Satellites and their uses

Satellite communication

Covering the whole of the Earth with communications satellites requires a minimum of three satellites evenly distributed.

Exploration of the solar system

Spacecraft have been sent to explore the planets, and some of these have been placed in orbit around those planets, becoming artificial satellites.

Artificial satellites have even been placed around the outer planets: Galileo (around Jupiter); Cassini (around Saturn).

Venus has been orbited by Magellan, and Mars has had a number of orbital missions including those which were a part of the Viking mission and more recently the Mars Global Surveyor.

The Moon has been explored by the Lunar Orbiters and others, but Mercury has only had fly-bys so far.

Global positioning system

The global positioning system (GPS) consists of a network of Earth-orbiting satellites that can be used to determine the position of any point on Earth with considerable accuracy.

The system works by using the synchronised signals from four GPS satellites to estimate the position in three dimensions and time. Using a hand-held commercial device, a GPS user can determine their position to within about 2 metre. For military applications it is accurate to less than 1 metre.

In North America, tractors have been linked to the GPS. An imaging satellite can detect where crops are nitrogen deficient. The information from the satellite is then used to vary the amount of fertiliser delivered to the crops at that point.

Weather satellites

Data from weather satellites is continuously updated and fed into large computer models that calculate the possible developments of weather patterns. The models are reliant on timely and accurate data but even then, because of the large amount of data involved, their predictions are only reliable for the next 48 hours. Good images are available from the web sites of the Meteorological Office and the American NOAA satellite.

Studying the universe

Among the first images of planets captured extra-terrestially were the photographs obtained by the Voyager satellites, many of which can be found on the Internet. The Hubble Space Telescope has provided images of better resolution and detail than those provided by any terrestrial telescope.

However, astronomers have also built ultra-violet, X-ray and gamma-ray satellites to detect radiation produced by stars and other astronomical objects. Most of this radiation is not detectable on the surface of the Earth because the atmosphere is not transparent to these wavelengths.

The data obtained from these satellites has helped to transform our understanding of the universe. For instance, astronomers have predicted that streams of matter falling into black holes will be heated to such an extent that they will produce X-rays as they fall towards the black hole. The detection of such sources in the constellation of Cygnus has confirmed their view that black holes exist even though they cannot be directly observed.

Finally, perhaps the major satellite in orbit around the Earth is the International Space Station. This is the largest artificial satellite ever to orbit the Earth.