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dorisreport             22-Feb-2006 19:34:08 Message No 0510
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Author: Laurent Soudarin <laurent.soudarin@cls.fr>
Subject: Corrective model for Jason-1 DORIS Doppler data available


Dear colleagues,

the corrective model for Jason-1 DORIS Doppler data designed by CNES (see 
description summary below)
is available on the CB ftp site under 
ftp.cls.fr/pub/ids/satellites/CORRECTIVE_MODEL_JASON1

This model was optimized and documented in order to be able to deliver an 
operational
module (software) that will be used by the processing centers of DORIS data.
You can get this model and the technical note which gives the principle of 
the model
and allow its implementation and its use.
Please, read the Readme.txt file before getting the model.


The sensitivity of the ultra stable oscillator (USO) of DORIS/Jason-1 to 
solar radiative phenomena
was demonstrated a few months after the start up of the instrument. This 
sensitivity causes a
fluctuation of the frequency when the satellite crosses the area of the 
South-Atlantic Anomaly (SAA).
The principal consequence is the impossibility of using the measurements of 
the DORIS beacons
located in the SAA for cm-precision positioning since the real frequency of 
the on-board oscillator
is varying rapidly in that area. Moreover, these DORIS measurements do not 
contribute (or little)
to the determination of the orbit of Jason-1 because they are eliminated 
during the pre-processing
on residuals criteria.
To correct for this sensitivity to the effects of solar radiation, a model 
of frequency evolution of the
USO was designed and validated by CNES (Lemoine and Capdeville, Corrective 
model for Jason-1
DORIS Doppler data. Journal of Geodesy, Doris special issue, submitted).
This model of frequency correction makes a significant improvement in the 
orbit adjustment.
It takes into account the geographical characteristics of the SAA region 
(1x1 degree SAA grid)
as well as parameters of the USO's response to this external stimulation: 
an amplitude,
a relaxation time-constant and a memory effect of the SAA disturbance.


Kind regards,

Laurent Soudarin and Hugues Capdeville



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