Harmoni-Planet: A holistic harmonization method for PlanetScope constellation imagery leveraging a graph-based greedy optimization strategy

The PlanetScope CubeSat constellation provides unprecedentedly high spatiotemporal resolution for Earth observations but is limited by radiometric inconsistencies resulting from sensor degradation and spectral configuration differences. Existing harmonization methods often rely on internal or external references, limiting harmonization to only a subset of spectral bands or restricting applicability to localized spatiotemporal scales. To address these limitations, we propose Harmoni-Planet, a novel harmonization method that leverages graph-based greedy optimization to achieve holistic radiometric consistency across all PlanetScope bands without requiring reference data. The method consists of two components: (1) graph construction, which integrates unharmonized images into a graph with nodes representing images and edges connecting intersecting images, and (2) graph optimization, which iteratively minimizes radiometric inconsistencies between each image and its intersecting images to optimize consistency. Harmoni-Planet was validated across four geographically diverse regions (the Nile, Beijing, Indonesia, and Greenland), achieving substantial mean absolute error (MAE) reductions of 53 %, 53 %, 57 %, and 25 %, respectively, and outperforming the CubeSat-enabled spatiotemporal enhancement method (CESTEM; 40 %, 41 %, 47 %, and − 69 %, respectively) and the official harmonization algorithm (25 %, 33 %, −9 %, and 9 %, respectively). Harmoni-Planet significantly improves the spatial and temporal comparability of imagery across all PlanetScope bands, regardless of whether images are acquired by the same or different generations of satellites. In addition, it supports flexible scene-based and strip-based implementations, effectively resolving both intra-strip and cross-strip inconsistencies. It also demonstrates robust potential for near-real-time harmonization of newly acquired imagery to accommodate the rapidly expanding data volume of the PlanetScope constellation. Furthermore, Harmoni-Planet supports seamless integration with standard third-party reflectance products such as Landsat-8 and Sentinel-2. Harmoni-Planet provides a practical solution to address cross-sensor radiometric inconsistencies, substantially improving the quality and reliability of PlanetScope data for diverse Earth observation applications.