The Atmospheric Correction Intercomparison eXercise (ACIX) is a collaborative initiative under the Committee on Earth Observation Satellites Working Group on Calibration & Validation (CEOS WGCV), jointly coordinated by ESA and NASA. ACIX‑IV Land represents the next phase of this activity, continuing to engage the community of atmospheric correction (AC) processor developers in a structured intercomparison framework.

The exercise will start in the occasion of the 1st ACIX‑IV workshop, to be held at ESRIN on 30 November – 1 December, which will bring together coordinators and participants to jointly define the protocol, datasets, and implementation approach. As in previous editions, this collaborative setup will ensure consistency and transparency throughout the exercise.

ACIX‑IV Land will rely on a combination of simulated, realistic scenes and real satellite observations. Activities will include the processing and analysis of simulated Sentinel‑2 and CHIME Top‑Of‑Atmosphere data observations produced by the DTE-S2GOS scene generator across a wide range of land cover types, seasonal conditions, and atmospheric scenarios. These will be complemented by the exploitation of Sentinel‑2 and Landsat‑8/9 observations acquired over well‑characterised reference sites.

Through the intercomparison of results produced by different processors, the exercise aims to evaluate performance across diverse conditions, identify strengths and limitations of current approaches, and highlight similarities and differences in algorithm behaviour. The outcomes are expected to support ongoing improvements in atmospheric correction methods and contribute to the development of increasingly robust, consistent, and harmonised land EO products.

The ACIX-IV Land timeline is presented below with the main steps and goals of the projects.