[daip] forwarded message from Harro Verkouter

[Jon Romney]

Thanks for the forward, Eric.

This clearly falls in the astro/geodetic area for which Craig
Walker is the VLBA's contact person.  However, during his
current prolonged absence I will make the following points --

--  This is not "just" a matter of convention among VLBI people.
There is a convention in the earth-rotation community, which all
VLBI systems should adhere to.  I believe it is to express the
polar X/Y coordinates in arcseconds.  However, I'm not sure,
and have made a point of *not* checking so that my suggestions
below may seem less self-serving.  If my belief is correct, then
the fundamental error is in the IDI spec.

--  The proper fix should involve changing whichever data are
inconsistent with the earth-rotation conventions.

--  Perhaps I've missed something, but I don't see why it would
be so difficult to operate on archive data.  The fix is very
simple, so it could be included in FITLD as Harro suggests.  It
could be applied every time at no significant computing load,
without any need to retro-edit existing files.

It would "only" be necessary to identify the files that need to
be fixed.  I believe the IDI files include sufficient informa-
tion to do that: the correlator or organization that wrote the
file, and possibly the date it was written.

Apologies if I've oversimplified the issue.

Jon


Eric Greisen wrote:
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> From: Harro Verkouter <verkouter at jive.nl>
> To: Eric Greisen <egreisen at nrao.edu>
> CC: Huib Jan van Langevelde <langevelde at jive.nl>
> Subject: Re: [daip] bug in AIPS 'tst"/ POLARX/Y
> Date: Thu, 10 Aug 2006 08:30:30 +0200
> 
> 
> Ok so I changed the subject to incorporate POLARX/Y.
> 
> To summarize: it is a matter of units. IDI says it should be 'meters', 
> VLBA s/w writes in units of 'arcseconds'. This breaks (at least) the 
> UVFIX task - UVWs are way off since apparently UVFIX *expects* 
> arcseconds rather than meters.
> 
> Our original question to you was: do you know if AIPS has an internal 
> unit associated with the POLARX/Y values and if so, which one is it? 
> Other tasks besides UVFIX may be affected by this.
> 
> Depending on if/what AIPS internally thinks we may or may not have to 
> fix FITLD to recompute the value of the POLARX/Y keyword in order to 
> represent it in the units which are internally used.
> 
> Browsing through 31DEC06 sources I found POLARX/Y referenced in a few 
> places and mostly in the comments it is taken to be in units of meters 
> save in the file APL/SUB/JPOLAR.FOR where it is assumed to be in units 
> of arcsec....
> 
> 
> The fix in itself would be a no-brainer for any of us - VLBA, JIVE, AIPS 
>   - I'd say.
> However, the more difficult thing we're faced with is the question of: 
> how can we fix this in such a way that it is somehow consistent and 
> hopefully not break existing archive data... it would be quite a 
> nuisance to have to retro-edit FITS files to modify the values of the 
> POLARX/Y keywords. That would definitely not be a no-brainer I hope 
> you'll agree with me.
> 
> By the way - answer these issues at your own convenience...
> 
> Cheers,
> 
> harro verkouter
> 
> Eric Greisen wrote:
> 
>>I can get the info eventually if you will remind me of the POLARX/Y
>>problem.  The VLBA is run by expert scientists not me - all aips does
>>is use whatever the correlator writes which may or may not be
>>correct.  Craig Walket, John Romney are ones who might change what is
>>written.  I cannot find the previous e-mail - either I lost it or it
>>was not from you and it does not say POLAR in its subject.
>>
>>Eric Greisen
> 
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