Subject: Re: estimating DORIS pole motion rates? 1-week preliminary results
From: Zuheir Altamimi <[email-masked]>(au lieu de Laurent Soudarin <[email-masked]>)
To: [email-masked]
Date: Thu, 23 Mar 2006 11:29:31 +0100

Dear Pascal,

Thank you for making these tests that I personally wanted to
see in order to evaluate DORIS performance in EOP determination.
 From your tests (thought it is over one week), it appears
clearly that DORIS is just too week to be able to accurately
estimate polar motion rates (and may be LOD). In my opinion,
it is up to IDS to decide on the appropriate strategy to be
adopted. Note that for similar reasons, ILRS decided not
to estimate polar motion rates, but ILRS ACs do estimate
LOD. It would be worth then to test if estimating LOD
would change your results or not.

On the other hand, I think it is worth testing over a longer
time span to have more convincing conclusion as you said.
 From the ITRF2005 pre-analysis, using LCA new submission
where polar motion rates and LOD are not estimated, I got
similar RMS as yours wrt IGS EOP series. But Laurent imposed
some linearity constraints at sub-day and day boundaries, and
he should absolutely test and demonstrate that these
linearity constraints do not somehow force EOPs to the a
priori values.
In any case FIXING polar motion rates to external values should
be prohibited and I don't think that you or Laurent fix polar
motion rates.

Best regards
Zuheir

Pascal Willis wrote :

>dear Laurent and Frank,
>here is a discussion that Frank could initiate at the next IDS meeting in 
>Venice.
>Just to start the discussion, I recomputed the most recent week of DORIS 
>data (Jan 1 to Jan 7, 2006) in 2 different ways:
>A) usual IGN/JPL: estimating daily pole motion + daily polar motion rate + 
>daily UTC-UT1 rate
>B) usual LEGOS/CLS: estimating daily pole motion
>First, I don't see any change in the station coordinates results for both 
>cases. To test for any improvement, I compute RMS of comparison for all 
>stations with IGN04D02 solution (long-term solution) after a 7-parameter 
>transformation.
>     N(mm)    E(mm)    V(mm)
>case A    18.5    21.5    12.7
>case B    18.4    21.1    13.0
>However, I see some significant improvement in the polar motion itself. To 
>test for improvement, I do daily difference of polar motion at 12:00 with 
>JPL/GPS and compute a RMS of the 7 values without removing any bias. I use 
>Bulletin B as a priori in my DORIS computations:
>     XP(mas)    YP(mas)    sqrt((XP**2)+(YP**2))
>case A    1.01    0.40    1.09
>case B    0.68    0.54    0.87
>it also removes the asymmetry I usually see between XP and YP and that 
>Laurent does not see at all in his results. It could be related to the 
>estimation of the daily polar motion rates that are not totally observable 
>with DORIS.
>So, if we want to go from typically 1 mas polar motion results for DORIS 
>to 0.5 mas that may be an easy way to go. The other possibility is also to 
>add more smoothing to our daily results (imposing continuity or linearity 
>at day boundary).
>Is any of these choices useful and desirable?
>I am ready to stop estimating polar motion rate for DORIS, but I would 
>like to be 100% sure that nothing will be lost for IERS EOP, IERS CPP, 
>future ITRF, IDS combination,... I would also like to be sure that all IDS 
>groups will follow the same rules.
>Do you think I should do some additional tests on a longer time span to 
>get a more reliable and convincing conclusion? Then it could make sense to 
>enlarge the distribution list to the IDS Analysis Forum and ask for feed-back.
>What is your opinion?
>Thanks in advance
>Pascal

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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
6-8 Avenue Blaise Pascal
77455 Champs-sur-Marne, FRANCE
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