Comparison of NASA Harmonized Landsat and Sentinel-2 (HLS) surface reflectance products with MODIS NBAR

Frequent surface reflectance measurements at fine spatial resolutions are crucial for land surface monitoring. The NASA Harmonized Landsat and Sentinel-2 (HLS) project addresses this need by combining data from Landsat and Sentinel-2 missions to provide 30-m surface reflectance products with a global average revisit frequency of 2-3 days. This study evaluates the consistency of the HLS Version 2 (V2.0) product against the established Moderate Resolution Imaging Spectroradiometer (MODIS) Nadir Bidirectional Reflectance Distribution Function-Adjusted Reflectance (NBAR) dataset. We found a high degree of consistency between HLS and MODIS NBAR, with strong positive correlations across all six common spectral bands (R 2 values: 0.939–0.986). Globally, mean absolute differences (MADs) were generally low, typically under 0.03 for visible bands and below 0.05 for NIR and SWIR bands. However, we observed band-dependent discrepancies, with higher discrepancies in the blue and SWIR1 bands. Spatially, substantial differences were found in polar regions, largely due to isolated snow, ice, and water which are fractional and hard to detect within the coarser MODIS pixels but are filtered out in the HLS data. This finding suggests a potential explanation for past disagreements in vegetation trend analyses involving the use of satellite data of different spatial resolutions, as coarser-resolution datasets may mischaracterize greening trends in areas with partial snow and ice cover. These insights are vital for helping users interpret HLS data more effectively and for refining multi-sensor data fusion methods. Our research also provides a valuable baseline for ongoing quality assessment of the HLS product for applications such as vegetation monitoring, land management, and disaster response.