Satellites have been looking down on our world for many decades, but there has always been one frustrating obstacle: at any one time, half of the globe is hidden in darkness and a sizeable proportion of what we can see is blanketed by impenetrable cloud. Impenetrable, that is, until space scientists found ways of placing radar instruments into orbit.
The initial breakthrough came in 1978 with the American Seasat, which provided a tantalising glimpse into the potential of such systems. Unfortunately, Seasat ceased to operate after only three months. Earth observation returned to the dark ages until ESA decided to take up the challenge. The new era truly began in 1991 when the 2.5-ton ERS-1 was inserted into a 780km- high polar orbit from which it could study the entire globe as the Earth rotated beneath it.
So what have European taxpayers received in return for their $860m investment in ERS-1? On paper, its achievements are certainly impressive. More than half a million radar images covering almost the entire planet have been placed in the archives. In addition, there has been almost continuous data return from its other main payloads. These include a scatterometer and radar altimeter which send back information on such features as wave height and wind speed, and a British-built radiometer that measures surface temperatures. Only one sensor, designed to provide precise information about the satellite's orbital position, has failed.
ESA officials admit that initial take-up of the new data was slow; no one knew what to do with it and how to handle it. This is now beginning to change and interest in the various data streams is steadily growing. "The demand for ERS-1 data has not only lived up to expectations but has exceeded them and is continuing to grow at a rate of 20 to 30 per cent a year," says a spokesman for the ESA.
Most of the data is provided free of charge to individual scientists or academic research institutions. Small firms and users in developing countries pay only for the reproduction costs, leaving just 7 per cent of the data to be distributed on a commercial basis. At a time when operational profits are becoming all-important in the space industry, such a lack of commercialisation seems perverse. The answer lies in ESA's original vision of the ERS-1 mission. "It was never conceived as a commercial service," says Chris Elliott, a consultant with Smith Associates in Guildford. "It was originally intended as a scientific service to look at the oceans and ice caps."
There have been several other obstacles in the way of using ERS data for commercial purposes. When the first satellite was launched, ESA was not geared up to provide rapid data distribution. This has largely been overcome by introducing a fast-delivery processing system at the Swedish receiving station in Kiruna.
Unfortunately, the satellites are unable to store the vast amounts of information needed to produce radar images. "The main problem is that we can only receive radar data when the satellite is viewed by a ground station," says Dr Andy Sowter of the National Remote Sensing Centre (NRSC) in Farnborough. "We have complete coverage in Europe, but the stations are generally operated elsewhere by local or national agencies. The data may spend many months sitting in their archives before it gets back to Europe."
Another problem is the limited number of opportunities to view a particular target. This is partly due to the design of the satellites. Their radar- imaging system only operates for about 20 minutes during each 100-minute orbit and its viewing angle is fixed. Furthermore, the satellites usually pass over the same place only once every 35 days. This repeat cycle is far too long for monitoring rapidly changing surface phenomena such as floods and oil spills.
Despite these drawbacks, ESA believes its radar satellites have considerable commercial as well as scientific potential. "We are expecting $1.3m from sales of ERS data this year," says Lanfranco Emiliani, the agency's director of earth observation. "We are trying to see if this can somehow be increased. We are under pressure from our [national] delegations to do this."
The future of earth observation by radar seems bright. "ERS-1 has broken new ground," comments Dr Sowter. "We are now ready to move on to different applications." The possibilities seem almost endless. ESA has almost 150 pilot projects under way to assess how useful the radar data might be. These include such wide-ranging studies as monitoring floods and ground water in arid regions; detecting ice floes and icebergs which threaten shipping; and policing ships by following their wakes on the ocean surface.
Dr Sowter believes there are two markets where radar data stand out: ocean monitoring and land-use monitoring in cloudy areas such as the tropics. Apart from picking up man-made oil slicks, the satellites can detect natural seepage from the ocean floor which tells companies where to drill for oil.
NRSC is using ERS-1 radar data in the search for suitable oil prospecting sites to the west of the Shetlands. Radar maps are being created for oil companies interested in exploring the relatively uncharted regions of Papua New Guinea and Venezuela. Valuable new research is also being carried out with the scatterometer and radar altimeter. Data are downloaded to the UK Meteorological Office within two hours of the original observation, and used to generate shipping and general weather forecasts. According to Steve Foreman, head of ocean forecast modelling at Bracknell, such data are also proving useful in routing ships and predicting coastal flooding.
Perhaps the most innovative use of the satellites is yet to come. Once ERS-2 becomes operational in July, the ESA intends to operate both craft in tandem for the following nine months. "It is rather like stereo," explains Dr Sowter. "They will generate two different views of the same area to get a 3-D picture."
Several hundred scientists are eagerly awaiting this unique opportunity to create the first highly accurate, digital, three-dimensional map of the Earth's continents.