Pictures taken from orbit by properly instrumented satellites can reveal an extraordinary amount of information about the Earth's surface. Such Earth-resources, or remote-sensing satellites have ushered in a revolution in many disciplines and fields of study. They have transformed the work of the mapmaker, geologist, town planner, environmentalist, forester and prospector, among others. The pioneering remote-sensing satellite of this type was ERTS-1, ERTS standing for Earth resources technology satellite. Launched in 1972, it was later renamed Landsat 1. Currently, only the last satellite in the series, Landsat 5 is fully operational. It has since been joined in orbit by a number of remote-sensing satellites from several countries. Two of the most advanced are the French SPOT ('satellite probatoire d'observation de la Terre') and the European Space Agency's ERS-1, which also carries imaging radar. An Earth-resources satellite operates in a near-polar orbit and views the whole of the Earth's surface as it spins round underneath. It carries imaging systems that scan areas of the surface, rather as the scanner in a TV camera scans a scene. The scanners in the satellite, however, view the surface not in ordinary white light, but in light of different wavelengths (or colours). Some of these are infrared wavelengths, which our eyes cannot detect. By 'seeing' in different wavelengths, the scanners can gather much more information about the surface than if they used ordinary visible light. This happens because every substance reflects different wavelengths in different ways - we say that each substance has a different spectral signature. The scanners can pick up these different signatures, whereas our eyes cannot. They are thus able to spot things that are invisible to the eye or ordinary photographic film. The scanning signals the satellites send back are computer-processed into visible images. Usually the images are printed in false colours, selected so that they emphasize particular features.
