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Date: Wed, 4 Jun 2008 17:04:22 +0200
To: IDS_Analysis_Forum <ids.analysis.forum@cls.fr>
From: Pascal Willis <willis@ipgp.jussieu.fr>
Subject: estimating DORIS Solar Radiation Pressure (SRP) coefficients
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Dear colleagues,

Please use the IDS Analysis Forum to continue discussion on this topic

following my presentation at the last IDS Analysis Working Group Meeting
(AWG) in Paris, it was decided that I would distribute some information on
our recent study done with Marie-Line Gobinddass (PhD student)
concerning the DORIS satellite solar radiation pressure (SRP) coefficient

I remind you that all presentations of this DORIS meeting are available at :

http://ids.cls.fr/html/report/meetings/AWG200803.html

For your information, we have just submitted an article on this SRP subject:

M.L. Gobinddass, P. Willis, O. de Viron, A. Sibthorpe, N.P. Zelensky, J.C.
Ries, R. Ferland, Y. Bar-Sever, M. Diament, Systematic biases in
DORIS-derived geocenter time series related to solar radiation pressure
mis-modeling, JOURNAL OF GEODESY, submitted

Here is some basic information concerning our study:

1) Results

The current SRP models do not possess the required accuracy for DORIS
geodetic applications. This is especially true for the SPOTs satellites, for
which only a basic SRP model is available

For geodetic applications, due to the strong correlation with 1/rev
empirical accelerations, it is recommended to multiply the current SRP
models by a single coefficient, close to 1.0 which is satellite dependant,
(see after for proposal). This coefficient must be fixed in the daily
computations.
Otherwise, systematic errors at periods of 118 days and 365 days will affect
the geodetic results. See the article for more details

2) Method used

We reprocessed all DORIS data, estimating the station coordinates, a single
SRP coefficient per day. However, 1/rev empirical accelerations were not
estimated and were fixed to 0.0 in order to estimate a really dynamic orbit

3) Proposed SRP coefficients

ENVISAT  = 0.97
JASON = 0.92
SPOT-2 = 1.08
SPOT-3 = 1.08
SPOT-4 = 1.13
SPOT-5 = 1.03 (0.86 after 2008/014)
TOPEX = 1.03 (0.96 before 1993/208)

The discontinuities found for TOPEX and SPOT-5 are clearly identified in our
computations and relate to changes in solar panel orientation for SPOT-5
or to
the type of measurement  for TOPEX (chained vs  unchained mode).

I understood that CNES will soon release some information concerning
SPOT-5 on this forum. From our computation, we estimate that the
change of orientation of the SPOT-5 solar panel toward the Sun is close to
34 degrees

However, in the case of TOPEX, the discontinuity cannot be explained at this
point

Best regards
Pascal
-- 
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Pascal Willis
Institut de Physique du Globe de Paris	Ph : +33-(0)1-44-27-24-84
Etudes Spatiales et Planetologie 	FAX: +33-(0)1-44-27-73-40
4, place Jussieu, Case 89 		Em : willis@ipgp.jussieu.fr
Paris 75252 Cedex 5, France		http://www.ipgp.jussieu.fr/~willis/
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