Subject: preliminary comparisons of weekly DORIS solutions : LCA et IGN/JPL - scale factor
From: Pascal Willis <[email-masked]>
To: IDS_Analysis_Forum <[email-masked]>
Date: Tue, 4 Jan 2005 10:06:23 -0800

Dear colleagues,

I did some preliminary investigations on the first DORIS weekly SINEX 
solutions available at CDDIS (from week 1148 = 02006 to week 1290 = 
04312)

It could be interesting in the future to be able to compare with the 
GRGS weekly solutions as done by Richard Biancale for his own 
combinations or with the Geoscience Australia solutions (or others).

It would also be interesting to see if a combined solution (even with 
only 2 DORIS solutions) provides better results or not.

1) Here is what I have done

I first constructed an external DORIS reference by using a GPS-only 
cumulative solution (IGS combined, position and velocity) and adding 
the GPS-DORIS local ties with proper weight to derive 
positions/velocities and full covariance matrix for DORIS station. 
Everything is then expressed in ITRF2000 (as much as possible).

For each week, I projected the LCA (resp the IGN/JPL) weekly solution 
and transformed it into the reference frame defined by my GPS 
reference expressed at the epoch of the measurement.

This is an easy way to look for accuracy independently of the number 
of DORIS sinex solutions available. Of course, the solutions relies 
very much on the accuracy of the local ties, but they are quite 
numerous for DORIS and usually better than 3 mm.

For each week and each time series I can then derive a scale factor 
and for station coordinates I can obtain a WRMS values over the whole 
network.

2) Here are the preliminary results

I only focused on scale stability and weekly residuals. A similar 
study should be conducted for the geocenter too as well as at EOP 
values.

2.1) Scale factor

Weekly scale towards ITRF2000 (see plot, can be inserted in MS Word 
as a picture)

scale		mean value (ppb)	standard dev (ppb)
LCA		 9.03			1.29
IGN/JPL		-3.26			0.89

The scale bias is much larger for LCA but of opposite sign.

After removing a bias, the IGN/JPL scale seems much stable with time 
(the LCA scale has much variability).

However, it can also be seen in the attached plot that around week 
1170 (corresponding to the change of DORIS constellation from 3 to 5 
satellites = new SPOT5 and ENVISAT satellites), there is a 
discontinuity in both the LCA and in the IGN/JPL scale, unfortunately 
of opposite sign and of different amplitude.

if I now only take the data with 5 satellites, I get the new statistics

scale		mean value (ppb)	standard dev (ppb)
LCA		 9.53			0.68
IGN/JPL		-3.55			0.65

The mean values are almost the same (as the period with 3 satellites 
was small compared to the period with 5 satellites) but the standard 
deviations are much smaller, showing that the previous standard 
deviation were showing mostly the (small) discontinuity due to SPOT5 
and ENVISAT with a somehow larger effect on the LCA series (as the 
discontinuity is larger).

(the end of the message will appear in a second email as the total 
email was too large)
-- 
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]