Earthwatching satellites.

Landsat, Spot, Topex-Poseidon, ERS, Polder, Argos, NOAA, Cosmos, Sarsat, Meteosat, Himawari, GOES, Tiros, Nimbus - mysterious words that often appear in news reports - are the names of artificial satellites.

The satellite era began on 4 October 1957, when the Soviet Union successfully launched Sputnik ("fellow traveller" in Russian) into orbit around the earth. An observation tool weighing almost 84 kg, Sputnik circled the earth for three months at altitudes ranging from 228 to 947 km.

Bolstered by progress in information technology, satellite technology has come a long way since then. Today we rely heavily on these eyes in the sky to tell us about the environment and help us to manage the earth's resources.

Eyes in the sky

Artificial satellites today observe the atmosphere, the oceans, and the earth's land surface from space. They provide information using two different techniques, data collection and remote sensing. With the first technique, they receive data from various points on the earth's surface and then beam it back to earth. With the second, they use their instruments to analyse information contained in electromagnetic radiation emitted by the earth.

The choice of orbit is extremely important. A satellite in geostationary orbit some 36,000 kilo-metres above the earth appears immobile since it moves …