[daip] FRING

[Jim Ulvestad]

There are several reasons for this message.  Often, there is a
single bad antenna that gives a poor starting point for the
solution, so that chi-squared is unable to get to a minimum
from its starting point.  I have found that this often happens
with Saint Croix, because the integration time is a bit
too long and the antenna loses coherence.  The snippet of
results you sent indicated that antenna 7 has much higher
SNR than the other antennas--is 7 the VLA or GBT?  If
so, you might try using 7 as the reference antenna.

This result can happen when you have low signal/noise,
so that an antenna starts off at a noise spike rather than
a true signal in the FFT.  Try increasing the SNR
by setting the parameters to average between IFs
and polarizations.  This is done by setting APARM(3)
and APARM(4) = 1.

Take a look at your SNPLT from the original FRING
run in order to find places where the solution did not
converge.  Then, run FRING on only that small piece
of data, and turn up the print level to APARM(6)=1.
This may show you a particular antenna that has very
low SNR (either resolution or poor coherence) or
a bad starting point because a noise spike is being found
out of the FFT.  You may then try running FRING without
that antenna and see if convergence is better.  If so,
you might want to flag the antenna.

Finally, you can set the window for the search window
to be fairly narrow by using DPARM(2) and DPARM(3).
If you already have set the clocks using the phase cals
or the FRING on a strong source.  If so, you can set a
small delay window; I would recommend DPARM(2)
of 60 to 100 (60 nsec to 100 nsec total width, meaning
+/- 30 to 50 nsec).  DPARM(3) also can be set to
restrict the window in fringe rate.  The value you would
use here is frequency-dependent, since the fringe rate
in milliHz is proportional to the observing frequency.
The fringe-rate error for VLBA antennas tends to
be atmosphere-dominated.  For 5 GHz, I'd guess
that DPARM(3)=20 or 30 is usually adequate.
But for 43 GHz, you may want to use 100 or more.

Finally, if you use low values of DPARM(2) and DPARM(3)
as I mention, you may want to reduce the SNR
threshold using APARM(7)---try setting it to 4.
It also may be that SOLINT is too long for the
atmospheric coherence, but you may be able to
shorten it some if you increase the SNR by using
APARM(3) and APARM(4) to average data.

Bottom line is that it appears to me that you have
fairly low SNR on at least some baselines, and
that this gives you some bad starting points in your
solutions.  Therefore, your goal should be to
identify which antennas have the poorest SNR,
to increase the SNR as possible or flag the low
SNR data, and narrow the initial search range down
to focus on where you really expect the fringes
to be.

A final thought--it may be that the source you are
running FRING on is fairly resolved.  Making an
initial map and self-calibration, then feeding this
image back into FRING as a source model (using
IN2NAME and NCOMP) might help things
converge better.

Best,

Jim

Shen Zhiqiang wrote:

>Hi,
>
>In running FRING, I saw many pieces of message as below
>
>FRING      Time=   0/ 20 58  4, Polarization =   2
>FRING     B= 01 - 04 IF=  1 R=     -22.5 D=      -5.9 SNR=  15.5
>FRING     B= 03 - 04 IF=  1 R=     -21.5 D=     -19.5 SNR=  10.4
>FRING     B= 04 - 07 IF=  1 R=     -22.5 D=     -16.6 SNR=171.1
>FRING     B= 04 - 08 IF=  1 R=     -22.5 D=     -16.6 SNR=  10.6
>FRING     FIT DID NOT CONVERGE FOR IF    1
>FRING      This probably means that the starting value for the
>FRING      delay or rate for one or more antennae is bad.  You
>FRING      may want to set search windows and try again.
>
>Obviously, this would lose some good detections. Is there any
>way to avoid this? What does it mean to set search windows?
>Are the windows used too small? 
>
>Again, this happened many times in a single run of FRING. 
> 
>Best regards,
>Zhiqiang Shen
>
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