[evla-sw-discuss] Telcal -> antsol -> cpslss

[Bryan Butler]

i would prefer to have this simply extended in the current C.  i don't 
want a new Java implementation to confuse the new TelCal which will be 
worked on soon (pending new hire).  if you extend it in the current C, 
then the person hired should pretty easily be able to work it into their 
new TelCal implementation, which will almost certainly be in Java.

	-bryan


Barry Clark wrote:
> At the heart of Telcal is the antsol routine, which solves for each
> antenna's amplitude and phase.  There are questions in my mind about
> what this routine should do in the Widar case.
> 
> 1.  Should it operate independently on each subband, or attempt a
> global solution for the whole baseband?
> You certainly get a better signal to noise, and thus can work on
> fainter sources by solving for the whole baseband.  I tend to think
> it is not worth the effort of implementing this rather complicated
> algorithm.
> 
> 2.  Should it simply sum all channels for a given baseline and subband,
> or should it simultaneously use the channelization to search for a delay
> error while solving for gains?
> The answer to this one depends on both the a priori accuracy to which
> we maintain our instrumental delays and on what we will use the antsol
> solutions for.  If, as now, we are fairly sloppy about maintaining
> instrumental delays, at sort of the 1-2 ns level, and if we want
> accurate instrumental responses saved for both tracking the instrument's
> behavior over the long term and for tracking source fluxes, we might
> need a multichannel antsol.  A delay error of a quarter turn across the
> band (one ns error in a 256 MHz subband, for instance) corresponds to
> about a 10% error in the amplitude.
> 
> So I thought I would look into generalizing antsol to a multichannel
> situation.  It is not clear to me if we should use it, because of the
> substantially higher computation required, but it might be worth looking
> at.
> 
> The routine at the bottom of the current antsol is called cpslss.  It
> takes a set of complex values, one per baseline, and solves for the 
> response of each antenna, in both amplitude and phase.  This routine
> has an amusing history.  It was originally written in Sail by Larry
> D'Addario for the Dec 10 data reduction system.  I (or I think it was I)
> translated it into Fortran for the Modcomp version of telcal, and Ken
> Sowinski translated it into C for the interim telcal.  In the process,
> almost all comments have been lost, if they were ever there in the first
> place.  And it is a rather obscure program.  It is an iterative
> solution, because the problem becomes non-linear in the low signal-to-
> noise case.
> 
> Anyway, just to see how it looks, I propose to generalize cpslss to the
> multichannel case.  The only obvious problem with doing so is that the
> convergence criterion used in the single value version makes no sense in
> the multichannel case.  I will, of course, write it in Java, making the
> fourth language for the algorithm.
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