.
.
A similar situation was in r1774 experiment one week before. The scatter of post-fit residuals was large at all baselines, but with ISHIOKA. Why the postfit residuals at ISHIOKA/SEGONG were normal (well, normal in VLBI2010 system means only a factor of two worse than it was 10 years ago)? The correlator output did not have cross-spectrum for IFs 12-16 for baselines with ISIHOKA. Therefore, group delays at ISHIOKA/SEGONG were computed using IFs 7-11.
I re-fringed data with PIMA and confirmed large scatter
for baselines with SEJONG. I disabled phase-cal at both ISHIOKA (the phase-cal
signal was not present, and I am not sure whether the unit is installed at all)
and at SEJONG. But I found another peculiarity: a saw-like pattern in group
delays at ISHIOKA:
.
Group delays computed with Fourfit did not show such a pattern.
I made step forward and ran two fringe fitting: with using IFs 7-11 and
with using IFs 12-16. The reference frequency for the first variant is
8.20499 GHz and 8.73299 GHz for the second variant. Using the estimates
of the group delay I transferred fringe phase from 8.73299 GHz down
to 8.20499
( φ2(f1) = φ2(f2) +
2π * (f2 - f1) * τgr ) and
made a plot of differences in fringe phases computed over IFs 12-16
minus fringe phases computed over IFs 7-11. The plot of dependencies of
these phase differences in time is shown below:
.
For comparison a similar plot for the baseline NYALES20/WETTZELL in
the same scale:
.
Similar plot at ISHIOKA/KOKEE reveals the saw-pattern, similar to that
we saw in the group delays:
.
After removal of a trend -1.007 × 10-3 rad/s, the residual
phase differences look normal:
.
This looks very strange. IFs 7-11 and IFs 12-16 are fed with different cables. It looks like phase in the cable that brings IFs 12-16 drifts with respect to the phase in the cable that brings IFs 7-11. But the most puzzling is that why we do not see this phenomena in Fourfits output.
.
Compare it with the plot at the beginning of the document (they are in the same scale). And the saw-like pattern at ISHIOKA is gone.
And yes, this trick helps to rescue SEJONG data in r1774 experiment as well.
.
Experiment r1773 experimetns with SEJONG shows changes during last three
hours, specifically after 2016.12.26_14:02. Phase cal shows jumps:
Post-fit residuals also shows jumps and large scattering. Re-fringing
witth pcal disabled for SEJONG does not bring residuals to normal.
Phase differences of IFs 7-11 versus IFs 12-16 at SEJONG/TSUKUB32 without
pcal applied shows linear trend and jumps after 2016.12.26_14:02
Applying phase-cal reduces trend by a factor of 4 but does not eliminate
jumps:
These jumps spoils X-band group delays. Here are X-band post-fit residuals:
but they do not spoil S-band group delay residuals:
Which group of Ifs: 7-11 or 12-16 is to blame? Here are X-band residuals computed
from IFs 7-11
and here are X-band residuals computed from IFs 12-16
They look normal.
We see phase shift between IFs 7-11 and 12-16. While the phase-shift is constant, it is calibrated when complext bandpass iscomputed. A drift of phase shift was mainly compensated with phase-cal. However the phase shift started to jump after 14:02 and these jumps were not compensated by phase-cal. The phase shifts did not affect group delay computed over 7-11 and 12-16 IFs, but it affected group delay computed over IFs 7-16.
Last update: 2017.02.16_21:15:44
Web page was prepared by Leonid Petrov
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