ACIX is an international collaborative initiative to inter-compare a set of atmospheric correction (AC) processors for high-spatial resolution optical sensors. The exercise will focus on Landsat-8 and Sentinel-2 imagery over a set of test areas. The inter-comparison of the derived Bottom-of-Atmosphere (BOA) products is expected to contribute to the understanding of the different uncertainty contributors and help in improving the AC processors.

Introduction
 

The 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, NASA and the European Space Agency are organizing this exercise on AC processors inter-comparison. The results of the exercise are expected to point out the strengths and weaknesses, as well as communalities and discrepancies of various AC processors, in order to suggest and define ways for their further improvement.

Objectives

The exercise aimed to bring together the developers of the available AC processors, who were invited to generate the corresponding BOA products. The input data were Landsat-8 and Sentinel-2 imagery of various test sites, i.e. coastal, agricultural, snow/artic areas and deserts. A common and harmonised inter-comparison procedure was agreed and followed by all the participants.
 

Objectives of the 2^nd workshop

• To better understand BOA reflectance uncertainty contributors by comparing the outputs of different AC schemes
• To identify and review the different uncertainty contributors
• To propose further improvements of the available AC schemes

Expected Outcomes of the 2^nd workshop

• Assessment of the relative differences among the inter-compared AC processors results
• Definition of key regions and key periods for validation and quality assessment
• Description of a coordinated plan for inter-comparison and validation activities

Schedule and Deadlines

The Atmospheric Correction Inter-comparison Exercise was performed in three phases:

1. Definition of the inter-comparison protocol: The participants were invited to propose a protocol for the AC processors inter-comparison. The number and type of the test sites, the number of scenes and the metrics for models inter-comparison were proposed before the 1st ACIX workshop. All the proposals were discussed in the workshop and all participants agreed on the final inter-comparison procedure.
2. Application of the AC processors: The participants applied their AC schemes on the test cases keeping the processing parameters constant. The results were submitted for analysis (3rd phase) to ACIX coordinators.
3. Analysis of the results: The ACIX coordinators analysed the submitted results from all participants and assessed the inter-comparison outputs based on the agreed metrics. All the results were announced to participants prior to the 2nd ACIX workshop, where they were presented and discussed.