[Difx-users] ITRF realisations {External}

Mon Jul 7 12:49:13 EDT 2025

SCHED is currently maintained enough to put in a little bit of work if 
more metadata on the frame used is requested in the VEX.
I don't envisage us being able to put in the work to do anything super 
complicated (non-linear etc).
We'd need a clear idea of how/where though :)

Jay

On 7/7/25 10:30, R. Craig Walker via Difx-users wrote:
> SCHED locations.dat file has date ranges.  SCHED uses the appropriate 
> entry.  This allows accounting for steps (earthquakes), but nothing 
> more complicated.  For example, there are three entries for MK-VLBA 
> with different date ranges.
>
> Cheers
> Craig
>
>
> On 7/7/25 5:08 AM, Leonid Petrov via Difx-users wrote:
>> Adam,
>>
>>   Vex schedule has some positions, although these are not necessarily
>> the positions actually used for scheduling. Sched (as as three other
>> scheduling programs) has no mechanism to account for non-linear site
>> motion, which in case of Japanese station may reach a meter level.
>>
>>   We have section ARRAY_GEOMETRY with a keyword STABXYZ.
>> Do you suggest to have two keywords for station positions?
>>
>> Leonid
>> 2025.07.07_07:06:00
>>
>>
>>> On 2025-07-06 23:32, Adam Deller wrote:
>>> Hi everyone,
>>>
>>> It sounds like there is a desire that some metadata relevant to the
>>> station positions propagates through from the sched station catalogue
>>> through to the vex file and thence into the FITS-IDI output. That
>>> seems a reasonable thing to aim for, regardless of what one thinks of
>>> the provenance of said station positions (and how one deals with the
>>> delay model that has been generated from them and applied at the
>>> correlator).
>>>
>>> Cheers,
>>> Adam
>>>
>>> On Sat, 5 Jul 2025 at 12:56, Leonid Petrov via Difx-users
>>> <difx-users at listmgr.nrao.edu> wrote:
>>>
>>>> Dear Chris,
>>>>
>>>>> but the sched catalogue as what most people use as the positions
>>>> used
>>>>> for Calc.
>>>>
>>>> Pity!
>>>>
>>>> Coordinates are not measurable quantities. The is is the
>>>> essence of geodesy. Path delay is invariant to a group
>>>> of rotations and translations. As a result, one cannot determine
>>>> station positions using only observations. One needs to apply
>>>> constraint equations with some arbitrary constants right hand sides.
>>>> You can select these constants whichever way you like, for set to
>>>> a telephone number of your spouse, but usually, these constants
>>>> are chosen in such a way that the origin of the coordinate system
>>>> is not too far from the place where nobody traveled -- I mean the
>>>> geocenter, and orientation is not too different then the EOP time
>>>> series maintained by Christian Bizouard. "Too far", "too different"
>>>> sounds subjective. In my view, 10 cm is not too far.
>>>>
>>>> Reference frame is just a synonym to station position catalogue.
>>>> If one determines position of ATCA pad XXX with respect, say
>>>> ATCA-104,
>>>> using total station, then that geodesist uses solution gsf_2025b,
>>>> then that guy can write: position of ATCA pad XXX in the gsfc_2025b
>>>> reference frame is A.
>>>>
>>>> ITRF is not consistent with anything. The latter versions are
>>>> worse
>>>> than the previous one. I do not recommend of using it all.
>>>>
>>>> I know users AIPS got used to a concept that the correlator would
>>>> provide them a perfect model and then they can work with residuals.
>>>> At NASA we have never used that approach at least since 1975.
>>>> Instead, we compute our model that is orders of magnitude more
>>>> accurate
>>>> than that the correlator used and add to theoreticals the quantity
>>>> new_path_delay minus old_path_delay. And we do not care about
>>>> TRF, EOP used by the correlator. I offered 5 or 6 times to the AIPS
>>>> maintainer to add this feature to AIPS and always heard "no!".
>>>> Personally, I have PIMA for VLBI data analysis instead of AIPS and
>>>> I am not affected by this problem.
>>>>
>>>>> 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.
>>>>
>>>> If you really want to have a precise theoretical model, use
>>>> open-source
>>>> NASA SGDASS that has station positions, source positions, EOP,
>>>> non-linear
>>>> station motion updated on a quarterly basis, various loading
>>>> applied,
>>>> the ionospheric model, path delay through the atmosphere computed
>>>> by integration of equations of wave propagation using the output of
>>>> NASA
>>>> numerical model, etc. The right place for this refined model is
>>>> the post-processing stage, not correlation. Though, you can run
>>>> difxvtd
>>>> if you like.
>>>>
>>>> Leonid
>>>> 2025.07.04_22:46:50
>>>>
>>>>> On 2025-07-04 22:26, Phillips, Chris (S&A, Marsfield) wrote:
>>>>> 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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>>>
>>> -- 
>>>
>>> !=============================================================!
>>> Prof. Adam Deller
>>> Centre for Astrophysics & Supercomputing
>>> Swinburne University of Technology
>>> John St, Hawthorn VIC 3122 Australia
>>> phone: +61 3 9214 5307
>>> fax: +61 3 9214 8797
>>> !=============================================================!
>>
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