## 1 Introduction  
The Sentinel-2 constellation constitutes a revolution in remote sensing in terms of data quantity, quality and availability. The high spatial and temporal resolutions of Sentinel-2 [1] have been demonstrated to be crucial for many applications that have been reported in the scientific literature and validated by operational applications covering a wide range of use cases, such as land-cover mapping, snow-extent mapping, biophysical parameter estimation, agriculture monitoring, etc. Sentinel-2 provides 13 spectral bands with spatial resolutions from 10 m to 60 m and a 5-day revisit cycle. The particularities of Sentinel-2 with respect to pre-existing comparable systems are:  
* in the temporal domain, a systematic acquisition plan (unlike tasked satellites, which acquire scenes on demand) with a high revisit frequency (5 days compared to the 16 days of Landsat);
* in the spatial domain, a higher resolution than Landsat (10 m to 20 m compared to 30 m);
* in the spectral domain, an increased number of bands with respect to both the classical blue, green, red, NIR band set and Landsat (4 visible, 1 NIR, 2 SWIR), with the novelty of 3 red edge (RE) bands, although a lack of thermal band with respect to Landsat. However, as we show in Section 1.4, very few published works have made full use of the spectral richness of Sentinel-2, and often these uses have not been demonstrated to be the only way to extract the target information. After 5 years of operation, the work on the new generation of Sentinel-2 satellites (S2 NG) has started, and one of the tasks is to identify the set of spectral bands. The question of <<Which additional spectral bands could be put on board S2 NG?>> has to be balanced with the one of the <<S2 possible useless bands>>, that is, the current available bands which could be removed for S2 NG. Adding spectral bands to a satellite bears a cost, which could impact the trade-off with other mission requirements, such as temporal revisit needing an additional satellite. Of course, all current Sentinel-2 bands contain _potentially useful_ information, since they sample different intervals of the electromagnetic spectrum, and, except for the pair B8-B8A (see Section 1.1), there is no significant overlap between the different spectral ranges. However, since there exists a high level of redundancy in the underlying observed nature, one can expect high degrees of correlation between the different bands, allowing us to _question the true usefulness_ of some bands. With 5 years of data collection and exploitation, it is now possible to quantitatively assess the usefulness of the different bands on board Sentinel-2. This could be done in terms of the quality of the results of downstream processing (biophysical parameter estimation, land-cover mapping, etc. ), but this would need to address a huge number of application domains with experiments and validation data without the guarantee of exhaustivity or chances of replicability. On the other hand, if we address the problem from the _information content_ point of view, we only have to deal with data at the sensor level. We therefore choose to pose the problem as a data reconstruction one: if one band can be reconstructed--within a predefined error margin--from the other bands, it can be removed from the satellite without quantitative loss of information. One could argue that what matters is the estimation of physical parameters and that imperfect reconstruction of a particular band can have no impact for many applications. This would allow going farther in terms of spectral band removal. We agree with this point of view, but all downstream processing entails the use of (imperfect by construction) models, and the closer we get to the sensor, the most application-independent the conclusions of the study will be. The aim of this paper is to leverage this interband correlation and assess which bands could be removed from future iterations of the Sentinel-2 constellation with a minimal impact on the usefulness of the acquired data.