This section provides a detailed description of the HDF5 data format used for the final generation of Seasat synthetic aperture radar (SAR) products. Although it does not cover a technical challenge, it is included for completeness of the processing description.
11.1 HDF5 Products
The Hierarchical Data Format (HDF) is the standard format of NASA’s Earth Observing System (EOS) mission. Its development by the HDF group is carried out through a subcontract under Raytheon Contract funded by NASA (The HDF Group, 2013).
The HDF5 datasets (.h5) provided by ASF are detected Seasat SAR images in ground range geometry. The layout of the Seasat HDF5 products is shown in Figure 1. It contains two groups: data and metadata.
The data group contains the SAR data as backscatter values in the HH layer. In order to provide basic geolocation information, two additional layers, latitude and longitude, are added. They contain geographic coordinates for every pixel in the image. The time variable completes the compliance to the Climate and Forecast (CF) metadata conventions (CF Conventions Committee, 2013).
The structure within the metadata group was modeled after the TerraSAR-X metadata (Astrium, 2013). The generalHeader section defines very basic metadata such as mission and data source. The instrument section summarizes radar parameters and settings. The satellite flight path with its orbital parameters is described in the platform section. The processing section provides vital information about Doppler as well as about processing parameters and flags. All product components are included in the productComponents section. The general product information is given in the productInfo section, while the SAR specific is stored in the productSpecific section. Finally, the setup section provides details about data ordering and processing.
Users are advised that the Seasat HDF5 products may have substantial geolocation errors. The ASF Seasat HDF5 products have the file extension .h5.
Figure 1: Data layout of the HDF5 files
11.2 XML Metadata
The same metadata structure that is stored in the HDF5 file is also saved as an XML file (.xml) and bundled with the ASF Seasat products. The XML (Figure 2) allows for simplified access and parsing of metadata information.
Figure 2: Seasat metadata in XML format
The development of XML metadata that is compliant to ISO 19115 and other related standards (NOAA EDM, 2013) is a work in progress. The SAR geometry is not comprehensively described with the current standard elements. In collaboration with Ted Habermann, now Director of Earth Science at The HDF Group, standards-compliant metadata for SAR data and best practices for future NASA missions are being developed. The current prototype structure is summarized in Figure 3. The ISO-compliant XML metadata files (.iso.xml), distributed as part of Seasat products, are considered experimental, and their structure is subject to change.
Figure 3: Prototype of an ISO 19115 compliant metadata structure for SAR data. Yellow backgrounds indicated mandatory elements, while green are conditional and blue optional.
11.3 GeoTIFF Products
GeoTIFF is a public domain metadata standard that allows georeferencing information to be embedded within a TIFF file. The GeoTIFF data products are geocoded to the Universal Transverse Mercator (UTM) map projection, using the zone that best represents the data’s geolocation. The original 12.5-m pixel size and the floating-point values of the ground range HDF5 products are kept in the GeoTIFF format. Because this product type does not require additional geographic information, it only contains a single layer of SAR data and is considerably smaller than the HDF5 product. Users are advised that the Seasat GeoTIFF products may have substantial geolocation errors. The ASF Seasat GeoTIFF products have the file extension .tif.
Written by Tom Logan, July 2013