The successful launch of ERS-2 on 21 April 1995, nearly four years after that of ERS-1 on 17 July 1991, not only ensures the data continuity indispensable for both the scientific research and operational applications initiated with ERS-1, but also opens a new chapter in research in the field of atmospheric chemistry. For the latter, ERS-2 is carrying a completely new instrument known as GOME, the Global Ozone Monitoring Experiment. Through the enhancement of another instrument, the Along-Track Scanning Radiometer (ATSR), already flying on ERS-1, ERS-2 will also provide an additional capability for the monitoring of changes in the Earth's vegetation cover.
The ESA Council's decision to finance the simultaneous operation of ERS-1 and ERS-2 in a so-called 'tandem mode' for a period of several months will allow uniform data sets to be collected, in particular from the identical Synthetic Aperture Radars (SARs) on board the two spacecraft. That will open completely new perspectives for many areas of scientific research as well as for operational and commercial applications.
In as little as four years, ERS-1 has revolutionised many areas of the Earth sciences and their practical applications. Data from ERS-1 sensors are helping scientists greatly to improve their understanding of the processes that control our environment. Such an understanding is the basis for models that can be used to forecast the effects of future natural and man-made changes.
As foreseen in the original mission objectives, ERS-1 data are being used extensively within the international scientific community for physical oceanography, polar science and climate research. Beyond these anticipated areas, the data have stimulated a much broader range of scientific utilisations than was originally thought. This is the case, for example, in the field of solid Earth and terrestrial sciences.
Thanks to the use of advanced observation techniques, primarily radar, ERS-1 provides both global and regional views of the Earth, regardless of cloud coverage and sunlight conditions. An operational near-real-time capability for data acquisition, processing and dissemination, offering global data sets within three hours of observation, has allowed the development of time-critical applications particularly in weather, marine and ice forecasting, which are of great importance for many industrial activities.
Every space agency that develops and launches a completely new type of satellite faces the same question: 'Will users have as much call for the data as was predicted in preparatory studies and surveys?' Four years after its launch, the demand for ERS-1 data has not only lived up to expectations, but has indeed exceeded them and is continuing to grow at a rate of 20% to 30% a year.
A few figures will serve to illustrate this point: the Proceedings of the two ERS-1 scientific symposia (held in Cannes in 1992 and Hamburg in 1993) contain a total of 410 original scientific papers. Many have also been published in recognised learned journals, including Nature.
At the first ERS-1 Application Pilot-Project Workshop, held in Toledo, Spain, in June 1994, more than 100 projects covering a wide spectrum of operational applications were presented. A significant number of those projects have now reached the operational stage, in particular in such fields as meteorology, ice forecasting and bathymetry.
The customer service at ESA's data handling centre ESRIN has dealt with some 15 000 orders for ERS SAR data from all corners of the Earth. This does not include the thousands of orders that went directly to non-ESA ground stations rather than to ESRIN. The large number of users in North America and the Asia-Pacific Basin is particularly striking.
The breadth of ERS users is enormous, ranging from individual scientists to multi-institutional research groups, and from small high-tech firms to large corporations and crucial public services such as meteorological offices.
Another important element is the availability of a well-maintained and accessible data archive. This is especially true in the case of radar data, which is unaffected by cloud cover and is in principle continuously usable. ERS-1 has provided more than half a million distinct radar images, each of an area of 100 km x 100 km, covering virtually the whole of the Earth's surface. They can be used either alone or in combination with optical images from satellites such as Spot and Landsat.
The high standard of performance in terms of satellite instrument operations and the reliable provision of well-calibrated data have stimulated and encouraged the use of both the SAR and the low-bit-rate data. This has been achieved through close and fruitful cooperation between the European and Canadian industry involved in the development of the ERS-1 satellite and ground segment, the scientific community, and the ESA project team.
Built in the same way as ERS-1 by a consortium led by Deutsche Aerospace, ERS-2 carries the same radar instruments, together with GOME and the enhanced ATSR. It will thus have to deal with an even more demanding range of tasks and an even greater number of users.
Following a worldwide Announcement of Opportunity issued in the spring of 1994, using very strict criteria, ESA selected a further 340 research teams interested in using data from both ERS satellites. A number of them are particularly eager to acquire data obtained when the two satellites are working in tandem. Such unique data are especially useful for SAR interferometry to generate accurate Digital Elevation Models (DEMs) which are of high value for many applications, such as hydrology and cartography, or to detect small (cm-level) movements in the Earth's crust, for example following earthquakes, prior to volcanic eruptions, or as a result of glacier flows. A large number of users are interested in the scientific exploitation of data from GOME, since it offers significant advantages over conventional instruments in terms of both measurement accuracy and spectral coverage.
The number of ERS receiving ground stations is also expanding. There are now mobile stations that allow data to be provided for areas not previously covered, such as Central and Eastern Africa, or incompletely covered with the existing stations, such as Antarctica.
ERS-2 will benefit greatly from the expertise and experience gained with ERS-1 in terms of data processing and dissemination, sensor calibration and data validation.
At the time of printing, ERS-2 is still in its commissioning phase but has already provided data of high quality, demonstrating its capability to continue the ERS service until the end of the 1990s, Then, the next generation of satellite, Envisat-1, with even more advanced sensors, will be launched and will take over the service to users until 2005.
With ERS-1 and ERS-2, ESA is playing a major role in the provision of the continuous, high-quality and reliable data that is needed for a better understanding of our complex home habitat Earth and its fragile environment.
ESA Bulletin Nr. 83.