# Comparison of SNAP-Derived Sentinel-2A L2A Product to ESA Product over Europe
## 1 Introduction
The Committee of Earth Observing Systems (CEOS) has identified four validation stages [23]. Here, we perform the first stage corresponding to comparisons using limited sites since we are comparing two implementations of the same algorithm and expect little or no differences a priori. Given that the goal of the Sentinel-2 mission is to provide satellite data to map land surface variables, the impact of different atmospheric correction algorithms on biophysical parameters should also be quantified. Validation, corresponding to comparison with coincident reference measurements of known (presumably better) uncertainty, is one approach to assess product performance. Product intercomparison is also useful to address the issue of limited reference measurements and to identify systematic patterns between products [24; 25]. Moreover, as [PERSON] [25] notes, intercomparison can be used to quantify improvement (or degradation) between algorithm versions. In consideration of these requirements, this study has two objectives:1. To validate (for AOT and WVP) and intercompare (for all L2A parameters) operational Sentinel-2 L2A products generated from the European Space Agency (ESA-L2A) with products generated offline using the SEN2 COR tool provided by ESA (S2C-L2A) using identical input Level 1B products and default settings. 2. To intercompare geophysical products generated using the European Space Agency Sentinel Level 2 Simplified L2 Product Prototype Processor [26] as well as widely used equations for spectral vegetation indices using, alternatively, ESA-L2A and S2C-L2A products as input. With respect to validation, our null hypothesis is that both ESA-L2A and S2C-L2A will exhibit similar agreement with in situ AOT and WVP as previously reported for S2C-L2A. With respect to intercomparison, our null hypothesis is that no difference should be observed for any product given that the algorithms are ostensibly identical. The method used for the inter-comparison capitalizes on previous studies of image comparison [25; 27; 28]. [PERSON] [25] proposed a framework for remote sensing fAPAR products inter-comparison. This framework included a number of pairwise comparison including regressions between products as pairwise difference metrics. We follow this approach here although we use standardized metrics used for other CEOS L2A product intercomparisons [20; 29] based on the Joint Committee Guides in Metrology [30]. The actual study was designed as a simple verification exercise since we expected a priori that there would be no evidence against the null hypotheses irrespective of the L1C-TOA product used. Our validation is limited to AOT and WVP since it is primarily to determine if the ESA and S2C algorithms are performing as previously reported. If the intercompared products validate against AOT and WVP in a manner similar to previously published experiments but exhibit differences in geophysical quantities approaching the magnitude of mission performance requirements [31], there would be a need for a more detailed investigation into both the ESA and S2C processing chains. The manuscript is organized as follows. The study sites and the used data are presented in Section 2. The processing steps are presented in Section 3. ESA and S2C maps of SLC, AOT and WVP and vegetation parameters are validated and intercompared in Section 4. Section 5 places the observed differences in context of the required and current accuracy of level 2A products derived from Sentinel 2 MSI.