## 1 Introduction
For Sentinel expansion, the ESA has identified six possible candidates with phase A/B under preparation for the expansions to the Copernicus space component [3], such as Sentinel-7 Anthropogenic CO\({}_{2}\) monitoring mission, Sentinel-8 High Spatio-Temporal Resolution Land Surface Temperature (LST) Monitoring Mission (companion to Sentinel-2 C/D), Sentinel-9 with two components: Polar Ice and Snow Topographic Mission, and Polar Weather payload on a Highly Elliptical Orbit, and Sentinel-10 with a Hyperspectral Imaging Mission. Other possible candidates for the expansion of Copernicus are Passive Microwave Imaging Mission, and L-Band SAR mission. In parallel, a recent study of the Copernicus Market [4] mentioned that the agriculture, ocean monitoring, oil, and gas are a potential market in terms of Copernicus impact and user benefits. The approach followed is to identify the user's needs, identifying the gaps and potential areas for improvement in the Copernicus EO infrastructure, taking into account the future instruments and missions. This form could analyse if the plans of the extension of Copernicus support the emergent needs. The European Commission (EC) has led a revolutionary programme aiming at securing and exploiting space infrastructure to meet future demands and societal needs. The H2020 Operational Network of Individual Observation Node (ONION) project identified the main needs of the space segment infrastructure of the Copernicus system and identified the key technology challenges to be faced in the future, taking into account the user requirements at the center of the design process. The ONION project analyzed the user needs and ranked the top 10 use cases [5]. Each use case is associated with a Copernicus service, and they are formed by a set of measurements required to meet the users' needs. The measurements are the geophysical products derived from satellite observations. In addition, the measurement gaps and user requirements were identified and defined by the ONION project (Table 1) [5; 6], taking into account if, in the coming decade, the Copernicus and contributing missions satisfy the user requirements. This work focuses on the identification of the potential sensor technologies and platforms to meet those needs detected. The capability of the different technologies is evaluated according to current trends in the design of small satellites. These technologies are presented in view of the novel developments in spacecraft and sensor miniaturization, reduced power consumption, measurement requirements, and data quality, in order to cover the user requirements [6], so as to obtain competitive and cost-effectiveness services. The 20 measurements with gaps detected [6] in the top ten use cases are: (1) Ocean surface currents, (2) dominant wave direction, (3) significant wave height, (4) horizontal wind speed over the sea surface, (5) sea ice type, (6) iceberg tracking, (7) sea ice cover, (8) sea ice extent, (9) sea ice drift, (10) sea ice thickness, (11) atmospheric pressure over the sea surface, (12) sea surface temperature, (13) ocean chlorophyll concentration, (14) ocean imagery and water leaving radiance, (15) color dissolved organic matter, (16) detection of water stress in crops, (17) estimation of crop evapotranspiration, (18) surface soil moisture, (19) crop growth and condition, and (20) monitoring system vessels. Marine for Weather Forecast, Sea Ice Monitoring, Fishing Pressure, and Agriculture and Forestry: Hydric Stress use cases involved all the measurements with observations gaps detected over Copernicus space infrastructure in the period 2020-2030. The Marine for Weather Forecast, Sea Ice Monitoring, and Fishing Pressure use cases are ranked as the emerging observation needs. These use cases required measurements that are of crucial importance for a wide range of activities from maritime traffic, fishery, environment, food and medicine supply for populations at high latitudes, as well as for oil and gas operations.