[RADS] TOPEX Side A/B bias ... again

Remko Scharroo remko at altimetrics.com
Mon Oct 6 17:26:32 CEST 2008

Dear RADS users,

Eric Leuliette and I worked on the TOPEX data again and had to  
conclude that the change made at the end of June was premature. Then  
we decided that there was a 6 mm bias between Side A and Side B. We  
determined this bias by trying to remove a discontinuity between the  
last cycles of Side A operation and the first cycles of Side B  

However, we figured out now that I made an error in the way the SSB is  
applied during the period of degradation of Side A. I first corrected  
SWH, and then recomputed the SSB. However, reports from NASA/WFF show  
that the degradation of Side A not only affects SWH, but also the  
range. And that the effect on the range more or less counterbalances  
the effect on SSB (through SWH). Hence, it is better to base the SSB  
on the original (corrupted) SWH.

Doing this, the average sea level increases to about 6 mm at the end  
of side A operations, more or less the bias that we computed for side  
B. This means that Side A and Side B were, in fact, unbiased.

In the current data that were uploaded at the beginning of the  
weekend, the TOPEX degradation is handled as follows.
- Compute SSB based on the original SWH, also during the period of PTR  
- Afterwards, apply a bias to the SWH as per Queffeulou [2003]
- Do NOT apply any range bias for Side B

This effectively raises sea level trend again by about 0.4 mm/year.

As a result all the reference frame offsets of the other satellites  
needed to be adjusted.

All of these are minor changes to the data files, so rsyncing should  
go relatively fast.


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