Use ALOS 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:

Where:

  • 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 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 view and uses the resolution level at which you are viewing the image. See Preview for details on the results. To preview a different area in your image, pan and zoom to the area of interest and re-enable the Preview option.
  5. Enable the Display result check box to display the output in the view when processing is complete. Otherwise, if the check box is disabled, the result can be loaded from the Data Manager.

  6. To reuse these task settings in future ENVI sessions, save them to a file. Click the down arrow and select Save Parameter Values, then specify the location and filename to save to. Note that some parameter types, such as rasters, vectors, and ROIs, will not be saved with the file. To apply the saved task settings, click the down arrow and select Restore Parameter Values, then select the file where you previously stored your settings.

  7. To run the process in the background, click the down arrow and select Run Task in the Background. If an ENVI Server has been set up on the network, the Run Task on remote ENVI Server name is also available. 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 ENVI Servers for more information.

  8. Click OK.