Atmospheric Correction Inter-Comparison Exercise II

Free and open data access policy to Landsat-8 and Sentinel-2 satellite imagery has stimulated the development of atmospheric correction (AC) processors for generating Bottom-of-Atmosphere (BOA) products. Several entities have started to generate (or plan to generate in the short term) BOA reflectance products at global scale for Landsat-8 and Sentinel-2 missions. To this end, the EuropeanSpace Agency (ESA) and National Aeronautics and Space Administration (NASA) have initiated the Atmospheric Correction Inter-comparison Exercise (ACIX) in the frame of CEOS Cal/Val Working Group.

ACIX is an international collaborative initiative to inter-compare a set of atmospheric correction (AC) processors for high-spatial resolution optical sensors. The first ACIX experiment started in June 2016 with the aim to bring together developers of state-of-the-art atmospheric correction (AC) processors and study the variations amongst the different approaches. The input data were Landsat8 and Sentinel-2A imagery over various sites of different land cover types around the world, i.e. agricultural, deserts, urban, snow and coastal areas. The description and conclusions of this first experiment are summarised in (Doxani et al, 2018). All the inter-comparison results can be found in the dedicated to ACIX I web site in CEOS Cal/Val portal.

The enhancements of the participating processors in ACIX I and the increasing interest from additional AC developers to be part of the experiment stimulated the continuation of ACIX and its second implementation (ACIX II). Similarly to the first exercise, ACIX II focus on Landsat-8 and Sentinel-2 imagery over a set of test areas. Concerning Sentinel-2, the products of both -2A and -2B missions are included in the input datasets. The test sites of ACIX II have been redefined and more representative cases, concerning land cover and aerosol types, are included comparing to the ones of ACIX I. Particular attention are also given to aquatic sites, i.e. coastal and inland waters, which were analysed as a separate sub-category.

Following the recommendations of ACIX participants and Earth Observation data users, an additional inter-comparison of cloud cover assessment was performed in parallel with ACIX, named Cloud Masking Inter-comparison eXercise (CMIX). Cloud screening is a crucial step of the radiometric pre-processing of optical remotely sensed data and an important uncertainty contributor to the retrieval of accurate surface reflectance within an atmospheric correction process. Therefore, it was considered essential to analyse these two processing chains concurrently.

The presentation will describe in details ACIX-II (land) and CMIX and discuss early results and lessons learned.