The Committee on Earth Observation Satellites (CEOS) works to ensure international coordination of civil space-based Earth observation programs and promotes the exchange of data to optimise societal benefit and inform decision making to secure a prosperous and sustainable future for humankind.
Since 2016, CEOS has developed the concept of CEOS Analysis Ready Data (CARD) in response to the rapidly evolving information technology landscape (Cloud, Big Data, machine learning); the growing user base for Earth observation data, particularly with those outside the field of remote sensing science; and the emergence of new Earth observation and data access capabilities in the private sector.
Starting with the land imaging domain and CEOS Analysis Ready Data for Land (CARD4L), CEOS has been working to support users and data providers by providing specifications that clearly define what constitutes ‘analysis-ready’ for a majority of a target user base. CARD4L are satellite data that have been processed to a minimum set of requirements and organised into a form that allows immediate analysis with a minimum of additional user effort and interoperability both through time and with other datasets.These specifications are accompanied by an assessment protocol which allows data producers to have their products ‘CARD’ certified.
Following the positive reception of CARD4L, CEOS is now working on a broadened concept of CEOS ARD – one that includes domains beyond land. This second and expanded phase is guided by the CEOS Analysis Ready Data Strategy 2021 and a team of representatives from the seven CEOS Virtual Constellations (covering atmosphere, land, precipitation and oceans), as well as the Working Groups on Calibration & Validation and Information Systems & Services.
This talk will cover the breadth of CEOS activity around Analysis Ready Data, the forward vision for CEOS ARD and analysis-ready data in general, and will identify ways that the broader community can become involved.
The CEOS Analysis Ready Data for Land (CARD4L) initiative enables users to access products that have already been processed to a certain level. This allows users to perform immediate data analysis and to address a variety of applications without the additional time and cost associated with the data pre-processing steps. The Land Surface Imaging Virtual Constellation (LSI-VC) within the Committee on Earth Observation Satellites (CEOS) has been leading the CARD4L initiative in collaboration with domain experts, EO data users, the calibration and validation community and data providers to implement the CARD4L framework. The framework provides concepts and broad specifications for EO data to be analysis ready and contains three core elements; a) Definition, b) Product Family Specification (PFS) and c) Product Alignment Assessment (PAA). This paper aims to provide an overview of the current status on the CARD4L framework with specific reference to optical data products. It will also introduce the processes and steps that Earth Observation data providers undergo to achieve CARD4L compliance for their products, optical ARD products that have been assessed as CARD4L compliant, and briefly outline CEOS plans to extend the CARD4L framework moving forward.
CEOS Analysis Ready Data for Land (CARD4L) has been devised with the primary objective of simplifying data handling and enhancing the utilization of Earth Observation – EO data. CARD4L for Optical/SAR/LIDAR data are those that have been processed to a minimum set of
user based requirements. This ensures that the data providers take the responsibility of fundamental data correction and processing tasks, so the users can take up spatial-temporal analysis and research of remote sensing data in a time-bound sense. The Product Family
Specifications (PFS) are the devised guidelines to the EO Data providers for catering to the minimum threshold requirements and the desired target requirements. The SAR product is declared to be CARD4L compliant once all the thresholding requirements are incorporated.
Synthetic Aperture Radar is an active coherent sensor and is unique with its day/night and all weather imaging capability and is increasingly used for characterizing the target area based on its interaction at different microwave frequencies .With a continually increasing user-base for Analysis Ready Data, CARD for SAR is of high significance. CARD4L Product Family Specifications (PFS) for SAR comprises four primary concepts pertaining to SAR, the Normalized Radar Backscatter (NRB), Polarimetric SAR, Geo-coded SLC and SAR Interferometry.
National Remote Sensing Centre-NRSC is an arm of Indian Space Research Organization –ISRO and is equipped with an Integrated Multi-Mission Ground Segment for Earth Observation Satellites -IMGEOS set-up for acquisition, processing and dissemination of data on a 24x7 basis from Indian and Global remote sensing satellite missions. NovaSAR (S Band-3.1-3.5GHz) is a joint technology demonstration initiative of SSTL (Surrey Satellite Technology Ltd.), UKSA, and Airbus DS. It was launched by ISRO in 2018. The primary imaging modes are Stripmap at 6m and ScanSAR with 25 m to 45 m ground resolution. The Tx/Rx polarizations are Single, Dual and Tri-pol. At present, all the user requests placed are acquired, processed at NRSC and disseminated to the users. The Level-1 NovaSAR products are geo-tagged and ellipsoid corrected. In order to improve the geo-location accuracies and to mitigate the SAR geometric distortion effects, SAR Simulation
algorithm with Radar image simulation using DEM–based geometry is developed in-house and implemented to generate terrain corrected Level-2 products .The sigma naught backscatter products are also derived from the level-2 product as Analysis-Ready data and are disseminated as a bundled product .
RISAT-1 comprises a C –Band SAR (5.35 GHz) developed and launched by ISRO in 2012.The primary imaging modes are Fine Resolution Stripmap (FRS1) at 3m,Medium/Coarse Resolution\ ScanSAR MRS/CRS with 18 m to 48m ground resolution .The Tx/Rx polarizations are Single, Dual for all modes. It also features a unique hybrid polarimetric mode .The circular FRS-1 mode is realized in the Circular Transmit Linear Receive-CTLR polarization configuration wherein, the transmission is Right Circular.
This work highlights the initiatives and progress taken up at NRSC for realizing the SAR CARD4L objectives. SAR-CARD4L realization for Normalized Radar Backscatter-NRB for NOVASAR is presented. As NRB requires geometrically and radiometrically corrected data,
Terrain Correction was applied on the Ground Range product. Therein, the corrected radar backscatter at S-Band is derived for various land-based themes like water body, farmlands, snow, and urban cover etc. and the results are analyzed.
For the SAR-CARD4L POLSAR, in–house software has been developed for polarimetric processing of RISAT-1 using the Raney model and derivation of the Stokes Matrices in order to analyze the Target scattering mechanism.Hybrid polarimetric parameters like Degree of
Polarization-m, relative phase (delta), axial ratio (AR), circular polarization ratio (CPR) are derived from the data channels .These parameters are in turn used to generate the data\ corresponding to Surface, Double and Volume Bounce scattering of the target area. Based on
the scattering mechanism, hybrid polarimetric decomposition techniques are implemented for carrying out polarimetric classification of the target. This paper also briefs on the roadmap for CARD4L compliance of upcoming SAR missions from ISRO’s perspective.
The Copernicus Sentinel-2 mission is nowadays fully operational, with the two satellites Sentinel-2A and Sentinel-2B down-streaming several terabytes of Earth Observation (EO) data every day, freely available to users. On top of the Level-1C (i.e. Top-Of-Atmosphere and orthorectified) products, also Level-2 (i.e. Surface Reflectance) products are released to the EO user community since March 2018. A prototype production of Sentinel-2-like surface reflectance products through a harmonization (i.e. Level-2H products) and fusion (i.e. Level-2F) process combining data from different sensors (e.g. Landsat OLI) is also on-going.
With a constantly growing data volume, optimizing use and uptake of Sentinel-2 data also by users with limited resources and expertise is a priority in order to maximize the return on investment of the overall Sentinel-2 mission. To do so, specific requirements have been adopted for Sentinel-2 products metadata and for the applied radiometric, atmospheric and geometric corrections in order to facilitate access to, understanding and processing by users of the satellite data itself. In parallel, in the last years a growing number of countries and international organizations have expressed a similar need for efficient and effective access, preprocess, and use of the growing volume of space-based data for local, regional, and national applications and decision-making. In order to answer to this global need, the Committee on Earth Observation Satellites (CEOS) has started the CEOS Analysis Ready Data for Land (CARD4L) initiative, where a CARD4L product is defined as “satellite data that have been processed to a minimum set of requirements and organized into a form that allows immediate analysis with a minimum of additional user effort and interoperability both through time and with other datasets”. Provision of CARD4L products by the several satellite data providers is expected to reduce the burden on global satellite data users and to allow immediate creation of Data Cubes and subsequent enhancement of the scientific value of satellite data. Therefore, in the present paper an overview of the Sentinel-2 Surface Reflectance products is provided and their compliance to the CARD4L Framework and requirements is analyzed.
CARD4L (CEOS Analysis-Ready Data for Land) is a joint effort by the Committee on Earth Observation Satellites (CEOS) 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 25 years of operational SAR missions. CARD4L is an opportunity to bridge that gap.
In a coordinated effort by the CEOS Land Surface Imaging Virtual Constellation (LSI-VC) and the CEOS Working Group on Calibration and Validation (WGCV) SAR Subgroup, four SAR-specific CARD4L Product Family Specifications (PFS) are considered:
1. Normalised Radar Backscatter (NRB). The NRB product has been subject to Radiometric Terrain Correction (RTC) and is provided in the gamma-0 backscatter convention. It is the most common SAR product and expected to be useful for, in particular, non-expert users.
2. 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:
o The normalised covariance matrix (CovMat) representation which preserves the inter-channel polarimetric phase(s) and maximizes the available information for users.
o Polarimetric Radar Decomposition (PRD) products, derived from coherent or incoherent polarimetric decomposition techniques. The selection of composition product(s) to be offered (e.g., Freeman-Durden, van Zyl, Cloude-Pottier, Yamaguchi-based) are at the discretion of each data provider.
3. Geocoded Single-Look Complex (GSLC). The CARD4L 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.
4. Interferometric Radar (INSAR). The CARD4L INSAR product specification covers a suite of three products generated by InSAR processing of (at least) two images captured of the same geographic area at different times:
o Wrapped interferogram: Image of differential phase signals between two SLC images
o Unwrapped interferogram: Image of differential phase signals where the wrapped fringes are summed (“unwrapped”) to give a continuous phase signal across the image
o Interferometric coherence: Image of phase coherence between the two images.
The CARD4L NRB and POL specifications have been endorsed by CEOS LSI-VC and can be accessed on the CEOS ARD website (ceos.org/ard). The GSLC and INSAR product specifications are at the time of writing (Nov 2021) under development.
The current family of SAR products from Sentinel-1, TerraSAR-X, ERS-1/2 and ENVISAT, primarily contains Level-1 Single Look Complex, Ground Range Detected or ellipsoid and terrain corrected products [1] [2] [3] [4], 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. In particular, users largely benefit from the open and free data policy of the ERS-1/2, ENVISAT and Copernicus programme (European space agency, European Commission). This has led to SAR 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 data exploitation but requires new strategies of data management and provision.
As a consequence, 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. 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). Within this context Analysis Ready Data were defined as: « satellite data that have been processed to a minimum set of requirements and organized into a form that allows immediate analysis with a minimum of additional user effort and interoperability both through time and with other datasets. » Different SAR product specifications are currently being defined to provide guidelines on how to best process and organize data to serve as many use cases as possible with the respective products [5].
In this context, ESA and DLR decided to collaborate in order to define a family of SAR ARD products for Sentinel-1, ERS-1/2 and ENVISAT, potentially to be extended to other SAR missions. These products should be calibrated the same way (RTC), denoised, projected and geolocated in order to allow immediate analysis by the users. The same gridding / tiling system (MGRS) and the same DEM (the Copernicus DEM) shall be used in order to allow Interoperability of the SAR data from different missions. The use of Cloud-Optimised GeoTIFF (CoG) rasters, of VRT files, of XML and STAC metadata will enable an efficient exploitation of these datasets into cloud-computing environments. Finally, using open-source code and libraries to generate these new products’ processors will represent a considerable step toward Open Science.
The status of the development of the ARD products from the different ESA and DLR missions will be presented, together will the plans for future missions like Sentinel-1 NG, High Resolution Wide Swath (HRWS), ROSE-L and BIOMASS.
REFERENCES
[1] ESA, “Sentinel-1 Product Specification”, version 3.9, 2021.
https://sentinel.esa.int/documents/247904/1877131/Sentinel-1-Product-Specification-18052021.pdf/c2f9d58d-217f-e21d-548d-97a2cbd71e2b?t=1621347421421
[2] Airbus, “TerraSAR-X Image Product Guide”, issue 2.3, March 2015.
https://www.intelligence-airbusds.com/files/pmedia/public/r459_9_20171004_tsxx-airbusds-ma-0009_tsx-productguide_i2.01.pdf
[3] https://earth.esa.int/eogateway/instruments/sar-ers/products-information
[4] ESA, “ENVISAT-1 Products Specifications Volume 8: ASAR Products Specifications”, issue 4, Ref: PO-RS-MDA-GS-2009, 20 January 2012.
https://earth.esa.int/eogateway/documents/20142/37627/Envisat-products-specifications-VOLUME-8-ASAR-PRODUCTS-SPECIFICATION.pdf/1fd5a0be-1634-06cc-9a1e-249874a6e3aa
[5] CEOS, “Analysis Ready Data For Land: Normalized Radar Backscatter”, version 5.5, 2021. https://ceos.org/ard/files/PFS/NRB/v5.5/CARD4L-PFS_NRB_v5.5.pdf.