[daip] VLBA autocorrelations / ACCOR corrections

[Jim Ulvestad]

Lincoln Greenhill wrote:
> 
> Hi Leonia,
> 
> You mention that ACCOR is supposed to fix problems that are as large
> as tens of percent.
> 
> Do you think  that the structure in the SN tables  output by ACCOR (i.e.
> systematic variations in gain on the order of < 1% on times scales of a
> few minutes) are not REAL?  Should one only use long solution intervals
> with ACCOR?

This issue has been argued about at various times in the past.  The
default
for ACCOR at some point was a 60-minute average, and I argued pretty
strongly for a 2-minute average; among other things, that provides an
opportunity to clip individual bad points using SNSMO, which would
distort
the whole solution in a 60-minute average.

The structure on time scales of a few minutes IS real.  This is caused
by
the automatic gain control--as a source rises or sets and changes
elevation,
the system temperature changes gradually, causing a trend in the ACCOR
solutions.  Then some threshold level is reached where the AGC kicks in
and changes the calibration slightly, at which time the solution appears
to jump back to near the previous level.  This sort of cycling repeats.
I assume that you see a sawtooth of one shape when the source is rising,
and essentially the opposite shape when the source is setting, though
I've
never actually verified this.

(Probably I didn't describe this in quite the correct language, but I
hope
you get the idea.)

jim

> 
> Regards,
> 
> Lincoln
> 
> Lincoln J. Greenhill      Harvard-Smithsonian Center for Astrophysics
> Radio & Geoastronomy Division, 60 Garden St, Cambridge, MA 02138, USA
> Telephone:  1 617-495-7194             FAX:  1 617-495-7345
> Internet:  greenhill at cfa.harvard.edu   http://cfa-www.harvard.edu/~lincoln
> 
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