Mapmakers,
town planners, mineral prospectors, farmers and foresters are among the
many groups of people who have benefited from another kind of satellite.
This is the Earth-survey, or Earth-resources satellite.
At the beginning of the Space Age, ordinary photographs of the Earth's surface from orbit revealed much useful information. Earthsurvey satellites are able to gather even more by scanning the surface in light of different wavelengths, such as infrared.
The best-known series of such satellites has been the US Landsats, of which five were launched, the first in 1972 and the last (Landsat 5) in 1984. Landsat 5 orbits at an altitude of about 700 km. Using an oscillating-mirror system, it scans the Earth's surface in 185-km square blocks in green and red visible light and at four infrared wavelengths using the Thematic Mapper and Multispectral Scanner.
The
French SPOT and the European Space Agency's ERS-1 are also Earth-survey
satellites. ERS-1 uses radar for scanning the surface and also monitors
weather and climate. The French Earth-resources satellite SPOT (satellite
probatoire pour 'observation de la Terre) has better resolution than Landsat.
It can spot details as small as 10 m - about the size of a bus. Like Landsat,
it scans at different visible and infrared wavelengths.
Information
comes back from the satellites in the form of electronic data. Computers
process the data and display it as images. They can manipulate the data
in various ways and display it in false-colour pictures or on film in a
number of different ways. The colours are chosen so as to pick out certain
features of the landscape. This is possible because every kind of feature
has a different "spectral signature". This means that it reflects different
wavelengths in its own particular way.
