The first satellite in the ERS series was launched in June 1991, and its successor (ERS-2) in April 1995. Since 1991, an almost global coverage of the Earth's surface has been attained with the satellite's SAR (Synthetic Aperture Radar) instrument.
The ERS satellites have Sun-synchronous, near polar, quasi- circular orbits with a mean altitude of 785 km and an inclination of 98.5°. Most of the ERS-1 mission has been performed with a 35-day cycle. ERS-2 only operates in a fixed repeat cycle of 35 days, which means that a particular site is covered every 16 days (figures for Equator latitude). Day and night orbits can be used to obtain a higher temporal coverage.
The coverage of one ERS-SAR image is 10,000 km² (100 x 100 km). A wide range of products has been made available by ESA through its Processing and Archiving Facilities (PAF's). Among these products, the PRI (Precision Image) can be considered to be the most adapted for general use in application fields such as agriculture, forestry or cartography. The pixel size of this product is 12.5 x 12.5 m. One can obtain the data either on Exabyte or CCT. However, Exabyte is the most widely used medium as Exabyte drives are now very cheap and can also be used for saving work following data interpretation. The size of one data set is basically 120 megabytes, which corresponds to an image size of about 8,000 x 8,000 pixels. Data are packed in BSQ (band sequential blocks). There are normally four files on one band, the third one corresponding to the data file.
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