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Sigma Nought and Beta Nought image from Radar image in IMAGINE

by Technical Evangelist ‎02-29-2016 06:53 AM - edited ‎09-20-2019 11:19 AM (3,872 Views)


How to create sigma-nought and beta-nought images in IMAGINE?
Customer is currently in emergency response mode for the Red river floods, and are
experiencing problems with our Radarsat-2 crosspol calibration that seems to be
sensor related. THey can get around this by calibrating. Does IMAGINE offer sigma
or beta nought calibration for Radarsat-2 data?


 Yes, IMAGINE has tools to create sigma-nought and beta nought images from radar
image data. Run IMAGINE Help > Search and search on these two phrases:

Radar Conversions
Radiometric Terrain Correction


The link for IMAGINE 2018.x is given below:




Beta Nought

Beta Nought (radar_conversion_beta_nought_symbol) is a measure of the backscatter that is returned to the antenna from a distributed scatterer on the ground. Beta Nought is in slant range geometry. As slant-range pixels cover different areas ("footprint" on the ground) which vary with the local incidence angle (look angle + DEM), Beta Nought contains a geometric impact on the radar backscatter, for example, bright pixels in foreshortened areas.

Beta Nought is calculated from the DN values of the SAR image by a simple sensor specific transformation. A Beta Nought image is useful to compare images of different SAR sensors or different times and provides rough information about surface scattering mechanisms and surface properties.

Conversion to Beta Nought

Radar images can be converted to Beta Nought format either during Import (for ERS-1, ERS-2, and Radarsat-1 sensors) or by using the Radar Conversions module (for all other sensors).

Not all sensor imaging modes are currently supported by the Radar Conversions module.

Sigma Nought

Sigma Nought (radar_conversion_sigma_nought_symbol) is the backscatter returned to the antenna from a unit area on ground. It is corrected for the local incidence angle, and is thus in the realm of ground range. Sigma Nought is the usual measure used to evaluate the reflectivity conditions of the surface, independent from geometric effects. A digital elevation model (DEM) is therefore required to transform the DN values of the input pixel or the corresponding Beta Nought values into Sigma Nought.

Sigma Nought is often used in scientific interpretation of SAR imagery. As the pixel values are absolutely calibrated, different SAR images in Sigma Nought notation can easily be compared. Additionally, experienced interpreters are able to retrieve information about surface scattering mechanisms and surface properties. In a Sigma Nought image, the topographic impact on the backscatter is minimized. The backscatter depends on surface backscatter properties and the image looks "flat". It varies with radar wavelength and polarization.

Conversion from Beta Nought to Sigma Nought

Radar images can be converted from Beta Nought to Sigma Nought format by using the Radiometric Terrain Correction module.


A typical workflow to create a radiometrically calibrated radar image in IMAGINE would be:

  1. Read original image (Import or direct read)
  2. Radar Conversion to Beta Nought
  3. Orthorectify Beta Nought image
  4. Radiometric Terrain Correction

Dialog Description

This dialog opens when you:

  • Click Raster tab > radar_rings Radar Toolbox > Utilities > Radiometric Terrain Correction
  • Click Radar Tools tab > radar_scopeUtilities > Radiometric Terrain Correction

Input file    Type the name of the input file to be corrected in the text field.

open.png    Click to open a File Selector.

The input file must be an orthorectified radar brightness (Beta Nought) image.

Input Unit    Select the units in which pixel values are reported. The default is Decibels.

Decibels    Input pixel values are given on a logarithmic scale.

linear    Input pixel values are given on a linear scale.

DEM File     Type the name of the DEM file.

open.png    Click to open a File Selector to navigate to the file.

Elevation Unit    Select the unit you want to use to measure elevation.

meters    Click this option to use meters as the unit to measure elevation.

feet    Click this option to use feet as the unit to measure elevation.

Output file     Type the name of the radiometric corrected output file in the text field.

open.png    Click to open a File Selector.

Incidence File     Type the name of the output incidence angle file.

open.png    Click to open a File Selector.

Input Parameters    Select the input parameters.

Scene Azimuth Angle    Enter the azimuth angle (in degrees) of the input image radar scene. Values range from -90 to +90 degrees.

Azimuth Angle is the rotation angle of image margin toward North.

Positive angle values -- counterclockwise image rotation

Negative angle values -- clockwise image rotation

Center Incidence Angle    Enter the radar incidence angle (off-nadir angle) at the input image center. This value may be derived from the original image header or the data provider.

View Direction    Select the look direction of SAR sensor corresponding to input image margins (with respect to North). The look direction may be derived from the input image itself by evaluating layover or shadow effects in the image.

Additional Parameters   

Sensor Flight Height Known    Enter the sensor flight height above ground in meters, if that information is available.

Output Options   

Ignore Background in Stats    Click this checkbox to omit background values when statistics are calculated for the output image.

Output Unit    Select the units to use for the radiometrically corrected output image. The default is Decibels.

Decibels    Click to report output pixel values on a logarithmic scale.

linear    Click to report output pixel values on a linear scale.

OK    Click to run this program with the options selected and close this dialog.

Batch    Click to open Batch Command Editor to schedule one or multiple processing jobs.

Close    Click to close this dialog.

Help    Click to open this Help document.