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7/31/2019 Roll Error Correction New
1/11
Correction of the SWOT roll errorbased on adjacent
measurements
-
Application:1-day vs 3-day orbit subsampling
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7/31/2019 Roll Error Correction New
2/11
Truth
Observations withroll error only
Observations withroll error + whitenoise (NPP)
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7/31/2019 Roll Error Correction New
3/11
S l o p e
( a r c s e c )
Along-track direction (km)
True slope (averaged across-track)Slope with roll error Slope
with roll error + white noise contributionTrue slope 1 day later
True slope 3 days later True slope 10 days later
No hope to have a good roll correction with cross-over
basedtechniques because cross-over time difference is generally
longer than3 days (and up to 10 days) except in some precise
locations.
Solution: use the adjacent tracks for which time difference is 3
days
or preferably 1 day (according to the orbit chosen) everywhere
.
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7/31/2019 Roll Error Correction New
4/11
A r c s e c
^ 2 / k m
/ c y c
l e White noise contribution Total slope noise1-day slope
difference
3-day slope difference
10-day slope difference
True slope
Same conclusions confirmed !
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7/31/2019 Roll Error Correction New
5/11
Adjacent zone tothe next track
Higher than 30N/S, the adjacent zone is large !
No adjacent zone below 20N/S (unless we exploitSWOT signal
farther than 60km away)
Adjacent tracks
L a
t i t u d e e
Distance to nadir (km)
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7/31/2019 Roll Error Correction New
6/11
True ocean
Swath n+3 Swath n+2 Swath nSwath n+1
SWOT with roll error
True ocean
Corrected SWOT signal
Swath n+3 Swath n+2 Swath nSwath n+1
SWOT with roll error
True ocean
Method
We minimize the coast function with respect to the slope of each
across-track measurement. Resolved iterativelyfor the n swath all
together .
J H 2
adj . J s
2 H dist adj .
0-day(unrealistic)
1-day
3-day
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7/31/2019 Roll Error Correction New
7/11
Noisy signal: Rollerror + white noise(NPP)
2D pre-smootingbefore rollcorrection
After rollcorrection
Note: We do first a 2D pre-smooting because at small scales, the
slope contribution from thewhite noise is bigger than the slope
error resulting from the roll (see spectra previous slides)
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7/31/2019 Roll Error Correction New
8/11
Slopecorrection froma roll error onlysignal (no pre-smoothing
stepnecessary)
Slopecorrection froma roll error +white noisesignal (with
thepre-smoothingstep)
1-day orbit
S l o p e
( a r c s e c )
S l o p e
( a r c s e c )
True slope
Corrected slopeObserved slope
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7/31/2019 Roll Error Correction New
9/11
3-day orbit
Slopecorrection froma roll error onlysignal (no pre-smoothing
stepnecessary)
Slopecorrection froma roll error +white noisesignal (with
thepre-smoothingstep)
S l o p e
( a r c s e c )
S l o p e
( a r c s e c )
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7/31/2019 Roll Error Correction New
10/11
Slopecorrection froma roll error onlysignal (no pre-smoothing
stepnecessary)
Slopecorrection froma roll error +white noisesignal (with
thepre-smoothingstep)
1-day slope difference
3-day slope difference Residual slope error with 3-day orbit
Residual slope error with1-day orbit
Considered slope noise (roll+ white noise contribution)
A r c s e c
^ 2 / k m
/ c y c
l e
A r c s e c
^ 2 / k m
/ c y c
l e
Residual slope error with 3-day orbit
Residual slope error with1-day orbit
1-day slope difference
3-day slope difference
True slope
True slope
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7/31/2019 Roll Error Correction New
11/11
