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.

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.

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

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.

  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 preview the settings before processing 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. To preview a different area in your image, pan and zoom to the area of interest and re-enable the Preview option. Depending on the algorithm being used by the tool, the preview result might be different from the final result of processing on the full extent, full resolution of the input image in the following scenarios: 1) If you zoomed out of the input raster in the view by 50%, or a percentage less than 50%, ENVI uses a downsampled image at the closest resolution level to calculate the preview, or 2) If the entire image is not visible in the view, ENVI uses the subset in the viewable area of the input image to calculate the preview.
  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 next to the OK button and select Save Parameter Values, then specify the path 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 next to the OK button 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.