Earthwatching Satellites

Article excerpt

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 at the same speed as the revolving globe. Geostationary satellites continuously observe a wide area but are hampered by their distance from the earth. A satellite in polar orbit, on the other hand, covers the entire planet without remaining permanently above a single region.

Geostationary satellites and satellites in polar orbit do not have the same uses, and their capacities complement each other. Both kinds are used in meteorology, for example, a field where satellites have proven to be of vital importance. Five geostationary satellites enable weather watchers to keep a constant eye on the earth's weather system in toto, while polar orbital satellites provide more precise measurements at different points on the earth's surface.

Satellites, whose weight ranges from a few kg to several tons, are launched by extremely powerful rockets. The United States, Russia, France, Japan, China, India and Brazil are among the countries that possess such launchers, and in some cases launch satellites for other countries. Canada and the Arab states, for example, have had satellites launched by the United States and France respectively.

Once in orbit, satellites record images by means of still photography, TV cameras, scanners or radar. Film from manned satellites is returned to earth for processing. Unmanned satellites either process the images themselves or transmit them to terrestrial receiving stations in different parts of the world. Each image is ordered in advance from the satellite by computer and identified in detail.

This battery of observational technology has great potential for environmental monitoring. In July 1991, the European Space Agency launched ERS-1 (an acronym for European Remote-Sensing Satellite), Europe's first environmental satellite, whose work was later carried on by its sister craft ERS-2. Radar sensors enable the satellites to "see" at night and through cloud cover. One system generates 100-km x 100-km scenes of regions over which the satellite flies and measures wind speed and direction and wave energy. Another measures wave height to within 2 cm. A third system is an infra-red detector that measures sea surface temperature to an accuracy of 0.3 [degrees] C.

High resolution images

"Our only way to obtain an overall vision of the state of ocean biology at any moment is satellite observation of the ocean's colours," explains John Withrow, a member of the Intergovernmental Oceanographic Commission (IOC). "Data on the colour of the ocean will enable us to monitor such important fields as biogeochemical cycles, the direct effects of biology on the physics of oceans, coastal resources and sustainable fishing. …