Wagers PaulSatellites used for television broadcasting are usually located in a geostationary orbit (35,786 km above the Earth’s equator). The advantage of this orbit is that the satellites rotate with a period equal to the Earth’s rotation, so the satellite is in a fixed position in the sky. Therefore, the dish of the receiving antenna can be pointed constantly at the satellite’s location, without the need to track the moving satellite.
Satellite television, like other types of communications relayed by satellite, starts with a transmitting dish located on the uplink.Template:Sfn Transmitting satellite dishes are very large, measuring between 9 and 12 meters in diameter. The larger diameter allows for more precise pointing of the antenna and increased signal strength to the satellite.Template:Sfn A transmitting dish is aimed at a specific satellite and transmits signals on a specific frequency band so that it can be received by one of the satellite’s transponders tuned to that frequency band. The transponder then relays the signal back to Earth at a different frequency (this process is known as backhaul, and is used to avoid interference with the transmitting signal from Earth), usually in the Template:Not translated (4-8 GHz), Ku-band (12-18 GHz), or both.Template:Sfn The line of transmission that goes from the satellite down to Earth is called the downlink.Template:Sfn
A typical satellite has up to 32 Ku-band transponders or 24 C-band transponders, or more in the case of hybrid Ku/C satellites.Template:Sfn Typical transponders have a bandwidth of 27 to 50 MHz each. Template:Sfn Each C-band geostationary satellite must be spaced 2° longitude from the next satellite to avoid signal interference; in the case of Ku-band, the distance can be 1°.Template:Sfn This means that there is a limit to the number of satellites, which cannot be more than 360/2 = 180 C-band geostationary satellites or 360/1 = 360 Ku-band geostationary satellites. Template:Sfn C-band signals are sensitive to ground interference, while Ku-band signals will be interfered with by rain (because water absorbs microwave radiation at this particular frequency very well).Template:Sfn The latter is further attenuated by ice crystals in storm clouds. Template:Sfn There are times when solar attenuation occurs due to the Sun being exactly behind the geostationary satellite that the antenna is pointing at.Template:Sfn The effect is due to solar radiation interfering with the satellite signal.
