Measure Radar Cross Section (RCS)
Lesson 1 of 1
Measure Radar Cross Section (RCS)
In this quick guide, you will:
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Learn why RCS is a valuable metric in determining the detection level of man-made objects, particularly aircraft. * •
Open and display a Single Look Complex (SLC) image through the SAR Radar Cross Section tool. * •
Draw Regions of Interest (ROIs) around helicopters in the SLC image. * •
Compute RCS and other metrics for the helicopters. * •
View RCS values and other metrics in a shapefile attribute table.
Sample Data
The exercises in this quick guide use a Capella SLC dataset of the Donskoye air base in Russia for demonstration. Download the ZIP file below and extract its contents to a directory on your computer.
[SAREssentials_RadarCrossSection.zip
26.8 MB
DownloadArrow down with horizontal line beneath it](assets/SAREssentials_RadarCrossSection.zip)
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File name: CapellaRussiaAirBase_slc. Also included are the associated ENVI header file (.hdr) and ENVI SARscape metadata file (.sml). * •
Acquisition date: 14 July 2023 * •
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: 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_C08_SP_SICD_HH_20230714095227_20230714095245.
Background
In simple terms, an object's Radar Cross Section (RCS) determines how detectable it is by radar systems. It represents how much electromagnetic energy an object reflects back to a radar receiver. RCS is a valuable metric used in the defense industry, as it can indicate the detection level of enemy aircraft by radar.
Factors that influence an object's RCS include its size, shape, material, orientation relative to the radar receiver, transmitter power, and receiver sensitivity.
For more information on RCS, see the resources at the end of this page.
For this quick guide, you will use the SAR Radar Cross Section tool to estimate the RCS of military helicopters at a Russian air base. The tool reads and displays SLC power images and lets you draw ROIs around individual objects in the images. The tool computes RCS and other metrics from the ROIs and creates shapefiles from them. It then adds the resulting values to the shapefile attributes, which you can see in the Attribute Viewer. ENVI also annotates the RCS values for each ROI on the screen.
Let's look at an example.
Run the SAR Radar Cross Section Tool
This scenario demonstrates how to start the SAR Radar Cross Section tool, then display the SLC image and draw ROIs around helicopters. You can also display the image and draw ROIs before starting the SAR Radar Cross Section tool, but those steps are slightly different.
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Start ENVI. - 2
In the Toolbox, expand the SAR Essentials folder and double-click SAR Radar Cross Section. The SAR Radar Cross Section dialog appears. - 3
Click the Browse button next to the Input Image field. A file selection dialog appears. - 4
Go the location where you extracted the sample data for this quick guide, and select the file CapellaRussiaAirBase_slc.sml. Then click Open. - 5
Click the Export tab. - 6
The RCS shapefile and associated annotations will be written to the directory specified in the ENVI Output Directory preference. To specify a different output folder, click the Browse button next to Output Folder and choose a different folder. - 7
Click the Next button. After a few seconds, the SLC image is added to the Layer Manager and displayed in the Image window. The RCS tool advances to the ROI Selection panel.
The image covers a WWII-era air base in Donskoye that the Russian Navy still uses for helicopter anti-submarine and search-and-rescue operations. The base houses Kamov Ka-27 and Ka-29 helicopters. The helicopters are visible as bright objects against dark concrete pads; for example:
For reference, here is a Google Earth image of helicopters at the air base:
Draw ROIs around Helicopters
The next step is to draw polygons around objects for which you want to measure RCS. Each polygon is an individual record within a given ROI. You can import existing ROI files (.xml) from a previous ENVI session or create new ROIs in the current session. For this exercise, you will draw new ROI polygons around helicopters.
For future reference, you can also create and draw multiple ROIs for different types of objects; for example, fixed-wing aircraft or vehicles.
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Click the Open ROI Tool button in the ROI Selection panel.
The Region of Interest (ROI) Tool appears. The Geometry tab is active, and the ROI type is Polygon. A new ROI layer is added to the Layer Manager. The cursor changes to a crosshair symbol so that you can begin drawing polygons.
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Zoom to any helicopter in the scene; for example:
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Click somewhere along the outline of the helicopter to define the first vertex. Then click to draw line segments around it. Double-click to complete and accept the polygon. The exact outlines of the helicopters are difficult to determine in this SLC image, so just do your best to try to capture their shapes.
The new ROI is added to the ROI layer, and the Record Count in the ROI Tool increments. Once accepted, the polygon is colored red. Here is a quick video animation showing an example:
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Continue to draw polygon ROIs around other helicopters in the image. They won't necessarily look like helicopters, but you will know they are helicopters if they are located on the concrete pads.
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In the ROI Tool, change the default name of the ROI from "ROI #1" to Helicopters. Press the Enter key to accept the name change.
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In the SAR Radar Cross Section dialog, click the Browse button next to the Input ROIs field.
The ROI Selection dialog appears. The "Helicopters" ROI is selected by default.
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Click OK to dismiss the ROI Selection dialog.
Compute RCS and Other Metrics
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Click the Next button in the SAR Radar Cross Section dialog. When processing is complete:
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Yellow annotations appear next to each ROI in the Image window. They display the calculated RCS values for each object. The annotation layer is named "rcs_ann.anz" in the Layer Manager. * •
The Report panel appears. * •
ENVI creates a shapefile from the collection of ROIs. It is named "rcs.shp" in the Layer Manager.
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Click the Finish button to close the SAR Radar Cross Section tool. - 3
Close the ROI Tool. - 4
Zoom to each helicopter and view its corresponding RCS value, shown in yellow.
Optional: Change the Text Size
If the annotation text is too small, you can change it as follows:
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In the Layer Manager, select the rcs_ann.anz layer to make it the active layer. - 2
Click the + button next to the rcs_ann.anz layer to expand it. Each annotation item is listed individually. - 3
Click the first "Helicopters" annotation item, press the Shift key on your keyboard, then select the last annotation item. This selects all of the items at once. - 4
Go the Property sheet in the lower-right corner of the ENVI application, and increase the Font Size property value. Here is a quick animation of these steps:
View Other Object Metrics
ENVI also calculates other metrics for each ROI record. Follow these steps to view the metric values:
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In the Layer Manager, right-click on the rcs.shp layer and select View/Edit Attributes. The Attribute Viewer appears with a table of computed values. - 2
Click and drag the lower-right corner of the Attribute Viewer dialog to expand it so that you can see all records. Here is an example:
Each row corresponds to an individual ROI record. The table provides the following metrics:
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tot_rcs_dB: Total integrated RCS, in decibels * •
mea_rcs_dB: Mean RCS, in decibels * •
std_rcs_dB: Standard deviation RCS, in decibels * •
max_rcs_dB: Maximum RCS, in decibels * •
tot_pix: Number of pixels in the ROI * •
tot_are_m2: Area of the ROI, in square meters * •
ilos_deg: Scene center line-of-sight (LOS) incidence angle * •
alos_deg: Scene center line-of-sight (LOS) azimuth angle
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Close the Attribute Viewer dialog.
This concludes the quick guide.
Additional Resources
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Draw Regions of Interest quick guide * •
MilitarySphere.com. "Unveiling the Intricacies of Radar Cross Section: A Comprehensive Guide." 28 June 2024. https://militarysphere.com/radar-cross-section. * •
Orbits Hub. "What is Radar Cross Section and Why Should You Care?" Accessed 15 July 2025. https://orbitshub.com/understanding-radar-cross-section-and-its-importance.
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