3.3 Calibration
Calibration is done to relate the pixel values directly with the backscatter thus quantifying the SAR image(Ballester-berman, Lopez-sanchez, & Fortuny-guasch, 2005; Hari Shanker Srivastava et al., 2008). Calibration vector on addition as an annotation product facilitates the conversion of backscatter values of intensity into the different backscatter coefficient values of σ0, radar brightness coefficient β0 and volumetric scatter coefficient γ0(Boerner, 2012; Hsieh et al., 2011).
Where,
β0= k×DN2 (2)
where β0 is RADAR brightness coefficient, k is calibration coefficient that relates the Backscatter to pixel values using simple mathematical formula and in turn provides radiometric correction to the imagery( Srivastava et al., 2009).
σ0= β0 × sin (ix,y) (3)
Where σ0 is RADAR backscatter coefficient expressed in degree decibels (-dB). And ix,y is local incidence angle at which the RADAR waves interact with the target(Tiwari, 2019; Veci, 2016).
γ0= β0× tan (ix,y) (4)
where, γ0 is the backscatter coefficient for volumetric Scatterers.
Calibration replaces the sensor level scaling of the imagery with user defined scaling. The Sentinel-1, L-1 products provide four Look Up Tables (LUTs) corresponding to the four backscatter coefficients and their Digital Number (DN) values(Zhou, Pan, Zhang, Wei, & Han, 2017).
The LUTs apply a gain depending upon the absolute calibration constant. The following formula applies radiometric calibration(Tapete, Cigna, & Donoghue, 2016)-
Values (i) =\(\frac{\left|\text{DN}\right|^{2}}{\text{Ai}^{2}}\ \) (5)
Where Values (i) = anyone of the backscatter coefficients or their DN values and Ai = one of βi, σi