**3. Survey of Earth Observation Sensors and Measurements Requirements to Cover the Future Gaps on Copernicus**

EO satellites have revolutionized the study of the environment, and are contributing to a more rational use of the natural resources, and environmental protection. The applications of the data supplied by these systems are enormous: disaster monitoring, weather forecast, maritime safety, marine resources monitoring, forestry, vegetation state, water cycle, energy budget, pollution control, water quality, climate change, and security; using radars, microwave and optical/IR radiometers, optical imagers or scanners. Table 4 presents the generic classification of the remote sensors. Instruments are classified in the following four categories: active or passive, either microwave or optical. Optical sensors measure the signals received around the visible part of the spectrum, from the Ultra-Violet (UV) to the Thermal Infrared (TIR). Microwave sensors use the signals in the microwave and millimetre-wave parts of the spectrum, typically from 1 GHz to 1 THz. Passive systems are based on the collection of the electromagnetic waves that are emitted/scattered by external sources, such as the Sun or other bodies. On the other hand, active systems such as radars and lidars, transmit an electromagnetic wave, either radio or laser, and measure the scattered/reflected signal from the Earth's surface or atmosphere. Microwave sensors do not rely on the Sun as source of illumination. These particular characteristics are especially important in Polar Regions that have extended dark periods in winter. In addition, microwaves are mostly unaffected by the cloud cover, except in some specific bands. This feature makes microwave sensors more suitable than optical sensors in these regions.

This section presents a survey of the selected EO technologies. In order to identify the potential EO sensors to improve the Copernicus space infrastructure, EO technologies are analyzed in depth based on the measurements with identified gaps, and the technological limitations. A total of 77 instruments have been surveyed, and their parameters (mass, power consumption, spatial resolution, swath, frequency bands, aperture, and orbit altitude) have been compiled from the Observing Systems Capability Analysis and Review (OSCAR) Tool [24], the Earth Observation Portal Directory [25], and companies websites (Appendix B). The best instruments in terms of data quality and suitable for the small platform are identified for each technology.


**Table 4.** Instrument categorization: potential instruments to complement the Copernicus system [6].

*<sup>a</sup>* multi-static radar using satellite navigation signals of opportunity (SoOp). *<sup>b</sup>* e.g., Direct Broadcast Satellite (DBS) television at Ku-band or X-band.
