## 3 Calibration Status of Sentinel-2
### Radiometric Calibration  
Use of the onboard solar diffuser is the principle absolute radiometric calibration approach and is fully described in [2]. The solar diffuser provides a bright and uniform image of the solar illumination, for which the incoming solar irradiance is well known based upon use of a relevant solar spectrum irradiance (Thuillier model recommended by the Committee on Earth Observation Satellites (CEOS) [13]) convoluted by the spectral band definitions, and accounting for a fine Earth-Sun distance calculation (based on Orekit [14]). As with Landsat OLI, knowledge of the detailed solar-diffuser bidirectional reflectance distribution function (BRDF) is the most sensitive point of the calibration method. A refined diffuser BRDF that fits a Rahman model better characterizes the diffuser panel. The overall gain trend for Sentinel 2 A and B are shown in Figure 4.  
The radiometric sensitivity of Sentinel-2 MSI presents a slight decrease with time. The absolute gain coefficients decrease by about 0.1% to 0.4% for S2B over six months in space, depending on the spectral band (Figure 4, use of the refined BRDF). For S2A, the trend of sensitivity loss reaches 0.4 to 1.0% over two years and half in orbit.  
Validation of the absolute calibration is based on vicarious measurements. Many methods are applied: the Rayleigh calibration method, the desert (PICS) calibration method, the in situ calibration method, and the inter-sensor calibration method. These methods are fully discussed in [2; 15].  
The Rayleigh calibration method is based on the comparison of the observed atmospheric molecular scattering over oceans with simulations. It allows an absolute calibration of the shortwave bands, from blue (B01) to red (B04). The desert calibration method is performed over 6 CEOS PICS in North Africa ([[http://calvalportal.ceos.org](http://calvalportal.ceos.org)]([http://calvalportal.ceos.org](http://calvalportal.ceos.org))): bright sites, spatially uniform and mainly very stable over time. This calibration method is based, too, on simulation of the observations [16]. The in situ calibration method is a collaborative task between the University of Arizona, the National Aeronautics and Space Administration (NASA) and S2-MPC using the well-known Railroad Valley calibration site in Nevada.  
Figure 5 illustrates the results for the spectral bands of MSI-A and MSI-B. Discrepancies between the different methods are lower than 5%, which is in the range of the Sentinel-2 specifications. The desert PICS validation method shows a consistency of the absolute calibration with discrepancies  
Figure 4: Refined time series of absolute gain coefficients for S2A and S2B based on the improved solar-diffuser bidirectional reflectance distribution function (BRDF) model.  
even lower than 3% for all the bands, except the 705 nm band for MSI-A. Results for the in situ validation, over the Railroad Valley site, are in the same range of discrepancies. For the Rayleigh calibration, a 4% discrepancy is obtained at the maximum for the B01 band, centered at 440 nm. In this spectral range, even if the Rayleigh scattering is the strongest, the sensitivity of the method to an uncertainty on the water reflectance is important.  
One can also notice the MSI-B reflectance is lower than the MSI-A reflectance for VNIR bands (see green squares on the bottom plots in Figure 5). The discrepancy is about 1% for most of the VNIR bands.  
Complementary analysis to improve the calibration approaches for better data interoperability of Sentinel-2 and Landsat-8 could be (i) to confirm their calibrations are computed with the same solar irradiance model and Earth-Sun distance; (ii) to increase, from both NASA and ESA, the amount of vicarious calibrations over the same sites, using the same spectral characterization of the targets, the same atmospheric parameters, the same radiative transfer code, as well as quasi-simultaneous observations.