High Matt,

For most antennas, the phase versus angle w.r.t. main lobe is not
given. The same is valid for the phase center. When they give it, you
should have a means to verify it.

As long as you make your array from same antennas, phase variation for
each antenna will be the same and will cancel. Of course this is not
true when you want to make time of arrival measurements for satellites
under different elevation angle.

Another thing that might be of importance is the polarization. This
also varies with antenna orientation.

If it really matters, I would suggest that you use a EM-field solver
to find the full radiation pattern (phase, magnitude and polarization)
for your antenna. If you consider patch like antennas, a NEC based
simulator is not the best choice, probably you have to divert to
expensive commercial packages. If money is available, you may hire
somebody. The learning curve can be lengthy to get trustful results
from simulation.

Technically spoken, it can be measured also, but you need a
combination of good measuring equipment and a person that is very well
into wave propagation. I'm not deep into GPS but is it possible to
calibrate the antennas with the GPS system?

Of course this does not answer your question, but I hope it will help
you a bit in finding an answer.

Wim
PA3DJS.