ACIX-II Land: the second implementation of the Atmospheric Correction Inter-comparison eXercise over Land

Abstract

The correction of the atmospheric effects on optical satellite images is essential for quantitative remote sensing applications. Open and free data access to Copernicus Sentinel-2 (EC/ESA) and Landsat 8 (NASA/USGS) missions increased significantly the scientific interest on atmospheric correction (AC) and several approaches have been introduced by involving different radiative transfer models, single or multitemporal images, various algorithms to estimate aerosol properties and water vapour content, constant or diverse aerosol models, various sources of ancillary data, etc. These methodologies are usually validated independently by developers and/or users based on a certain number of sites with available reference data and/or are compared with results of other AC processors. In order to investigate all the AC aspects and issues in an integrated way, a benchmark exercise (Atmosperic Correction Inter-compariosn eXercise, ACIX) was initiated in 2016 in the frame of CEOS Working Group on Calibration & Validation (WGCV) with the aim to compare the state-of-the-art AC processors. ACIX is a voluntary and open-access initiative to which every AC processor’s developer is invited to participate. ACIX-I was an initial attempt to study the variability of AC performances over diverse atmospheric and land cover conditions using Landsat 8 and Sentinel-2A input data. It was highly appreciated by the participants and considered as a useful tool to discover not only the assets and flaws of the approaches, but also ways to improve them. Thus, a second implementation of the experiment was requested to inter-compare the enhanced versions of the participating processors, but also to be expanded by including additional AC processors. In this second implementation, ACIX was split in two categories: Land and Aqua, with focus on the processors performing over land and water correspondingly. In this presentation, attention is given only to the Land part of the exercise. The sites for the inter-comparison analysis over land were defined by investigating the full catalogue of AERONET sites for available measurements within 30min (±15min) from the satellites’ overpass. Eventually a total of 123 and 110 AERONET sites, which were distributed globally and representing various land cover types, made the site list for Sentinel-2A, -2B and Landsat 8 acquisitions correspondingly. Based on these available AERONET measurements, Aerosol Optical Depth (AOD) and Water Vapour (WV) retrievals were validated with the help of various statistical metrics. Regarding Surface Reflectance (SR) validation, as there is not yet any global network of systematic ground-based measurements, alternative approaches had to be adopted. Therefore, simulated SR reference dataset was computed over all the test sites by using the 6SV full radiative transfer code, with the required aerosol and water vapour information to have been acquired from AERONET. Moreover, measurements from the calibration dedicated network RadCalNet over La Crau (France) and Gobabeb (Namimbia) were involved in the SR validation. The observations in this case were processed to the same sun and sensor geometry, as well as spectrally integrated to the corresponding sensor spectral bands of Sentinel-2 and Landsat 8. The analysis results varied depending on the AC product compared, the reference dataset and the metrics. In this presentation an overview of the analysis and results will be given and discussed