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 YNTHESIZE 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)
Overview of This Tutorial
This tutorial demonstrates the u 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, u adaptive filters, perform slant-to-ground range transformation, u texture analysis, and create an output image-map.
Files Ud in This Tutorial
ENVI Tutorial Data DVD: envidata\ndv_sirc
File Description
ndv_l.cdp L-band SIR-C subt in ENVI compresd data product (.cdp) formatinstructed
i Region of interest (ROI) file
Background: SIR-C/SAR
SIR-C is a polarimetric SAR instrument that us 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 cond radar system, XSAR, was also flown on this mission, but the data are neither discusd nor procesd here.) Additional information about SIR-C is available on the NASA/JPL Imaging Radar Home Page at southport.v/.
Prepare SIR-C Data
The data ud in this tutorial are a subt 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 ba of mountains. The data were preprocesd by reading and subtting from tape and multilooking (averaging) to 13 m square pixels. The data are provided in ENVI compresd 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 l
earn more about the process. Skip to the Synthesizing Images ction if you are not interested in data preparation.
Optional: Read a SIR-C CEOS Data Tape
1.From the ENVI main menu bar, lect File→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. Click OK. The tape is scanned to
determine what SIR-C files it contains. A dialog appears to let you lect the desired datats. By default, ENVI reads all of the data files on the tape.
3.If you do not want to read all of the data files, click Clear, then lect the check box next to each desired file.
Click OK.
4.You can independently subt and multilook the lected 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 click Spatial Subt or Multi-Look to 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 form filename_c.cdp and filename_l.cdp for the C- and L-bands, respectively. The SIR-C data are
read from the tape, and one compresd scattering matrix output file is created for lected each datat.
Optional: Multilook SIR-C Data
Multilooking is a method for reducing speckle noi 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 az
imuth and range resolutions. The SIR-C file ud in this tutorial was a single-look datat 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, lect Radar→Polarimetric Tools→Multilook Compresd Data→SIR-
C Multilook. An Input Data Product Files dialog appears.
2.Click Open File and lect an input file. ENVI detects whether the file contains L- or C- band data and displays
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the filename in the appropriate field of the dialog. Click OK.
3.Select the file to multilook by lecting the check box next to the name. You can lect multiple files.
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 desired Samples (range) and Lines (azimuth) values.
6.Enter a ba filename and click OK.
Synthesize Images
The SIR-C quad-polarization data provided with this tutorial and available on tape from JPL are in a non-image, compresd format. Accordingly, images of the SIR-C data must be mathematically synthesized from the compresd scattering matrix data. You can synthesize images with any transmit and receive polarization combinations you want.
1.From the ENVI main menu bar, lect Radar→Polarimetric Tools→Synthesize SIR-C Data. An Input
Product Data Files dialog appears.
2.Click Open File. A file lection dialog appears.
3.Navigate to envidata\ndv_sirc and lect ndv_l.cdp. Click Open. 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 combinations—HH, VV, HV, and TP—are listed in the Select Bands to Synthesize list of the Synthesize Parameters dialog. By default, all of the bands are lected to be synthesized.
1.Enter ndv_l.syn in the Enter Output Filename field.
2.Click the Output Data Type drop-down list and lect Byte. This scales the output data to byte values. (If you以礼待人
will be performing quantitative analysis, the output should remain in floating-point format.) Click OK. 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 ud to synthesize an image. The ellipticity angle falls between -45 and 45 degrees and determines the “fatness” of the ellip. 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 main menu bar, lect Radar→Polarimetric Tools→Synthesize SIR-C Data. The file
ndv_l.cdp should still appear in the Selected Files field. Click OK. The Synthesize Parameters dialog appears.
2.Enter -45 in both the Transmit Ellip and Receive Ellip fields and 135 in the Transmit Orien and Receive
Orien fields.
3.Click Add Combination. This will produce a right-hand circular polarization image.
4.Enter 0 in both the Transmit Ellip and Receive Ellip fields and 30 in the Transmit Orien and Receive Orien
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fields.
5.Click Add Combination. This will produce a linear polarization with an orientation angle of 30 degrees.
6.Click Clear under the list of polarization combinations to turn off synthesis of the standard polarization bands,
which have already been generated.
7.Select the Yes radio button for Output in dB? This will produce images that are in decibels with values typically
between –50 and 0.
8.In the Enter Output Filename field, enter ndv_l2.syn and click OK. 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, lect [L-TP] under ndv_l.syn and click Load Band. The SIR-C, L-band, total-
power image appears in a new display group.
2.From the Display group menu bar, lect Enhance→Interactive Stretching. A histogram plot window
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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.Enter 5 in the left Stretch field and 95 in the right field.
5.From the histogram menu bar, lect Stretch Type→Gaussian. Click Apply. A Gaussian stretch is applied小学生老舍读本
with a 5% low and high cutoff.
6.Generate and compare linear and square-root stretches.
7.To display a color composite, lect the RGB Color radio button in the Available Bands List. Select
[L-HH], [L-VV], and [L-HV] in quential order under ndv_l.syn.
8.Click Display #1 and lect New Display. Click Load RGB to display the HH band in red, VV in green, and HV
in blue. The color variations in the images are caud by variations in the radar reflectivity of the surfaces. The bright areas in the sand dunes are caud 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.Clo the histogram plot window and Display #2 when you are finished. Keep Display #1 open for later exercis.
Define ROIs for Polarization Signatures
You can extract polarization signatures from a SIR-C compresd scattering matrix for a region of interest (ROI) or a single pixel in a polarimetric radar image. Define ROIs by lecting pixels or by drawing lines or polygons within an image.
1.From the Display group menu bar, lect Overlay→Region of Interest. An ROI Tool dialog appears.
2.Four ROIs were previously defined and saved for u in extracting polarization signatures for this tutorial. From
the ROI Tool dialog menu bar, lect File→Restore ROIs. A file lection dialog appears.
3.Select i. A dialog box appears, stating that the regions were restored. Click OK.
4.Regions named veg, fan, sand, and dert pvt appear in the table in the ROI Tool and are drawn in the display
group.
5.To draw your own ROI, lect ROI_Type→Polygon, Polyline, or Point from the ROI Tool menu bar.
6.Click New Region, enter a name, and choo a color.
Draw polygons by clicking the left mou button in the display group to lect the endpoints of line
gments, or by holding down the left mou button and moving the cursor for continuous drawing. Click
the right mou button once to clo the polygon and a cond time to accept the polygon.
成就需要理论Draw polylines in the same manner as polygons. Click the left mou button to define the line endpoints
and click the right button to end the polyline and a cond time to accept the polyline.
Point mode is ud to lect individual pixels. Click the left mou button to add the pixel currently under
the cursor to the ROI.
You can lect multiple polygons, lines, and pixels for each ROI.
7.Repeat Step 6 to draw a cond ROI. You can save the ROIs to a file and restore them later by lecting File →
Save ROI from the ROI Tool dialog menu bar.
Extract Polarization Signatures
Polarization signatures are 3D reprentations of the complete radar scattering characteristics of the surface for a pixel or average of pixels. They show the backscatter respon at all combinations of transmit and receive polarizations and are reprented 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 compresd 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, lect Radar→Polarimetric Tools→Extract Polarization Signatures→
SIR-C. The filename ndv_l.cdp should appear in the Input Data Product Files dialog. If not, click Open File and lect this file. Click OK. The Polsig Parameters dialog appears.
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2.Select the four ROIs (veg, fan, sand, and dert pvt) by clicking Select All Items.
3.Select the Memory radio button and click OK. 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 reprent ellipticity and orientation angles, respectively. You can lectively plot the vertical axis as intensity, normalized intensity, or dB by lecting Polsig_Data from the Polarization Signature Viewer dialog menu bar.
