Polarimetric SAR

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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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    http://www.exelisvis.com/Learn/Resources/Tutorials.aspxhttp://southport.jpl.nasa.gov/http://www.exelisvis.com/Learn/Resources/Tutorials.aspx
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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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