Overview of This Tutorial

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ENVI Tutorial: Polarimetric SAR Processing and Analysis Table of Contents OVERVIEW OF THIS TUTORIAL.....................................................................................................................................2 Background: SIR-C/SAR......2 PREPARE SIR-C DATA................3 Optional: Read a SIR-C CEOS Data Tape.............................................................................................................3 Optional: Multilook SIR-C Data ...........................................................................................................................3 SYNTHESIZE IMAGES..................4 Default Polarization Combinations.......................................................................................................................4 Other Polarization Cos.........................................................................................................................4 Display Images..................5 Define ROIs for Polarization Signatures ...............................................................................................................6 Extract Polarization Signatures6 Adaptive Filters .................................................................................................................................................8 Slant-to-Ground Range Transformation9 Preview CEOS Header...........................9 Resample Image......... ...

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• Publié par: Difef
• Nombre de lectures: 69
• Langue: English

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             ENVI Tutorial: Polarimetric SAR Processing and Analysis                

       

 

 

 

Table of Contents O VERVIEW OF T HIS T UTORIAL 2.....................................................................................................................................   Background: SIR-C/SAR.2   P REPARE SIR-C   D ATA ........................3.......................................................................................................................   Optional: Read a SIR-C CEOS Data Tape.3   Optional: Multilook SIR-C Data.3   S YTNEHISEZ  I MAGES ..................4...............................................................................................................................   Default Polarization Combinations.4   Other Polarization Combinations.4   Display Images.5   Define ROIs for Polarization Signatures.6   Extract Polarization Signatures.6   Adaptive Filters.8   Slant-to-Ground Range Transformation.9   Preview CEOS Header..........................................................................................................................................................9   Resample Image.................................................................................................................................................................9   Texture Analysis.10   Create Color-coded Texture Map........................................................................................................................................10   Image-Map Output.11  

 

Tutorial: Polarimetric SAR Processing and Analysis

Overview of This Tutorial This tutorial demonstrates the use of ENVI’s 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 CD-ROM: Tutorial Data CD #1 Path: envidata\ndv sirc  _    Required Files File Description ndv_l.cdp  L-band SIR-C subset in ENVI compressed data product (.cdp) format pol_sig.roi  Region of interest (ROI) file

Background: SIR-C/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 October 1994 (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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Tutorial: Polarimetric SAR Processing and Analysis

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 and multilooking (averaging) to 13 m  square pixels. The data are provided in ENVI compressed data product (.cdp) format. This non-image format is similar to the tape format and cannot be viewed until images are synthesized for specific polarizations.     The first two functions described in this example—reading the data tape and multilooking—were 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 the Synthesizing Images section if you are not interested in data preparation.

Optional: Read a SIR-C CEOS Data Tape 1.From the ENVI main menu bar, selectFile → Tape Utilities → Read Known Tape Formats → 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 reads all of the data files on the tape.   3.do not want to read all of the data files, click Clear, then select the check box next to each desired file.If you 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 SubsetorMluitL-kooparameters for the data file. Enter anto enter output filename. Each input file must have an output filename. By convention, the output filenames should take the formmena.c_cdpelifandf _ands, respectively are ta ilename l.cdpfor the C- and L-b The SIR-C da . 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 for completeness.   1.From the ENVI main menu bar, selectRadar → Polarimetric Tools → Multilook Compressed Data → SIR- C Multilook. An Input Data Product Files dialog appears.    2.ClickOpen Fileand select an input file. ENVI detects whether the file contains L- or C- band data and displays the filename in the appropriate field of the dialog. ClickOK.   3.to the name. You can select multiple files.Select the file to multilook by selecting the check box next   4.Enter any one of three values—number of looks, number of pixels, or pixel size—and the other two are calculated automatically. Integer and floating-point number of looks are supported.   5.Enter the desiredSmalpse(range) andLines(azimuth) values.   6.Enter a base filename and clickOK.

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Tutorial: Polarimetric SAR Processing and Analysis

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.the ENVI main menu bar, selectFrom Radar → Polarimetric Tools → Synthesize SIR-C Data. An Input Product Data Files dialog appears.   2.ClickOpen File. A file selection dialog appears.   3.Navigate toevndita\adn_visrcand selectpdc.l_vnd. ClickOpen. When the filename appears in the Selected Files L: field, clickOK. The Synthesize Parameters dialog appears.

Default Polarization Combinations Four standard transmit/receive polarization combinations—HH, VV, HV, and TP—are 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.Enterys.ln_vdnin theEnter Output Filenamefield.   2.Click theOutput Data Typedrop-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.the ENVI main menu bar, selectFrom 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 theTransmit EllipandReceive Ellipfields and135in theTransmit Orienandeceive R Orien fie lds.   3.ClickAdd Combinationproduce a right-hand circular polarization image.. This will   4.Enter0in both theTransmit EllipandReceive Ellipfields and30in theTransmit OrienandReceive Orien  fields.   5.ClickAdd Combination. This will produce a linear polarization with an orientation angle of 30 degrees.   6.ClickClearunder the list of polarization combinations to turn off synthesis of the standard polarization bands, which have already been generated.   7.Select theYesin dB? This will produce images that are in decibels with values typicallyradio button for Output between –50 and 0.   8.In theEnter Output Filename field, enterys.2nndv_land clickOK. After processing is complete, two bands corresponding to the polarization combinations are added to the Available Bands List.

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Tutorial: Polarimetric SAR Processing and Analysis

Display Images _ 1.In the Available Bands List, selectL[-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, selectnhcaeEn → 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.Enter5in the leftrtSetchfield and95in the right field.   5.From the histogram menu bar, selectStretch Type → ssainauG. ClickApply. A Gaussian stretch is applied with a 5% low and high cutoff.  

    6.Generate and compare linear and square-root stretches.   7.To display a color composite, select theRGB Colorradio button in the Available Bands List. Select [L-HH],]VV-L[, andH-L[V]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.

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Tutorial: Polarimetric SAR Processing and Analysis

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.   1.From the Display group menu bar, selectOyalrev → Region of Interest. An ROI Tool dialog appears.   2.Four ROIs were 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 dialog appears.   3.Selectpioig.rol_sA dialog box appears, stating that the regions were restored. Click. OK.   4.Regions named veg, fan, sand, and desert pvt appear in the table in the ROI Tool and are drawn in the display group.    5.To draw your own ROI, selectT_IOepyR → nPogoly,Pylolnie, orPointfrom the ROI Tool menu bar.   6.ClickNew Region, enter a name, and choose a color.   •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 for continuous drawing. Click the right mouse button once to close the polygon and a second time to accept the polygon.   •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.   •Point mode is used to select individual pixels. Click the left mouse button to add the pixel currently under the cursor to the ROI.   •You can select multiple polygons, lines, and pixels for each ROI.   7.Step 6 to draw a second ROI. You can save the ROIs to a file and restore them later by selecting FileRepeat →  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 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 dialogs, as shown on the next page. To extract your own polarization signatures, perform the following steps.   1.From the ENVI main menu bar, selectRadar → Polarimetric Tools → Extract Polarization Signatures →  SIR-C. The filenamel_vdnpdc.should appear in the Input Data Product Files dialog. If not, click Open File and select this file. ClickOK. The Polsig Parameters dialog appears.   2.Select the four ROIs (veg, fan, sand, and desert pvt) by clickingSelect All Items.   3.Select theeMomyrradio 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 as 2D gray scale images. The X and Y axes represent ellipticity and orientation angles, respectively. You can selectively plot the vertical axis as intensity, normalized intensity, or dB by selecting Polsig_Data from the Polarization Signature Viewer dialog menu bar.

 

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Tutorial: Polarimetric SAR Processing and Analysis

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, change the Z-axis by selectinggiD_taaPols → edizoNmrla  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. This representation shows the difference in pedestal heights and shapes better, but it removes the absolute intensity differences.   •Alternately, select Polsig_Data→Co-Pol and Cross-Pol to toggle between co-polarized and cross- polarized signatures.   6.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.

    7.Click-and-drag any axis to rotate the polarization signature.   8.You can optionally output the signatures to a file or printer by selectingFile → Save Plot AsorFile → Print from the Polarization Signature Viewer dialog menu bar.   9.the Polarization Signature Viewer and ROI Tool dialogs when you are finished.Close

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Tutorial: Polarimetric SAR Processing and Analysis

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 main menu bar, selectRadar → 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.the Select by toggle button to choose Band.In the Gamma Filter Input File dialog, click   3.SelectH]-HL[under._slydnvnand clickOK. The Gamma Filter Parameters dialog appears.   4.Accept the default values, and select theMeyromradio button. ClickOK.   5.In the Available Bands List, clickDisplay #1and selectNew Display. Select theGray Scaleradio button, select the new band name (ammaG), and clickLoad Band.   6.From the Display group menu bar, selectnEehanc → [Image] Square Root.   7.In the Available Bands List, clickDisplay #2and selectDisplay #1. Select[-LHH]underndv_l.nys, and clickLoad Band.   8.From the Display #1 menu bar, selecthaEnenc → [Image] Square Root.    9.From any Display group menu bar, selectTools → 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 (l.v_nsydn) open for the next exercise.   

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Tutorial: Polarimetric SAR Processing and Analysis

Slant-to-Ground Range Transformation A radar system 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 distortion in the range direction. The true, or ground  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, the correction 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.the ENVI main menu bar, selectFrom 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.Select ndv_pen Report dialog appears. Scroll down and note that the line ader. A l.cdpand clickOCEOS He 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 resample the image to square 13.32 m pixels, thus removing slant range geometric distortion.

Resample Image 3.the ENVI main menu bar, selectFrom Radar → Slant to Ground Range → SIR-C. A file selection dialog appears.     4.Selectpddvn.c_land clickOpenSlant Range Correction Input File dialog appears.. The   5.Selectndv l.synand clickOK. The Slant to Ground Range Correction Dialog appears. ENVI automatically _ populates the Instrument height (km), Near range distance (km), and Slant range pixel size (m) fields with information from the CEOS header.   6.Enter13.32in theOutput pixel size (m)field to generate square ground-range pixels.   7.From theResampling Methoddrop-down list, selectliBiarne.   8.In theEnter Output Filenamefield, enterndvgmi.rg_. 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 imageydsn._lv(n), Band 2 corresponds to L-VV, etc.   9.In the Available Bands List, clickDisplay #1and selectNew Display.   10.Select a band from the resampled image and clickLoad Band. The resampled image appears in Display #2. Make sure Display #1 (vdnnys.l_) shows the corresponding polarization band.   11.Compare the two images.   12.When you are finished comparing images, close Display #2. Keep Display #1 (.l_vdnsyn) open for the next exercise.

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