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 07:56:59 -0600

Dear IDS Analysis Forum-

I forward below the message of Luca Cerri (CNES)
concerning the testing of ten-day gravity fields from
the GRGS and the difference they seem to create with
a mean model in orbits of Jason-1/2 and Envisat.

Best regards
F. Lemoine
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________________________________________
De : Cerri Luca [[email-masked]]
Date d'envoi : lundi 7 février 2011 12:49
 
Objet : Long term trends in the gravity field

Dear all, I hope you are doing well.

I have recently tested the time series of the 10-day gravity fields from
GRGS
http://grgs.obs-mip.fr/index.php/fre/Donnees-scientifiques/Champ-de-grav
ite/grace/release02
on Envisat and Jason1 over the timespan sept.2009-aug.2010 .

- The time series significantly diverges from the mean model that
we are still using in our "GDRC" POD standards (which includes drifts
only
in the deg. 2,3,4 zonals and in C21/S21 from iers2003 mean pole).
Our POD standards are neglecting important trends in the mean gravity
field
(Greenland ice sheet, GIA and hydrology).

- C21/S21 of GDRC standards are obtained using the mean pole from
iers2003
which is significantly different from the mean pole of iers 2010 .
The time series of C21/S21 from the 10-day fields follows quite well
the mean pole iers2010. Although the difference of C21/S21 is large,
it has little impact on the orbit of either Envisat and Jason.

- However, the impact on the orbit due to changes in other harmonics
is significant:

Mean radial orbit difference from Sept 2009 to Aug 2010
on dynamic DOR+SLR orbits (1/rev evey 24 hours, Drag every 6 hours)
(see attached plots)
                        West     East
Envisat
Grace10day - GDRC      -15 mm  +15 mm

Jason1
Grace10day - GDRC       +7 mm   -7 mm

- SLR fits are also improved when using the 10-day fields.

- The above radial orbit difference would compensate the SSH difference
observed by the CLS calval team between Envisat and Jason.
(see Faugere et al, slides 6 and 7
http://www.aviso.oceanobs.com/fileadmin/documents/OSTST/2010/oral/19_Tue
sday/Tuesday_afternoon/faugere.pdf)

Given the above facts we plan to update our POD standards using a mean
model
that includes drifts (and ITRF2008). These drifts are obtained
using 8 years of Grace data (2003-2010) and will be in the next mean
field which is being finalized by the GRGS/GFZ team.

This will not solve everything, because some harmonics have a noticeable

non linear behaviour (C31/S31 for instance) and are not well represented

by the mean model.
So we might also make the operational GDR orbit a bit more
reduced-dynamic,
within the limits imposed by DORIS observability.
I'm looking into possible options.

Of course, any comment is welcome, and we are open to discussion.

Thank you !

Best regards

Luca
CNES POD Team

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