Signals are beamed up to the comsats and received back from them by huge dish aerials at transmit/receive ground stations. These are linked into each country's communications system by cable or microwave radio links.
Intelsat (International Telecommunications Satellite Organization) is the biggest worldwide satellite communications network. It has over 110 member nations, and launches and maintains powerful comsats, such as Intelsat 6, over the Atlantic, Pacific and Indian Oceans. The comsats are in geostationary orbit: they circle over the Equator at a height of 35,900 km. In this orbit they circle the Earth every 24 hours. In other words they keep pace with the Earth as it turns, and therefore appear fixed in the sky.
Geostationary
orbit
Most satellites that need to reach geostationary orbit 35,900 km high
are first launched into low orbit. Then their on-board motor fires to send
them into a transfer orbit. When they are 35,900 km high, the motor fires
again to direct them into a circular geostationary orbit.
Russia maintains a large comsat network known as Orbita, which uses Molniya satellites. They do not circle in geostationary orbits, but in orbits that are eccentric, or highly elliptical. These orbits take them as high as 40,000 km over Russia but as low as 600 km on the other side of the Earth. In this way they are "in sight" of Russian ground stations for most of the time.
Weather forecasting has been revolutionized by the use of satellites. They are able to scan the whole Earth and the atmosphere continuously, day and night. They can show how weather systems are developing anywhere in the world, in places where there are no ground weather stations. They take cloud pictures, measure water and air temperature, and relay weather data.
Some weather satellites circle in geostationary orbit, where they view nearly a whole hemisphere. The US GOES and European Meteosat are examples. Other satellites are launched into a polar orbit, over the North and South Poles. They can scan the whole Earth every 12 hours as it spins beneath them. The US NOAA series of satellites are in polar orbit.
A
satellite dish (left) in a remote village in India. It receives signals
from a communications satellite in geostationary orbit, 35,900 km high.
The satellite beams down regular television programmes for entertainment
and also specialist programmes for education and instruction in, for example,
farming and family health.
A
GOES weather satellite and an image (left) taken by GOES 4 in geostationary
orbit over the eastern Atlantic Ocean. It shows a hurricane (David), spiralling
over the Caribbean islands of Hispaniola and Puerto Rico. The central core
of the hurricane measures more than 400 km across.
The
first Sputnik
Russia thrust the world into the Space Age on 4 October 1957, when
it launched Sputnik 7 with a modified Sapwood intercontinental ballistic
missile (ICBM). It was an aluminium sphere measuring 58 cm across and weighing
83 kg. It sent back simple radio signals from its four long aerials. Its
orbit took it as low as 220 km above the Earth, and the whiff of atmosphere
there gradually caused it to slow down. It fell back to Earth after 92
days.
BENEFITS
OF SPACE PROBES
Meteorological satellites can monitor the changing patterns of the weather and plot ocean currents, which play a major role in determining the Earth's climate. Data gathered by monitoring such vast expanse as this Russian ice floe can be used to predict climate change. Resource satellites are used for geological and ecological research. For example, they map the distribution of plankton - major part of the food chain - in ocean waters.
