**1. Introduction**

The Copernicus system, previously known as Global Monitoring for Environmental Security (GMES), is a revolutionary program of the European Union (EU) to address the end-user requirements over six thematic services: Atmosphere, Marine, Land, Climate Change, Emergency Management, and Security. Copernicus is supported by the space and in situ components. The space segment is based on a set of Earth Observation (EO) satellites known as the Sentinels and some contributing missions. Contributing missions with space infrastructure are the Earth Explorer missions [1] operated by the European Space Agency (ESA), the meteorological missions operated by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), and EO missions operated by the European Union (EU), third countries, and commercial providers.

Currently, there are seven Sentinels satellites in orbit: Sentinel-1A and Sentinel-1B with C-band Synthetic Aperture Radar (SAR) for land and ocean observation, Sentinel-2A and Sentinel-2B with high resolution optical imager called Multi-Spectral Imager (MSI) for land and vegetation observation, Sentinel-3A and Sentinel-3B with a suite of instruments such as Synthetic Aperture Radar altimeter (SRAL), and medium resolution optical imager: Ocean and Land Colour Imager (OLCI) and Sea and Land Surface Temperature Radiometer (SLTR) for ocean and land observation, and Sentinel-5P with cross-nadir scanning sounder called Tropospheric Monitoring Instrument (TROPOMI) for atmospheric chemistry and aerosol studies. Future Sentinel missions that will be launched in the next decade are Sentinel-4 for atmospheric chemistry as hosted payload over Meteosat Third Generation-Sounding (MTG-S); Sentinel-5 will be launched as hosted payloads over MetOp-Second generation (MetOp SG) for atmospheric chemistry, aerosol and spectral irradiance studies; and Sentinel-6 will be launched in a Low Earth Orbit (LEO) inclined over the equator for ocean altimetry as an international program between ESA, the National Aeronautics and Space Administration (NASA), the National Centre for Space Studies (CNES), EUMETSAT, and the National Oceanic and Atmospheric Administration (NOAA). Additionally, the third and fourth units of Sentinel-1C/D, Sentinel-2C/D, and Sentinel-3C/D will have planned to launch for the continuity of these programs.

At present, Earth Explorer missions are: Soil Moisture and Ocean Salinity (SMOS) launched on 2 November 2009 for sea surface salinity and soil moisture monitoring; this is considered as a potential gap because this mission has no continuity; Atmospheric Dynamics Mission—Aeolus (ADM-AEOLUS) launched on 22 August 2018, with an Atmospheric Laser Doppler Instrument (ALADIN) for contribution to aerosol observation and wind profile. Future Earth Explorer missions are: EarthCARE mission with a suite of instruments such as a Atmospheric Lidar (ATLID), Broad-Band Radiometer (BBR), Cloud Profiling Radar (CPR), and Multi-Spectral Imager (MSI) for cloud, aerosol, and radiation process studies; Biomass mission with a interferometric and polarimetric P-band SAR for biomass and glacier topography study; and FLEX mission with a FLORIS instrument for photosynthetic activity monitoring. Additionally, the ESA has chosen two potential Earth Explorer candidates missions [2], the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) with measure in the 15–100 micron range, and Sea-Surface Kinematics Multi-scale (SKIM) monitoring with a multi-beam radar altimeter with a wide swath. These two candidates considered will spend the next two years being studied thoroughly and only one will be implemented.

State of the art of the meteorological contributing missions of Copernicus are MetOp in Low Earth Orbit (LEO), and Meteosat Second Generation (MSG) in Geostationary orbit (GEO). For the incoming decade (2020 to 2030), these programs will have continuity because new missions will be launched such as Meteosat Third Generation (MTG) and MetOp Second Generation (SG).

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 CO2 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. Another high priority use case with observation gaps (Table 1) is the Agriculture and Forestry: Hydric Stress. The key measurements to cover for this use case are important to study the hydrological cycles, agriculture production, climatology, and meteorology. With the objective to cover these 20 measurements with gaps, we designed a methodology that focuses on the critical technologies to complement Copernicus observation gaps.

The methodology applied to select the appropriate sensors and platforms is sketched in Figure 1. First, a survey of the commercial small platform capabilities is presented in terms of mass, payload power, communications, pointing knowledge, and control. Second, the state-of-the-art sensors in terms of mass, power consumption, swath, and data rate is presented. Each sensor or technology is then studied to cover the observation gaps. Based on the survey of the instrument capabilities and data quality, a summary of the existing, and emerging in EO sensors is given, including the scientific and technological limitations in terms of spatial resolution, accuracy, and swath. Within these bounds, the potential instruments are selected according to the available commercial small platforms. The reference instruments are evaluated based on the variables with gaps that can be measured using a scoring method. This scoring method assigns a high score to the sensors that present lower power consumption, lower mass, and high data quality (better accuracy, smaller spatial resolution, and/or wider coverage). Finally, the most relevant instrument technologies compatible with small platforms are identified to complement the existing Copernicus Services for the selected use cases.


#### **Table 1.** The top ten use cases.


**Table 1.** *Cont*.

**Figure 1.** Design process to select payload and platform according to the requirements.

#### **2. Survey of Commercial Small Platforms**

This section presents the results of a comprehensive survey of commercial Low Earth Orbit (LEO) small platforms for EO, in order to properly select the platforms for each technology. To do this, the capabilities and limitations of the small commercial buses are taken into account. A total of forty-two commercial platforms from eighteen different companies have been identified, and their information has been compiled from company websites and conferences proceedings (Appendix A).

These small platforms cover a wide range of payload mass and power. They are categorized into three groups nano-, micro-, and mini-satellites. Table 2 summarizes their typical parameters. These platforms support payload masses from 1 kg to 600 kg [9], payload powers (orbital, average) from 1 W to 1500 W [10], downlink up 15 Mbps (S-band) [11], 100 Mbps (X-band) [12], and 1.2 Gbps (K-band) [13]. In this context, the recent evolution of the capability of micro- and mini-class platforms, and the payload miniaturization have demonstrated being a true competitor of large spacecrafts for some applications. Table 3 summarizes the capabilities of CubeSat EO platforms (3U, 6U, and 27U). Nanosatellites are now becoming popular thanks to the CubeSat standard. Typical CubeSat missions can be implemented in 1 to 3 years, with typical budgets from 200 K to1M\$ USD, including launch.

On the other side, ESA has promoted the development of a generic Small Geostationary Platform [14] (SmallGEO or SGEO) industrialized by OHB [15]. This flexible and modular platform has a lifetime of up to 15 years, a payload mass of up to 400 kg, and a payload power of up to 4 kW [16]. This platform was originally proposed to help European industries in the commercial telecom satellite market. However, the Earth Observation domain can also benefit from the capability of this platform in terms of available power and payload mass. In this way, an analysis of the EO technologies that are appropriate for use in small platforms is conducted in the next section.




**Table 3.** Summary of survey of commercial CubeSat platforms capabilities.
