Cobalt geographical delays offset

It has been discovered that the geographical delays applied to the data stream by the Cobalt correlator were off by 1 second. A fix was implemented, but observations performed between December 19, 2013 and June 29, 2015 are affected. Due to the magnitude of the offset, only long baseline data reduction can potentially have issues. For data reduction strategies using the standard long baseline pipeline, calibration solution transfer for up to 2 degrees in HBA (using appropriate calibrators) shows no adverse effects due to this issue. Also, standard long baseline processing in LBA shows not to be affected by this issue.

In beam-formed observations, the issue resulted in a slight mis-pointing of tied-array beams:  The pointing direction of the tied-array beam was calculated for one second later than the actual time, meaning that the beam would have been pointing slightly west of the specified position.  This means that the actual pointing direction would have a slightly lower right ascension than specified.  Hence a value of 15arcsec cos(declination) should be subtracted from the specified pointing to correct for this issue.

For all relevant timing issues, please consult the timing delays system history page.
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