Transcript Presentation_OSTST07_Calval_J1_MA

Jason-1 GDR data
Performance assessment
M.Ablain, S.Philipps, J.Dorandeu, - CLS
N.Picot - CNES
OSTST Hobart 2007 – Performance assessment Jason-1 data
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Introduction
• CalVal activities – under CNES contract (SALP)
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Work routinely performed at CLS
Jason-1 data have been analyzed and monitored since the beginning of the mission
Monitoring of all altimeter and radiometer parameters
SSH performances at crossovers and along-track SLA
Specific studies : Impact of new retracking algorithms, new orbits, new geophysical
corrections, …
• Objectives
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1 – SSH Performances since the beginning of the mission
2 – GDR version ‘B’ reprocessing assessment
3 – Impact of new JMR correction
4 – Mean sea level
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Jason-1
TOPEX
1 - SSH performances
• Data used : GDR ‘A’ from cycle 1 to
135 and in version ‘B’ from cycle 136
onwards.
• The cyclic mean of SSH crossover
differences is stable and similar with
T/P one
Mean SSH crossover differences [cm]
• The map of mean differences shows
weak signal (< 2 cm) only with GDR
‘B’
=> Small hemispheric bias (1 cm) is
detected thanks to the very good
quality of new orbits
-3 cm
-3 cm
Mean SSH crossover differences from cycle 136 onwards [cm]
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1 - SSH performances
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Jason-1
TOPEX
GDR ‘A’
GDR ‘B’
Cyclic standard deviation of SSH
differences is very good :
– 5.2 cm RMS at crossovers for GDR‘B’
– Standard deviation decreases from cycle
136 onwards : GDR ‘A’ => GDR ‘B’
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Similar results for the SLA along track :
– 10.3 cm RMS along-track
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TOPEX performances slightly lower
than for Jason-1:
STD of SSH crossovers [cm]
Jason-1
TOPEX
– TOPEX SSH high frequency content is
lower than Jason-1 due to different
ground processing
– After applying similar retracking
algorithms (MLE4 & LSE), both missions
have the same high frequency content
 see dedicated Faugere’s talk
STD of along-track SLA [cm]
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2 - GDR reprocessing in version ‘B’
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GDR ‘A’ - Jason-1
GDR ‘B’ – Jason-1
Reprocessing of the GDR in version B :
– JPL (cycles 1 to 21) and CNES (cycles 128
to 135), in early 2006.
– Reprocessing of the GDR in version B by
JPL is on going from cycle 22 to 127
– Half of the cycles have already been
reprocessed (just before OSTST).
mean SSH crossovers [cm]
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The main evolutions in the GDR ‘B’ :
– new retracking : order 2 MLE-4,
– new precise orbit based on a GRACE gravity
model
– new geophysical corrections (tidal models,
DAC, Sea State Bias).
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GDR ‘A’ - Jason-1
GDR ‘B’ – Jason-1
Cyclic mean crossovers slightly more stable
Standard deviation improved from 6.1 cm to
5.2 cm with GDR ‘B’ data.
STD of SSH crossovers [cm]
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2 - GDR reprocessing in version ‘B’
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SSH differences at crossovers are very
homogenous with GDR ‘B’ :
– Large structures of high positive or negative
differences are observed using GDR ‘A’.
– These patches are removed using GDR ‘B’’
 New orbits using EIGEN-CG03C gravity
field explain this improvement.
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-3 cm
+3 cm
Mean SSH crossovers using GDR A orbits [cm]
Thanks to this new orbit quality, small
signals can now be detected such as an
hemispheric bias : +/- 1 cm :
– This signal corresponds to a pseudo time-tag
bias of about 0.3 ms
– A time shift of 0.173 ms has been added in
the GDR ‘B’ L1-B processing (seems wrong
sign), but the sign of this correction has been
assessed
 The origin of this weak signal remains
unknown.
-3 cm
+3 cm
Mean SSH crossovers using GDR B orbits [cm]
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3 - JMR correction
New JMR correction (GDR ‘B’)’
Former JMR correction (GDR ‘A’)
Drift, cycles 27 to 32
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GDR ‘A’ JMR corrections exhibits several
anomalies :
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a jump of 9 mm at cycle 69,
a drift of 5 mm between cycles 27 to 32,
60 day signals of almost 5 mm amplitude due to yaw
mode transitions.
Jump, Cycle 69
2002
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New JMR correction provided by PODAAC (in
GDR ‘B’) :
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2005
2006
Daily differences between JMR and ECMWF
model and in comparison to other missions
The jump at cycle 69 is very well corrected,
The drift is not completely removed
yaw mode transitions are reduced but still visible
A stronger signal is detected at the end (6mm)
Daily wet tropophere mean values are noisier
for Jason-1 than for the other missions =>
This apparent noise is linked to signals
generated by yaw mode transitions.
2002
2003
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2004
2005
2006
4 – JMR Correction
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Slope (JMR – Model) is -0.26 mm/year
and local slopes can reach 5 mm/year.
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Until now, the ECWMF model was used to
calculate the Jason-1 MSL (for stability) :
 Use of JMR leads to an increase of
0.26 mm/year of the MSL
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Considering that the JMR correction is not
completely well tuned, the use of the
ECMWF model is probably preferable for
MSL estimations (model stability has to be
monitored)
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Finally this analysis shows that the wet
troposphere correction is a major item in
the error budget of the Jason-1 MSL.
JMR – ECMWF Model = -0.26 mm/year +/- 0.02 (LSR)
2002
2003
2004
2005
2006
Global and local slopes of differences between
JMR and ECMWF model
-4 mm/year
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+4 mm/year
4 – Mean Sea Level
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T/P and Jason-1 MSL have to be fitted
together since T/P mission is finished,
applying :
– homogeneous corrections
– Jason-1 / T/P SSH bias
MSL slope (TP+J1) = 2.9 mm/year +/- 0.03 (LSR)
 MSL slope of 2.9 mm/year
Some discrepancies exist between the
two missions:
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 Studies are shared between
GOHS/LEGOS and CLS to estimate a
more realistic error budget
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2005
2003
2001
1999
1997
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TMR correction is used for T/P instead of the
ECMWF model for Jason-1,
T/P and Jason-1 orbits are not homogeneous
over the whole period.
Other corrections could impact the MSL as for
example the ECMWF pressure fields in the T/P
data.
1995
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- IB correction applied
- Seasonal signals removed
- GIA not applied
1993
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Conclusion
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Good performances of Jason-1 GDR data :
5.1 cm RMS at crossover, and stable
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Data performances better with GDR ‘B’:
Variance gain = 21 cm²  35 %
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0 cm²
100 cm²
SSH variance at crossovers using GDR “A” [cm²]
Some improvements will be provided in next
GDR release :
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New SSB (Labroue, Venice 2007)
JMR corrections
New Orbits
New geophysical corrections :DAC (MOG2D
HR)
0 cm²
100 cm²
SSH variance at crossovers using GDR “B” [cm²]
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