Subject: Re: estimating EOP rates for DORIS or not / IDS and IERS goals
From: Zuheir Altamimi <[email-masked]>(au lieu de Laurent Soudarin <[email-masked]>)
To: [email-masked]
Date: Thu, 23 Mar 2006 11:30:23 +0100

Dear Pascal,

Thank you for the test you did over 1 year of data which clearly shows
what is expected. Below I add some more comments as my contribution
to the discussion, prior to your Venice meeting that I could not attend,
unfortunately.

 From my point of view, given the DORIS EOP noise level, even when the
PM rates are not estimated, which is still about 10 times larger than
GPS and about 2-3 times larger than SLR and VLBI, the recommendation
for IDS would be not to estimate the PM rates, because DORIS will not
really substantially contribute to the EOP combination. But of course
this topic needs more investigation within IDS, and research should
continue in order to assess the best strategy and approaches. Improving
the quality of the DORIS daily EOP offsets will however improve the
DORIS TRF alignment to GPS, SLR and VLBI.

In the contrary, I am totally against fixing the PM rates to known
series, because this will completely denature the DORIS data. And
I don't understand why we need to do that for DORIS.
In the current configuration of TRF&EOP combination, at least
for ITRF2005, we match the PM offsets of the different techniques
at the standard noon epochs. Because satellite techniques already
provide the PM offsets at noons, we then do not need to make use
of PM rates (though they are included in the combination if they
are available). Only in case of VLBI there is no choice and so
the PM rates should be provided in order to be able to combine
VLBI EOPs with satellite techniques at noon epochs.

There is also the fundamental question about the physical meaning
of the "linear daily PM rate" which still need more thoughts and
discussion, not only for DORIS, but also for the other techniques.
Sub-daily EOP fluctuations may or may not behave linearly, and
given their magnitude (presumably less than 100 µas), only GPS
is, given it is precision, more sensitive to these EOP changes.

Best regards
Zuheir

Pascal Willis wrote :

>Dear colleagues,
>thanks to all those who participated in the discussion.
>I reprocessed 1 year of DORIS data without estimating polar motion rates 
>and I confirm the first results: EOP RMS towards JPL/GPS solution (no 
>bias, no drift removed) go down to 0.2 - 0.8 mas, leading to a factor of 2 
>improvement, as expected.
>I also agree with Markus that if we don't estimate EOP rate, it will 
>create problems when further computations are done, this is exactly why we 
>estime polar motion and rate, per day, using 24 hours of data and no 
>additional constrain at JPL.
>     * Markus you will have to talk to the ILRS groups who decided some
>       time ago not to estimate these parameters anymore.
>
>I don't think that Analysis Centers are ready to submit 2 solutions (1 for 
>IDS to show best results and 1 for IDS with complete estimation) unless 
>one can be derived directly from the other (no reprocessing) adding some 
>type of constrains.
>It may be obvious, but people analysis these results have the possibility 
>to use both polar motion and polar rate and fix the rate to an known 
>series before analysis the polar motion results. This would allow 
>distribution of complete SINEX results (polar motion and rate) but showing 
>better polar motion results, if the technique (DORIS or SLR) does not have 
>the data strength to really estimate the polar motion rate per day.
>Concerning the DORIS LCA solution, none of the hypothesis that I proposed 
>seem to be correct (see complete answer from Jean-Michel Lemoine). 
>However, I think that connecting the EOP estimation every 6 hours and 
>forcing some smoothing (imposing condition on the rate) may add some 
>rather tight constrains.
>     * In my opinion, if the improvement does not come from this
>       smoothing, the LCA group has to explain how they can get a factor
>       of 2 improvements from their last solution. Not estimating the
>       rate would be one way (see above), but it is not the case. Some
>       tests could be conducted to show if the constrain used by LCA is
>       tight or not.
>
>At least, we have something to discuss at the Venice meeting, and maybe 
>also to better explain in the Position paper from Frank Lemoine, so that 
>people can understand the consequences of these choices.
>Best regards
>Pascal
>--
>Pascal Willis
>Jet Propulsion Laboratory, MS 238-646     Ph : 1-818-393-4748
>4800 Oak Grove Drive                      FAX: 1-818-393-4965
>Pasadena CA 91109                         Em : [email-masked]

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Zuheir Altamimi                           Email : [email-masked]
Institut Geographique National            Phone : 33 1 64 15 32 55
ENSG/LAREG                                FAX   : 33 1 64 15 32 53
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