How the RADAR Sigma Nought calibration is calculated in ENVI ?

 

This Help Article discusses how the Radar Sigma Nought calibration is calculated in ENVI. This option can be launched:

- from the ENVI Classic menu bar by selecting:  Radar -> Calibration -> Sigma Nought

or

- by using the Radar > Sigma Nought Calibration tool in the ENVI Toolbox.

The calculation of the RADAR Sigma Nought calibration in ENVI is based on the equations available in the following document from ESA:

        https://earth.esa.int/pub/ESA_DOC/ers_sar_calibration_issue2_5f.pdf


Specifically, ENVI uses equation 1 from the above link to calibrate the data, which is the "Simplified Equation for the derivation of sigma nought in PRI products":

   

Where:

   - N is the number of pixels within the Area Of Interest (AOI) i.e. the group of pixels corresponding to the distributed target in the image,
   - i and j are the range and azimuth locations of the pixels within the distributed target containing N pixels,
   - DN_ij is the digital number corresponding to the pixel at location (i,j),
   - α is the average incidence angle within the distributed target,
   - α _ref is the reference incidence angle, i.e. 23.0 degrees.


During the Sigma Nought calibration ENVI calculates the incident angle on each image pixel. Regarding the ESA website ENVI could then based the calibration on the equation 2, which is called the "Comprehensive equation for the derivation of sigma nought in PRI products" and which is a combination of the equations 2a and 2b. Indeed the equation 2a uses the incident angle calculated on each image pixel:

   

Where:

   - A_ij is the amplitude corresponding to the pixel at location (i,j) in the product.
   - α_i is the incidence angle of a pixel at range coordinate i. It is calculated from the image geometry.


Nothing is wrong in the current ENVI process which is using a strict application of equation 1. The main difference between these two ways is that ENVI does not use the incident angle values calculated on each image pixel but an average value of the incident angle within the distributed target. One argument to use the strict application of the equation 1 is that ENVI does not have all of the necessary parameters to use the full, comprehensive equation 2 - such as the powerloss along the range direction.

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Reviewed by BC on 09/17/2014