[daip] FRING

Mon Jan 5 20:32:07 EST 2004

The White Mountains have an elevation as high as about 
16 degrees when seen from the Owens Valley VLBA antenna.
Use SNPLT to plot the system temperature for Owens
Valley during the run.  (INEXT='TY', OPTYPE='TSYS'
I think).  When the source is rising early in the run,
you may see system temperatures near 300 K.  This will
indicate that you are looking at the mountains.  If this
is the case, flag the data for a timerange up until the
system temperature seems to have dropped to a much lower
value and stabilized.

I always set DPARM(4)=0 and let FRING find it from the data.
Despite the FRING warning, I don't actually know what happens
when you set it to the wrong value.

I have always had trouble setting APARM(5)=2.  This used to
be recommended in some cases, but it didn't seem to work
very well, and we changed the documentation so that we no
longer recommended it.

I usually set APARM(5)=0.  If you have solved for the clock
well using the phase-cals or FRING on a strong source, 
the different IF channels should
have the same phase (check this using POSSM on a strong
source, applying the calibration after the phase-cals or the
short FRING have allowed you to take out the clock.
Then, the multi-band delay should be low, and the single-band
delay should be the same for all the IFs, and very small.
This would enable you to average the IFs in FRING.

I haven not used APARM(5)=1 in several years, so I cannot
comment usefully on the results.

Best,

Jim

On Mon, 5 Jan 2004, Z.-Q. Shen wrote:

> Hi Jim,
> 
> Thanks for your diagnostic message. It is very helpful.
> 
> After some testing runs, I found the setting of APARM(4)=1 helps.
> In this case, I used APARM(5)=1 to solve for multi-band delay over
> 8 IFs. This would increase the SNR for each spectral data point, right?
> But I wonder if we can solve for single-band delay (i.e., APARM(5)=0 or 2)
> with APARM(4)=1. I did try APARM(5)=2 and found more solutions
> compared to APARM(5)=1 solutions, but these solutions seem to be not
> good with quite large scattering. I did not try APARM(5)=0 because of very
> limited SNR.
> 
> This is VLBA only observations at 3mm. Antenna 7 is OV. I suppose
> that high SNR is related to the noise spike. The observing log said that
> OV GPS Delta was drifting at - 0.1 us per hour. You also mentioned in
> the VLBA Scientific Memo No. 25 that a common cause is when OV is
> looking at the White Mountains. I wonder where is White Mountains
> to the OV station.
> 
> I have another question regarding running FRING. This is about DPARM(4).
> It is said that we should use DAPRM(4) to specify the correlator integration
> time, which is 1 sec in my data. But when I accidentally set it to 2 sec,
> I did not see any difference compared to the results from FRING with the
> correct DPARM(4)=1. My question is how important is the DPARM(4) in
> FRING.
> 
> Thank you and a happy new year to you.
> 
> Eric
> 
> At 21:35 2003-12-23, Jim Ulvestad wrote:
> >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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> 
> 