WHEN THE SCIENCE FICTION AUTHOR Arthur C Clarke published a seminal article laying down the principles of satellite communication in Wireless World in 1945, the moon and perhaps a few small asteroids were Earth's only' satellites--and no-one even knew about the asteroids. Artificial satellites, though the norm today, were the stuff of Clarke's fantastic imagination. But in the years that followed, science was quick to catch up.
Today, satellites are an integral part of our lives. Communications, Earth monitoring and mapping have all come to rely heavily upon these awkward-looking metal contraptions in the sky. And with the technology constantly being updated, the uses of satellite data and the extent of its application are growing. Increasingly, it is our imaginations that are stretched by technology, rather than the other way round, as it was in Clarke's day.
The first satellite, Sputnik-1, was launched by the Soviet Union in 1957, famously kick-starting the Space Race. In 1964, the USA launched Syncom-3, the first successful geostationary satellite. These orbit in a band of space 35,788 kilometres above the Earth's surface--an area known fittingly as the Clarke Belt. They move in sync with the Earth, always remaining in the same position overhead. Each can cover about 42 per cent of the Earth's surface, so three correctly placed satellites will be able to see the entire globe. Today, such satellites are mainly used for communications and television, allowing viewers' dishes to face the same spot in the sky and receive Sky.
Weather satellites also inhabit the Clarke Belt, but for most Earth monitoring, a sun-synchronous, or polar, orbit is favoured. Orbiting the Earth in about 100 minutes at an altitude of 700-900 kilometres, these can build up a complete picture of the planet every two to three weeks. These are the satellites that can be seen silently hurtling across the night sky.
Thanks to improved sensors, the real fruits of remote sensing--the term given to viewing the Earth from afar, whether from a satellite or an aircraft--are only now being realised. "It has revolutionised how we understand the world," says Jonathan Raper, Professor of Geographic Information Science at City University, London.
There are essentially two types of sensor for Earth monitoring: passive and active; the former receives information, while the latter sends out a beam of microwave radiation and records how long it takes to bounce back. A single satellite may have several different type of sensor for monitoring different aspects of the Earth. Envisat, launched on 1 March 2002 into a polar orbit, is the world's largest environmental satellite and boasts ten instruments designed to gather data on land, sea, water, ice and temperature.
The capabilities of a satellite sensor are determined by the ranges of wavelengths recorded and the precision with which the amount of reflected energy can be measured. A sensor recording in a single waveband produces a monochromatic image, while multispectral images are produced from sensors that simultaneously record multiple wavebands. Because humans are only capable of viewing images in red, green and blue, any three of the wavebands available must be chosen to highlight the particular features of interest. When any other combination is used, the resulting image is called a false-colour composite.
These images are very sensitive to the presence of healthy vegetation, providing an unbiased universal means to analyse and monitor vegetation change. That said, wheat fields and deciduous woodland appear different in winter, making the job of identifying what is on the ground more difficult. The Great Britain map of land cover of 1990 was the first complete map of its kind since the 1960s and the first to be comprehensively created using satellite data. The sensors aboard the US Landsat satellites classified 25 different types of land cover, all down to 25-metre accuracy. …