Development of an ESA Sentinel-1 Normalized Radar Backscatter product

Abstract

The current family of Copernicus Sentinel-1 (European Space Agency ESA / 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 a wide range of 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 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 offers opportunities for accelerated gain of knowledge but requires new strategies for data management and provision. The definition of Analysis Ready Data (ARD) addresses this challenge with the aim of homogenizing the data offerings from various sources whilst facilitating performant cloud access and high data quality. The Committee on Earth Observation Satellites (CEOS) defines a set of satellite ARD standards detailing higher level satellite data products [2] and thus contributes to a federated platform and data infrastructure. These standards are expected to be built upon in a new Open Geospatial Consortium (OGC) Standards Working Group on ARD, which is currently being formed. Specific to Synthetic Aperture Radar (SAR) data, three CEOS ARD product family specifications (PFS) currently exist: Normalised Radar Backscatter (NRB), Polarimetric Radar (POL), and Ocean Radar Backscatter (ORB). Of the three, NRB has been introduced first and one fully CEOS ARD certified product has already been made available [3]. Recently, ESA has defined its own prototype Sentinel-1 NRB product to pave the way for a potential contribution to the Copernicus data catalog [4]. The initial development was carried out in cooperation with the University of Jena and is now being continued at DLR. If feasible, this new NRB product might replace the existing GRD product, which would then be available only on demand. The CEOS ARD compliant NRB product consists of radiometrically terrain corrected backscatter data, several ancillary image layers referred to as per-pixel metadata in the CEOS ARD nomenclature, and extensive metadata in OGC EO XML [5] as well as STAC JSON format [6]. The latter is seen as particularly valuable for large scale cloud or High-Performance Computing (HPC) exploitation with respect to general data discovery as well as direct utilization in popular data cube related tools like xarray and dask. Here, we present the recent progress in the definition of the ESA Sentinel-1 NRB product, the evolution of the open-source prototype processor, and the extension of the existing product towards ORB. While NRB is expected to significantly increase the usability over land areas, the benefit of ORB over GRD for ocean use cases is currently still being investigated. Furthermore, it is of current interest in how far both products, NRB and ORB, can be distributed as one or whether they should be seen as two completely separate products. The results of ongoing experiments will be presented. Furthermore, a demonstration of the product content and its data cube capabilities will be given. Last but not least, the status of the systematic processing plans of DLR for this year will be presented, which will give insight into the overall data volume and processing effort to be expected for creating such a product. The experience gathered from this processing campaign will contribute to the refinement of the Sentinel-1 NRB product specification for consideration in the Copernicus data catalog.