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