ERS-SAR image of earthquake region in Turkey

The tandem mission did not yield the only unexpected results. "Using ERS interferometry data covering periods following large earthquakes, we have learned that, after an earthquake, physical processes take place in the rocks adjacent to the fault, producing a regional stress readjustment over several years. These post-seismic processes produce small and slow surface shifts that have been observed for the first time using ERS, and these quantitative observations are essential to understand the physics of the Earth's crust," explains Gilles Peltzer, of CalTech's Jet Propulsion Laboratory.

Peltzer is clear about the new capabilities that ERS offered the seismology community: "ERS provided the first complete images of the surface deformation associated with geophysical processes. These images are spectacular and will remain the first ever produced," adds Peltzer.

Other scientists in different disciplines are equally impressed. "The ERS mission's most significant achievements to date are its detailed mapping of the marine geoid and seafloor topography, and the use of SAR interferometry to map deformations in the Earth's crust," says Dr Anny Cazenave, of France's CNES Laboratoire d'Etudes en Géophysique et Océanographie Spatiale.

Surface winds around Antarctica

"The ERS programme has shown us that satellite altimetry is a new and very valuable tool to monitor surface continental waters with important applications to continental hydrology, regional climate variability and water resources policy," adds Dr Cazenave.

According to Dr Per Knudsen and Dr Ole Andersen, of Denmark's KMS, "The ERS mission gave us altimetry data which enabled us to study the marine gravity field in very high detail and we were able to map the height of the entire ice sheet in Greenland - including the altitude of the centre, which had never been measured before."

"In atmospheric science, the ERS programme's most significant achievements have been the global measurement from space of minor trace gases responsible for ozone depletion, measurements of air pollution (trace gases in the troposphere) from space, and exploiting the possible synergy of different sensors on the same platform, which is an important lesson for Envisat," explains Ankie Pieters from KNMI, the Royal Dutch Meteorological Institite. "ERS showed us that it is possible to use global space measurements to support global change studies, leading to political decisions on international agreements and treaties, and changes in the behaviour of society - companies adopting green labelling, for example," Pieters adds.

Oil slick lying along the coast of Oporto, Portugal

ERS has not only made a major contribution to research. Data from ERS instruments is being used operationally in a number of applications. "ERS-SAR provided a unique data source, which enabled national organisations to prototype and validate the operational infrastructure for oil spill detection and sea ice mapping services. ERS SAR data was also important for establishing agreements with end users to demonstrate the capabilities, and these information services would not have the performance levels seen today without the benefit of ERS SAR," explains Jan-Petter Pedersen from Tromsoe Satellite Station.

"Another very important contribution of ERS was to make SAR data available to the science community on an unprecedented scale. ERS helped to build scientific acceptance of the use of SAR for Earth System monitoring." comments Professor Werner Alpers from the University of Hamburg.

Another major success of the ERS mission has been the operational use of data from the scatterometer, which measures the 'roughness' of the ocean surface, from which wind speed and direction can be calculated, and oil leaks detected. "In our wave forecasting service we use ERS SAR imagery to measure the spacing between wave packets. We can then forecast local properties at our customers'operations sites. Without the ERS SAR imagery, such measurements would be prohibitively costly," explains Hafedh Hajji of France's MeteoMer. Adrian Huntley, of Infoterra in the UK adds, "The ERS SAR archive offers a unique capability to distinguish genuine seepage slicks due to their persistence between acquisitions. This is a major factor in our service to the offshore oil and gas industry".

Envisat in testing

Experience with the ERS programme has shaped its successor, Envisat. But ERS also leaves a legacy of data still to be explored. "We've seen a pattern in the evolution of the science from the ERS mission," comments Professor Alpers. "Initial research has focussed on what you can do with the data, followed by validation of the use of data for deriving earth science measurements, and then analysis of earth system processes using the data.

The continuing contribution of the ERS mission will be the use of the archive as a 10 year snapshot of earth system parameters, such as land cover, ice cover, global sea surface temperature measurements for ocean warming and air-sea exchange, ocean variability records, and finally global wind fields and wind stress," Ankie Pieters adds. "Combining Envisat and ERS atmospheric measurements will give us a ten year data set of atmospheric constituents, which is the minimum timeframe necessary for long-term trend measurements."

"We're still getting new science from ERS data," comments Gudmandsen, "for example, we're beginning to be able to monitor subsidence and land movements of only a few millimetres using new techniques."

Deforestation in Rondonia, Brazil

"The ERS mission has shown conclusively how valuable remote sensing data can be," concludes Alpers, "we can directly observe climate change and its effects, monitor changes in global forest cover through clouds, measure the variation of sea levels globally, track pollution in the atmosphere and the sea, observe global ocean currents, measure the true shape of the Earth and watch it changing - without ERS some of these things would have been impossible, and others would have needed hundreds of thousands of measurement stations."