**6. Conclusions**

This work is the first study reviewing the instrument requirements based on the end-users needs. The gaps of the future European EO spaceborne infrastructure were identified to select the potential technological areas for complementing and refining the Copernicus infrastructure. The exploration of the requirements and future European EO technology led to detecting and identifying the measurements with gaps and the emerging need to monitor the polar regions. Based on these results, future instruments, services and technology areas to upgrade were identified. Some of the most important characteristics, from which all the Copernicus services can benefit, are latency time and lower revisit time. Specifically, reductions of both revisit time from 24 h to 3 h and product delivery in the marine services from 24–48-h to 1 h would support, e.g., marine services, enabling polar navigation, enhanced marine real-time weather forecast, oil and gas exploration and oil spill remediation.

**Author Contributions:** E.L. and A.C. identified the measurement with gaps. P.S., A.M., H.M. and I.L. defined the user requirements. E.L. and A.C. identified the potential technologies to cover the measurement gaps. E.L. wrote the paper. A.C., H.P. and P.S. revised it.

**Funding:** This project was funded by the EU H2020 ONION project, under Grant Agreement 687490. It has also received support from the projects AGORA(ESP2015-70014-C2-1-R) of the Spanish Ministry of Economy and Competitiveness and the ICREAAcademia Award from the Catalan Government.

**Conflicts of Interest:** The authors declare no conflicts of interest.
