A Combined CEOS-ARD Format Specification for Synthetic Aperture Radar

CEOS Analysis-Ready Data (CEOS-ARD) - previously referred to as CARD4L - is a joint effort by the Committee on Earth Observation Satellites (CEOS) agencies to streamline data flows and enable interoperable products between sensors and data providers, and, specifically, to broaden the Earth Observation user community by provision of data products that do not require expert knowledge to ingest and analyse. This last point is perhaps particularly relevant for Synthetic Aperture Radar (SAR), where the potential to contribute to today's great environmental challenges with unique information is significant, but with the SAR user community remaining small and expert-oriented even after 30+ years of operational SAR missions. CEOS ARD for SAR is an opportunity to bridge that gap. Since the launch of the CEOS-ARD initiative, three product specification format (PFS) documents, related to different ARD-SAR products, have been approved and released by the CEOS Land Surface Imaging Virtual Constellation (LSI-VC) [1]: - Normalised Radar Backscatter (NRB). The NRB product has been subject to Radiometric Terrain Correction (RTC) and is provided in the gamma-nought backscatter convention. It is the most common SAR product and expected to be useful for, in particular, non-expert users. - Polarimetric Radar (POL). The POL product format is an extension of the NRB format, required in order to better support Level-1 SLC polarimetric data, including full-polarimetric modes (RADARSAT-2, ALOS-2, SAOCOM and future missions), and hybrid or linear dual-polarimetric modes (i.e., Compact Polarimetric mode available on RCM, SAOCOM and the upcoming NISAR mission). The POL product can be defined in two processing levels: The normalised covariance matrix (CovMat) and Polarimetric Radar Decomposition (PRD) products. - Ocean Radar Backscatter (ORB). The ORB product is a simplified version of the NRB. The main differences are the geometric correction with respect to a geoid and the provision of measurements in sigma-nought backscatter convention, which is recommended for most ocean applications. Response and feedback from the SAR community and space agencies have thus far been very positive. Data providers appreciate having guidelines for developing their ARD service while however the publication of a large number of SAR CEOS-ARD PFSs has been raised as a potential source of confusion. Since the endorsed SAR PFSs share about 75% of the metadata information and use the same documentation structure, a single "ensemble" PFS document has been produced, which comprises the specifications for all CEOS-ARD SAR products that has been endorsed (NRB, POL, and ORB), in order to simplify the documentation and reduce the confusion. The PFS has been given the generic name "CEOS-ARD for Synthetic Aperture Radar". It is to be submitted for endorsement by the LSI-VC in August, 2023. It will include the upcoming GSLC (also to be submitted for endorsement in August, 2023) and the InSAR (in development) products: - Geocoded Single-Look Complex (GSLC). The CEOS-ARD GSLC product describes the complex radar reflectivity on the surface with all propagational phases removed, so that the amplitude and phase values represent properties of the surface and not the instrument. GSLC data are presented in a common, often user-defined, ground based coordinate system (e.g. UTM, geographical coordinates, etc.), rather than in radar slant range coordinates, to facilitate use by non-radar-specialists. - Interferometric Radar (InSAR). The CEOS-ARD InSAR product specification covers a suite of three products (coherence, wrapped and unwrapped interferogram) generated by InSAR processing of (at least) two acquisitions. This presentation will highlight the common metadata items and supplemental ones for each CEOS-ARD SAR product inside a harmonized structure. [1] CEOS Analysis Ready Data website; https://ceos.org/ar