The Harmonized Landsat Sentinel-2 (HLS) project is an extension of research conducted at NASA's Goddard Space Flight Center in Greenbelt, MD, that takes input data from the joint NASA/USGS Landsat 8 and the ESA (European Space Agency) Sentinel-2A and Sentinel-2B satellites to generate a harmonized, analysis-ready surface reflectance data product with observations every two to three days.
The HLS project is a major outcome of the Satellite Needs Working Group assessment in 2016. In that assessment, federal agencies and end users identified a need for more frequent Landsat-like observations to track short-term changes in vegetation and other land components to support agricultural monitoring and land cover classification at moderate to high resolution in both the visible and thermal components of the electromagnetic spectrum. Spectral similarities between the Landsat 8 Optical Land Imager (OLI) and the Sentinel-2 Multi-Spectral Imager (MSI) present an opportunity to harmonize data from both sensors to generate higher-frequency imagery products for land surface monitoring and applications.
Previous versions of HLS data products produced by the HLS Science Team at Goddard had limited spatial coverage — only covering North America and other select global locations. The current version of the HLS algorithm is a cloud-based software stack that expands the spatial coverage to include all land masses globally, outside of Antarctica.
Two data products are generated as part of the HLS project: the L30 data product generated with Landsat 8 data, and the S30 product generated using Sentinel-2 data. Outputs from these data products are still being validated for science quality, and only provisional S30 data currently are available through NASA's Land Processes Distributed Active Archive Center (LP DAAC). Provisional L30 data will be available through NASA's LP DAAC in early-2021. Feedback or questions about the provisional HLS data products can be made in the Earthdata Forum for HLS.
Improving Land Monitoring Capabilities
HLS data products greatly improve current publicly-available remote sensing land monitoring capabilities, particularly in terms of the frequency of land surface observations through time. The harmonization of HLS ensures that the Landsat 8 collection (30-meter spatial resolution with a 16-day repeat period) and the Sentinel-2A/B collection (10 to 20-meter spatial resolution with a five-day repeat period) can be used as if they were a single collection. Through HLS, land surface observations can be acquired at an unprecedented 30-meter spatial resolution every two to three days.
The frequent revisit times of HLS allow for time series of land surface applications at the field/plot scale, which supports a wide range of applications, including agricultural health, insect infestations, and natural hazard impacts. Planned missions for Landsat 9 (scheduled for launch in late-2021) and Sentinel-2C (scheduled for launch in 2023) also are expected to be incorporated into HLS data products, further improving re-visit times and land monitoring capabilities.
Collaborating with Internal and External Partners
The HLS project is a collaborative effort between NASA and other external agencies, including the USGS and ESA.
At NASA, the HLS algorithm is developed by the HLS Science Team and is ported and operated by the Interagency Implementation and Advanced Concepts Team (IMPACT) located at NASA's Marshall Space Flight Center in Huntsville, AL. The data products are archived at and distributed by LP DAAC, which is a partnership between NASA and the USGS. Coordination between these partners on the data life cycle ensures that HLS data are high-quality (research quality once the provisional tag is removed), freely available, and accessible to end users with two to three day latency.
External HLS partners (USGS and ESA) provide access to the input data at scale for global HLS data to be produced within the latency targets identified above. In addition, USGS provides the source code for atmospheric correction so the Level 1 input top of atmosphere reflectance can be converted to the output L30 and S30 surface reflectance data products.
Data Availability Timeline
HLS data products are currently in a provisional state as science validation is ongoing. The official release of a new version of research-quality HLS products is expected in early- to mid-2021. The HLS project generates both new data transmitted to the ground station and historical data from the Landsat 8 and Sentinel-2 archives. Initial HLS data products available in 2021 will include new data as they come in from the satellite (i.e., forward processing).
Shortly after the release of HLS standard data products, historical processing of the L30 and S30 archives, back to 2013 and 2015 respectively, is expected to begin with a target completion date of early 2022. As discussed above, Landsat 9 and Sentinel-2C are expected to launch within the next few years and data from these satellites will be incorporated into the HLS data products as they become available from each sensor.