Satellites can be used for a number of tasks including communications and astronomy.
COMMUNICATIONS
Communication satellites can receive both television and telephone signals, transmitted from one continent to another. An example of a powerful communication satellite used for this purpose is the Intelsat 4A. While in its geostationary orbit, precisely 24 hours is taken for it to circle the Earth 36,000 kilometres (22,500 miles) above the equator. The radio signals received are of a high frequency, and therefore they can cut through the Earth's atmosphere. This in turn allows excellent communications over large distances.
LAND MAPPING
Before the start of the Space Age, precise ground mapping was a long and meticulous process. With the introduction of satellites like Landsat, it has become a very easy business. Landsat is the name of a succession of extremely prosperous satellites launched to look at Earth's resources. The Landsat satellite revolves in polar orbit approximately 650 kilometres (400 miles) high. It can receive images not only at visible light wavelengths, but also at invisible light wavelengths, for example infra red.
Landsat has two scanning systems called the multispectral scanner and thematic mapper. They construct a picture in a similar way to a television scanning system. The images of a landscape acquired at various wavelengths can be transformed into true colour images. However, more often than not, they are presented with false colours. By a careful choice of colours, it is possible to highlight specific features. As an example, this system can be utilised to display rocks that might hold unfamiliar mineral deposits.
ASTRONOMY
The Infra-Red Astronomy Satellite was launched in 1983. It contained a reflecting telescope that was specially cooled. This satellite had the ability to find infra-red rays coming from various sources of heat, which could not be detected from Earth.
Satellites in space almost always utilise solar cells to produce the power they require for operation. These cells have the ability to convert the energy from sunlight directly into electricity. Each cell consists of thin wafers of silicon, 0.15 millimetres (0.006 inches) thick. Many thousands of these individual cells are required just to provide a few hundred watts of power. These cells can also charge small battery packs, which come into use when the satellite travels into the Earth's shadow. The solar cells can either be mounted on `wings' or `paddles' positioned in such a way that they protrude from the satellite. In some cases, the whole body of the satellite can be covered by these cells.
