[daip] Feature request for FITLD for VLBA data

[Walter Brisken]

Hi AIPS,

I have learned of an interaction of a previous version of DiFX with FITLD 
that yields troublesome processing within AIPS.  I believe a simple, and 
very save, change to FITLD could seamlessly correct for this.

The problem is that the IDI FITS file contains multiple FREQIDs which are 
in fact identical.  Having separate FREQIDs precludes proper operation of 
a number of tasks in AIPS and makes calibration nearly impossible.  A 
reasonably small example file is

  /home/parallax/VLBA/BZ065B/VLBA_BZ065B_bz065bpass1_BIN0_SRC0_0_170303T211055.idifits

which has an AIPS friendly sym link

  /home/parallax/VLBA/BZ065B/GEO1.FITS

Running FITLD with default adverbs (except for DATAIN) yields the abridged 
output shown below.

The proposed fix is for AIPS to renumber all equivalent FREQIDs to a 
common FREQID.  I think the hard part is already done in FITLD: 
identification of the matching frequency IDs.  So perhaps when a file is 
being loaded, a mapping from FITS-FREQID to AIPS-FREQID could be 
established, and then the data and relevant tables could be remapped 
on-the-fly during read.  This could be optional behavior if so as to allow 
the existing behavior (maybe there is a legitimate use of this).

 	Thanks,

 	Walter


--------------------------------

Abridged output from FITLD:

FITLD1: Task FITLD  (release of 31DEC18) begins
FITLD1: Create MULTI       .UVDATA.   1 (UV)  on disk  1  cno    1
FITLD1: Sky frequencies (MHz) before re-ordering occurs:
FITLD1: Incoming FREQID #   1
FITLD1:  22174.25   22161.25   22122.25   22057.25
FITLD1:  21979.25   21888.25   21758.25   21732.25
FITLD1: Incoming FREQID #   2
FITLD1:  22174.25   22161.25   22122.25   22057.25
FITLD1:  21979.25   21888.25   21758.25   21732.25
FITLD1: Incoming FREQID #   3
FITLD1:  22174.25   22161.25   22122.25   22057.25
FITLD1:  21979.25   21888.25   21758.25   21732.25
.
.
.
FITLD1: Incoming FREQID #  17
FITLD1:  22174.25   22161.25   22122.25   22057.25
FITLD1:  21979.25   21888.25   21758.25   21732.25
FITLD1: !!!!! WARNING: FREQIDS   1 AND   2 ARE IDENTICAL
FITLD1: !!!!! WARNING: FREQIDS   1 AND   3 ARE IDENTICAL
FITLD1: !!!!! WARNING: FREQIDS   1 AND   4 ARE IDENTICAL
.
.
.
FITLD1: !!!!! WARNING: FREQIDS  15 AND  16 ARE IDENTICAL
FITLD1: !!!!! WARNING: FREQIDS  15 AND  17 ARE IDENTICAL
FITLD1: !!!!! WARNING: FREQIDS  16 AND  17 ARE IDENTICAL



-- 
-------------------------
Walter Brisken
Director
Long Baseline Observatory
(575)-835-7133 (office)
(505)-234-5912 (cell)