Satellite television lets viewers watch more channels than would normally be available using an ordinary aerial. A satellite can broadcast using high bandwidths, so there are hundreds of channels available through satellite.

In the UK, there are two satellite television providers. Sky Digital uses the Astra satellite system to provide both free-to-air television channels and subscription channels. Freesat also uses the Astra satellite system but is only a free-to-air unencrypted service managed by the BBC and ITV.

Free-to-air channels do not require a subscription and so can be viewed by anybody who has a satellite dish and set-top box. Subscription channels are available to viewers who pay extra to view those channels. These channels include sports, movies, entertainment, music, children's programmes and foreign TV. Viewers can also buy programmes on a pay-per-view basis. This means that the viewer pays to watch a single programme, such as a football game, a concert or a movie.

Set-top boxes let the viewer see an electronic programme guide so that they can see what is currently being broadcast and what will be broadcast in the near future. Some set-top boxes include a hard disk so that programmes can be recorded simply by selecting them from the electronic programme guide.

How satellite television works

Traditionally, television has been broadcast using powerful aerials that transmit radio waves. The signals are picked up using aerials. There are a number of problems with this system, including the limited number of frequencies available to transmit channels and the need to be in near line of sight with the transmitter on the Earth.

It would probably surprise most people to know that satellite television first started broadcasting in the former Soviet Union in 1967. Satellite television companies send television signals from the Earth to a satellite in geosynchronous orbit around 35,000km above the Earth. The satellites then broadcast the television signal using a method called direct broadcast satellite. Most DBS systems encrypt the signal that is being sent to the home, so that specialist equipment is required to receive the signal and homes will only receive the channels they have paid for. This signal is also compressed so that more channels can be broadcast using one satellite frequency, A satellite dish is a specialised aerial that receives the microwave signals from the satellite in space. A receiver, more commonly known as a set-top-box, is then required to decode and decrypt the signals. The correct decryption key is required to decrypt the signal meaning that viewers can only receive the channels they have subscribed to. The receiver will also decompress the signal and extract the individual channels.

A common misconception is that emailing, shopping and games can be played using satellite communication signals. While these features may be available on some set-top-boxes, it is actually a telephone line that is used to send data from the user's home back to the television company as television satellite dishes are only capable of receiving data.

Satellites can be used to monitor weather patterns across the globs. The most common use of satellites is to view photographs taken of cloud formations which can then be used to predict the direction of weather fronts. Satellites are also able to collect other information such as snow and ice cover, the effects of pollution, fires, smog, dust, haze and movements of hot and cold water in oceans.

How weather satellites work

Weather satellites are either in geostationary orbit (fixed position above the equator) or polar orbiting. Geostationary orbit is similar to geosynchronous orbit in that the satellite is in a fixed position above the Earth, but instead it follows the line of the equator. In Europe, the Meteosat series of satellites are used over the Atlantic and Indian oceans. The geostationary satellites are used by television companies to provide pictures of cloud formations. Polar-orbiting satellites follow an orbit between the two poles of the Earth, from north to south and south to north. They are only 850km above the Earth compared with 35,000km for geostationary satellites. Therefore, the photographs provided by polar-orbiting satellites are of a much higher quality. The polar-orbiting satellites use thermal and infrared images that can be used to calculate land and water temperatures by seeing the cloud types and heights.

Global position systems were first used by the US military to help locate troops and vehicles on the ground. In 1983, the US GPS system was made available for civilian use. There is now another GPS system operated by Russia (GLONASS) and India plan to have their IRNSS system operating by 2012 with the European Galileo project due to be operational by 2013.

GPS uses include map making, land surveying, navigation for ships and aircraft, and use by the military. The system has become very popular with car satellite navigation systems, which use GPS satellites to identify their location. This is then shown on an interactive map. The navigation software includes instructions telling the driver where to go at each junction. As the navigation software uses the GPS to identify exactly where the vehicle is, if a wrong turn is made then the software will recognise this and identify a new route. To start a new journey, a driver needs to input the destination - this is usually selected from a list of favourites, or entered using a postcode or street address and name of town. It's also possible to enter points of interest such as railway stations, airports and city centres. The navigation software calculates the route to the destination taking account of the current position given by the GPS. The software is also able to tell the driver how long it will take to reach the destination, the distance left to the destination and what the driver's current speed is. All of this is achieved by the software constantly being told the vehicle's current location by the GPS receiver.

GPS can be used for many social activities, including orienteering and a game called geocaching. The latter is a worldwide game in which participants have to find a hidden 'cache'. Participants look at the geocaching website to find caches that are hidden in a particular area. They are then given the latitude and longitude co-ordinates of the cache and use a GPS device to find it.

How GPS works

The GPS consists of 24 satellites in orbit as part of the NAVSTAR constellation. Each satellite orbits the Earth twice a day and there are always at least four satellites in line of site from any point on the Earth at any time. A GPS receiver will look for communication to at least three satellites and then use a method called trilateation to calculate the distance to each satellite and determine its location.