[Difx-users] ITRF realisations {External}

[R. Craig Walker]

Locations.dat includes a text entry "FRAME".  At least in the older 
version I have on my computer, that tells where the data came from - in 
the case I'm looking at, which solution from which group.  But that is 
just text and you would need to understand the details of the solution 
to know which ITRF is involved.  So the information is (or should be) 
there to chase down the frame, but it would be good to make it more 
formal.  It doesn't seem that FRAME got passed to the VEX file.

Years ago, we tried rather hard to be sure that the station and source 
catalogs being used for Sched were the same as were being used for the 
IOP observation.  It took a while to get it right, but I think we did.  
I can't speak for the last 10 years since I retired.

Cheers,
Craig

P.S. Chris, your previous email came in between when I started to 
compose mine and when I sent it - without checking for anything new.  So 
it was not a response to yours.


On 7/4/25 8:37 PM, Phillips, Chris (S&A, Marsfield) wrote:
>
> Hi Craig,
>
> Yes I think the main issue is if correlators use the location.dat from 
> sched as telescope positions, the bookkeeping is not enough to know 
> which ITRF frame was used. The rest of the sched/calc mechanisms are fine.
>
> Cheers
>
> Chris
>
> *From: *Difx-users <difx-users-bounces at listmgr.nrao.edu> on behalf of 
> R. Craig Walker via Difx-users <difx-users at listmgr.nrao.edu>
> *Date: *Saturday, 5 July 2025 at 09:32
> *To: *difx-users at listmgr.nrao.edu <difx-users at listmgr.nrao.edu>
> *Subject: *Re: [Difx-users] ITRF realisations {External}
>
> Sched, at least when I was the author and maintainer, never claimed to
> do geometric calculations at a level appropriate for correlation.  For
> scheduling of scans, typically many seconds long, one can be more
> casual.  So Sched never required the user to provide EOP.  The
> correlators needed to get measured EOP values for best results.  What
> Sched does provide to the correlator and get from the users are the
> station and source coordinates in whatever reference frames are being
> used.  For this, it is important that the coordinate values be in the
> same system used for the EOP measurements that the correlator will use,
> whatever those are.  After all, the EOP are a major factor in defining
> the rotation between the terrestrial and celestial reference frames at
> the time of the observations.  So when scheduling, you should learn
> which ITRF realization is being used for the EOP observations and use
> that for astronomy observations.  For geodetic observations, it may not
> matter because the model will be bucked out and replaced anyway.
>
> Cheers,
>
> Craig
>
>
> On 7/4/25 6:49 AM, Leonid Petrov via Difx-users wrote:
> > Chris,
> >
> >   I looked at sched-12.0. I found that internally it uses UTC(t)
> > function as time. It assumes UT1(t) = UTC(t), see for instance
> > schgeo.f. This can introduce an error that is equivalent to an
> > error in position of mid-latitude sites at a level of several
> > hundreds meters.
> >
> >   In that context difference in several centimeters between different
> > station catalogues is irrelevant.
> >
> > Leonid
> > 2025.07.04_08:47:11
> >
> >> On 2025-07-04 01:02, Phillips, Chris (S&A, Marsfield) via Difx-users
> >> wrote:
> >> Hi all,
> >>
> >> I have just learned that ITRF (which we use as the XYZ coordinate
> >> system of our telescopes) has multiple realisations (ITRF2000,
> >> ITRF2014 etc).
> >>
> >> There was approx a 9cm change between around ITRF2005, which is huge.
> >>
> >> The “Frame” detail in Sched is light on details and vex2difx make
> >> no mention of frame.
> >>
> >> Does anyone know what ITRF frame Sched expects?
> >>
> >> Thanks
> >>
> >> Chris
> >> _______________________________________________
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