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ENVI Classic Tutorial
Polarimetric SAR
Processing and Analysis
Polarimetric SAR Processing and Analysis 2
Files Used in this Tutorial 2
Background: SIR-C and SAR 2
Prepare SIR-C Data 3
Optional: Read a SIR-C CEOS Data Tape 3
Optional: Multilook SIR-C Data 3
Synthesize Images 5
Default Polarization Combinations 5
Other Polarization Combinations 5Display Images 6
Define ROIs for Polarization Signatures 7
Extract Polarization Signatures 8
Adaptive Filters 10
Slant-to-Ground Range Transformation 11
Texture Analysis 13
Image-Map Output 13
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Polarimetric SAR Processing and Analysis
This tutorial demonstrates the use of ENVI Classics tools for analyzing polarimetric synthetic
aperture radar (SAR) data. You will multilook Spaceborne Imaging Radar-C (SIR-C) from Death
Valley, California; synthesize images, define ROIs for (and extract) polarization signatures, use
adaptive filters, perform slant-to-ground range transformation, use texture analysis, and create an output
image-map.
Files Used in this Tutorial
Download data files from the Exelis website.
File Description
ndv_l.cdp L-band SIR-C subset in ENVI Classic compressed data product (.cdp) format
pol_sig.roi Region of interest (ROI) file
Background SIR C and SAR
SIR-C is a polarimetric SAR instrument 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 October1994 (SRL-2), collecting high-quality SAR data over many sites around
the world. (A second radar system, XSAR, was also flown on this mission, but these data are neither
discussed nor processed here.) Additional information about SIR-C is available on the NASA/JPL
Imaging Radar Home Page at http://southport.jpl.nasa.gov/.
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Prepare 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 were preprocessed by reading and subsetting from
tape andmultilooking(averaging) to 13 m s quare pixels. The data are provided in ENVI Classic
compressed data product (.cdp) format. This non-image format is similar to the tape format and cannot
be viewed until images are synthesizedfor specific polarizations.
The first two functions described in this examplereading the data tape and multilookingwere already
applied to the SIR-C data. The steps are provided here only for completeness if you want to learn more
about the processes. Skip to "Synthesize Images" on page 5 if you are not interested in data preparation.
Optional Read a SIR C CEOS Data Tape
1. From the ENVI Classic main menu bar, select File > Tape Utilities > Read Known TapeFormats > SIR-C CEOS. The SIRC Format - Load Tape dialog appears.
2. Enter the tape device name and accept the default record size of 65,536. ClickOK. The tape is
scanned to determine what SIR-C files it contains. A dialog appears to let you select the desired
datasets. By default, ENVI Classic reads all of the data files on the tape.
3. If you do not want to read all of the data files, c lick Clear, then select the check box next to each
desired file. ClickOK.
4. You can independently subset and multilook the selected data files as they are being read from
tape. However, you should perform multilooking on disk (unless you have insufficient disk space)
as this function is extremely slow from tape.
5. Select a filename, then clickSpatial SubsetorMulti-Lookto enter parameters for the data file.Enter an output filename. Each input file must have an output filename. By convention, the output
filenames should take the formfilename_c.cdpandfilename_l.cdpfor the C- and L-
bands, respec tively. The SIR-C data are read from the tape, and one compressed scattering matrix
output file is created for selected each dataset.
Optional Multilook SIR C Data
Multilooking is a method for reducing speckle noise in SAR data and for changing the size of a SAR
file. You can multilook SIR-C data 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 with 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 forcompleteness.
1. From the ENVI Classic main menu bar, select Radar > Polarimetric Tools > Multilook
Compressed Data > SIR-C Multilook. An Input Data Product Files dialog appears.
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2. ClickOpen File and select an input file. ENVI Classic detects whether the file contains L- or C-
band data and displays the filename in the appropriate field of the dialog. ClickOK.
3. Select the file to multilook by selecting the check box next to the name. You can select multiple
files.
4. Enter any one of three valuesnumber of looks, number of pixels, or pixel sizeand the other
two are calculated automatically. Integer and floating-point number of looks are supported.
5. Enter the desiredSamples(range) and Lines(azimuth) values.
6. Enter a base filename and clickOK.
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Synthesize Images
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 with any transmit
and receive polarization combinations you want.
1. From the ENVI Classic main menu bar, select Radar > Polarimetric Tools > Synthesize SIR-C
Data. An Input Product Data Files dialog appears.
2. ClickOpen File. A file selection dialog appears.
3. Selectndv_l.cdp. ClickOpen. When the filename appears in the Selected Files L: field, click
OK. The Synthesize Parameters dialog appears.
Default Polarization Combinations
Four standard transmit/receive polarization combinationsHH, VV, HV, and TPare listed in the
Select Bands to Synthesize list of the Synthesize Parameters dialog. By default, all of these bands are
selected to be synthesized.
1. Enterndv_l.synin the Enter Output Filename field.
2. Click the Output Data Type drop-down list and selectByte. This scales the output data to byte
values. (If you will be performing quantitative analysis, the output should remain in floating-point
format.) ClickOK. After processing is complete, four bands corresponding to the four
polarization combinations are added to the Available Bands List.
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. From the ENVI Classic main menu bar, select Radar > Polarimetric Tools > Synthesize SIR-C
Data. The file ndv_l.cdpshould still appear in the Selected Files field. ClickOK. The
Synthesize Parameters dialog appears.
2. Enter-45in both the Transmit EllipandReceive Ellipfields and135in the Transmit Orien and
Receive Orienfields.
3. ClickAdd Combination. This will produce a right-hand circular polarization image.
4. Enter0 in both the Transmit EllipandReceive Ellipfields and 30in the Transmit Orien and
Receive Orienfields.
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5. ClickAdd Combination. This will produce a linear polarization with an orientation angle of 30
degrees.
6. ClickClear under the list of polarization combinations to turn off synthesis of the standard
polarization bands, which have alrea dy been generated.
7. Select the Yes radio button forOutput in dB?This will produce images that are in decibels with
values typically between 50 and 0.
8. In the Enter Output Filename field, enterndv_l2.synand clickOK. After processing is
complete, two bands corresponding to the polarization combinations are added to the Available
Bands List.
Display Images
1. In the Available Bands List, select[L-TP]underndv_l.synand clickLoad Band. The SIR-
C, L-band, total-power image appears in a new display group.
2. From the Display group menu bar, select Enhance > Interactive Stretching. A histogram plot
window appears, which shows the current stretch (between the vertical dotted lines on the input
histogram) and the corresponding DN values in the text fields.
3. Drag the dotted vertical lines to change the stretch, or enter the desired DN values into the
appropriate fields.
4. Enter5 in the left Stretchfield and95in the right field.
5. From the histogram menu bar, select Stretch Type > Gaussian. ClickApply. A Gaussian stretch
is applied with a 5% low and high cutoff.
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6. Generate and compare linear and square-root stretches.
7. To display a color composite, select theRGB Color radio button in the Available Bands List.
Select [L-HH], [L-VV], and[L-HV]in sequential order underndv_l.syn.
8. ClickDisplay #1and selectNew Display. ClickLoad RGBto display the HH band in red, VV
in green, and HV in blue. 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.
9. Adjust the stretch as desired (Gaussian and square-root stretches work well on all three bands).
10. Close the histogram plot window and Display #2 when you are finished. Keep Display #1 open for
later exercises.
Define ROIs for Polarization Signatures
You can extract polarization signatures from a SIR-C compressed scattering matrix for a region of
interest (ROI) or a single pixel in a polarimetric radar image. Define ROIs by selecting pixels or by
drawing lines or polygons within an image.
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1. From the Display group menu bar, select Overlay > Region of Interest. An ROI Tool dialog
appears.
2. Four ROIs we re previously defined and saved for use in extracting polarization signatures for this
tutorial. From the ROI Tool dialog menu bar, selectFile > Restore ROIs. A file selection dialogappears.
3. Selectpol_sig.roi. A dialog box appears, stating that the regions were restored. ClickOK.
4. Regions namedveg, fan, sand, anddesert pvtappear in the table in the ROI Tool and are
drawn in the display group.
5. To draw your own ROI, selectROI_Type > Polygon, Polyline, orPointfrom the ROI Tool menu
bar.
6. ClickNew Region, enter a name, and choose a color.
l Draw polygons by clicking the left mouse button in the display group to select the endpoints
of line segments, or by holding down the left mouse button and moving the cursor forcontinuous drawing. Click the right mouse button once to close the polygon and a second
time to accept the polygon.
l Draw polylines in the same manner as polygons. Click the left mouse button to define the
line endpoints and click the right button to end the polyline and a second time to accept the
polyline.
l Point mode is used to selec t individual pixels. Click the left mouse button to add the pixel
currently under the cursor to the ROI.
l You can se lect multiple polygons, lines, and pixels for each ROI.
7. Repeat Step 6 to draw a second ROI. You can save the ROIs to a file and restore them later by
selectingFile > Save ROI from the ROI Tool dialog menu bar.
Extract Polarization Signatures
Polarization signatures are 3D 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 rec eive polarizations and are represented as either co-polarized or cross-polarized. Co-
polarized signatures have the same transmit and receive polariza tions. 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
dialogs, as shown on the next page. To extract your own polarization signatures, perform the following
steps.
1. From the ENVI Classic main menu bar, select Radar > Polarimetric Tools > Extract
Polarization Signatures > SIR-C. The filename ndv_l.cdpshould appear in the Input Data
Product Files dialog. If not, clickOpen File and select this file. ClickOK. The Polsig Parameters
dialog appears.
2. Select the four ROIs (veg, fan, sand, anddesert pvt) by clickingSelect All Items.
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3. Select the Memoryradio button and clickOK. Four Polarization Signature Viewer dialogs
appear, one for each ROI. The polarization signatures are displayed as 3D wire mesh surface
plots and a s 2D gray scale images. The X and Y axes represe nt ellipticity and orientation angles,
respectively. You can selectively plot the vertical axis as intensity, normalized intensity, or dB byselecting Polsig_Data from the Polarization Signature Viewer dialog menu bar.
4. Polarization signature statistics appear at the bottom of each Polarization Signature Viewer
dialog. 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 heightof the polarization signature. The rougher
surfaces have more multiple scattering and therefore 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.
5. In any given Polarization Signature Viewer dialog, c hange the Z-axis by selectingPolsig_Data >
Normalized from the Polarization Signature Viewer dialog menu bar. This normalizes the
signature by dividing by its maximum; the signature is plotted between 0 and 1. Thisrepresentation shows the difference in pedestal heights a nd shapes better, but it removes the
absolute intensity differences.
6. Alternately, select Polsig_Data > Co-Pol andCross-Polto toggle between co-polarized and
cross-polarized signatures.
7. Use the left mouse button to drag a 2D cursor on the polarization signature image on the right side
of the plot. Note the corresponding 3D cursor in the polarization plot.
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8. Click-and-drag any axis to rotate the polarization signature.
9. You can optionally output the signatures to a file or printer by selectingFile > Save Plot As or
File > Printfrom the Polarization Signature Viewer dialog menu bar.
10. Close the Polarization Signature Viewer and ROI Tool dialogs when you are finished.
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 the filter
to adapt to different textures within the image.
1. From the ENVI Classic main menu bar, select Radar > Adaptive Filters > Gamma. A Gamma
Filter Input File dialog appears with a list of open files. You can apply a filter to an entire file or
to an individual band.
2. In the Gamma Filter Input File dialog, click theSelect bytoggle button to choose Band.
3. Select[L-HH]underndv_l.synand clickOK. The Gamma Filter Parameters dialog appears.
4. Accept the default values, and select the Memoryradio button. ClickOK.
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5. In the Available Bands List, clickDisplay #1and selectNew Display. Select the Gray Scale
radio button, select the new band name (Gamma), and clickLoad Band.
6. From the Display group menu bar, select Enhance > [Image] Square Root.
7. In the Available Bands List, clickDisplay #2and selectDisplay #1. Select[L-HH]underndv_
l.syn, and clickLoad Band.
8. From the Display #1 menu bar, selectEnhance > [Image] Square Root.
9. From any Display group menu bar, select Tools > Link > Link Displays. The Link Displays
dialog appears. ClickOKto link the gamma-filtered L-HH image (Display #2) with the original
L-HH image (Display #1).
10. Click in an Image window to toggle between the two images, using the dynamic overlay feature.
The figure below shows a portion of the original image (left) and the gamma-filtered image
(right).
11. Close Display #2 when you are finished. Leave Display #1 (ndv_l.syn) open for the nextexercise.
Slant to Ground Range Transformation
A radar s ystem looks to the side and records the locations of objects using the distance from the sensor
to the object along the line of sight, rather than along the surface. An image collected using this
geometry is referred to as a slant range image. Slant range radar data have a systematic geometric
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distortion in the range direction. The true, orground range, pixel sizes vary across the range direction
because of the changing incident angles. This makes the image appear compressed in the near range,
relative to what it would look like if all of the pixels covered the same area on the ground.
Slant-to-ground range correction for SIR-C is performed on synthesized images. In other words, thecorrection is not performed on the entire SIR-C compressed data product file. However, this file does
store the required information in the CEOS header about the sensor orientation.
Preview CEOS Header
1. From the ENVI Classic main menu bar, select Radar > Open/Prepare Radar File > View
Generic CEOS Header. A file selection dialog appears. You must select the original
unsynthesized data file from which to extract the necessary information.
2. Selectndv_l.cdpand clickOpen. A CEOS Header Report dialog appears. Scroll down and
note that the line spacing (azimuth direction) is 5.2 m, while the pixel spacing (slant range
direction) is 13.32 m. Close the CEOS Header Report dialog when you are finished reviewing it.
Next, you will use the Slant-to-Ground-Range function to resa mple the image to square 13.32 mpixels, thus removing slant range geometric distortion.
Resample Image
1. From the ENVI Classic main menu bar, select Radar > Slant to Ground Range > SIR-C . A file
selection dialog appears.
2. Selectndv_l.cdpand clickOpen. The Slant Range Correction Input File dialog appears.
3. Selectndv_l.synand clickOK. The Slant to Ground Range Correction Dialog appears. ENVI
Classic automatically populates the Instrument height (km), Near range distance (km), and Slant
range pixel size (m) fields with information from the CEOS header.
4. Enter13.32in the Output pixel size (m)field to generate square ground-range pixels.
5. From the Resampling Methoddrop-down list, select Bilinear.
6. In the Enter Output Filename field, enterndv_gr.img. ClickOK. The input image is
resampled to square 13.32 m pixels. Four new bands appear in the Available Bands List. Band 1
of the resampled image corresponds to the L-HH band of the original, slant-range image (ndv_
l.syn), Band 2 corresponds to L-VV, etc.
7. In the Available Bands List, clickDisplay #1and selectNew Display.
8. Select a band from the resampled image and clickLoad Band. The resampled image appears in
Display #2. Make sure Display #1 (ndv_l.syn) shows the corresponding polarization band.
9. Compare the two images.
10. When you are finished comparing images, close Display #2. Keep Display #1 (ndv_l.syn)
open for the next exercise.
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Texture Analysis
Texture is a measure of the spatial variation in the gray levels in the image, as a function of scale. ENVI
Classic calculates texture based on a processing window size you specify. The texture measuresdemonstrated in this tutorial are Occurrence Measures, including data range, mean, variance, entropy,
and skewness. These terms are explained in ENVI Classic Help. Texture is best calculated for radar
data with no resampling or filtering applied.
1. From the ENVI Classic main menu bar, select Radar > Texture Filters > Occurrence
Measures. A Texture Input File dialog appears.
2. Click the Select Bytoggle button to choose Band. Select[L-HH]underndv_l.synand click
OK. An Occurrence Texture Parameters dialog appears.
3. Deselect all of the Textures to Compute options except forData Range.
4. Set the Processing Window:RowsandColsto 7 and 7.
5. In the Enter Output Filename field, enterndv_hh.texand clickOK.
Create Color Coded Texture Map
1. In the Available Bands List, clickDisplay #1and selectNew Display.
2. SelectData Range underndv_hh.texand clickLoad Band.
3. From the Display #2 menu bar, selectEnhance > [Image] Square Root.
4. From any Display group menu bar, select Tools > Link > Link Displays. The Link Displays
dialog appears. ClickOKto link the original image (Display #1) with the colored texture image
(Display #2).
5. Click in an Image window to toggle between the two images.
6. Double-click inside an Image window to display the Cursor Location/Value tool. Examine the
data values in the textured image, and compare these to the original image.
7. From the Display #2 menu bar, selectTools > Color Mapping > Density Slice. A Density Slice
Band Choice dialog appears.
8. Select the Data Range band and clickOK. A Density Slice dialog appears.
9. Accept the default ranges by clickingApply.
10. Try creating your own density-sliced image and view the results.
11. Keep Display #2 open for the next exercise.
Image Map Output
In this exercise, you will create a map of your color-coded textured image and add a border and map
key.
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1. From the Display #2 menu bar, selectOverlay > Annotation. An Annotation dialog appears.
2. From the Annotation dialog menu bar, select Options > Set Display Borders .
3. In the Display Borders dialog, enter100in the upper field, and leave the remaining fields 0.
4. ClickBorder Color and selectItems 1:20 > White. ClickOK. This adds a 100-pixel white
border at the top of the image.
5. Move the Image box in the Scroll window to the top of the image containing the border.
6. Enter a map title in the empty field in the Annotation dialog. Set theSize value to16. Click the
Color box once to select black.
7. Click in the Image window to show the map title, then move it inside the white border to the far
left. Right-click to lock the map title in place. You can place multiple text items on the image in
this manner, and you can change their font size, type, color, and thickness as desired.
8. From the Annotation dialog menu bar, select Object > Color Ramp.
9. EnterMin andMaxvalues of0 and 255respectively, setInc to 4, and set the font Size to 14to
annotate the color ramp.
10. Click in the Image window to show the map key, move it inside the white border to the far right,
then right-click to lock it in place. The following figure shows a sample map; your results may be
different.
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11. Save the image to a PostScript file by selectingFile > Save Image As > Postscript File from the
Display #2 menu bar. An Output Display to PostScript File dialog appears.
12. Leave the default values, and enter an output filename or accept the default name ofndv_
hh.ps. ClickOK. Or, output the map directly to your printer by selectingFile > Printfrom theDisplay #2 menu bar.
13. When you are finished, select File > Exit from the ENVI Classic main menu bar.
Copyright Notice:
ENVI Classic is a registered trademark of Exelis Inc.
QUAC and FLAASH are registered trademarks of Spectral Sciences, Inc.
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