[Difx-users] On delay modelling

[Mugundhan vijayaraghavan]

Dear All,

I have a few queries about how delay modelling is carried out in VLBI for
compensating the same.

1.) The geometric delay is calculated as tg=*b.s*, where *b *is the
baseline vector and *s *is the source vector. Lets say I have two antennas,
both located about 100 kms apart. How do standard VLBI delay modelling
software calculate this delay ? Based on some preliminary reading I
understood that the baseline distance are first calculated referenced to
the earth center, if this is done, are delays estimated assuming the earth
center to be the phase reference ? How is this earth centered reference
then transformed to the celestial frame ? because both *b *and *s *must be
in the same coordinate system for carrying out a dot product operation,
right ?

2.) In some books/articles i find a reference to a RA and Dec of Baseline ?
What does this physically mean ? I'm not able to visualize this clearly.
any help will be greatly appreciated !

3.) In the complete delay model, tm, which is the sum of geometric
delay+clock delay+ionospheric/atmospheric delay+fixed delays due to analog
component, the fastest varying component will be geometric delay only, once
this is compensated, if the other quantities are contributing to some
excess time varying delay, this will be seen as a residual fringe. Now, for
clock delay, is this estimated using the allan deviation of the clock being
used? Lets say my clock loses 10^-9 seconds in 30 minutes, and if I sample
my signal at 16 MHz which is ~ 62.5 ns, will I be able to integrate the
data without any degradation due to clock upto 30 minutes ?

4.) There is also an associated baseline velocity component which will lead
to a time difference between the wavefronts received at both the antennas.
Is this baseline velocity the same as the orbital velocity of the earth ?
Or is this modelled differently ?

I would greatly appreciate if the experts here clarify my doubts. Kindly do
point me to references that may lead to clarification of these doubts too !

Thanking you,
With best regards,

Mugundhan V.
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