[Difx-users] ITRF realisations {External}

[Phillips, Chris (S&A, Marsfield)]

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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