AIRSAR images must be synthesized before you can use them in ENVI processing routines. Use Synthesize AIRSAR Data to synthesize standard and specific transmit and receive polarization and total power images from compressed Stokes files.

The input AIRSAR data must be in JPL Stokes matrix format (standard file extension .stk). For easier processing, name the three files with the following convention: filename_p.stk, filename_l.stk, and filename_c.stk.

You must enter the data parameters as described in Entering Missing AIRSAR Parameters).

You can generate byte images to conserve disk space when quantitative analysis is not an issue.

See the following sections:


  1. From the Toolbox, select Radar > AIRSAR > Synthesize AIRSAR Data. The Input Stokes Matrix Files dialog appears. Use this dialog to read the compressed Stokes matrix files.
  2. Click Open File. The Enter Filename dialog appears.
  3. Select a Stokes matrix filename. If the file has a valid AIRSAR header with all of the required parameters, ENVI adds it to one of the Selected Files fields in the Input Stokes Matrix File dialog (depending on whether the file is P-, L-, or C-band data). If you followed the suggested file naming convention described previously, ENVI automatically adds the correct filenames for the other two files to the appropriate fields. Otherwise, repeat the file selection process for each of the bands.
  4. If your radar data filenames do not appear in your directory, set the filename filter to *.* using the Files of Type drop-down list. To clear the names in the Input File dialog at any time, click Reset or click Cancel to quit without processing.

    If the Stokes matrix input file does not have a valid AIRSAR header, or if values are missing from the header, the AIRSAR Header Parameters dialog appears. Enter the missing AIRSAR parameters.

  5. Click OK. The Synthesize Parameters dialog appears.
  6. Select the synthesizing parameters.
  7. Select output to File or Memory.
  8. Click OK. All of the synthesized bands are placed in a single file. ENVI adds the synthesized images to the Layer Manager.

Enter Missing AIRSAR Parameters

In the AIRSAR Header Parameters dialog, all of the known parameters are listed in the dialog. The dialog often appears because JPL has changed header parameters for a particular processor version. Typically, the COMP SCALE FACTOR (also known as GENFAC) or the Band might be missing from the header.

  1. Enter the Offset to Data in bytes.
  2. Enter the COMP SCALE FACTOR. A factor of 1.0 is typically suitable.
  3. Enter the Number or Samples and Number of Lines in the corresponding fields.
  4. Select the Band to which the settings apply.
  5. Click OK. The Input Stokes Matrix Files dialog appears.

Select Synthesizing Parameters

Use the Synthesize Parameters dialog to select specific send and receive polarization combinations to synthesize into ENVI images. The standard polarizations HH, VV, HV (where H=horizontal, V=vertical), and a total power (TP) image are generated for each of the three frequencies (bands) by default. These bands appear in the Select Bands to Synthesize list.

  1. From the Select Band Combinations to Synthesize list, select the check boxes next to bands to include.
  2. Enter the transmit and receive ellipticity and orientation angles in the Transmit Ellip/Orien and Receive Ellip/Orien fields.
  3. Click Add Combination. Ellipticity values range from -45 to 45 degrees with an ellipticity of 0 producing linear polarizations. Orientation values range from 0 to 180 degrees with 0 representing horizontal and 90 degrees for vertical polarizations.
  4. Select the desired bands, C, L, and /or P by selecting the box to the left of the band name.
  5. Click Add Combination. The selected images are listed in the Additional Images field.
  6. To perform spatial subsetting of the image to synthesize directly from the Stokes matrix data, click Spatial Subset.
  7. From the Output Data Type drop-down list, select one of the following:
  • Floating Point: To output the data in floating-point. If you are creating output to a floating-point data type to mask bad data values, enter the minimum and maximum data values to use, in units of intensity, in the Intensity Min and Max fields.
  • In the Output in dB field, select Yes to output the converted image to decibels, or select No to output the converted images as intensity values.

    When creating output images in dB, the Intensity Min and Max values are still entered in units of intensity as the output is bounded by these values prior to computing dB (for example, if you bound the data by 0 to 1 then a dB image will be bounded by -Inf to 0).

    Synthesized values less than the Intensity Min are set to zero (0) and values greater than the Max are replaced by the value one (1).

  • Byte: To output the data in byte format. Use this setting when quantitative analysis is not required. In the Std Multiplier field that appears, enter the standard deviation multiplier used to calculate a minimum and maximum for scaling. The default value of 1.5 works for most AIRSAR and SIR-C scenes.
  • Some additional fields vary, depending on your selection.


van Zyl, J.J., H.A. Zebker, and C. Elachi: Imaging radar polarization signatures: theory and observation. Radio Science 22(4):pp. 529-543, 1987.

Zebker, H.A., J.J. van Zyl, and D.N. Held. Imaging radar polarimetry from wave synthesis. Journal of Geophysical Research 92(31): pp. 683-701, 1987.