Phase centre stability

Hi to all,
I need to calculate the phase center stability (1-sigma error) of an antenna. I have calculated the phase center position at different pointing angles, but I need to compute the variations of the phase center around these computed positions.

I have been studied some papers and I have seen that the phase center stability is determined during the calibration process, however I need to compute this data theoretically.

Can anyone tell me how to make this please?
Thank you so much in advance.

http://ipnpr.jpl.nasa.gov/progress_r...2-132/132H.pdf