Subject: positioning results with Jason/SAA model
From: Laurent soudarin <[email-masked]>
To: [email-masked]
Date: Fri, 21 Oct 2005 16:53:53 +0200

Dear colleagues,

Because of the Cryosat launch failure, the renewal of the DORIS 
constellation is postponed to at least 2008 (Jason-2).
By then, there are strong chances that we lose an instrument again (Spot2 
is going over its 16 years !).
So it is critical to use the capacity of all the operating instruments for 
the IDS objectives.
Until now, Jason was not used for the geodetic products because of the 
South Atlantic Anomaly (SAA) effect on the on-board Ultra Stable Oscillator 
(USO).
A model of frequency evolution of the USO was designed and validated by 
CNES for the first DORIS receiver (called chain 2). An other one is 
currently being developed for the chain 1 (back-up receiver switched on on 
June, 28 2004).

This model was implemented within GINS and I used it to process the 
DORIS/Jason data, including data of the SAA stations, of course (7 months 
from the period Dec 03 - May 04). Here are some conclusions of this processing:

-Orbits computations show improvement of the orbits and decreasing of the 
residuals (~0.025 mm/s).

-Comparisons of the station coordinate solutions for Topex and Jason are 
similar except for 7 SAA stations:
HBK, Ascension, St Helen, Punta Delgada, Libreville, Santiago, Kourou. The 
SAA effect is not completely absorbed, but it has no effect on the orbit 
and on the other stations.

-When these 7 stations are removed from the Jason matrices, the 6-satellite 
combination leads to a gain of 5 to 10 mm in term of positioning precision 
wrt the 5-satellite combination (Spot-2,-4,-5, Topex, Envisat).

This is shown on the attached plot where the monthly precision is 
represented for 3 combinations:
5-satellites (yellow),
6-satellites keeping the SAA stations in Jason matrices (blue),
6-satellites without the SAA stations in Jason matrices (red).

These are rather encouraging results.
I think it's particularly important to focus on this point during the next 
IDS workshop.

Best regards
Laurent