基于ENVI的极化SAR数据处理流程介绍

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 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

ENVI Tutorial Data DVD: envidata\ndv_sirc

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 /.

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, select 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 select the desired datasets. 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 select the check box next to each desired file.

Click OK.

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 click Spatial Subset 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 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, select Radar→Polarimetric Tools→Multilook Compressed Data→SIR-

C Multilook. An Input Data Product Files dialog appears.

2.Click Open File and 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. Click OK.

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 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 base filename and click OK.