[daip] CVEL help

[Eric Greisen]

Vicas Dana wrote:
> Dear Eric,
> 
> Your recent email regarding CVEL bugs has alarmed the radio component
> of the LITTLE THINGS team. Before we sound the alarm for the whole
> team, we need to understand the situation properly and for that we
> need your help. LITTLE THINGS team has been using a frozen version of
> Aips, 31DEC08. From your message we understand that the first bug is
> been around since 2009 only, while the second bug, the more serious
> one seems possible to have affected us too. Could you please suggest
> some ways to check if we have been affected? We have constantly been
> checking the result of CVEL with POSSM, but was that enough?

The things to check are 1. Did you run CVEL on single-source files? and 
2. Did these files have an NX table attached?  In the 31DEC08 version, I 
would not be certain that an NX table would be attached to a 
single-source file, but SPLIT began doing that in the 08 version.

The bug applies only to single-source data sets with an NX table.  There 
was no bug for multi-source files (all have NX table) and no bug for 
single-source files with no NX table.

I agree that you should check.  Note however whether CVEL actually did 
anything for you.  It would have to if you did not Doppler track as you 
observed and you combined data from different configurations.  The 
correction for the Earth's rotation within a day is very small wrt the 
channel widths you would have used.

31DEC08 actually would have had a different "bug" with single-source 
files that HAD been Doppler tracked.  The knowledge of that tracking 
would have been lost in SPLIT (it is buried in the CL table) and so CVEL 
would shift data that had been tracked which is not a good idea. 
31DEC09 contains a frequency offset table written by SPLIT to preserve 
any offsets for single-source CVELs.

Eric Greisen