Reliability
Are communication satellites reliable?
Satellites offer a well-established form of distribution that has been used for many years. The technology is proven. Once a satellite has been launched, its reliability is very high. If a transmitter in the satellite fails, there are several backup transmitters on board which can be activated by a command from the ground control station. If anything should happen to the control station, there are other identical control stations at different locations that can be used instead.
Antennas for satellite control located at SIRIUS Satellite Control Station at Esrange, outside Kiruna in the far north of Sweden.
The transmitter stations (uplinks) on the ground that are used to send television, radio and data traffic to the satellite have their own backup systems. If an uplink station fails, there are alternative stations at other sites. A terrorist attack on a transmitter station or a control station cannot disable the satellite system.
If someone aims a jamming transmitter at the satellite, or if a legal transmitter is wrongly adjusted, there are ways of dealing with this. An uplink station for the 18 GHz band costs about SEK 10 million (about EUR 1.1 million) to set up and requires a transmitting dish about 5 metres in diameter. Jamming transmitters can be traced relatively easily and the satellite operator then contacts the authorities in the country where the interference is coming from to shut down the signal. Interference between satellite systems is well regulated by international agreements that oblige the parties involved to adhere to certain power levels, coverage areas and frequency bands.
The satellite is locked into its orbit by means of a station-keeping system which ensures that it is always correctly positioned. In the unlikely event of the satellite leaving its correct position, there are backup systems to correct this.
The rockets used to launch satellites have been developed and refined over the years and their quality is now very high, so the risk of something going wrong during a launch is very low. However, there have occasionally been problems with newly-designed rockets (the ARIANE 5, for example). Every satellite operator must allow for this, and have contingency plans in order to maintain their existing and planned services.
A satellite has a predetermined life, limited by the amount of fuel it has for correcting its orbital position. This makes it possible to plan in good time to launch new satellites and to send old satellites into another orbit as they approach the end of their life. Detailed rules and recommendations for dealing with old satellites have been drawn up in international agreements.
The positions of the satellites in space, the range of transmission frequencies used, and the geographical areas satellites may cover are also strictly regulated by international agreements. The coordinating body is the ITU (International Telecommunications Union), which a member of the United Nations system of organisations. The transmitters and other communication equipment on a television satellite obtain their power largely from the satellite's solar panels. For periods when the satellite is in the Earth's shadow, which happens for about one hour at midnight for a period every spring and autumn, here are batteries on board to maintain communication. This avoids the enforced interruptions in transmissions that would otherwise occur during these "eclipse periods".
Receiving equipment
A parabolic antenna ("dish") lasts as long as a conventional or Yagi television receiving antenna (aerial). The reliability of satellite reception is very high. The expected availability is about 99.95 % over a year. This means that reception problems amount to a not more than four hours per year in all for the dish recommended by the satellite operators for a certain geographical area. The reason is that heavy rain (usually in connection with thunder) damps the signal, making it too weak for satisfactory reception. Choosing a larger dish increases the availability. There will be no changes in frequency range or transmitter power, so that viewers will be able to use their dish and LNB for a long time, although they may wish to change their satellite receiver for other reasons.
Radiation
There is no radiation at all from a dish used only for reception. There is none from an ordinary television aerial either. The radiated power of satellites is 60-70 watts per satellite transponder (one transponder is used for six to eight television channels) and the satellites are about 36,000 kilometers above the surface of the Earth. Compare this with terrestrial television transmitter masts, with a power output of about 60,000 watts for an analog television channel. The radio waves from a terrestrial transmitter are far more powerful than those from a satellite.
