How to calculate phase values from SIFT / AKAZE key points for coregistration

Hi ,
I am new to SAR technology. I am working on coregistration(Terrasarx images) using SIFT/AKAZE feature extraction techniques. I extracted matched key points from master and slave images and also calculated homography/RANSAC.
How to calculate phase values? I also want to do measure the error (pixel level) and compared to Geofloki results?
can anyone help me in this regard?