Detect Moving Targets in a Dynamic Aperture Image
Lesson 1 of 1
Detect Moving Targets in a Dynamic Aperture Image
In this quick guide, you will:
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Learn about dynamic aperture images. * •
Create a dynamic aperture image whose bands can be animated to show targets in motion.
Sample Data
The exercises in this quick guide use a Capella Single Look Complex (SLC) image for demonstration. Download the ZIP file below, and extract the contents to a dedicated directory on your computer.
[SAREssentials_MovingTargets.zip
624.8 MB
DownloadArrow down with horizontal line beneath it](assets/SAREssentials_MovingTargets.zip)
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File name: Capella_Shanghai_slc. Also included are an associated ENVI header file (.hdr) and ENVI SARScape file (.slc). * •
Acquisition date: 07 May 2024 * •
Processing notes: The source datasets were Sensor Independent Complex Data (SICD) files in National Transmission Imagery Format (NITF), which require 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 SLC images that are 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_C09_SP_SICD_HH_20240507211544_20240507211611.
Background
SLC images acquired in spotlight or dwell mode can reveal moving objects such as aircraft, vehicles, and vessels. In spotlight mode, a SAR sensor’s antenna beam focuses on a specific target for several seconds as it passes overhead. The time it takes to receive backscattered echoes from the target during the flyover is called dwell time or aperture. Similar to spotlight mode, dwell mode focuses on a target for an even longer time period.
The SAR Dynamic Moving Target Detection tool divides an entire aperture spectrum into a user-specified number of sub apertures (up to 25). Each sub aperture is assigned a unique band in the resulting dynamic aperture image. You can use ENVI's band animation tools with the dynamic aperture image to watch objects move in real time.
Dynamic aperture images are best created from sensors that operate in dwell mode with longer dwell times such as 10 to 30 seconds. However, satisfactory results can also be obtained in spotlight mode with at least 5 seconds of dwell time. Be aware that the spatial resolution of a dynamic aperture image degrades with an increasing number of sub apertures.
Run the Dynamic Moving Target Detection Tool
For this exercise, you will import a Capella SLC image that covers a small area in the Shanghai harbor. The image was acquired in spotlight mode with a dwell time of 27 seconds. Here is the acquisition time metadata:
Below is a geocoded image created from the SLC image. Moving vessels are rendered as blurred, or defocused, objects. A dynamic aperture image will confirm that these objects are indeed moving.
To automatically detect and refocus blurred objects, use the SAR Inverse-SAR Refocusing tool.
Note: The SAR Dynamic Moving Target Detection tool only accepts SLC images in slant-range or ground-range geometry. You will import a SLC image, not the geocoded image shown here.
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In the Toolbox, go to the SAR Essentials > Moving Target folder and double-click SAR Dynamic Moving Target Detection. The SAR Dynamic Moving Target Detection dialog appears. The Input tab is active. - 2
Click the Browse button next to Input Image. A file selection dialog appears. - 3
Select Capella_Shanghai_slc.sml and click Open.
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Click the Optional tab. - 6
Click the DEM Option drop-down list and select Sea Level.
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Click the Export tab. - 8
Drag the Number of Sub-Apertures slider to a value of 20. This will create an image with 20 bands, resulting in a smoother animation at the expense of slightly reduced spatial resolution. - 9
The dynamic aperture image 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. - 10
Leave the Grid Size field empty. - 11
Select the Yes option for Precise Geolocation.
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Click the Next button. Processing takes several minutes to complete. When it is finished, the Report panel is displayed. A three-band color composite of the dynamic aperture image is added to the Layer Manager and displayed in the Image window. Its file name is dynamic_moving_target_detection_cube. - 13
Click the Finish button to close the SAR Dynamic Moving Target Detection tool. - 14
Click the Zoom to Full Extent button in the Toolbar.
Only three bands can be displayed at a time in the Image window. This color composite is not very useful for visual interpretation. Instead, you will animate the bands in the dynamic aperture image. The full image consists of 20 bands.
Animate the Dynamic Aperture Image
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In the Layer Manager, right-click on dynamic_moving_target_detection_cube and select Band Animation > Raster Series. The Band Animation dialog appears, and a new raster series layer is added to the Layer Manager and displayed in the Image window. Animation begins immediately. You can see the vessels moving throughout the harbor. The vessels near the top of the image are drifting sideways, while the ones near the bottom of the image are mostly moving in a linear direction.
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Click the Stop button in the Band Animation dialog to pause the animation.
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Click and hold the left mouse button over the "wheel" in the Band Animation dialog, then drag in either direction. This displays each frame (band) incrementally. Each one represents a unique viewing angle at a specific point in time during the spotlight acquisition.
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Click the icon circled in red below and select Save Video Animation.
The Save Video dialog appears.
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Optionally choose a file name and directory for the output AVI file, and click OK. - 6
When you are finished animating the dynamic aperture image, close the Band Animation dialog. The raster series layer is removed from the Layer Manager and Image window.
This concludes the quick guide.
Additional Resources
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SAR Essentials: Detect Moving Targets in a Color Sub Aperture Image quick guide * •
SAR Essentials: Estimate the Velocity of Moving Objects quick guide * •
SAR Essentials: Use Inverse SAR to Refocus Moving Objects quick guide
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