A quiet anniversary slipped by a day or two ago. Ten years ago, on July 17th, 1991, the European Space Agency launched its first European Remote Sensing Satellite, ERS-1. Over the following decade, ERS-1 and its sister spacecraft and ultimate successor, ERS-2, have provided environmentalists and geophysicists throughout the world with an embarras de richesses in terms of scientific data.
The ERS satellites were the first of a new generation, differing from any predecessors in the sophistication of their instrumentation and the relatively low altitude of their orbit. Their payload was dominated by three radar instruments, which operate at a microwave frequency unaffected by clouds or rain, and the spacecraft were designed to orbit the Earth 14 times a day at a height of about 500 miles.
The Synthetic Aperture Radar (SAR) has provided pictures of exceptionally high quality which appeal to a wide variety of scientific and commercial users. At sea, for example, it can be used for tracking ships, or for monitoring the distribution of ice floes; over land the pictures are used for mapping, for forest and crop studies and for glaciology.
The Scatterometer is the instrument most useful to meteorologists. Radar signals bounced back from the rippling ocean allow the satellite to construct a moving three-dimensional picture of the sea's surface.
This allows the height, speed and direction of ocean waves to be calculated, information which is valuable in its own right, but which also allows the speed and direction of the surface wind to be inferred. Successive scans over a relatively short period provide meteorologists with a snapshot of the entire global wind pattern.
The third important ERS instrument is the Radar Altimeter. It directs a concentrated microwave beam vertically downwards and measures the height of the satellite above the ground very accurately.
This information allows, for example, the volume of the Greenland and Antarctic icecaps to be monitored continuously, thereby contributing to our understanding of the global warming problem; it also makes possible the detection of tiny deviations in the elevation of the Earth's surface, important data for monitoring seismic and volcanic activity.
ERS-1 operated successfully from 1991 until March 2000, when it died having circled the Earth more than 45,000 times and having lived three times longer than expected. ERS-2 was launched in 1995 and is still going strong in orbit.
Together the satellites have provided a wealth of scientific data to give new insights into the shape of our planet, the chemistry of our atmosphere, the behaviour of our oceans and the effects of mankind's activity on our environment.