Subject: JASON/DORIS oscillator - SAA effect on stations positioning
From: Pascal Willis <[email-masked]>
To: Jean-Paul Berthias <[email-masked]>
Cc: "John C. Ries" <[email-masked]>, Nikita Zelensky <[email-masked]>, Bruce J Haines <[email-masked]>, Martine Feissel <[email-masked]>, Gilles Tavernier <[email-masked]>, Pierre Sengenes <[email-masked]>, Flavien Mercier <[email-masked]>
Date: Wed, 23 Apr 2003 10:39:03 -0700

Dear Jean-Paul,

following my previous messages, new stations seem to be affected by 
the SAA effect (see figure 1). In my presentation at the IDS Workshop 
in Paris, I presented the PDMB stations as a possible outlier. In 
fact, this is not the case, as now Toulouse seems now to be also 
affected. PDMB is effectively affected by the same effect.

My guess is that all stations who see the JASON satellite while in 
the SAA region will be affected at some point in time.

Of course the effect is less visible for POD as dynamic models are 
more important and also because the CNES preprocessing tend to reject 
more or more of these data as the effects become larger even on the 
POD processing. One more time an additional parameter per station 
(station drift) will not solve the problem but only mitigate it.

I am worried about 3 things :

	- the phenomena does not seem to decrease (see figure 2, 
showing results for Arequipa). The effect is still linear in time 
since the launch of the satellite. This may contradict CNES early 
predictions about a possible stabilization of the effect. The only 
improvement I see is due to the CNES data preprocessing that tends to 
reject data when the effect becomes to large (equivalent more than 
0.5 to 1 m in position). For example, AREB which used to have 4,000 
points per month has now less than 1,000. The consequences can be 
seen in the 3rd figure, as the estimate standard deviation grow 
larger (there is less and less data) at an increasing rate.

	- Every time I get new data (now up to cycle 43 ), the region 
of the world affected by the SAA effect still keeps growing. I am not 
sure that we have yet detected all the possible stations affected by 
the phenomena. As stated before, the limit is all stations that 
possess a DORIS measurement while the JASON satellite is in the SAA 
region. There may still be more to find.

	- actual CNES preprocessing tends to delete more and more 
data in this region of the world (these data are not even kept with a 
bad preprocessing flag in the file at CDDIS). My concern is that more 
and more data maybe lost in the future. At some point the lack of 
data over an entire region of the world may affect POD results. We 
may have to investigate this issue : assuming that all data in figure 
1 are lost, what would be the effect on the MOE and POE orbits of 
JASON?

Best regards
Pascal

NB: To create such a map, I compare my JASON weekly stations point 
positioning with a global solution (position/velocity) obtained over 
the last 10 years of data including all satellites except JASON. I 
then look for erroneous velocities with a zeroing effect at the 
launch of the JASON satellite and in the direction predicted by the 
error on the satellite clock acceleration.