Spectral and spatial harmonization of Landsat ETM+, OLI, and Sentinel-2 MSI using time-series harmonic analysis and blur kernel estimation

The integration of Landsat and Sentinel-2 observations into a virtual constellation offers unprecedented potential for near-daily medium-resolution (10–100 m) Earth monitoring. However, effective joint use of these datasets remains limited by intrinsic spectral and spatial inconsistencies arising from differences in sensor design, spectral band configurations, and spatial resolutions. To address these challenges, this study introduces the Spectral and Spatial Harmonization Model (SSHM)----a unified two-stage framework for harmonizing Landsat-7/8/9 and Sentinel-2 A/2B observations. In the first stage, SSHM estimates location-specific blur kernels for coarsening Sentinel-2 imagery to 30-m resolution, followed by cross-sensor spectral harmonization using time-series harmonic analysis and pixel-wise linear transformations. In the second stage, Landsat data are refined to 10-m resolution using an iterative optimization approach that jointly enforces temporal continuity with 10-m Sentinel-2 time series and spatial consistency with 30-m Landsat data using the pre-estimated blur kernels. Experimental results across 22 globally distributed study sites show that SSHM effectively reduces sensor-induced spectral discrepancies—by 5% between Landsat OLI and Sentinel-2 MSI, 11% between Landsat OLI and ETM+, and 22% between Sentinel-2 MSI and Landsat ETM + —outperforming existing harmonization approaches while maintaining superior robustness to observational noises. Moreover, SSHM outperforms conventional resampling and representative spatiotemporal fusion algorithms in refining Landsat to 10/20-m resolution, reducing reflectance discrepancies: 25% in the blue band, 40% in green, 43% in red, 50% in near-infrared (NIR), 67% in shortwave infrared 1 (SWIR1), and 71% in SWIR2 (compared to nearest neighbor sampling). These advances of SSHM enable the generation of more consistent and spatiotemporally coherent 10/30-m harmonized Landsat and Sentinel-2 reflectance products that are essential for monitoring fine-scale, rapid, and heterogeneous land surface changes.