At the precision needed for 3-mm observing, the ATCA antennas are not identical
in terms of their physical construction. In particular the offset
between the elevation and azimuth axis will differ between antennas.
VLBI astronomers, who have used heterogeneous arrays of antennas, have
coped with this issue for years. For the ATCA at 3mm, what is presumed to
be a difference in this offset introduces a instrumental phase error, that
needs to be corrected.