Earth observation (EO) data delivered by remote sensing satellites provide a basic service to society, with great benefits to the civilian. The data is nowadays ubiquitously used throughout society for a range of diverse applications, such as environment and resource monitoring, emergency management and civilian security. Over the past 50 years, the EO data chain that has been mastered involves acquisition process of sensor data on-board the satellite, its compression and storage on-board, and its transfer to ground by a variety of communication means, for later processing on ground and the generation of the downstream EO image products. The EO-ALERT project, an H2020 European Union research activity, addresses the challenge of a “high speed data chain” and the need for increased EO data chain throughput. EO-ALERT proposes the definition and development of the next-generation EO data and processing chain, based on a novel flight segment architecture that moves optimised key EO data processing elements from the ground segment to on-board the satellite. The objective is to deliver the EO products to the end user with very low latency for increased throughput. This paper presents the activities performed during the first part of the EO-ALERT project, focussing on the definition of the user requirements for the EO-ALERT EO data processing chain, based on the identified market needs and application scenarios, now and in the future. Several application scenarios for EO data are reviewed and those which need and drive a high speed data processing chain are identified, considering commercial and institutional applications and focusing on those applications with a low latency need. Considering the innovative on-board processing chains implemented in the EO-ALERT project, the end users consider that the corresponding technology advances can enable very competitive and effective applications in the maritime surveillance and the disaster management, especially to cope with extreme weather events. These applications require a high responsiveness to events, reducing the response time to few hours, or even to minutes, after an emergency situation arises. Ship detection application includes, among others, traffic surveillance, protection against illegal fisheries, oil discharge control and sea pollution monitoring. One of the main objectives in maritime surveillance in Europe is to improve the European capability in the field of ship detection in both open and coastal waters for the purposes of security, providing observations on very short notice and near real-time detection. Flexibility and responsiveness for the provision of EO data are very important for maritime surveillance and security services. Two European EO missions have been selected to develop and test the on-board data processing chain for the ship detection scenario: the TerraSAR-X satellite mission embarking an X-band Synthetic Aperture Radar instrument able to image day and night under almost all meteorological conditions; and the Very High Resolution and agile optical satellite, Deimos-2 for detection and classification of small ships. There are many different segments of society that benefit from an early detection and warnings of the extreme weather events. Defence, police and emergency authorities use extreme weather warnings to reduce loss of human life and property by providing people with instructions, building containers for foods, evacuating people from risky areas, just to cite a few. Also, environmental and road authorities need these kinds of warnings to reduce loss of natural resources and human life. Aviation needs information about storm occurrence and exact position in order to plan flight routes to reduce loss of human life, property and fuel. The shipping and fishing trades take advantage of this information to re-plan and take less risky routes. The industry energy sector uses wind speed warnings, associated to convective storms, to reduce windmill breakdown. Two meteorological phenomena are going to be detected in the extreme weather scenario: convective storms and surface winds overseas and oceans. The main objective is the provision of early warnings for these two phenomena from satellite information. Considering the specific needs for the meteorological scenario, the following current sensors have been considered: SEVIRI on board Meteosat Second Generation to be used operationally, the radar on board TerraSAR-X to be used operationally and SLIM6 on board Deimos-1 to be used for research purposes. A high-level mission analysis for the above mentioned assets is presented, encompassing: coverage analysis, ground station contact analysis and data latency. For ship detection, a scenario that combines optical images from Deimos-2 and SAR images from TerraSAR-X is studied, providing information on possible common observation windows. An experimental campaign at the end of the project is intended to validate the technology performances in the experiment scenarios. The objective is to acquire the necessary in-situ data at the same time that SAR and optical remote sensing images are acquired on the region of interest, so as to provide a holistic knowledge of the monitored area and data for validation of the processing chain performances. @en