## 2 Background and International Context  
### ESA/Copernicus Current and Future Land-Focused Optical Sensors  
ESA, in the frame of the European Copernicus programme [4], has developed two satellite families in the optical domain, namely Sentinel-2 (S2) [5] and Sentinel-3 (S3) [6], to derive the bio-geophysical land variables needed by the land downstream services. Additionally, Copernicus contributing missions, in particular, Proba-V [7], operationally provided real time data to the Copernicus Global Land Service (CGLS) [8] and Copernicus Climate Change Service (C3S) [9] up to June 2020, bridging the gap until the full operations of the Sentinel-3 twin satellites constellation. The payload carried by these missions allows complementary observations of the Solar-reflected and Earth-emitted spectral radiation at different spatiotemporal and spectral resolutions. More specifically, these missions have the following main characteristics:  
* Sentinel-2 is a two-satellite constellation operational since March 2017 and ensuring a 5-day repeat cycle at the Equator in a sun-synchronous orbit at 786 km altitude with a local time at the descending node of 10:30 AM [5]. The Multi-Spectral Imager (MSI) on board S2 is a push-broom instrument and has a wide swath of 290 km with 13 spectral bands, covering the Visible (VIS), Near-Infrared (NIR) and Short-Wave Infrared (SWIR) domains at different spatial resolutions at the ground, ranging from 10 m to 60 m. The four bands at 10 m resolution in the VIS and NIR range ensure compatibility with SPOT 4 and 5 and meet the user requirements for land cover classification. The 20 m bands include the spectrally narrow red-edge bands for vegetation status monitoring, and the two SWIR bands for improving snow/ice/cloud discrimination. The bands at 60 m resolution are mainly dedicated to atmospheric correction and cloud screening. * Sentinel-3 is a twin-satellite constellation operational since April 2018 in a sun-synchronous, near-polar orbit at 814 km with a local time at the descending node of 10:00 AM [6]. The Ocean and Land Colour Instrument (OLCI) on board S3 is a spectrometer imaging in push-broom mode and consisting of five cameras in a fan-shaped arrangement. OLCI provides a native 300 m resolution across a swath of 1270 km with 21 spectral bands in the 400-1020 nm spectral range. S3 also carries the Sea and Land Surface Temperature Radiometer (SLSTR) on board, a dual view conical scanning imaging radiometer designed to retrieve global coverage sea and land surface temperature, active fire monitoring, ice surface temperature, clouds and atmospheric aerosols in support of various Copernicus services. SLSTR provides six solar VIS/NIR/SWIR reflectance bands at 500 m resolution and three thermal infrared bands and two dedicated fire bands at 1 km resolution. * The Proba-V satellite was launched in March 2013 in a quasi-sun-synchronous orbit at 820 km, with a mean local time in the range of 10:45 AM +/\(-\)15 min [7]. The VEGETATION instrument on board Proba-V consists of three overlapping cameras covering a 2250 km swath for near-daily global coverage of landmasses and coastal zones. The Proba-V cameras acquire data into four spectral bands: Blue, Red, NIR and SWIR. The Proba-V operational phase was terminated in June 2020 to limit the impact of orbital drift on mission's archive multi-temporal consistency [10]. By 1 July 2020, the Proba-V mission entered an Experimental Phase scenario with reduced acquisition planning but with the perspective to expand its observation capabilities thanks to the launch of a series of Cubesat Companion satellites [11]. The ensemble of the ESA's operational land products derived from those missions and currently provided to the user community is summarised in Table 1, with a focus on those bio-geophysical variables retrieved from the observations in the solar spectrum domain, which is the main subject of this paper.