Sentinel-1 Bursts

Sentinel-1 Interferometric Wide (IW) mode and Extra-Wide Swath (EW) mode single look complex (SLC) data are collected using a form of ScanSAR imaging called Terrain Observation with Progressive Scans SAR (TOPSAR). In these modes, data is collected by cyclically observing, then switching the antenna beam between multiple adjacent sub-swaths. Within each observation of a sub-swath, a short sequence of radar pulses are transmitted in rapid succession, imaging a contiguous portion of the target surface. These short sequences of radar pulses are called “bursts”, and are the fundamental unit of SLC data collected using the TOPSAR technique.

Illustration from Sentinel Online, ESA

Data from many bursts are packaged together to create the standard Sentinel-1 IW and EW SLC products. These products are produced by creating sub-swath images that contain a series of overlapping bursts, where each burst has been processed as a separate SLC image. Sub-swath images from all available swaths (three swaths for IW mode and five swaths for EW mode) are packaged together to create the final SLC products. While data from many bursts are packaged together in the standard SLC products, SLC data from a single burst can be utilized independently.

To enable single-burst applications, ASF has developed an extraction service that allows users to download single-burst IW and EW SLC products. The burst SLC data are available as a GeoTIFF and supplemental metadata files from the parent SLC product are available as a single XML file.

Burst SLC products

Burst SLC products are virtual i.e., they are created on-demand whenever a user makes a download request for a given product. The burst SLC data products are created by extracting the portion of SLC data corresponding to a given burst from the parent swath SLC file in the SLC SAFE archive. Information on where each burst is located within the parent swath SLC file is obtained from the annotation metadata files that are also found within the SLC SAFE archive. The data undergoes only one alteration post-extraction: setting burst data to zero (the data’s NoData value) in regions marked as invalid. The invalid region is defined using the firstValidSample, and lastValidSample fields found within the burst sections of the annotation metadata files. For more information on these fields, we refer you to the Sentinel-1 Product Specification Document. This operation is performed to ensure compatibility with existing SAR processors that expect invalid regions to already be removed when ingesting burst SLC data. Once this operation is complete, the burst SLC data is written to a GeoTIFF file and is distributed to the user.

Burst Metadata Files

The burst metadata files were constructed by combining all of the metadata xml files found in the burst’s parent SAFE file that share a polarization with the burst. This includes the annotation, noise, rfi, and calibration metadata files for all swaths. We made the decision to include metadata for all swaths, because we are aware of some workflows that require metadata from neighboring bursts. We made an effort to modify the original metadata files as little as possible, and instead chose to combine the original metadata file contents in a single file.

The XML structure combines separate manifest, product, noise, rfi, and calibration files into a single format. The root element, “burst,” includes a “manifest” element that contains the parent SAFE manifest file. The “metadata” section contains “product”, “noise”, “rfi”, and “calibration” elements, each encapsulating content from the respective original XML files. For example, the “product” sections include the annotation xml file contents from the parent SAFE. As noted above, this metadata file contains data from one polarization and one swath so there will be three “product” elements for IW bursts, and five for EW bursts. This consolidation simplifies data retrieval, providing an organized and unified representation of information that was previously distributed across individual XML files.

burst
    manifest: ORIGINAL_XML
    metadata
        product
            swath
            polarisation
            content: ORIGINAL_XML
        noise
            swath
            polarisation
            content: ORIGINAL_XML
        rfi
            swath
            polarisation
            content: ORIGINAL_XML
        calibration
            swath
            polarisation
            content: ORIGINAL_XML
        {STRUCTURE ABOVE REPEATED FOR EACH SWATH}