[Launch Date: Estimated October 2013]
The Sentinel-1 series of satellites will address the issue of data continuity for SAR data at large. The immediate priority is to ensure such continuity for ... C-band data. Under the current scenario, provision of ENVISAT data to feed SAR-based services is likely to cease in the 2008-2010 timeframe. In order to meet the need for continuity, and taking into account the availability of Radarsat-2, the first Sentinel 1 satellite should be launched before the end of the Envisat operations.
The experience with ERS, Envisat and Radarsat constitutes the basis for the Sentinel-1 mission requirements and concept.
Summary Source: http://www.esa.int/esaLP/SEMZHM0DU8E_LPgmes_0.html