[daip] Re: Atmospheric opacity solutions with APCAL

[Amy Mioduszewski]

Dear Greg,

Eric asked me to answer your questions since I am the VLBI/APCAL
expert in the AIPS group.  The basic answer is that you should
not expect the model for the opacity to agree at elevations of
lower than ~10 degrees.  It does not surprise me at all that the
fits do not agree with points at 2 degrees elevation.  APCAL does
a simple sec(z) approximation of the atmosphere.  It is true that
the Tsys at very low elevations does approximate Tr + Tatm, but
this is not used in the model.  Without a much better approximation
for the atmosphere than sec(z), I don't know how we could use it.

Cheers,

Amy