Estimate the Velocity of Moving Targets
Lesson 1 of 1
Estimate the Velocity of Moving Targets
In this quick guide, you will:
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Learn what conditions must be met to estimate velocities of moving targets. * •
Use the SAR Velocity Estimation tool to determine velocities of moving vessels at sea.
Sample Data
The exercises in this quick guide use a Sentinel-1 power image for demonstration.
[SAREssentials_VelocityEstimation.zip
8.6 MB
DownloadArrow down with horizontal line beneath it](assets/SAREssentials_VelocityEstimation.zip)
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File name: Sentinel1_Skagen_pwr. Also included are an ENVI header file (.hdr) and ENVI SARscape metadata file (.sml). * •
Acquisition date: 16 July 2019 * •
Processing notes: This is a spatially subsetted, multilooked, power image derived from a Sentinel-1 Interferometric Wide (IW), Single Look Complex (SLC) dataset. We used the Basic Data Processing tool in SAR Essentials to create the power image. * •
Source: European Union, Copernicus Sentinel-1 imagery. The source ID is S1A_IW_SLC__1SDV_20190716T053200_20190716T053227_028138_032D98_EDA1.SAFE.
Background
To understand how velocity estimation works, you should be aware of some basic principles. Objects that move toward the range direction are typically offset in an image from their true locations. The range direction is perpendicular to a satellite's flight path, which is also called the azimuth.
For example, look at the moving train in the color sub-aperture image below. We know it is moving because of its rainbow colors. (See the SAR Essentials: Detect Moving Targets quick guide for more information.) Tracks are displayed as dark-red lines. When the train travels in the same direction as the satellite's azimuth, the white streak is close to the train's true position on the tracks. However, as the train deviates and turns more along the range direction, the image shows a large offset between the train and the tracks.
You can take advantage of this positional offset to estimate the train's velocity. Thus, velocity estimation requires the following conditions:
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An object must have a strong range component, meaning that it is moving along the range direction (perpendicular to the satellite's flight path) and some positional offset is evident. * •
The image should contain a visual clue that indicates the object's true position. This can include a road for moving vehicles, a railway for a train, or a wake left behind by a vessel.
If you try to estimate the velocity of an object that is moving along the azimuth direction (parallel with the satellite's flight path), you will receive an error message that its velocity cannot be determined.
Let's look at an example that uses moving vessels at sea.
Run the SAR Velocity Estimation Tool
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Start ENVI. - 2
In the Toolbox, go to the SAR Essentials folder and expand the Moving Target folder. - 3
Double-click SAR Velocity Esimation. The SAR Velocity Esimation Tool appears.
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Click the Open menu option in the SAR Velocity Esimation Tool menu bar, and select Open Image. The SAR Select Image dialog appears. - 5
Click the Browse button next to Input Image. A file selection dialog appears. - 6
Go to the location where you saved the sample data for this quick guide. - 7
Select the file Sentinel1_Skagen_pwr.sml and click Open. - 8
Click OK in the SAR Select image dialog. After a few seconds, the image is added to the Layer Manager and displayed in the Image window. The following screenshot shows the image displayed at its full extent.
The bright objects are moving vessels. They are slightly offset from their wakes (i.e., true locations), which are displayed as straight lines. Here, the azimuth direction is vertical and the range direction is horizontal.
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Optional: Click the Units menu option and select Knots. - 10
Zoom to a single vessel, then click the Start button. The cursor changes to a crosshair symbol. - 11
Click on one end of a wake, then click again on the other end. A dashed line is drawn between both points. The cursor changes to a yellow circle. - 12
Click in the middle of the vessel.
The velocity and directional arrow are displayed next to the vessel:
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Click the Start button, then the Stop button. This resets the drawing mode. - 14
Repeat the same steps with the vessels shown in the image below. Your estimates may be slightly different than what is shown here:
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Zoom to the vessel shown in the red image magnifier below:
Notice that the wake is located to the left of the vessel, and it runs vertically along the azimuth direction. This is different from the other wakes in the image.
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Click the Start button. - 17
Click on one end of the vertical wake, then again on the other end. A dashed line is drawn between the two points. - 18
Click in the center of the vessel. After a few seconds, an error message appears:
The message indicates that velocity cannot be determined because the line you drew along the wake is parallel with the azimuth direction. Again, objects must be moving along the range direction if you want to calculate their velocities.
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Click OK to dismiss the error message. - 20
Click the Stop button. - 21
Optional: To erase previously drawn annotations, click the Annotations menu option and select Remove Latest for single items or Remove All for all items. - 22
In the Layer Manager, expand the SAR Measurements item to view individual annotation items. Each annotation set consists of three items: (1) A velocity estimation value, (2) a "Direction" arrow, and (3) a "Displacement" line showing the direction of displacement between the object and its true location; for example:
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To save the annotations for a future ENVI session, right-click on the SAR Measurements item in the Layer Manager and select Save As. Then specify an output file name with an .anz extension.
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: Use Inverse SAR to Refocus Moving Objects quick guide
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