Subject: anomalous TZ geocenter value from DORIS/SPOT4 data in 1998
From: Pascal Willis <[email-masked]>
To: IDS_Analysis_Forum <[email-masked]>
Date: Fri, 21 Jan 2005 07:32:33 -0800

Dear colleagues,

in the Position Paper presented at the last IDS meeting, there was 
some discussion concerning an erroneous geocenter value (TZ = -60 cm) 
that was found when using the DORIS/SPOT4 data in 1998.

Pascal Willis, Jean_Francois Cretaux, DORIS data Analysis Strategies, 
Position Paper, IDS Plenary Meeting, May 3-4, 2004, Paris, France
http://lareg.ensg.ign.fr/IDS/events/prog_2004.html

I think this problem is now better understood. At least we have an 
hypothesis that we can test.

1) presentation of the problem

As it can be seen from Figure 1, the Z geocenter (translation) 
derived from the DORIS/SPOT4 data alone presents a large systematic 
error (around -60 cm) in 1998 just after its launch (IGN/JPL results).

This problem is not visible in the multi-satellite solution using 
SPOT2 and TOPEX only data.

However this problem is visible in the SPOT2-SPOT4-TOPEX solutions at 
a lesser level [ -20 cm = (-60 + 0 + 0 / 3) ]. This is why the 
current IGN/JPL solutions at CDDIS do not use the SPOT4 data in 1998.

2) correlation with a phase center correction

After several considerations, it became apparent that a TZ error 
could be created by an erroneous phase center correction in the 
cross-track component. As it would not cancel between ascending and 
descending passes.

We have then reprocessed the SPOT4 data from 1998 and 1999 in POD 
mode in a very dynamic mode (no once per rev, station position fixed 
to IGN04D02 solution) using additional parameters for phase center 
correction offsets.

Figure 2 shows the results for the cross-track component

Here is a zoom of for the end of 1998

It can be seen that

after January 11, 1999 (included) the estimated parameter 
(cross-track correction) is compatible with a zero hypothesis (after 
cycle 32)

before January 11, 1999, except for a few days in late 1998 (in fact 
cycle 29), there is a clear offset of about 13 cm

We have then reprocessed at JPL the DORIS/SPOT4 1998 data without 
using the phase center correction available in the data files but 
computing them using the X,Y,Z offsets provided by CNES at the CDDIS 
data center. The TZ geocenter offset disappears.

We have also reprocess the DORIS/SPOT4 data in 1998 using the CNES 
correction in the data files but also adding a 13 cm offset 
correction in the cross-track phase center correct. The -60 cm offset 
also disappear in the TZ geocenter estimation.

3) consequences

It seems that DORIS/SPOT4 cycles 1 to 31 (except cycle 29) were 
processed using an erroneous value of the phase center correction 
offsets. Our estimation derived form this study of the cross-track 
error is 13 cm.

If CNES could reprocess one or two DORIS/SPOT4 cycles in 1998 using 
the current models, we could be able to verify our hypothesis.

We would then need to estimate the gain in precision that we could 
obtain from these early SPOT4 DORIS data for station position, 
geocenter variations and EOPs.

If this hypothesis is validated by CNES and if we can prove that CNES 
reprocessing of these earlier data could bring improvement in the 
DORIS results, we should start a discussion of how and when to  do 
it, specially in view of the ITRF2004 deadline (submission of time 
series before the end of February 2005).

Best regards
Pascal

-- 
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]