only when in orbit.
Containing a chain of electronics, transponders are located inside the satellite, which are the key devices of operation for communication satellites. Transponders help to clean up any distortion the radio signals may have acquired as it traveled through the atmosphere. Its next function is to convert the radio signal into the frequency needed to transmit back to Earth. It is at this time that the signal is also amplified.
Satellite footprints refer to a particular pattern. Spotlights have different shapes and sizes of beams, as does a communication satellite. The radio waves that are transmitted by a satellite fall to Earth in a particular pattern. Any antenna within this footprint can send and receive signals, whether to or from the satellite.
Constellation Fleet
A fleet of satellites, such as Iridium and Globalstar, were launched in the late 1990's into LEO, low-Earth orbits. LEO's are much closer to the Earth than the GEO satellites. Because of this factor, they are smaller and use cheaper equipment to send messages. The LEO, in comparison to the GEO satellites, is beneficial for providing cheaper applications, such as, the cell phone. Iridium satellites send signals between one another allowing for greater flexibility of a mobile communication system. Globalstar, one of the constellations in this fleet of satellites, has 48 man-made satellites; Iridium has 66 man-made satellites.
Communication satellites have seen great improvements since the year 1954. It was in that year that the United States Navy sent a message from Washington, D.C. to Hawaii by bouncing a signal off the Moon's surface. Bell Telephone and NASA launched an aluminized balloon that reflected signals across North America. It's name was Echo, and it was launched in 1960. The first GEO satellite for commercial traffic was launched in 1965 with the name of Early Bird, or Intelsat 1.
Thanks to the continuing work of many scientists, businessmen, teachers, and lay-people, communication techniques continue to improve and provided greater service to every nation on Earth.
Sources:
1. Editors. Secrets of the Universe. International Master Publishing: US. 1999
2. Couper, Heather and Nigel Henbest. Space Encyclopedia: "Biographies." DK Publishing Book: NY, 1999
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