Use Inverse SAR to Refocus Moving Objects

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Lesson 1 of 1

Use Inverse SAR to Refocus Moving Objects

In this quick guide, you will:

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Open and display an Umbra Single Look Complex (SLC) image of vessels at sea. * •

Identify and mark vessels that are moving. They are rendered as unfocused, white lines. * •

Refocus the moving vessels to reveal their true shapes, while also estimating their velocities.

Sample Data

The exercises in this quick guide use an Umbra Single Look Complex (SLC) file for demonstration. Download the ZIP file below and extract its contents to a directory on your computer.

[SAREssentials_InverseSAR.zip

225.5 MB

DownloadArrow down with horizontal line beneath it](assets/SAREssentials_InverseSAR.zip)

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File name: Umbra_GwangyangPort_slc. ENVI header files (.hdr) and ENVI SARScape files (.sml) are also included. * •

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.

Background

The SAR Inverse-SAR Refocusing tool detects and refocuses moving targets in a SLC image. Objects that are moving in the azimuth direction are displayed as unfocused, white streaks in high-resolution images. Once they are refocused, the objects' true shapes are revealed. The following graphic shows an example of two moving objects that are determined to be vessels:

In the detection step, the tool automatically detects moving objects and marks them with red polygons. This saves considerable time, as opposed to manually identifying and marking objects yourself. The tool creates a shapefile containing one polygon for each feature that needs refocusing. Automatic detection provides a good starting point. If needed, you can use ENVI's vector editing tools to modify the initial shapefile before proceeding with the refocusing step.

After the refocusing step, a new polygon shapefile is created for the detected objects, and separate rasters are created for each refocused object. The velocities of each object are displayed on-screen.

For best results, follow these guidelines:

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The SAR Inverse-SAR Refocusing tool is optimized to detect objects with minimal surrounding clutter; for example, vessels in an open sea or vehicles positioned on a road surface with low backscatter. Avoid using images that contain a mixture of vegetation, man-made objects, and other features that exhibit medium and high backscatter. The background should be homogenous, either water or land but not both. * •

You can optionally create a small spatial subset that encompasses your area of interest or to restrict analysis to a homogenous area, as described in the previous bullet point. You can do this directly in the SAR Inverse-SAR Refocusing tool by selecting a pre-defined shapefile or Google Earth KML/KMZ file to use for spatial subsetting. * •

The image resolution must be sufficiently high to allow for effective refocusing. * •

The tool currently supports focused SICD data with the Polar Format Algorithm (PFA) or Range Doppler (RG_DOP) algorithm. The image metadata usually indicates what algorithm was used to focus the SICD data. In the case of non-SICD data focused with these algorithms or data focused with different or undefined algorithms, refocusing will still work but the speed will not be shown in the attributes.

Tip: To check if your images are suitable for the SAR Inverse-SAR Refocusing tool, run the SAR Compatibility Check tool in the SAR Essentials folder of the Toolbox. See the SAR Essentials: Check Data Compatibility quick guide for more information.

**Run the SAR Inverse-SAR Refocusing Tool**

The SICD file you will use in this exercise covers a small area of open sea near Gwangyang Port in South Korea. In the map below, the image extent is shown with a blue border:

1

Go to the Toolbox and expand the SAR Essentials > Moving Target folder. - 2

Double-click SAR Inverse-SAR Refocusing. The SAR Inverse-SAR Refocusing tool appears. The Input tab is active. - 3

Click the Browse button next to the Before Image field. - 4

Select the file Umbra_GwangyangPort_slc.sml and click Open. - 5

Leave the Spatial Subset field empty. This is for selecting a pre-defined shapefile of the area of interest. You will process the entire image instead. - 6

For the Target Background drop-down list, keep the default selection of Water.

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Click the Export tab. - 8

The refocused images 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

Click the Next button. When processing is complete, the Refocusing panel appears and the SICD file is displayed at 100% resolution. - 10

Rotate the scroll wheel on your mouse, or use the Zoom tools in the Toolbar, to zoom out until you can see red boxes around some vessels.

The red boxes indicate objects that were determined to be moving. The detected objects, or records, are added to a new shapefile named isar_detection.shp.

Optional: Edit Detected Records

Automatic detection results are accurate most of the time, as with our current example. However, false detections may occur, particularly if part of an object is moving while the rest of it is stationary. In rare cases, the detection algorithm will not correctly filter objects with high directive backscatter, and it will flag them as moving objects.

If you encounter these scenarios, you can use ENVI's vector editing tools to modify the isar_detection.shp file.

You do not have to perform these steps in this exercise because the results are accurate. These instructions are for future reference.

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Click the Vectors drop-down list in the Toolbar and select Edit Vector. The cursor changes to a crosshair symbol.

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To delete a record, click on it to outline it in blue. Then right-click and select Delete.

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To add a new record:

(a) Click the Vectors drop-down list.
(b) Select Create Vector.
(c) Draw a polygon around the desired object by clicking to define vertices.
(d) To close the polygon, right-click and select Accept.
(e) Right-click again and select Save.

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When you are finished editing vectors and you are confident that the polygons accurately represent moving objects, right-click on isar_detection.shp in the Layer Manager and select Save.

Complete the Process

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In the Layer Manager, right-click on isar_detection.shp and select View/Edit Attributes. The Attribute Viewer dialog appears. - 2

Review the shapefile attributes, which provide helpful information about the detection results:

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IDPolygon: The identification number of each polygon record * •

ReliabIndx: Reliability index; the number of sub-aperture images along which the object moves * •

AzBaricMot: Azimuth Baricenter Motion; a rough estimate of the azimuthal velocity when a velocity estimate is not available after refocusing * •

ISARRes: The minimum detectable velocity (in meters per second), which depends on the azimuth resolution of the sensor

3

Close the Attribute Viewer. - 4

Click the Next button in the SAR Inverse-SAR Refocusing tool. The refocusing process begins.

When processing is complete:

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The tool advances to the Report panel. * •

A new shapefile named isar_refocused.shp is added to the Layer Manager and displayed in the Image window. * •

Separate rasters of each refocused object are added to the Layer Manager and displayed in the Image window. They are named Polygon_x_slc, where x is 0, 1, 2, 3, etc. * •

Estimated velocities are displayed as yellow annotations next to each refocused object. The annotation layer is named isar_refocused_ann.anz.

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Click the Finish button to close the SAR Inverse-SAR Refocusing tool. - 8

In the Layer Manager, uncheck the following layers to hide them:

isar_detection.shp
Umbra_GwangyangPort_slc

Evaluate Refocused Objects

1

Zoom to each refocused object, evaluate its appearance, and note the estimated velocity (in yellow). Objects are displayed at full resolution with no multilooking. As a result, the objects are elongated along the azimuth direction. Future versions of SAR Essentials will produce multilooked, geocoded rasters so that objects will have more natural shapes.

The estimated velocities are also recorded in the shapefile (isar_refocused.shp). Follow the steps you learned earlier to view the shapefile attributes. Velocities ("AzVel") may be positive or negative depending on whether the object's motion vector is aligned with the satellite's direction of motion.

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: Detect Moving Targets in a Dynamic Aperture Image quick guide * •

SAR Essentials: Estimate the Velocity of Moving Objects quick guide

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