**1. Introduction**

Copernicus is a program that powers the European Earth Observation (EO) capacity to meet the user needs and be highly competitive globally. Copernicus addresses six thematic services: land, marine, atmosphere, climate change, emergency management and security. Each service relies on a product portfolio that is derived from space and in situ infrastructure. The European Space Agency (ESA) has developed the space segment, a series of missions called the Sentinels, specifically tailored to the operational needs of the Copernicus program. Additionally, the Sentinels' missions are supported by contributing missions, such as the Earth Explorer missions by the ESA and the Meteorological Satellites (EUMETSAT) and include missions from European Union (EU) and non-EU member states.

Sentinel-1 is equipped with a C-band Synthetic Aperture Radar (SAR) for land, ocean and emergency services. This is based on a constellation of two polar orbiting satellites, in the same orbital plane with a 180◦ orbital phase difference. Currently, Sentinel 1-A and Sentinel 1-B are operational satellites, and Sentinel 1-C and Sentinel-1D are future missions to ensure data continuity. The first Sentinel-1 satellite was Sentinel-1A, and it was launched on 3 April 2014. Sentinel-1B was launched on 25 April 2016. Sentinel-1C will be launched in 2021 and Sentinel-1D in 2023. Each Sentinel-1 is expected to have at least seven years of lifetime.

Sentinel-2 is equipped with a Multi-Spectral Imaging (MSI) sensor, to cover the land and emergency services of Copernicus. Constituted by the A/B/C/D series, at present, there are two satellites in polar orbit (Sentinel-2A and Sentinel-2B). Sentinel 2-A was launched on 23 June 2015, and Sentinel 2-B was launched on 7 March 2017. Planned missions to provide data continuity are Sentinel-2C and Sentinel-2D, which will be launched in 2021 and 2022.

Sentinel-3 is equipped with seven instruments for land- and ocean-monitoring services. The two multispectral sensors are named the Ocean and Land Color Imager (OLCI) and Sea and Land Surface Temperature Radiometer (SLSTR). It also has a Synthetic aperture Radar Altimeter (SRAL) that requires a micro-wave radiometer for water vapor correction, a Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS), a Laser Retro-Reflector (LRR) and a GPS receiver for orbitography correction. It is composed by four satellites series (A/B/C/D), with two operational polar orbit satellites at the present time. Sentinel-3A and Sentinel-3B were launched on 16 February 2016 and 25 April 2018, respectively. The future Sentinel-3C and Sentinel-3D will be launched in 2023 and 2024.

Sentinel-5p, also known as Sentinel-5 Precursor, is equipped with a Tropospheric Monitoring Instrument (TROPOMI). This mission brings support for atmospheric services. It is based on a single satellite in polar orbit, launched on 13 October 2017.

Sentinel-4 and Sentinel-5 are planned as hosted payloads for a mission operated by EUMETSAT, to ensure the atmospheric and climate change services of Copernicus. Sentinel-4 is a spectrometer called the Ultra-violet, Visible and Near-infrared sounder (UVN), which will be onboard the Meteosat Third Generation-Sounder (MTG-S). MTG-S is composed of two series (MTG-S1 and MTG-S2), in geostationary orbit. MTG-S1 and MTG-S2 are scheduled for launch in 2023 and 2031, respectively. Sentinel-5 is a sounder called the Ultra-violet, Visible and Near-infrared Sounder (UVNS) onboard the MetOp-Second Generation (MetOp-SG, with the series A1, A2 and A3), in polar orbit. MetOp-SG A1/A2/A3 will be launched in 2021, 2028 and 2035, respectively.

Sentinel-6, also called the Joint Altimetry Satellite Oceanography Network-Continuity of Service (JASON-CS), will be developed and implemented through a partnership between EUMETSAT, ESA, National Aeronautics and Space Administration (NASA), and National Oceanic and Atmospheric Administration (NOAA). A radar altimeter package like the one in Sentinel-3 will be equipped in two Sun-synchronous series satellites (JASON-CS-A and JASON-CS-B) with a seven-year lifetime each. Currently, the launches of JASON-CS A and B are planned for 2020 and 2025, respectively.

In recent years, European Commission (EC) has led the Horizon 2020 program-supporting mission aligned with major EU policy priorities. In the context of Copernicus, the priorities are to contribute to the evolution of its services and to satisfy the end-user needs. The H2020 ONION (Operational Network of Individual Observation Nodes) project played an important role in defining the technological EO requirements based on the user needs and future measurement gaps of the Copernicus system in the horizon 2020–2030. Each use case is linked to a Copernicus service, and they are integrated by a set of measurements required to fulfil the users' needs. The measurements are the geophysical product estimated from satellite acquisitions. The measurements with gaps are the measurements detected with an observation gap (in terms of spatial resolution, and/or revisit time, and/or accuracy, and/or temporal continuity, and/or data latency) in the Copernicus space infrastructure in the time period from 2020–2030. The main objective of the ONION project was to place the user requirements at the center of the design process, as well as to identify solutions to meet these needs. This project has helped to understand the challenges for the evolution of the new Copernicus missions.

From the knowledge of the end-user needs, this project has provided an important scientific basis to address the measurement requirements, the instrumentation and remote sensing technologies that have to be explored to cover the next decade of the measurement gaps of the Copernicus system, where monitoring of the polar regions is an emerging need, with improved revisit time and latency time for marine weather forecast and sea ice monitoring use cases.

The methodology used in this work is described in Figure 1. First, the top 10 use cases were ranked according to the end-user needs [1], and end-user requirements were defined [2]. Second, a database of the future Copernicus instruments and contributing missions was generated to analyze the measurement gaps in the horizon 2020–2030. The gaps were detected based on the ability of these sensors to monitor each measurement defined in the use cases. Measurement gaps were analyzed in terms of the spatial resolution, revisit time, precision and temporal continuity, as well as the data latency for products requiring near real-time data (Section 2). Based on the results of the gap analysis in the time frame 2020–2030, monitoring of the polar regions arose as the top emerging need. Accordingly, Section 3 describes the importance of observing the polar regions. Section 4 presents the potential instrumentation required to cover the emerging needs, based on the measurement characteristics. Section 5 presents a discussion based on the limitation of current technologies and the challenges addressed to next generation of the sensors, to ensure all the measurements with gaps in the polar region are covered by the Copernicus space segment. Finally, the conclusions are presented.

**Figure 1.** Methodology applied to define the end-user requirements and measurement gaps.
