Subject: correcting DORIS/JASON data using LEGOS/SAA model
From: Pascal Willis
To: jean-michel lemoine -grgs-61332 <[email-masked]>
Cc: Willy Bertiger <[email-masked]>, Bruce J Haines
Date: Tue, 30 Dec 2003 13:45:42 -0800

<[email-masked]>, Yoaz Bar-Sever <[email-masked]>

Dear Jean-Michel,

Happy New Year for 2004!!!

Here are some very good news for you. Congratulations for your clever 
way to handle the JASON/SAA problem.

following your latest email, I corrected the DORIS/JASON data using 
your correction file (JASON satellite drift tabulated per epoch).

For test purposes, I only processed 8 days in 2003 (94 to 101 = April 
4 to 11, 2003) in free network mode, using Gipsy/Oasis on a daily 
basis.

1)JASON-only positioning solution

The estimated stations positions agree quite well with my own 
reference (using 10 years of all DORIS data) combined over the 8 day 
period :

	N	E	V	everything in mm
  WRMS   38.6   57.8   51.9	JASON-only solution

In my opinion, this is very encouraging with only 1 satellite.

2)JASON + other satellites positioning solutions

I then combined the above 8 day solution with the regular weekly 
solution that I had previously computed using all satellites but 
JASON. This is only a combination at the normal matrix level.

Here are the results (comparison with my position/velocity reference) :

	N	E	V	everything in mm
  WRMS   17.0   16.7   19.3	all satellites but JASON (previous processing)
  WRMS   13.7   14.6   12.8	all satellites including JASON

This is the first time that I can see a real improvement in the 
positioning results using the DORIS/JASON data. In my opinion, it 
totally validates your approach of the SAA effect.

It could be interesting to continue these tests on other periods of 
time as some weekly solution give better results than others 
(sometimes closer to 10 mm) without using JASON data.

3) combining all satellites at the data level

In this second approach, I do not combine the normal matrix of 
stations positions coming from JASON with the normal matrix of 
position coming from all satellites but JASON, but I really process 
the data together, putting tight constraints on clocks and 
tropospheric parameters.

The first good news is that the DORIS residuals are now much smaller 
and closer to what we expect (3.8 mm/s for JASON)

Here are the new results over 8 days :

	N	E	V	everything in mm
  WRMS   16.1   14.5   15.7	all satellites (at DORIS data level)

The results are definitively better with JASON but not as good as 
with the normal matrix approach. This might mean that the way I 
presently apply the correction does not account for all of the SAA 
effect.

In my opinion, your corrections should also be used by the SOD to 
preprocess the DORIS measurements as there are presently more and 
more DORIS/JASON data rejected. This is an important and logistically 
difficult issue that you could discuss internally in Toulouse in 
order to help the JASON/SWT team get sufficient DORIS/JASON data in 
the future.

I would appreciate if we could conduct some more extensive tests on a 
much longer period of time to have a more definitive opinion on these 
results.

I could also send you a complete TOPEX/JASON clock drifts estimations 
series that could help you refine your model.

If you have different versions of your model, it could be easy for me 
to tests several of them to verify how different they are.

See also my technical question in my previous message about the time 
scale used in your file.

Best regards
Pascal

NB: Please feel free to distribute this message. As you did all the 
work you should take the initiative on that.
-- 
Pascal Willis
Jet Propulsion Laboratory, MS 238-646     Ph : 1-818-393-4748
4800 Oak Grove Drive                      FAX: 1-818-393-4965
Pasadena CA 91109                         Em : [email-masked]