[LEAPSECS] Fwd: IERS Message No. 354: Recent changes to the IERS 14 C04 series / Bulletin B

[Michael.Deckers]

On 2018-05-07 12:41, Rob Seaman wrote:
>
> Anybody have more details about this? How it happened or what it might 
> mean for practical timekeeping?
>
> Rob
>
> --
>
>
>
> -------- Forwarded Message --------
> Subject: 	IERS Message No. 354: Recent changes to the IERS 14 C04 
> series / Bulletin B
> Date: 	Mon, 7 May 2018 10:57:14 +0200 (CEST)
> From: 	central_bureau at iers.org
> To: 	messages at iers.org
>
>
>
> ************************************************************************
> IERS Message No. 354                                        May 07, 2018
> ************************************************************************
>
>
> 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/.


     I am just guessing what is meant. Here is my tentative 
de-Frenchification:

             [From its production in|Since] February 2017, [|the] 14 C04 
nutation
             [|data for the deviation of the observed celestial 
intermediate pole CIP
             from the pole of the 2006 nutation series] was [only based 
upon|derived
             only from] the IVS combined solution [|for the CIP,] 
[according to|following]
             a recommendation issued by representatives of IVS and IERS.

             [But,|Also,] on March 3, 2018 when Bulletin B [|for 2018 
February] was made
             it [turned out|was discovered] that [|the] IVS combined 
solution had not
             been [updated since|kept up to date after] January 13. So, 
celestial pole
             offsets (CPO) were [set to|determined to be] zero after 
this date [|2018-02-13].
             In order to fix this problem, on March 3 we [run|ran] 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 [|onwards]. With this new solution, 
the pole
             coordinates and UT1-UTC were slightly changed.

             There [was a also|also has occurred] a serious flaw in UT1 
values
             [till|before] January 2018, where UT1 [intensive 
values|values derived
             from intensive VLBS observations] [are no more 
accounted|were no longer
             taken into account] after we wrongly follow[|ed] an 
[advise|advice]
             of an IVS/IERS representative. Because of [the error|this 
erroneous]
             interpolation, [|the] UT1 solution was seriously 
[downgraded|degraded in]
             between IVS dates.

             Whereas the [precision|uncertainty] of UT1 [intensive|data 
taken from
             intensive VLBR observations] is about 30 micros[|econsds] 
([against|as opposed to]
             10 micros[|econds] 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 [|evidence of] an 
anomalous increase of SLR LAGEOS
             1/2 orbital residuals [using|with respect to] the 14 C04 
[series]. Then we
             discovered that these anomalies were precisely located at 
the dates
             where UT1 intensive[|s] had been ignored, and [|had been] 
replaced by
             [a pure interpolated|] values [between|interpolated solely 
from]
             neighbouring R1/R4 sessions.

             According to [the|a] 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|to] UT1 [intensive|deduced from 
intensive VLBR observations]
             back [to|since] 1996. The old version, [updated|computed] 
until [2018/04/16|2018-04-15]
             was put in the directory 
ftp://hpiers.obspm.fr/iers/eop/eopc04/eopc04.2017/.

     Michael Deckers.