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The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the EU-ESA Global Monitoring for Environment and Security programme. The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. In particular, as part of the GMES space component, ESA is currently undertaking the development of 3 Sentinels mission families. Each Sentinel is based on a constellation of 2 satellites in the same orbital plane. This configuration allows to fulfil the revisit and coverage requirements and to provide a robust and affordable operational service. The launch of the 2nd satellite is scheduled 18 months after the launch of the 1st spacecraft of the constellation. The lifetime of the individual satellite is specified as 7 years, with consumables allowing mission extension up to 12 years. The lifecycle of the space segment is planned to be in the order of 15-20 years. The strategy for Sentinel procurement and replacement over this period is being elaborated, but will likely result in a need for 4-5 satellites of each type if the desired robustness for the service that GMES will provide is to be achieved. This paper will describe the operational and observational capabilities of the Sentinel-1 mission based on the user requirements, including potential emergency requests. An example of a pre-defined mission timeline for each and every cycle will be given.