Subject: [ids.analysis.forum] TR : Long term trends in the gravity field (forwarded)
From: "Lemoine, Frank G. (GSFC-6980)" <[email-masked]>
To: "[email-masked]" <[email-masked]>
Date: Tue, 8 Feb 2011 08:14:22 -0600

Dear IDS Analysis Forum-

I forward below the mail from John Ries,
sent in response to Luca's previous message on the effect
of long-term trends in the gravity field on the orbits
of Jason-1 & Envisat. John Ries discusses the recent
behaviour of C21/S21.

F.

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De : John C. Ries [[email-masked]]
Date d'envoi : lundi 7 février 2011 16:12
Objet : Re: Long term trends in the gravity field

C21/S21 is interesting. When the mean pole motion was relatively
linear (as it was from around 1976 to 1999), C21/S21 were reasonably
well predicted from the mean pole. But since around 2000, the mean
pole has diverged significantly from linear motion, hence the update
to the 2010 conventions for the mean pole. The interesting part is
that C21/S21 as observed from SLR and GRACE are not quite following
the new mean pole path...it's as if there is some 'memory'.

In the attached plots, you will see the original IERS2003 model for
C21/S21 based on the linear model (solid black line), and the
IERS2010 model for C21/S21 if one used the new mean pole to predict
C21/S21 (dashed red line). The actual estimates are shown along with
a model I created that I cludged assuming some memory of the original
straight line path. As you can see, using the new mean pole for
computing C21/S21 is almost as bad as using the old one, though I
expect the old one to continue to diverge with time.

So using observed values for C21/S21 is probably the best thing to
do, if you have something to use. If one does use the new mean pole,
it looks like the error is currently only on the order of 5e-11.

Best regards,
JR

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