**7. Conclusions**

This study has reviewed the state of the art in EO sensors and platforms and has presented a methodology to select the best instruments' technologies and platforms required to complement the Copernicus system in the time frame 2020–2030. Suitable instruments for small platforms have been analyzed using several attributes, and they have been ranked using a quantitative scoring method. Results show that the most relevant payloads capable of filling the measurements gaps are: GNSS-R at 10 km spatial resolution, X-band imaging SAR at 1 km spatial resolution, and multispectral Optical instrument with bands in the VIS (10 m of spatial resolution), NIR (10 m), MWIR (1 km), and TIR (1 km).

The high temporal resolution of one hour required can only be achieved if a sufficiently large number of spacecrafts are used; then, the architecture selection could be analyzed and optimized [31,71]. A distributed or Federated Satellite System (FSS) will help to reduce the temporal gaps. The possibility to create strategic alliances to establish distributed or federated architectures between different missions and agencies must be carefully evaluated to safe costs. Federated Satellite System (FSS) concepts could also be applied to future instrument technologies to cover the gaps, taking into account different satellites program and space agencies.

**Author Contributions:** E.L. performed the survey of the instruments and commercial platforms; E.L., A.C., H.P., P.R., S.T., J.C. and S.P. identified the potential instruments to cover the measurements gaps; E.L. developed the quantitative method; A.C. revised the quantitative method; E.L. wrote the paper; A.C., H.P., P.R., S.T., J.C. and S.P. revised it.

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

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

#### **Appendix A**

This section presents all the commercial LEO small platforms that have been considered in the survey with their corresponding references. Then, the commercial platforms are assessed in terms mass, power consumption, communications, pointing control, and knowledge. Tables A1–A3 summarize each platform and manufacturer with the available capability to support a wide range of available payload mass and power. These small platforms were categorized into three groups' nano-, micro-, and mini- platforms based on the criteria of the International Academy of Astronautics [110]. Nano-satellites have a mass smaller than 10 kg, micro-satellites have a mass between 10 kg and 100 kg, and mini-satellites have mass in the range from 100 kg to 1000 kg.




Surveyofavailablemicro-satelliteplatformsforEarthObservation.

*Remote Sens.* **2019** , *11*, 175


