The ESA Sentinel-1 Normalized Radar Backscatter Product

Kurzfassung

The current family of Copernicus Sentinel-1 (European Space Agency / European Commission) products primarily contains Level-1 Single Look Complex (SLC) and Ground Range Detected (GRD) products [1], which inherit their definition from the European heritage Synthetic Aperture Radar (SAR) satellite missions ERS-1, -2 and ENVISAT. These products have over the years proven to be reliable high-quality data sources since the start of Sentinel-1A in 2014 and users largely benefit from the open and free data policy of the Copernicus program. This has led to Sentinel-1 products being routinely used in several operational applications as well as enlarging the user base of SAR data in general. However, the rapid increase of data volume is presenting a challenge to many users who still want to exploit this wealth of information but lack the resources for the processing needed to convert these Level-1 products to interoperable geoinformation. Cloud exploitation of data offers opportunities for accelerated gain of knowledge but requires new strategies of data management and provision. In this context, the term Analysis Ready Data (ARD) has been coined and several activities have indicated the potential of enlarging the Copernicus product family by such ARD products. However, no agreement has yet been reached on the definition of ARD as different user communities perform very different analyses and thus have different understandings of analysis readiness. With the aim to standardize different categories of ARD, the Committee on Earth Observation Satellites (CEOS) has set up the CEOS Analysis Ready Data for Land initiative (CARD4L) [2], which has recently been renamed to simply CEOS ARD (CARD) to accommodate ocean products. Several product specifications have been defined to provide guidelines on how to best process and organize data to serve as many use cases as possible with the respective products. One such product is Normalized Radar Backscatter (NRB) [3]. However, the CARD NRB specification can also be thought of as a guideline rather than a specification. It gives recommendations on what information should be included in a dataset but gives no concrete specifications on how this information should be created. For this reason, data providers are required to translate these guidelines into actual products with the software of choice to then reach out to CEOS for assessment of CARD compliance. Recently, ESA has defined its own prototype Sentinel-1 NRB product [4]. The aim is to be fully aligned with the CARD certification, offering a high-quality radiometrically terrain corrected SAR backscatter product with all relevant general metadata and per-pixel metadata (ancillary data layers). It is intended as a global and consistently processed product achieving the highest possible quality. It is designed to be complete to satisfy requirements of users with a prioritized focus on the Copernicus Services. Ultimately, it intends to significantly lower the effort for processing for the SAR user community. Core characteristics are the structuring into the Military Grid Reference System (MGRS) tile grid for interoperability with Sentinel-2 ARD products, storage in Cloud Optimized GeoTIFFs and the provision of Spatiotemporal Asset Catalogue (STAC) metadata. We present the characteristics of this S1-NRB product, give insight into current work towards extending the product to Ocean Radar Backscatter (ORB), and outline the DLR plans of globally generating the product on the terrabyte High Performance Data Analytics (HPDA) platform. These activities are expected to contribute to the further definition of a Copernicus Sentinel-1 NRB product. This type of product is now baseline in the frame of the Copernicus Expansion and Sentinel Next Generation and is hence planned for further future missions.