[Difx-users] vex2difx freqId list

Wed May 16 08:14:23 EDT 2018

Hi Jan,

My understanding of this is that freqId is a global parameter, not station specific, while the frequency lists are station specific, not global. This poses an obvious  ambiguity where different stations have different frequency structures.

This has been discussed at a previous difx meeting, but IIRC the fix was not quite trivial and the demand for a generalisation was insufficient to force a change.

In your scenario, selecting just freqId=2,3 (or perhaps freqId=11,12) might do what you want,  as I'm not quite sure how the selection logic works in detail, but have often found that it does the 'sensible' thing in poorly defined scenarios.

If not, a cheat I have used in the past is to set the frequencies I wish to discard to some nonsense value such that they do not form a correlatable baseline. This vex hacking can be a bit messy, but has a very predictable result.

Regards,
Cormac

Sent from Blue<http://www.bluemail.me/r?b=12976>
On 16 May 2018, at 19:27, Jan Wagner <jwagner105 at googlemail.com<mailto:jwagner105 at googlemail.com>> wrote:

Hi again,

got another potential user or interpretation mistake related to v2d.
This cropped up while trying to fix something in v2d+vex for
RadioAstron correlation. Basically we have stations with 16 MHz
recorded bands that overlap only a 1 x 64 MHz band out of VLBA 4 x
64 MHz. The other 3 x 64 MHz should supposedly be thrown away.

We tried to create 16 MHz zooms into just that 64 MHz VLBA band, and
discard the other 64 MHz bands via including only desired bands in
the v2d SETUP "freqId" parameter ("select which bands to
correlate"). However, vex2difx then complains about mismatched
bandwidth correlation. Despite the correct (I think) freqId band
selection.

With 'vexpeek' the original list of bands is:

# 16 MHz stations
# 22236 MHz +16M = 22236 MHz to 22252 MHz
Subband[0]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=U, pol=R]
Subband[1]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=U, pol=L]
# 22236 MHz -16M = 22220 MHz to 22236 MHz
Subband[2]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=L, pol=R]
Subband[3]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=L, pol=L]
# 22268 MHz +16M = 22268 MHz to 22284 MHz
Subband[4]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=U, pol=R]
Subband[5]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=U, pol=L]
# 22268 MHz -16M = 22252 MHz to 22268 MHz
Subband[6]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=L, pol=R]
Subband[7]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=L, pol=L]

# 64 MHz VLBA stations
# 22156 MHz +64M = 22156 to 22220 MHz ; *discard*
Subband[8]=[2.2156e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
Subband[9]=[2.2156e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
# 22220 MHz +64M = 22220 to 22284 MHz, keep
Subband[10]=[2.222e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
Subband[11]=[2.222e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
# 22396 MHz +64M = 22396 to 22460 MHz ; *discard*
Subband[12]=[2.2396e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
Subband[13]=[2.2396e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
# 22460 MHz +64M = 22460 to 22524 MHz ; *discard*
Subband[14]=[2.246e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
Subband[15]=[2.246e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]

The ZOOM section has zooms for only the desired one band out of 4 x
64 MHz VLBA:

addZoomFreq = freq at 22220.00/bw at 16.0/noparent at true
addZoomFreq = freq at 22236.00/bw at 16.0/noparent at true
addZoomFreq = freq at 22252.00/bw at 16.0/noparent at true
addZoomFreq = freq at 22268.00/bw at 16.0/noparent at true

The SETUP section has selects only the desired 16 MHz bands and the
one 64 MHz band:

freqIds = 0,1,2,3,4,5,6,7,10,11

However, vex2difx still complains about mismatched bandwidths. The
complaint disappears only when I add three more 16 MHz zooms so that
a piece of each three undesired 64 MHz VLBA bands is included.

Apparently the "freqIds" list is either ignored, or not parsed
correctly, or is checked too late?

The wiki describes the SETUP parameter "freqId" as "a comma
separated list of integers that are freq table indexes to select
which bands to correlate; default is to correlate all. Note: this
should be used to select parent bands for zoom frequencies if
difx2fits is to be run."

From the actual behaviour though it seems that "freqId" does not
actually "select which bands to correlate"?

cheers,
Jan

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