## 2 EO Satellite and In Situ Observations for Earth Surface
### Copernicus Sentinels  
The Copernicus Sentinels program developed by the European Space Agency consists of a family of missions designed for the operational monitoring of the Earth system with continuity up to 2030 and beyond (Figure 1 and Table 1). The Sentinels' concept is based on two satellites per mission necessary to guarantee a good revisit and global coverage and to provide more robust datasets in support of the Copernicus Services. On-board sensors include both radar and multi-spectral imagers for land, ocean and atmospheric monitoring:  
Sentinel-1 is a polar-orbiting, all-weather, day-and-night radar imaging mission for land and ocean services. Sentinel-1A was launched on 3 April 2014 and Sentinel-1B on 25 April 2016. Sentinel-2 is a polar-orbiting, multi-spectral high-resolution imaging mission for land monitoring to provide, for example, imagery of vegetation, soil and water cover, inland waterways and coastal areas. Sentinel-2 can also deliver information for emergency services. Sentinel-2A was launched on 23 June 2015 and Sentinel-2B followed on 7 March 2017. Sentinel-3 is a polar-orbiting multi-instrument mission to measure sea-surface topography, sea- and land-surface temperature, ocean colour and land colour with high-end accuracy and reliability. The mission will support ocean forecasting systems, as well as environmental and climate monitoring. Sentinel-3A was launched on 16 February 2016 and Sentinel-3B has been launched on 25 April 2018. Sentinel-5 Precursor--Sentinel-5P--polar-orbiting mission is dedicated to trace gases and aerosols with a focus on air quality and climate. It has been developed to reduce the data gaps between the Envisat satellite--in particular the Sciamachy instrument--and the launch of Sentinel-5. Sentinel-5P is orbiting since 13 October 2017. Sentinel-4 is devoted to atmospheric monitoring that will be embarked on a Meteosat Third Generation-Sounder (MTG-S) satellite in geostationary orbit and it will provide European and North African coverage. Sentinel-5 will monitor the atmosphere from polar orbit aboard a MetOp Second Generation satellite. Sentinel-6 will be a polar-orbiting mission carrying a radar altimeter to measure global sea-surface height, primarily for operational oceanography and for climate studies. Future Sentinels missions include polar-orbiting satellites dedicated to support an anthropogenic CO2 monitoring capacity. In the longer term, thermal Infrared imagers with focus drought monitoring and on polar regions are being planned, and a hyper-spectral instrument will enable more precise agricultural monitoring. Special focus here is given to the Sentinel-3 mission [46], jointly operated by the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and which tackles precisely the scales of interest of our study and has operational requirements. The objectives of Sentinel-3 are to measure sea-surface topography, sea- and land-surface temperature and ocean- and land-surface color in support of ocean forecasting systems, and for environmental and climate monitoring. The Sentinel-3 satellites series ensures global, frequent and near-real time ocean, ice and land monitoring, with the provision of observation data in routine, long term (up to 20 years of operations) and continuous fashion, with a consistent quality and a high level of reliability and availability. The Sentinel-3 mission addresses these requirements by implementing and operating the following instruments, building on experience and heritage from the ERS and ENVISAT missions:  
A dual frequency, delay-Doppler Synthetic Aperture Radar Altimeter (SRAL) instrument supported by a dual frequency passive microwave radiometer (MWR) for wet-tropospheric correction, and a Precise Orbit Determination package. This combined package provides measurements of sea-surface height and topography measurements over sea ice, ice sheets, rivers and lakes.