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IERS Message No. 354                                        May 07, 2018
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Recent changes to the IERS 14 C04 series / Bulletin B


Dear IERS users,

From its production in February 2017, 14 C04 nutation was only based
upon the IVS combined solution according to a recommendation issued by
representatives of IVS and IERS. But, on March 3, 2018 it turned out
that IVS combined solution had not been updated since January 13, when
Bulletin B was made. So, celestial pole offsets (CPO) were set to zero
after this date.

In order to fix this problem, on March 3 we run again the C04
combination by taking all VLBI solutions, of which the last UT1/CPO
determination went back to February 12. So we had to update the C04
series from January 13. With this new solution, the pole coordinates and
UT1-UTC were slightly changed.

There was a also a serious flaw in UT1 values till January 2018, where
UT1 intensive values are no more accounted after we wrongly follow an
advise of an IVS/IERS representative. Because of the error
interpolation, UT1 solution was seriously downgraded between IVS dates.
Whereas the precision of UT1 intensive is about 30 micros (against 10
micros for R1/R4 UT1), the error introduced by interpolation between two
IVS dates is probably much larger. We came to this conclusion, after
Frank Reinquin (CNES) put forward an anomalous increase of SLR LAGEOS
1/2 orbital residuals using the 14 C04. Then we discovered that these
anomalies were precisely located at the dates where UT1 intensive had
been ignored, and replaced by a pure interpolated values between
neighbouring R1/R4 sessions.

According to the decision of the IERS Directing Board of April 8, 2018
the 14 C04 solution for UT1 was modified on April 16, 2018 by including
the contribution of UT1 intensive back to 1996. The old version, updated
until 2018/04/16 was put in the directory
ftp://hpiers.obspm.fr/iers/eop/eopc04/eopc04.2017/.

Kind regards,
Christian Bizouard
Chair, IERS Earth Orientation Centre


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