Process Single Look Complex (SLC) Intensity Data
Lesson 1 of 1
Process Single Look Complex (SLC) Intensity Data
In this quick guide, you will:
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Learn what steps are needed to prepare a SLC dataset for visual interpretation and mapping. * •
Open and display an Umbra SLC dataset of the Port of Gwangyang, South Korea. * •
Use the SAR Basic Data Processing tool to create a multilooked, filtered, and geocoded image from the Umbra SLC dataset. * •
Open a Capella SLC dataset of the Italian and Swiss Alps. * •
Use the SAR Basic Data Processing tool to create a classification image from the Capella SLC dataset that reveals SAR shadows and different degrees of layover.
Sample Data
The exercises in this quick guide use two different SLC images for demonstration. Download the ZIP file below and extract its contents to a directory on your computer.
[SAREssentials_BasicDataProcessing.zip
4.3 GB
DownloadArrow down with horizontal line beneath it](assets/SAREssentials_BasicDataProcessing.zip)
South Korea Dataset
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File name: Umbra_SouthKoreaPort_slc (with associated .hdr and .sml files) * •
Acquisition date: 05 January 2024 * •
Processing notes: The source dataset was a Sensor Independent Complex Data (SICD) file in National Transmission Imagery Format (NITF), which requires the ENVI NITF/NSIF Module to read. Since not all users have the ENVI NITF/NSIF Module, we used the "Imported data" option in the SAR Basic Data Processing tool to create an SLC image in SARscape format instead of SICD. * •
Source: Umbra Open Data Program(opens in a new tab), Attribution 4.0 International (CC BY 4.0)(opens in a new tab) license. The source ID is 2024-01-05-01-53-37_UMBRA-07_SICD.
Alps Dataset
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File name: Capella_Alps_slc (with associated .hdr and .sml files) * •
Acquisition date: 01 October 2024 * •
Processing notes: The source dataset was a SICD/NITF file. Since not all users have the ENVI NITF/NSIF Module, we used the "Imported data" option in the SAR Basic Data Processing tool to create a SLC image in SARscape format instead of SICD. * •
Source: Capella Open Data(opens in a new tab), Attribution 4.0 International (CC BY 4.0)(opens in a new tab) license. The source ID is CAPELLA_C13_SP_SICD_HH_2024100118414. * •
Also included is a 0.5-meter Digital Elevation Model (DEM) from swissALTI3D(opens in a new tab). It covers the same geographic area as the Capella image. When creating geocoded images, you should ideally use a DEM whose resolution and acquisition time closely match those of the SLC data. The ground and azimuth resolution of the Capella SICD image are approximately 0.4 meters, so the swissALTI3D DEM will work well.
Background
SLC imagery can be difficult to visually interpret because of its slant-range geometry and speckled appearance. You can use the SAR Basic Data Processing tool to smooth and georeference (or geocode) SLC imagery for easier visual interpretation and mapping in a Geographic Information System (GIS) application.
With this tool, you can apply any combination of processing steps to SLC data:
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Spatial subsetting: You can select a Google Earth KML/KMZ file or shapefile to define an subset area. This can significantly reduce processing time with large source files. You will not perform spatial subsetting in this quick guide. * •
Multilooking: SLC images represent slant-range geometry where pixel spacing is different in the range and azimuth directions. Multilooking averages the azimuth and range resolution cells, which reduces speckling noise at the expense of degrading the spatial resolution. * •
Spatial filtering: SAR images typically have a speckled appearance due to multiple backscatter sources within a given cell. Multilooking is often used as a first step to reducing speckling; however, you can apply additional filtering to further smooth SAR images. * •
Geocoding: Georeference SLC data to a Geographic WGS-84 projection. * •
Layover and shadow mask creation: Create a classification image of areas exhibiting layover and SAR shadowing.
You do not need to run the SAR Basic Data Processing tool before using other SAR Essentials tools. Most tools and workflows already involve some amount of processing and geocoding. You really only need to use this tool to create a multilooked, filtered, and geocoded image; or if you want to create a file that is in SARscape (_slc) format for use with ENVI SARscape.
Open and Display an Umbra SLC Image
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Select File > Open from the ENVI Menu bar. An Open dialog appears. - 2
Go to the location where you extracted the sample data, then go to the South Korea directory. - 3
Select the file Umbra_SouthKoreaPort_slc. - 4
Click Open. The image is added to the Layer Manager and displayed in the Image window. - 5
Wait for ENVI to build the necessary pyramid files, as seen in the Status bar. - 6
Click the Zoom to Full Extent button in the Toolbar.
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Click the stretch type drop-down list in the Toolbar and select Gaussian. This brightens the image.
The image shows a highly oblique perspective as the SAR sensor acquires data from a side-looking angle during its traversal. The image is further oriented with North down.
Objects appear elongated due to different sensor resolutions in the range and azimuth directions. Later when you create a multilooked, filtered, and geocoded image, the pixels will be square. This improves image quality and makes interpretation easier.
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Zoom into the image and note the highly speckled, "salt-and-pepper" appearance of features such as the vessel below. This is a result of complex backscattering in the SLC image.
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Select File > Data Manager from the Menu bar. The Data Manager appears.
The image contains SLC data where each pixel records the amplitude and phase of the backscattered signal. The actual signal is recorded in the Real and Imaginary bands, but those are not used for visual interpretation.
Magnitude is the same thing as amplitude or intensity. It refers to the strength of the reflected signal at the sensor. With SLC images, the Magnitude band is the most useful for visual interpretation; thus, it is displayed by default.
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Close the Data Manager.
Even though the magnitude image is technically suitable for visual interpretation, its geometry makes interpretation difficult. The image represents slant-range geometry, it has a speckled appearance, and it is not georeferenced or oriented with respect to North. Next, you will use the SAR Basic Data Processing tool to correct these issues.
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In the Layer Manager, uncheck the Umbra_SouthKoreaPort_slc layer to temporarily hide it.
Run the SAR Basic Data Processing Tool
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In the Toolbox, expand the SAR Essentials folder and double-click SAR Basic Data Processing. The SAR Basic Data Processing dialog appears. The Input tab is active. - 2
Click the Add File(s) button (the green "plus" icon). A file selection dialog appears. - 3
Select Umbra_SouthKoreaPort_slc.sml and click Open.
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Click the Optional tab. - 5
Click the DEM Option drop-down list and select Sea Level. - 6
Accept the default settings for Output Coordinate System, Polarization, and Enable mosaicking of Sentinel-1 Data. - 7
Click the Filtering Method drop-down list and select Refined Lee. This is a SAR-specific filter that enhances edges and lines while using spatial averaging to reduce speckling noise.
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Click the Export tab. - 9
In the Generate Products list, select Geocoded data only. - 10
Leave the remaining fields empty.
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Click the Next button. Processing takes several minutes. When it is finished, the Report panel is displayed. - 12
Scroll through the report and note the characteristics and geometry of the original SLC image.
Refer to the SAR Essentials: Check Data Compatibility quick guide for more information on interpreting these reports.
The geocoded image and processing files are written to the directory specified in the Output Directory preference. To verify the location, select File > Preferences from the Menu bar. In the Preferences dialog, click the Directories item on the left side and look for the Output Directory preference setting. You can change this directory as needed, but you must close and restart ENVI for it to take effect.
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Click the blue Help button in the lower-left corner of the SAR Basic Data Processing dialog. A popup window appears with a simple description of the tool. - 14
Click the Read more link at the bottom of the popup dialog. A new browser window or tab opens with the associated help topic for the tool. Keep this page open for the next exercise.
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Click the Finish button. The Report panel is dismissed. - 16
Click the Zoom to Full Extent button in the Toolbar. - 17
Click the stretch type drop-down list in the Toolbar and select Square Root. This brightens the geocoded image.
This is how the image appears after multilooking, filtering, and geocoding. It has a nadir perspective. We added an image magnifier annotation (in red) in the image below. Note that the buildings have more accurate shapes when compared to the original SLC image.
Next, you will rerun the SAR Basic Data Processing tool and create a layover/shadow classification image.
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In the Layer Manager, right-click on the View item and select Remove All Layers.
Create a Layover and Shadow Classification Image
This exercise uses an Umbra SLC dataset of the Swiss and Italian Alps for demonstration. The following true-color image shows the approximate coverage area:
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Select File > Open from the Menu bar. An Open dialog appears. - 2
Go to the location where you saved the sample data for this quick guide, then go to the Alps directory. - 3
Select the file Capella_Alps_slc and click Open. The SLC image is added to the Layer Manager and displayed in the Image window. This image also has a distorted perspective, much like the Umbra image that you viewed earlier. The following screenshot shows the image at its full extent:
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In the Toolbox, double-click SAR Basic Data Processing. The SAR Basic Data Processing dialog appears. - 5
Click the Add File(s) button next to the Input File List field. A file selection dialog appears. - 6
Select Capella_Alps_slc.sml and click Open.
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Click the Optional tab. - 8
Click the DEM Option drop-down list and select Use Input DEM. - 9
Click the Browse button next to the Input DEM field. A file selection dialog appears. - 10
Select the file SwissALTI3D_DEM.dat and click Open. - 11
Select the Yes option for Subtract Geoid. - 12
Click the Geoid Type drop-down list and select EGM2008. - 13
Delete the default value of -32768 for Data Ignore Value for DEM. - 14
Click the Filtering Method drop-down list and select Refined Lee. - 15
Leave the remaining fields alone.
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Click the Export tab. - 17
Use the Ctrl key on your keyboard to multi-select Geocoded data and Layover/Shadow Mask. - 18
Leave the remaining fields blank.
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Click the Next button to create the geocoded image and the layover/shadow classification image. When processing is complete, the Report panel is displayed. - 20
Click the Finish button to close the SAR Basic Data Processing tool.
Interpret the Layover and Shadow Classification Image
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In the Layer Manager, uncheck the following layers to hide them:
Capella_Alps_geo_info.anz (annotation layer)
Capella_Alps_geo_ls_mask_info.anz (annotation layer)
Capella_Apls_slc (the original SLC image)
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Right-click on the Capella_Alps_geo layer and select Zoom to Layer Extent. This is the geocoded image.
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Uncheck the Capella_Alps_geo layer (the geocoded image) to hide it. The layover and shadow classification image is displayed.
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Uncheck classes 0 through 4 to hide them. Layers 5-7 are enabled, but they do not contain any pixels. This means the SAR Basic Data Processing tool did not identify any radar shadows in the Capella image. - 5
Turn on the following layers, one-by-one, by clicking their check boxes:
2: Layover Active
3: Layover Near Passive
4: Layover Far Passive
You can optionally check and uncheck the geocoded image (Capella_Alps_geo) while viewing each layer. - 6
Read the "Layover and Shadow Mask Technical Note" in the SAR Basic Data Processing help topic to learn more about each layover and shadow type. The help topic provides detailed, technical information about how each type is calculated.
This concludes the quick guide.
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