[mmaimcal]Instrumental Delay offsets

[John Conway]

Hi,

 Actually for the antenna location problem and required
delay stability there are two critical timescales. One is the
timescale and size of delay variations over a cycle around 9
sources. One such cycle wil take 15 minutes, and 4 or 5 such
cycles would be done  in each calibration run. The second
timescale would be of order weeks, the interval between
antenna position observations on a given antenna.

My main question is whether my simulations of antenna position
location should solve for a  constant electronic delay or not
or whether this was stable/or measured so I did not have
to solve for it from the astro obs. I have been
assuming that I will have to solve for this term
but that that it was stable relative
to the atmosphere in the hour or so it took to
do the measurements. Is this a correct working assumption?
I should of course as part of memo specify what is needed
in terms of delay stability for the antenna location problem.

As I mentioned earlier whether one solves for the instrumental
delay or not makes a significant difference in the rms of
the antenna z component. For this reason in geodectic VLBI some lengths
are gone to to estimate the analogue delay prior to the digitisation. A
phase cohherent comb of 1MHz tones is injected at the front end
and the phase detected after digitisation. The detection can be done
both at the antenna or at  the correlator. I assume ALMA will
not have such a capability, or will it?

  John

On Wed, 26 May 2004, John Conway wrote:

> > > In an earlier version of the cal plan I have
> > > (Butler et 2003-08-07) there is in section 2.3
> > > a specification  of systematic and random offsets of 11.9 fs(?)
> > > and 54.5 fs(?). I assume the units are fs not microns
> > > (although its not clear). Its also not clear what the
> > > stability times are meant to be for the systemtic and
> > > random components.
> >
> > From Mark Holdaway
> >
> > These numbers are from the fast switching work (LAMA Memo 803),
> > and are in fs.  The systematic component has a 300 s time scale
> > (time between cross-frequency calibrations if we are calibrating
> > at 90 GHz and obsering at say 650 GHz).  Consider the 54.5 fs to be
> > on timescales of ~2-20 seconds.
> >
>
> Hi,
>
>  In LAMA Memo 803 section 7.1 'Requirments on Instrumental stability'
> I believe you specify...
>
> 78fs per antenna random jitter on timescales of a few seconds
>
> and a drift in the fre scaled delay difference over 300sec to be
> 32fs or  better
>
> These are not exactly the numbers we discuss above, which are
> somewhat less. Maybe there is another specification in LAMA memo
> 803 which I missed (its quite a long memo).
>
> Your memo deals with stabilties on timescales up to  300sec,
> - because of long slews, cable unwrapping etc,  a cycle around say nine
> sources for antenna position determionatio will take
> somewhat  longer than  that (say 15min) so that it the
> relevant timescale, and the spec  needed
> in  on the drift in delay at a single freq (probably 90GHz).
>
> Are we still at the stage where specifications can still be made?
> or is hardware being designed and build already on the existing
> specs. If new Specs for instrucmnetal stability are possible I should
> carefully consider if a new spec is needed for basleine determination
> and what it should be.
>
>   John
>
>
>