Polarimetric SAR Processing and Analysis Using ENVI - Tutorial of ENVI Software - Completely GIS, GPS, and Remote Sensing Lecture Material - facegis.com
Polarimetric SAR Processing and Analysis Using ENVI

Overview of This Tutorial

The following tutorial demonstrates the use of ENVI's radar data analysis functions. Data from the Spaceborne Imaging Radar-C (SIR-C) for Death Valley, California are used in this example. The data were obtained by SIR-C onboard the Space Shuttle Endeavor in April 1994.

Files Used in This Tutorial

You must have the ENVI TUTORIALS & DATA CD-ROM mounted on your system to access the files used by this tutorial, or copy the files to your disk.

The files used in this tutorial are contained in the NDV_SIRC subdirectory of the ENVIDATA directory on the ENVI TUTORIALS & DATA CD-ROM.

The files listed below are required to run this exercise. Several new files will be generated during the processing steps.

Required Files

NDV_L.CDP	L-band SIR-C subset in ENVI Compressed Data Product .cdp format.
POL_SIG.ROI	Saved Regions of Interest (ROI).
TEXTURE.DSR	Saved Density Slice Range .dsr file.

Generated Files

NDV_L.SYN	Synthesized images (~2.5 Mb, also generates .hdr file).
NDV_L2.SYN	Synthesized images in dB (~5 Mb, also generates .hdr file).
NDV_GAM.IMG	Gamma filter result (~0.6 Mb, also generates .hdr file).
NDV_GR.IMG	Slant to ground range result (~0.9 Mb, also generates .hdr file).
NDV_HH.TEX	Texture filter result (~2.5 Mb, also generates .hdr file).
ENVI.PS	Output ENVI postscript file (~3.8 Mb).

Background: SIR-C/SAR

SIR-C is a polarimetric synthetic aperture radar that uses two microwave wavelengths: L-band (24 cm) and C-band (6 cm). The SIR-C radar system was flown as a science experiment on the Space Shuttle Endeavor in April (SRL-1) and October 1994 (SRL-2), collecting high quality SAR data over many sites around the world (a second radar system--"X-SAR"--was also flown on this mission, but these data are not discussed or processed here). Additional information about SIR-C is available on the NASA/JPL Imaging Radar Home Page on the World Wide Web at http://southport.jpl.nasa.gov/ .


Analyzing SIR-C Data

The data used in this tutorial are a subset of L-band "Single Look Complex" (SLC) SIR-C data that cover the northern part of Death Valley, including Stovepipe Wells, a site of active sand dunes and extensive alluvial fans at the base of mountains. These data have been pre-processed by reading/subsetting from tape and multilooking (averaging) to 13 m square pixels. The data are provided in a special ENVI "Compressed Data Product ( .cdp ) format. This is a non-image format similar to the tape format and can not be viewed until images are "synthesized" for specific polarizations.

The first two functions described in this example--reading the data tape and "multilooking"-- have been pre-applied to the SIR-C data. We include the sections here for completeness in dealing with SIR-C data. Skip to the section "Synthesize images - Start The Actual Work Here" if you are not interested in reading about data input and preparation.

Read a SIR-C CEOS Data Tape

  • The file used for this exercise has already been read from tape and saved in a file. Instructions on reading a SIR-C tape are included here only for completeness.
  • Select Radar Tools->Read CEOS Data->Read SIR-C CEOS Tape to read a SIR-C CEOS data tape into ENVI.
  • Note that this utility can also be accessed by selecting Utilities->Tape Utilities->Read Known Tape Formats->SIR-C CEOS

The SIRC Format--Load Tape dialog will appear. See the Tape Reading section of the ENVI User's Guide for details on the SIR-C tape-reading function. To read a tape:

  1. Enter the tape device name and let the record size default to 65,536.
  2. Click "OK".

The tape will be scanned to determine what SIR-C files it contains and a dialog will appear, allowing you to select the desired data sets. By default, ENVI will read all of the data files on the tape.

  1. If you do not want to read all of the data files, click "Clear" and then click on the box next to each desired file. When the files are selected, click "OK".
  2. The selected data files can be subset and multilooked independently as they are being read from tape. We recommend, however, that multilooking be performed on disk files unless insufficient disk space is available, as this function is extremely slow from tape.
  3. Click on a filename, then click on "Spatial Subset" or "Multi-Look" to enter parameters for the data file and enter an output filename.

Each input file must have an output filename. By convention, the output filenames should take the form filename_c.cdp and filename_l.cdp for the C- and L-bands, respectively.

The SIR-C data will be read from the tape and one compressed scattering matrix output file created for each data set selected.

Multilook SIR-C Data

Multilooking is a method for reducing speckle noise in SAR data and for changing the size of a SAR file. SIR-C data can be multilooked to a specified number of looks, number of lines and samples, or azimuth and range resolutions.

  • The SIR-C file used in this tutorial was a single look dataset that had a range resolution of 13 m and an azimuth size of 5 m. Multilooking has already been performed in the azimuth direction to make 13 m square pixel sizes. Instructions on Multilooking are included here only for completeness in dealing with SIR-C data. To multilook a dataset:
  • Select Radar Tools->Polarimetric Tools->Multilook Compressed Data->SIR-C Multilook.
  • When the Input Data Product Files dialog appears, click "Open File" and select the input file.

ENVI will detect whether the file contains L- or C- band data and display the file name in the appropriate field of the dialog.

  1. Click "OK".
  2. Select the file to multilook by selecting the box next to the name.

Multiple files can be selected.

  1. Enter any one of three values--number of looks, number of pixels, or pixel size--and the other two will be calculated automatically.

Both integer and floating point number of looks are supported.

  1. Enter the desired values in the appropriate box for both "Samples (range)" and "Lines (azimuth)".
  2. Enter the base file name in the appropriate text box and click "OK".

Synthesize Images - Start The Actual Tutorial Work Here

The SIR-C quad-polarization data provided with this tutorial and available on tape from JPL are in a non-image, compressed format. Accordingly, images of the SIR-C data must be mathematically synthesized from the compressed scattering matrix data. You can synthesize images of any transmit and receive polarization combinations desired.

  1. Select Radar Tools->Polarimetric Tools->Decompress-Synthesize Images->Synthesize SIR-C Images.
  2. When the Input Product Data Files dialog appears, click "Open File" to display a standard file selection dialog and navigate to the NDV_SIRC subdirectory of the ENVIDATA directory .
  3. Select the file NDV_L.CDP from the list. When the filename appears in the "Selected Files L:" field, click "OK".

The Synthesize Parameters dialog appears.

Polarimetric SAR Processing and Analysis Using ENVI - facegis.com

Figure 1: The Synthesize Parameters dialog.

Default Polarization Combinations

Four standard transmit/receive polarization combinations--HH, VV, HV, and TP--will be listed in the "Select Bands to Synthesize" list in the Synthesize Parameters dialog. By default, these bands are selected to be synthesized.

  1. Enter the output filename NDV_L.SYN in the "Enter Output Filename" field.
  2. Select "Byte" from the "Output Data Type" pulldown menu. This will scale the output data to byte values. Make the appropriate selections and click OK.
  3. If you will be doing quantitative analysis, the output should remain in floating point format.

An ENVI Status Window will appear and after a short wait, the file NDV_L.SYN will be created, and four bands corresponding to the four polarization combinations will be added to the Available Bands List, which appears automatically.

Other Polarization Combinations

The transmit and receive ellipticity and orientation angles determine the polarization of the radar wave used to synthesize an image. The ellipticity angle falls between -45 and 45 degrees and determines the "fatness" of the ellipse. The orientation angle is measured with respect to horizontal and ranges from 0 to 180 degrees.

You can synthesize images of non-default polarization combinations by entering the desired parameters as follows.

  1. Select Radar Tools->Polarimetric Tools->Decompress - Synthesize->Synthesize SIR-C Images.

The file NDV_L.CDP should still appear in the "Selected Files" field.

  1. Click "OK".
  2. Enter -45 and 135 in the "Transmit Ellip" and "Orien" fields respectively and -45 and 135 in the "Receive Ellip" and "Orien" fields respectively.
  3. Click "Add Combination".

This will produce a right hand circular polarization image.

  1. Enter 0 in both the "Transmit Ellip" and "Receive Ellip" fields and 30 in both the "Transmit Orien" and "Receive Orien" fields.
  2. Click "Add Combination".

This will produce a linear polarization with an orientation angle of 30 degrees.

  1. Click "Clear" under the list of standard polarization combinations to turn off synthesis of the standard polarization bands which have already been generated.
  2. Select "Yes" next to the "Output in dB?" label.

This will produce images that are in decibels and therefore have values typically between -50 and 0.

  1. Enter the output filename NDV_L2.SYN and click "OK".

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Figure 2: Synthesize Parameters Dialog with non-standard orientation and ellipticity angles.

The file NDV_L2.SYN will be created, and two bands corresponding to the two polarization combinations will be added to the Available Bands List.

Display Images

  1. Click on the band named "[L-TP]: NDV_L.SYN" in the Available Bands List, and click "Load Band".

The SIR-C L-band total power image will be displayed in a new window.

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Figure 3: L-Band SIR-C Total Power Image with Gaussian stretch applied.

  1. Investigate the image using the Scroll and Zoom windows.
  2. Click the right mouse button in the Main Image display and select Functions->Display Enhancements->Interactive Stretching.

A window containing a histogram plot of the data in the image window appears.

The histogram plot shows the current stretch with the red and green lines on the input histogram and the corresponding DN values in the text boxes.

  1. Click the left mouse button on and drag the red and green lines to change the stretch or enter the desired DN values into the appropriate fields.
  2. Enter 5% in the field next to the red bar and enter 95% in the field next to the green bar.
  3. Select "Gaussian" from the "Stretch Type" button menu (initially labelled "Linear") and click "Apply".

This will perform a gaussian stretch with a 5% low and high cut-off.

  1. Also try to compare the linear and square-root stretches.
  2. To display a color composite image, select the "RGB Color" toggle button in the Available Bands List.
  3. Click on the "[L-HH]: ndv_l.syn", "[L-VV]: ndv_l.syn", and "[L-HV]: ndv_l.syn" bands.
  4. Click "New" to start a new display window.
  5. Click "Load RGB" to display the HH band in red, VV in green, and HV in blue.
  6. Adjust the stretch as desired (Gaussian and Square Root both work well on all three bands).
  7. Display the other synthesized bands as desired.

The color variations in the images are caused by variations in the radar reflectivity of the surfaces. The bright areas in the sand dunes are caused by scattering of the radar waves by vegetation (mesquite bushes). The alluvial fans show variations in surface texture due to age and composition of the rock materials.


Define ROIs for Polarization Signatures

Polarimetric SAR Processing and Analysis Using ENVI - facegis.comPolarization signatures can be extracted from the SIR-C compressed scattering matrix for a Region of Interest (ROI) or a single pixel in a polarimetric radar image. ROIs are defined by selecting pixels or by drawing lines or polygons within an image.

  1. Select Functions->Region of Interest->Define Region of Interest in the window containing the grayscale L-TP image.

The Region of Interest Controls dialog appears.

Four ROIs were previously defined and saved for use in extracting polarization signatures for the purposes of this tutorial.

  1. Restore the pre-saved ROIs by selecting File->Restore ROIs and selecting the filename POL_SIG.ROI .

A dialog box will appear stating that the regions were restored.

  1. Click "OK".

Regions named veg , fan , sand , and desert pvt will appear in the "Available Regions of Interest" list and will be drawn in the image window.

Regions can be drawn in both the image and zoom windows and can consist of any combination of polygons, lines, and pixels.

You can also draw your own Regions of Interest using ENVI's standardized ROI tools.

  1. Select the type of region to draw by clicking in the box next to "Polygon", "Polyline", or "Point".
  2. Draw polygons by clicking the left mouse button to select the endpoints of line segments or holding down the mouse button and moving the cursor for continuous drawing.
  3. Click the right mouse button once to close the polygon and a second time to accept the polygon
  4. Polylines are drawn in the same manner as polygons.
  5. Click the left mouse button to define the line endpoints and click the right button to end the polyline and again to accept the polyline
  6. Point mode is used to select individual pixels.
  7. Click the left mouse button to add the pixel currently under the cursor to the ROI.

Multiple polygons, lines, and pixels may be selected for each ROI.

  1. Click "New Region" to define another ROI, enter a name, and choose a color.
  2. Draw a second ROI.

ROIs can be saved to a file and restored at a later time by choosing File->Save ROI in the ROI Controls dialog.


Extract Polarization Signatures

Polarization signatures are 3-D representations of the complete radar scattering characteristics of the surface for a pixel or average of pixels. They show the backscatter response at all combinations of transmit and receive polarizations and are represented as either co-polarized or cross-polarized. Co-polarized signatures have the same transmit and receive polarizations. Cross-polarized signatures have orthogonal transmit and receive polarizations. Polarization signatures are extracted from the compressed scattering matrix data using the ROIs for pixel locations. Polarization signatures are displayed in viewer windows; the figure below shows an example.

Polarimetric SAR Processing and Analysis Using ENVI - facegis.com

Figure 5: Polarization Signature Viewer.

To extract your own polarization signatures:

  1. Select Radar Tools->Polarimetric Tools->Extract polarization Signatures->SIR-C.

The filename NDV_L.CDP should appear in the dialog.

  • If not, click "Open File" and select the filename.
  • Click "OK".

The Polsig Parameters dialog will appear.

  1. Select the four ROIs-- veg , fan , sand , and desert pvt --by clicking in the box next to each name or by clicking "Select All".
  2. Click "Memory" and click "OK".

Four Polarization Signature Viewer dialogs will appear, one for each ROI. The polarization signatures are displayed as both 3-D wire mesh surface plots and as 2-D gray scale images.

  • The X and Y axes represent ellipticity and orientation angles and the vertical axis can be selectively plotted as intensity, normalized intensity, or dB by selecting the desired option from the Polsig_Data pulldown menu.
  • Select Options->Show Statistics in the Polarization Signature Viewer window to display polarization signature statistics.
  • Change the precision of the Z-axis values by entering 4 in the "Z Prec" field and clicking "Apply".

Notice the range of intensity values for the different surfaces. The smoother surfaces-- sand and desert pvt --have low Z values. The rough surfaces-- fan and veg --have higher Z values. The minimum intensity indicates the "pedestal height" of the polarization signature. The rougher surfaces have more multiple scattering and therefore have higher pedestal heights than the smoother surfaces. The shape of the signature also indicates the scattering characteristics. Signatures with a peak in the middle show a Bragg-type (resonance) scattering mechanism.

  1. Change the Z axis by selecting Polsig_Data->Normalized from the pulldown menu.

This normalizes the signature by dividing by its maximum and plots it between 0 and 1. This representation shows the difference in pedestal heights and shapes better, but removes the absolute intensity differences.

  • Alternately select Polsig_Data->Co-Pol and Cross-Pol to toggle between co-polarized and cross-polarized signatures.
  • Use the left mouse button to drag a 2-D cursor on the polarization signature image on the right side of the plot. Note the corresponding "3-D" Cursor in the polarization plot.
  • The polarization signatures can be annotated by selecting the Options->Annotate Polsig.
  • The signatures can be output by selecting File->Output in the Polarization Signature Viewer window.
  • When you have finished examining the polarization signatures, select File->Cancel All to close all signature windows.

Use Adaptive Filters

Adaptive filters are used to reduce the speckle noise in a radar image while preserving the texture information. Statistics are calculated for each kernel and used as input into the filter allowing it to adapt to different textures within the image.

  1. Select Radar Tools->Adaptive Filters->Gamma.

The Gamma Filter Input File dialog will appear with a list of open files. The filters can be run on an entire file or an individual band.

  1. Click on the arrow toggle button next to the label "Select By File" to toggle to "Select By Band" and list the bands in the files.
  2. Select the band "[L-HH]:NDV_L.SYN" and click "OK".

The Gamma Filter Parameters dialog will appear.

  1. Use a filter size of 3 and the "default number of looks" of 1 and output the result to memory by selecting the "Memory" toggle button.
  2. Click "OK".

The resulting image name will appear in the Available Bands List as "Gamma ([L-HH]: NDV_L.SYN)".

Display the Filter Result

  1. Display the result of the filter by selecting the "Gray Scale" toggle button and selecting the band name in the Available Bands List.
  2. Click "New" to start a new display window and click "Load Band".
  3. Select Functions->Display Enhancements->Default (quick)->Quick Square Root in the Main window to perform a Square Root stretch on the image.

You can use dynamic overlays to compare the results of the Gamma filter to the original image.

  1. Display the original band by selecting "[L-HH]: NDV_L.SYN" in the Available Bands List, clicking "New" and "Load Band".
  2. Perform a quick Square Root stretch on this image as well.
  3. Select Functions->Link->Link Displays.
  4. The Link Displays dialog will appear.
  5. Make sure that "Yes" appears only next to the names of the two displays that contain the Gamma filtered and original images and click "OK".

Dynamic overlay is activated automatically when the windows are linked.

  1. Click the left mouse button in either image to cause a small portion of the second linked image (the overlay) to appear in the first image (the base).

The overlay is active in both windows simultaneously and in the Zoom window.

  1. Move the overlay by clicking and holding the left mouse button and moving the cursor.
  2. Change the size of the overlay by pressing the middle mouse button and dragging the corner of the overlay to the desired location.
  3. Compare the Gamma Filter image to the original data.
  4. Turn the dynamic overlay feature off by selecting Functions->Link->Unlink Displays or Dynamic Overlay Off in the Main window.

Perform Slant-to-Ground Range Transformation

Slant range radar data have a geometric distortion in the range direction. The true or ground range pixel sizes vary across the range direction because of the changing incidence angles. This geometric distortion is corrected by resampling the slant range data to create ground range pixels that are a fixed size. The slant-to-ground range transformation requires information about the instrument orientation. For SIR-C data, the necessary information is found in the CEOS header.

  1. Select Radar Tools->Slant to Ground Range->SIR-C.
  2. When the Enter SIR-C Parameters Filename dialog appears, select the file NDV_L.CDP .

The Slant Range Correction Input File dialog will appear.

  1. Select the file NDV_L.SYN and click "OK".

The Slant to Ground Range Correction Dialog will appear and all of the pertinent information will be filled in from the CEOS header in the .CDP file.

  1. Enter 13.32 in the "Output pixel size" field to generate square ground-range pixels.
  2. Select "Bilinear" as the resampling option, enter NDV_GR.IMG as the output filename, and click "OK".

The input image is resampled to 1152 13.32m sized square pixels.

  1. Display this image and compare it with the slant range image.

Use Texture Analysis

Texture is the measure of the spatial variation in the grey levels in the image as a function of scale. It is calculated within a processing window of user-selected size. The texture measures demonstrated in this tutorial are Occurrence Measures, including Data Range, Mean, Variance, Entropy, and Skewness. These terms are explained in the ENVI User's Guide and on-line help. Texture is best calculated for radar data that has not had any resampling or filtering applied.

  1. Select Radar Tools->Texture Filters->Occurrence Measures.
  2. When the Texture Input File dialog appears, click the "Select by Band" toggle button and select the band "[L-HH]: NDV_L.SYN" and click OK.
  3. In the Occurrence Texture Parameters dialog, deselect all of the Textures to Compute except for Data Range, change the Processing Window Rows and Cols to 7 x 7, enter an output filename NDV_HH.TEX, and click OK.

An ENVI Status Window will appear and after a short wait the new band will be listed in the Available Bands List

Create Color-coded Texture Map

Display the resulting Data Range texture image by clicking on the band name Data Range:[L-HH] in the Available Bands List and clicking "Load Band".

  1. Perform a quick square root stretch on the image.
  2. Select Functions->Color Mapping->Density Slice in the Main Image display.
  3. Apply the default ranges by clicking on Apply. Examine this image with respect to the original data using image linking and dynamic overlays.
  4. Now select Functions->Interactive Analysis->Cursor Location/Value in the Main window and look at the Data Range values while roaming around the image.
  5. Try creating your own density sliced image or use the predetermined color-coded density slice TEXTURE.DSR saved for your use by selecting File->Restore in the Density Slice dialog, selecting the file TEXTURE.DSR and clicking Open, and then Apply.

This density slice shows the various textures in the image in distinct colors.

Display and density slice some of the other texture measures and use dynamic overlays to compare to each other and the original data.


Create an Image-Map for Output

  1. With the Data Range texture measure displayed, add a border around the image to place a key and text annotation.
  2. Select Functions->Overlays->Annotation in the main window.
  3. When the Annotation dialog appears, select Options->Set Display Borders.
  4. Enter 100 in the upper text boxes, select a border color of White, and click "OK".

This will make a 100 pixel white border at the top of the image.

  1. Position the Main Image indicator box in the Scroll window at the top of the image to display the portion of the image with border.
  2. Place a title at the top of the image by entering a title into the text field in the center of the dialog.
  3. Enter 16 in the "Size" field to change the font size and change the color to Black.
  4. Place the title on the image by using the left mouse button to position it and clicking the right button to fix the location.

Multiple text items can be placed on the image in this manner and the font size, type, color, and thickness can be changed.

  1. Put a color table ramp in the top border of the image for a color-code key by selecting Object->Color Ramp in the Annotation dialog.
  2. Enter "Min" and "Max" values of 0 and 255 respectively, set "Inc" to 4, and the Font size to 14 to annotate the Color Ramp.
  3. Position the color ramp using the left mouse button and fix the location by clicking the right mouse button.
  4. Add any other objects desired to the image using the annotation dialog.
  5. Save the image to a PostScript file by selecting Functions->Output Display->Postscript File.
  6. Enter an output filename or let the name default to ndv_hh.ps . and click "OK".
  7. Optionally, output directly to your printer by selecting Functions->Output Display->Printer.

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Figure 6: Color-coded texture image.

Exit ENVI

When you have finished your ENVI session, click "Quit" or "Exit" on the ENVI Main Menu.

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