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Communications Satellites

1. Introduction

A communications satellite is any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals.

2. History and Development

Some of the first communications satellites were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so they could be picked up by receiving stations around the world. Echo 1, launched by the United States in 1960, consisted of an aluminized plastic balloon 30 m (100 ft) in diameter. Launched in 1964, Echo 2, was 41 m (135 ft) in diameter. The capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas.

Satellite communications currently make exclusive use of active systems, in which each satellite carries it own equipment for reception and transmission. Score, launched by the United States in 1958, was the first active communications satellite. It was equipped with a tape recorder that stored messages received while passing over a transmitting ground station. These messages were retransmitted when the satellite passed over a receiving station. Telstar 1, launched by American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan and could also relay several hundred voice channels. Launched into an elliptical orbit inclined 45 to the equatorial plane, Telstar could only relay signals between two ground stations for a short period during each revolution, when both stations were in its line of sight.

Hundreds of active communications satellites are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. One frequency band used, 500 MHZ wide, is divided into repeater channels of various bandwidths (located at 6 GHZ for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission). A band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non-mobile) ground stations. An 80-MH Z-wide band at about 1.5 GHZ (up-and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission.

Date: 2015-01-12; view: 1221

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