ASF Provides Open Source Software for Increasing the Usability of SAR Data
by Patrick Denny, User Tool Development Lead, Alaska Satellite Facility
Synthetic Aperture Radar (SAR) technology has been around for quite some time now, but only in recent years has it become a common data type. The long path to popularity can most likely be attributed to a couple characteristics associated with SAR data. First, SAR data are typically provided in one of several CEOS formats. Since most image viewing software will not load such a format, many users are stopped here. Second, since SAR data are acquired from a side-looking geometry instead of the typical nadir geometry that most optical data are acquired from, it does not easily mesh with other data layers.
ASF has developed an open source tool, Convert, under a derivative of the BSD license, to help make SAR data more accessible to a wider variety of researchers. This software can ingest CEOS format data, process it to an orthorectified, map-projected state, and export it to several formats easily readable by imaging and GIS software.
The Convert package comes with a graphical user interface (GUI) and a set of command line tools. Both offer a means to the same end—SAR data to a more usable format for the user. The GUI provides an easy-to-use interface that can be learned in one sitting. The command line tools are available for more advanced users and provide the ability to add Convert’s functionality to a script.
Since it is common for scientists to want to process large quantities of data with similar parameters simultaneously, this tool was created with a batch processing concept. Users are able to select as many data sets as they like and load them into a queue that will be processed sequentially according to parameters setup by them in the Convert interface.
Convert’s primary functions can be divided into four categories 1) data ingest, 2) digital terrain correction, 3) geocoding, and 4) export of finished products
1. Data ingest: As CEOS data sets are selected and loaded into the Convert processing queue, a thumbnail of each scene is generated and the metadata is made viewable. Calibration parameters can be applied upon import in the sigma-, beta-, or gamma-naught projections in order to correct for the radar’s antenna pattern. Users may choose to have the calibrated image in the power or decibel scale.
2. Digital terrain: The second facet of Convert is interaction with a user-provided digital elevation model (DEM). Due to the side-looking geometry of SAR, it is typically necessary to “terrain correct” or orthorectify the data in areas of high relief. This step removes distortions such as layover and shadow which characterize SAR data and is necessary when preparing SAR data for use with other data layers. The Convert software does an exceptional job at it. In addition to geometric terrain correction, radiometric terrain correction is available. This can be used to adjust the radiometric backscatter values so that the local topography is in effect removed. In the case that the topography is somewhat flat, terrain correction may not be necessary; instead the accompanying DEM can be used to help refine the geolocation of the SAR data simply by matching the DEM and SAR and adjusting the metadata appropriately. This technique can correct for offsets such as those seen in JERS datasets, in which a clock error resulted in azimuthal offsets of up to 25 km.
3. Geocoding: Geocoding is transforming the SAR data into a map-projected state from its original slant-range geometry. There are three datums to choose from and five available map projections, each with several predefined parameter sets. The user may also define their own set of parameters for a map projection in order to attain minimal distortion. Splines are used to interpolate between a grid of precise projection points which results in fast processing and accurate results.
4. Export function: The final stage in the Convert processing stream is the export of data to a common imagery format. To preserve the full integrity of the data and geospatial metadata, the GeoTIFF format is available. Other available image types are generally used for display on posters or in image editing software. For ALOS PALSAR polarized data, the output image can be created in RGB color or each band can be exported as an individual grayscale image.
Once processing is complete, the item is moved from the processing queue to a list of completed processes, accompanied by an expandable thumbnail of the completed image. Users now have several options; the data can be moved back to the input queue for reprocessing, the ASF format metadata can be viewed, the processing log can be viewed, or if the output happens to be JPEG, there is a light weight image viewer built in for a quick inspection of the output.
The primary benefit of this software is the ability for remote-sensing and GIS professionals to easily use SAR data. The intuitive interface of this free, open-source software will enable users to convert ASF Level 1 SAR scenes into compatible GIS layers. Overlay with other data sets such as Landsat, MODIS, or commercial imagery is made feasible by the high geometric accuracy of Convert’s GeoTIFF product.
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