[Difx-users] ITRF realisations {External}

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

Hi Leonid,

My concern with sched was that the catalogue have no frame info recorded into the positions. Sched does not do much with positions, so I agree for sched it does not matter but the sched catalogue as what most people use as the positions used for Calc.

I have chatted to Cormac offline, who points out that the absolute difference between frames would only affect stuff like ionospheric corrections, which will make difference at all. However the different frames have many cm difference between them so if inconsistent frames are used in the catalogues, that *will* have an effect.

So what frame used likely does not matter, but for normal astronomical use cases my worry is that we as a whole are not careful enough.

Cheers
Chris

From: Leonid Petrov <Leonid.Petrov at lpetrov.net>
Date: Friday, 4 July 2025 at 19:49
To: Phillips, Chris (S&A, Marsfield) <Chris.Phillips at csiro.au>
Cc: Difx-users <difx-users at listmgr.nrao.edu>
Subject: Re: [Difx-users] ITRF realisations {External}
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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