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.
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.
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.
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).
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/ .
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.
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:
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.
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.
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.
ENVI will detect whether the file contains L- or C- band data and display the file name in the appropriate field of the dialog.
Multiple files can be selected.
Both integer and floating point number of looks are supported.
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.
The Synthesize Parameters dialog appears.
Figure 1: The Synthesize Parameters dialog.
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.
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.
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.
The file NDV_L.CDP should still appear in the "Selected Files" field.
This will produce a right hand circular polarization image.
This will produce a linear polarization with an orientation angle of 30 degrees.
This will produce images that are in decibels and therefore have values typically between -50 and 0.
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.
The SIR-C L-band total power image will be displayed in a new window.
Figure 3: L-Band SIR-C Total Power Image with Gaussian stretch applied.
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.
This will perform a gaussian stretch with a 5% low and high cut-off.
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.
Polarization 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.
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.
A dialog box will appear stating that the regions were restored.
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.
Multiple polygons, lines, and pixels may be selected for each ROI.
ROIs can be saved to a file and restored at a later time by choosing File->Save ROI in the ROI Controls dialog.
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.
Figure 5: Polarization Signature Viewer.
To extract your own polarization signatures:
The filename NDV_L.CDP should appear in the dialog.
The Polsig Parameters dialog will appear.
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.
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.
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.
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.
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.
The Gamma Filter Parameters dialog will appear.
The resulting image name will appear in the Available Bands List as "Gamma ([L-HH]: NDV_L.SYN)".
You can use dynamic overlays to compare the results of the Gamma filter to the original image.
Dynamic overlay is activated automatically when the windows are linked.
The overlay is active in both windows simultaneously and in the Zoom window.
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.
The Slant Range Correction Input File dialog will appear.
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.
The input image is resampled to 1152 13.32m sized square pixels.
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.
An ENVI Status Window will appear and after a short wait the new band will be listed in the Available Bands List
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".
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.
This will make a 100 pixel white border at the top of the image.
Multiple text items can be placed on the image in this manner and the font size, type, color, and thickness can be changed.
Figure 6: Color-coded texture image.
When you have finished your ENVI session, click "Quit" or "Exit" on the ENVI Main Menu.