Detect Moving Targets in a Color Sub Aperture Image
Lesson 1 of 1
Detect Moving Targets in a Color Sub Aperture Image
In this quick guide, you will:
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Learn about color sub aperture images. * •
Create a color sub aperture image that reveals moving vessels as colored streaks.
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 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 metadata 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 Moving Target Detection tool in SAR Essentials creates a color sub aperture image. For spotlight and dwell-mode acquisitions, the entire azimuth spectrum is split into three segments called sub apertures. Each sub aperture is associated with a specific antenna squint, or viewing angle. Each one is assigned a color to create a red, green, and blue (RGB) composite image.
Colored streaks in a sub aperture image indicate objects moving along the azimuth direction (the satellite flight direction), or having a motion component in that direction, as the following figure shows:
Image courtesy of sarmap SA.
Uniformly colored sub aperture images also help to distinguish man-made objects from the surrounding natural environment. Pixels with bright colors often indicate man-made, anisotropic (directional) objects that exhibit strong backscattering. Their colors and brightness differ according to their direction relative to the squint angle of the sensor.
Image courtesy of Umbra Space.
Run the 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 color sub 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 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, expand the SAR Essentials > Moving Target folder. - 2
Double-click SAR Moving Target Detection. The SAR Moving Target Detection dialog appears. The Input tab is active. - 3
Click the Browse button next to Input Image. A file selection dialog appears. - 4
Go to the location where you saved the sample data for this quick guide, and select the file Capella_Shanghai_slc.sml. Then 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
The color sub 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. - 9
Leave the Grid Size field empty. - 10
Select the Yes option for Precise Geolocation. This will create a geocoded color sub aperture image.
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Click the Next button. Processing takes a few minutes to complete. When it is finished, the Report panel is displayed. The geocoded color sub aperture image is added to the Layer Manager. Its file name is moving_target_detection_cube. - 12
Click the Finish button to close the SAR Moving Target Detection tool. - 13
Click the Zoom to Full Extent button in the Toolbar.
Let's take a closer look at this image.
Interpret the Color Sub Aperture Image
White features in the image represent stationary objects. Features with a red/green/blue color scheme in the open sea correspond to moving vessels. Movement along the azimuth direction causes defocusing and rainbow-like artifacts.
Other vessels exhibit a combination of forward movement, rotation, and drifting. Movement varies along the azimuth direction, which results in different rainbow patterns at different range positions.
Additionally, any motion along the range direction will shift the object’s apparent location in the azimuth direction.
Let's look at other examples.
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Click the Cursor Value button in the Toolbar.
The Cursor Value dialog appears, and red crosshairs appear in the Image window.
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In the Go To field of the Toolbar, enter pixel coordinates 2727p, 2876p. The red crosshairs center over a colored streak on a road near the harbor. The colored streak corresponds to a vehicle that is traveling along the road in a linear direction. It does not exhbit the same "broad" color effect as the vessels that were drifting in a sideways direction.
The vehicle’s motion has both azimuth and range components. The azimuth motion produces the rainbow effects, while the range motion shifts the vehicle’s position outside the road.
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In the Go To field of the Toolbar, enter pixel coordinates 1326p, 3049p. The red crosshairs center on some vessels docked along the pier. Colored streaks on both sides of the vessels indicate that the vessels may be rocking back and forth, or that their side lobes exhibit a strong anisotropic component. Examples like this must be interpreted with caution.
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Close the Cursor Value dialog. The red crosshairs are removed from the Image window. - 5
In the Layer Manager, right-click on the View item and select Remove All Layers.
This concludes the quick guide.
Additional Resources
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SAR Essentials: Detect Moving Targets in a Dynamic Aperture Image quick guide * •
SAR Essentials: Estimate the Velocity of Moving Targets quick guide * •
SAR Essentials: Use Inverse SAR to Refocus Moving Objects quick guide
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