How NASA merges satellite data to watch the Earth change every single day
NASA's Harmonized Landsat and Sentinel-2 project creates a seamless, global view of our planet by combining data from multiple satellite fleets. By treating disparate sensors as a single, unified system, researchers can now monitor land surface changes at 30-meter resolution with unprecedented frequency, revolutionizing how we track agriculture and hazards.
The Harmonized Landsat and Sentinel-2 (HLS) project, managed by NASA's Goddard Space Flight Center, solves a fundamental problem in remote sensing: individual satellite missions often lack the revisit frequency required to track rapid environmental changes. While Landsat 8 and 9 provide high-quality imagery, they have a 16-day repeat cycle. By integrating these with the European Space Agency's Sentinel-2A, 2B, and 2C satellites—which have a five-day repeat period—the HLS project creates a harmonized, analysis-ready product that effectively functions as a single, high-frequency constellation.
To achieve this, the HLS algorithm performs complex atmospheric corrections and adjusts for differences in sensor view geometry and spectral bandpass. This process, which utilizes the NASA Earth Exchange computer cluster, ensures that data from the Landsat Operational Land Imagers and the Sentinel-2 Multispectral Imager can be used interchangeably. The result is a global land surface reflectance product, excluding Antarctica, that provides observations at 30-meter spatial resolution every 1.6 days. This capability was identified as a critical need during a 2016 Satellite Needs Working Group assessment.
The project has evolved significantly since its inception. While early versions of the HLS data set were limited to 82 test sites covering roughly 7% of the global land area, the current cloud-based software stack now covers all global land masses. The project continues to expand, with Sentinel-2C data incorporated following its 2024 launch and a new suite of vegetation index products introduced in February 2025. Future developments include the anticipated completion of low-latency products by 2027, further enhancing the ability of scientists and farmers to monitor everything from crop health and insect infestations to the immediate aftermath of natural disasters.
Source: Data in Harmony: NASA's Harmonized Landsat and Sentinel-2 Project
