Use Radar Backscatter to calibrate a radar amplitude raster to sigma nought backscatter, in decibels. It currently works with ALOS-1 or ALOS-2 PALSAR Level-1.1, 1.5, 2.1, and 3.1 data.

You can also write a script to perform backscatter calibration using the RadarBackscatter task.

For ALOS-2 Level-1.5, 2.1, and 3.1 data, sigma nought is calculated as follows:

For ALOS-2 Level-1.1 data, sigma nought is calculated as follows:


  • I is the imaginary part of the value
  • Q is the real part of the value
  • calibration factor is obtained from the ALOS metadata
  • DN refers to a single pixel value

ENVI calculates sigma nought on a pixel-by-pixel basis without any ensemble-based averaging.

Reference: Japanese Aerospace Exploration Agency (JAXA), ALOS-2/PALSAR-2 Level 1.1/1.5/3.1 CEOS SAR Product Format Description, May 23, 2014.

Follow these steps:

  1. From the Toolbox, select Radar > ALOS PALSAR > Radar Backscatter.
  2. Select an ALOS PALSAR amplitude raster and perform any optional spatial subsetting, then click OK.
  3. To write the output to disk, select the File radio button and specify a filename and location. To produce output in memory only, select the Virtual radio button.

  4. Enable the Preview check box to see a preview of the settings before you click OK to process the data. The preview is calculated only on the area in the Image window. See Preview for details on the results. To preview a different area in your image, pan and zoom to the area of interest and reenable the Preview option.
  5. Enable the Display result check box to display the aggregated classification raster in the Image window when processing is complete. Otherwise, if the check box is disabled, the raster can be loaded from the Data Manager.
  6. To run the process on a local or remote ENVI Server, click the down arrow and select Run Task in the Background or Run Task on remote ENVI Server name. The ENVI Server Job Console will show the progress of the job and will provide a link to display the result when processing is complete. See the ENVI Servers topic for more information.

  7. Click OK.