Basic SAR Processing and Analysis - Tutorial of ENVI Software - Completely GIS, GPS, and Remote Sensing Lecture Material -
Basic SAR Processing and Analysis Using ENVI

Overview of This Tutorial

This tutorial is designed to give you a working knowledge of ENVI's basic tools for processing single-band SAR data (such as RadarSat, ERS-1, JERS-1).

Files Used in this Tutorial

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 RSAT_SUB subdirectory of the ENVIDATA directory on the ENVI TUTORIALS & DATA CD-ROM.

Required Files

The files listed below are required to run this exercise.

LEA_01.001	Radarsat Leader File
BONNRSAT.IMG	Radarsat Image Subset
BONNRSAT.HDR	ENVI Header for Above
RSI_F1.IMG	Frost Filter Result
RSI_F1.HDR	ENVI Header for Above
DSLICE.DSR	Density Slice level save file
RSI_F2.IMG	Laplacian Filter Result
RSI_F2.HDR	ENVI Header for Above
RSI_F3.IMG	Laplacian Filter result with 0.9 addback
RSI_F3.HDR	ENVI Header for Above
RSI_FUS.IMG	Simulated Fused TM and Radarsat
RSI_FUS.HDR	ENVI Header for Above
RSI_MAP.JPG	Radarsat map composition example


Concepts for processing SAR with ENVI

Most standard ENVI processing functions are inherently radar capable including all display capabilities, stretching, color manipulations, classification, registration, filters, geometric rectification, etc. Specific Radar menu items are also included under the Radar Tools menu for specific ENVI routines that are particularly useful for radar processing. Many of these can also be accessed from their functional areas on the ENVI main menu. ENVI provides standard and advanced tools for analysis of detected radar images as well as advanced SAR systems such as JPL's fully polarimetric AIRSAR and SIR-C systems. ENVI can process ERS-1/ERS-2, JERS-1, RADARSAT, SIR-C, X-SAR, and AIRSAR data as well as any other detected SAR data set. In addition, ENVI is designed to handle radar data distributed in the CEOS format, and should be able to handle data from other radar systems that distribute their data in this format.

RadarSat Data

The ENVI software completed a RADARSAT Endorsement Review on 02/29/95. BSC demonstrated ENVI's Radarsat capabilities using ENVI 2.5 on a Pentium 133 running Microsoft Windows NT 3.51. All ENVI SAR capabilities are fully cross-platform portable, running identically on supported UNIX, Intel PC, and Macintosh/Power Macintosh systems. The demonstration consisted of the following components:

  • CEOS Header Browsing
  • Ingest Verification
  • Path Image (SGF) from CD-ROM
  • Path Image - ScanSAR (SCN) from CD-ROM
  • Sample Point Target Grid (Beam 51, SGF) from 8mm Tape
  • ENVI Features Demonstration - histograms, contrast stretching, band math, filters, display, hard copy output, etc
  • Radiometric Enhancements
  • Speckle Reduction
  • Texture Analysis
  • Edge Enhancement
  • Registrations - Image-to-image, Vector, Map
  • IHS Data Merge

Radarsat's review showed that ENVI already had most of the RADARSAT Level 1 and Level 2 capabilities including the following:

Level 1

  • Existing features are all radar capable
  • Specific required features are already available in ENVI 2.0 at no extra cost
  • Speckle reduction filters - Lee, Local Sigma, Bit Errors
  • Texture Analysis - Data Range, First Moment, Second Moment, RMS
  • Edge Enhancements - high pass, laplacian, directional
  • Radiometric enhancements - linear, piecewise linear, gaussian, arbitrary, LUTs
  • Handling of 16 bit data files

Level 2

  • Image-to-Image registration
  • Image-to-Vector registration
  • Image-to- Map registration
  • Mosaicking
  • RGB to HIS conversion and data merging

Radarsat-Specific Routines added to ENVI 2.5

ENVI RADARSAT-specific ingest routines and other modifications were added for ENVI version 2.5 including:

  • RADARSAT CEOS Reader for CD-ROM, Disk, and Tape with 16 bit or 8 bit ingest
  • Compression on ingest
  • RADARSAT CEOS Header/Trailer Browser
  • Support for the following Platforms: UNIX (SUN, SGI, HP, IBM, DEC)

Intel PC (Windows 3.1, Windows NT, Windows 95), MacIntosh and Power MacIntosh

ENVI Version 2.5 was certified as Radarsat Compatible on 02/29/95 and SAR support has subsequently been improved with every version upgrade.

Single-Band SAR Processing Scenario

This section of the tutorial describes a typical single-band SAR processing scenario from data input through processing and analysis, to publication-quality and/or map output. Data used for the example are a subset of a Radarsat 1 Path Image, Fine Beam 2, December 17, 1995, Bonn, Germany.

Read CEOS Data, Display Data, and Review Quality

ENVI provides the tools to read Generic CEOS data tapes and Radarsat Data from both Tape and CD-ROM.

  • For tape reading, select Radar Tools->Read CEOS Data->Read Radarsat CEOS Tape
  • To read original RadarSat data from disk or CD select Radar Tools->Read CEOS Data->Read Radarsat Data
  • For the purposes of this tutorial, a Radarsat image subset has already been extracted. From the ENVI Main Menu, select File->Open Image File and choose the file BONNRSAT.IMG by clicking on the filename then Open.
  • Load the image into an ENVI display window by choosing the band name in the Available Bands List and clicking on Load Image at the bottom of the dialog.

The figure below shows the image subset of the Radarsat Image of Bonn, Germany, with a 2% Linear Stretch applied. These data were acquired during the Radarsat commissioning phase and should not be used for scientific analysis or interpretation. Data are copyright, Radarsat, 1995.

Basic SAR Processing and Analysis Using ENVI -

Figure 1: Radarsat subset of Bonn, Germany with 2% linear stretch applied


Review CEOS Header

Many SAR data sets are distributed in CEOS format. ENVI provides generic tools to read CEOS headers and display CEOS header information on-screen. ENVI also has tools specifically designed to read Radarsat CEOS headers, which contain additional information.

  1. Select Radar Tools->Review CEOS Header->Radarsat.
  2. Choose the RadarSat leader file LEA_01.001 or one of the other files to extract the CEOS information and display on-screen.

Basic SAR Processing and Analysis Using ENVI -

Figure 2: CEOS Header Information.


Contrast Stretching (Square Root)

Radar data typically cover a large range of data values. As seen above, default linear stretches do not do a very good job of contrast enhancing most radar images. ENVI's square-root stretch provides a means of spreading radar data out better over a given range of grayscales than other types of stretches, thus permitting improved display of Radar images. The figure below shows a Square-Root Stretch of the Radarsat Image of Bonn Germany. Compare to the linear contrast stretch above.

  1. To perform a square-root stretch with ENVI, display the image to be stretched and click the right mouse button in the Main Image display to toggle the Functions menu and select Functions->Display Enhancements->Default (quick) Stretches->Quick Square-Root.

The stretch will be applied to the data based on the statistics of the data in the Main Display window.

  1. Display the image in a new window and select Functions->Link->Link Displays to link the square-root image back to the 2% linear stretch image. Click and drag using the left mouse button to compare the two images using the dynamic overlay.

Basic SAR Processing and Analysis Using ENVI -

Figure 3: Radarsat image with square root contrast stretch applied.


Remove Speckle using Adaptive Filters

Adaptive filters provide a means of removing radar speckle from images without seriously affecting the spatial characteristics of the data. The Frost Filtered Image shown in below is a considerable improvement over the unfiltered data. The Frost filter, an exponentially damped circularly symmetric filter that uses local statistics, is used to reduce speckle while preserving edges in the data. The pixel being filtered is replaced with a value calculated based on the distance from the filter center, the damping factor, and the local variance. Compare to the images above and to the dynamic overlay image below. To perform the Frost Filter on the Radarsat data:

  1. Select Radar Tools->Adaptive Filters->Frost.
  2. Choose BONNRSAT.IMG as the input image and use the default filter size (3x3) and Damping factor (1.0). Enter an output filename and click OK, or load the presaved file RSAT_F1.IMG to a second display using a square root stretch.
  3. Select Functions->Link->Link Displays to link the Frost filtered image back to the 2% linear stretch image. Click and drag using the left mouse button to compare the two images using the dynamic overlay

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Figure 4: Frost Filter Result.

Density Slice

Density slicing provides a means of visually enhancing radar differences based on image brightness. The Density Sliced Image below has four levels, with higher radar backscatter in the warmer colors.

  1. In the Frost Filtered image display, click the right mouse button in the Main Image display to toggle the Functions menu and choose Functions->Color Mapping->Density Slice.
  2. Enter the desired ranges and colors in the Density Slice dialog by choosing File->Restore Ranges, selecting the file DSLICE.DSR and then Open, and then clicking "Apply" in the Density Slice dialog to density slice the image.
  3. Use Dynamic Overlays to compare to the grayscale images above.

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Figure 5: Density sliced Radarsat image.

Edge Enhancement

A Laplacian Filter can be used to enhance edges in SAR data and other data types. This is a convolution filter with a kernel (for a 5 x 5 filter) of:

0 0 -1 0 0
0 -1 -2 -1 0
-1 -2 16 -2 -1
0 -1 -2 -1 0
0 0 -1 0 0
  1. From the ENVI Main Menu, select Filter->Convolution->Laplacian. Use BONNRSAT.IMG and the default parameters to create a new output file or view the presaved file RSI_F2.DAT.

Applying the kernel in this fashion strongly enhances the edges and causes loss of most of the radiometric information.

  1. Choose the filter as above, but enter the value of 0.9 for the Add Back Value. Use BONNRSAT.IMG and the default parameters to create a new output file or view the presaved file RSI_F3.DAT.

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Figure 6: Edge Enhancement results; left image is Laplacian, right is with 0.9 addback.

  1. Compare the result to the filtered image and to the original data using dynamic overlays


Data Fusion

One of the strengths of SAR data is its highly complementary nature with respect to other data sets. Radar data provides a wealth of spatial information that isn't present in many other types of image data. Conversely, however, SAR data doesn't have much compositional information, which is typically expressed in multispectral optical data sets. This situation leads naturally to the use of combined SAR/Optical data.

The most common means of combining data sets is the use of Intensity, Hue, Saturation (IHS) transforms to combine a multispectral, color-composite image with a monochromatic SAR sharpening band. ENVI provides a simple tool to conduct data merging using IHS.

No optical data set was available that corresponded to the Bonn Radarsat data so you are not able to perform this function with these data. The example and figure below, however, illustrate a simulated image showing IHS-Merged Data of an unrelated Landsat Thematic Mapper data set. This image is designed to give you an idea of how a merged SAR/Optical dataset might appear. ENVI also provides a Color Normalization (Brovey) transform for data fusion.

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Figure 7: Simulated Radarsat/TM Data Fusion.

  1. Given 3-band color composite image, select Transforms->Image Sharpening-->IHS and then enter the three bands for the color composite into the Select Input RGB Input Bands dialog.
  2. Select the single band SAR image.

If the two images have the same spatial dimension, ENVI goes ahead with the fusion. If the two data sets are both georeferenced, but with different pixel sizes, ENVI resamples low resolution image to match the high resolution image, then fuses the two data sets.

Image-Map Output

The final output from any image processing within ENVI is usually a map-oriented, scaled image-map for presentation or visual analysis and interpretation. Radar data can be used in map composition like any other data set. If map registration is desired, ENVI includes full image-to-image and image-to-map registration capabilities. Please see the Map Registration tutorial or the ENVI User's Guide or on-line help for more information. ENVI also provides all of the tools to produce fully annotated publication-quality maps. This includes pixel, map, and geographic (latitude/longitude) grids; scale-bars; declination diagrams and north arrows; text and symbols; polygons, polylines, and geometric shapes (circles, rectangles); map keys and legends; and image insets. For additional information on map composition, please see the Map Composition tutorial in this volume, or the ENVI User's Guide or on-line help.

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Figure 8: Radarsat Map Output.



ENVI is fully radar capable (Radarsat Level 2 Certified); most of ENVI's routines work transparently with SAR data. ENVI also provides a series of specialized tools for analysis of both single-band and polarimetric, multifrequency Radar data. A typical scenario might consist of Reviewing the CEOS header, reading the CEOS data, displaying and contrast stretching, removing speckle using an adaptive filter, density slicing, edge enhancement, data fusion, and map composition. These tools provide end-to-end SAR processing capabilities, including tape/CD-ROM input, processing, analysis, and publication output within a single software system.