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ENVI

Calibrate to Sigma Nought

Calibrate to Sigma Nought (σ0)

Use Sigma Nought Calibration to calibrate ERS or RADARSAT data to radar backscatter coefficient (σ0). This function requires a DEM with the same spatial dimensions and pixel size as the input radar data. The DEM image is used to accurately calculate the incidence angle over the image. The radar backscatter coefficient values are output in decibels (dB). This function works for the following radar products:

  • ERS Pulse Repetition Interval (PRI)
  • RADARSAT Georeferenced Data (Level-1)
  • SAR Georeferenced Fine (SGF)
  • SAR Extra-Fine Resolution (SGX)
  • ScanSAR Narrow (SCN)
  • ScanSAR Wide (SCW)
  • Single Look Complex (SLC)

The steps for calibrating data to sigma nought vary by product type.

See the Radar Backscatter topic for instructions on calibrating ALOS PALSAR data to sigma nought.

ERS PRI data


See ERS-1 SAR Radiometric Calibration for the method used to calibrate ERS radar data.

  1. From the Toolbox, select Radar > Sigma Nought Calibration. The Select Radar Input File dialog appears.
  2. Select an ERS data file and perform optional spatial subsetting, then click OK. The Select Input DEM File dialog appears.
  3. Select the corresponding DEM data file and click Open. The DEM file must be the same dimensions and pixel size as the input radar file.
  4. If ENVI cannot find the leader file, the Enter Leader File dialog displays. Select the file and click Open. The Sigma Nought Parameters dialog appears.
  5. The ERS Incidence Angle and ERS Absolute Calibration values will be automatically read from the leader file. If they are not found in the leader file, enter the center of image incidence angle and absolute calibration constant in the appropriate fields.
  6. Select output to File or Memory.
  7. Click OK.

RADARSAT-1


  1. From the Toolbox, select Radar > Sigma Nought Calibration. The Select Radar Input File dialog appears.
  2. ENVI reads RADARSAT-1 data in CEOS format. Select the data file (typically named dat_01.001) and perform optional spatial subsetting, then click OK. The Select Input DEM File dialog appears.
  3. Select a DEM file with the same pixel size and spatial dimensions as the RADARSAT image, and click OK. The Sigma Nought Parameters dialog appears.
  4. ENVI will automatically read the Near Range Angle and Far Range Angle from the leader file. If theses values are not found in the leader file, enter the near range angle and far range angle, in degrees, in the appropriate fields.
  5. Select output to File or Memory.
  6. Click OK.

RADARSAT-2


  1. From the menu bar, select File > Open As > Radar Sensors > RADARSAT.
  2. The Enter RADARSAT Filenames dialog appears.

  3. Select a RADARSAT-2 image file in GeoTIFF format (image_<n>.tif, one for each polarization channel). Click Open.
  4. From the Toolbox, select Radar > Sigma Nought Calibration. The Select Radar Input File dialog appears.
  5. Select the image file you just opened and perform optional Spatial Subsetting, then click OK. The Select Input DEM File dialog appears.
  6. Select a DEM file with the same pixel size and spatial dimensions as the RADARSAT image, and click OK. The Sigma Nought Parameters dialog appears.
  7. ENVI will automatically read the Near Range Angle and Far Range Angle from the product.xml file. If theses values are not found, enter the near range angle and far range angle, in degrees, in the appropriate fields.
  8. Select output to File or Memory.
  9. Click OK.



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