Mesage sent 2005 October 6

Previously it was announced that the VLBA correlator had been using
predicted values for Earth Orientation Parameters (EOP) for
approximately the last 2 years.  The Earth rotation and pole position
are not easily predicted, even over only 2 weeks, so these values are
significantly worse than the Rapid Service values that are normally
used.  This can adversely affect projects that depend on an accurate
correlator model, such as phase referencing.

With this email, we announce that the ability to fix EOP is now
available in the AIPS task CLCOR and that there is a new VLBA Test
Memo Number 69 that explains how to fix data, presents details of what
happened, shows how the poor EOP affects data, and gives results of
test observations.

To use the new CLCOR capability, use the 31DEC05 version of AIPS and
be sure that it has been updated (midnight job) since 2005 September
30. To read the VLBA Test Memo, go to
http://www.vlba.nrao.edu/memos/test/ and click on the type (ps, pdf,
html) of file you would like to read for Memo 69.

Note that before this problem began, and since it was fixed,
occasional job scripts have used poor EOP values, usually because they
were prepared before the Rapid Service values were loaded into the
data base.  See the Memo 69 for instructions on how to check the EOP
used for specific projects.

Cheers,

Craig Walker

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    R. Craig Walker            Array Operations Center
    cwalker@nrao.edu           National Radio Astronomy Observatory
    Phone  505 835 7247        P. O. Box O
    Fax    505 835 7027        Socorro NM 87801   USA
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