Calibration: Beta0, Sigma0, Gamma0 and Local Incidence Angle

Dear Mr. @mengdahl,
many thanks for your reply.
It’s hard for me to see where angles are involved in the Calibration equation shown by the SNAP Help (see the image below):

Could you please help me to find (in the Calibration formula for S-1) the reference to Incidence Angle?

BR