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ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
1
STRATEGY for ENVISAT RADAR ALTIMETRY
CROSS-CALIBRATION and VALIDATION
J. Benveniste (1) and
the ENVISAT RA-2/MWR Cross-Calibration and
Validation Team
(1) European Space Agency,
Earth Observation Applications Department
ESRIN, Via Galileo Galilei, I-00044, Frascati (RM), Italy,
(e-mail: Jerome.Benveniste@esrin.esa.it)
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
2
Presentation Overview
· Objectives
· Relative Calibration definition
· Cross-Calibration and Validation Strategy
· The Cross-Calibration and Validation Team
· Requirements
· Experience from ERS-2 Commissioning Working Group
· Conclusion
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Objectives
· Cross-calibrated data products:-
Determine the relative bias between ENVISAT RA-2 (resp. MWR)
and ERS-2 RA (resp. MWR) at the mm level, or/and other
altimeters (T-P, GFO, Jason).
Rationale: to ensure ERS-ENVISAT data products consistency
· Validated data products:-
Geophysical validation of the processing models and the data
products.
Rationale: to authorize distribution of validated data products to all
users, within 6 months after launch.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
4
Relative Calibration
· Relative calibration, so-called cross-calibration, consists of estimating
the offset, plus its associated error, of each measured parameters by
differentiating geophysical level data products from two sources:
Envisat and a reference (e.g.,
ERS-2, T-P, GFO, Jason, buoys, models
).
These offsets, or relative biases, are the ensemble averaged deltas
between the two measurement sources.
Rationale: When the two measurement sources are of same nature
(e.g., ERS-2 RA and ENVISAT RA-2), the two mission-long time
series can be united with high precision.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Cross-Calibration and Validation Strategy
(1)
· (i) Cross-calibration of the RA-2 main geophysical parameters against
ERS-2 (and/or TP, GFO, Jason):-
Range, Significant Wave Height and Wind Speed (Sigma0)
» Global comparison at cross-overs and along the collinear tracks of
millions of globally distributed data (significant error reduction)
» Global statistical comparison against ERS-2, Models, In-situ data
· (ii) Calibration of MWR geophysical retrieval algorithms:-
against radiosondes (+spaceborne) measurements of water vapour.
cross-calibration against ERS (and/or other radiometers) of MWR
brightness temperatures and water vapour.
· (iii) Preparation of long-term drifts detection:-
Global tide gauge network for RA-2 range monitoring.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Cross-Calibration and Validation Strategy
(2)
· (iv) Product validation
Verify with real data the consistency of the product package
(document, media, format and actual dataset), global valid data
coverage, outliers, flags. Tune data editing criteria.
Quantifying the inherent validity and accuracy of the range and
altitude measurements, geophysical corrections, of the SWH and
Wind speed measurements.
Determination of orbit error in the RA-2/MWR data products (via
crossover analyses). Estimation of time tag errors by minimization
of crossover height differences.
The validated data will be used for the cross-calibration activities:
This will enhance the quality and thoroughness of the validation.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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RA2/MWR Cross-Cal and Val Team Composition
· Experts in the Members States have proposed to contribute to the
RA-2/MWR Cross-Calibration and validation via the AO proposals.
· A selection of AOs have been retained to contribute to the ESA core
cross-calibration and validation planned activity to reach the
objectives.
· Guest participants from non Member States have also been invited to
collaborate in the CCVT.
· Expert Support Laboratories, Ground Segment processor expert
· ESA Staff to coordinate Team and subgroups and to support activities
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Special Tasks
· Impact assessment of the novel automatic mode switching on
geophysical applications.
· Cross-calibration of low and medium resolution modes on ERS-2 ice
mode.
· Sea State Bias estimation in both Ku and S band.
· Dual frequency ionospheric correction validation
Using the novel S Band data.
· Dual frequency rain flag validation
Using the novel S Band data.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
9
Requirements
(1)
· Flying Formation Configuration
ENVISAT will fly a 35-day repeat orbit and the same ground track as
ERS-2, 30 minutes ahead. This meets the cross-calibration
requirements
(shorter than the decorrelation time of fastest meteocean signals)
.
Minimal relative cross-track distance to reduce the influence of cross-
track geoid gradient: 100m
(better was achieved for ERS-1/2)
·
Operation in low/medium resolution over ocean: 3 days in same part of 3 cycles.
· Data products
Near Real Time (FDGDR // URA/QLOPR)
Off-Line (IGDR, GDR // OPR ; SGDR // WAP)
Other altimeter/radiometer data
In situ data and models
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Requirements
(2)
· Validated Orbit
Best available validated orbit will be used, including validated
DORIS tracking and laser ranging data.
· Wet tropospheric path delay correction
Model correction will be used until the MWR has been calibrated
and validated
» The ERS-2 MWR took 50 days to stabilise.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Results from ERS-2/ERS-1 Cross-Calibration
(1)
· URA, QLOPR and OPR products were validated
· The calibration values for each geophysical parameter estimated by
the different institutes converged to similar values within the specified
error limits
This provides high degree of confidence in the results
This shows the robustness of the calibration strategy
· Simultaneous flight of altimeters on ERS-1 and ERS-2 permitted to
discover jumps (up to 4 cm) between RA switch-offs.
ERS RA cal. scheme (SPTR) allows to derive a correction file
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Results from ERS-2/ERS-1 Cross-Calibration
(2)
· Sigma
0
in URA/QLOPR and OPR were adjusted by global statistical
comparisons
There is therefore no bias between ERS-2 and ERS-1
Wind speed was validated against ECMWF model
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Results from ERS-2/ERS-1 Cross-Calibration
(3)
· Controversy on SWH
Buoys Vs. WAM
Calibration coefficients eventually agreed!
» But it was recommended not to apply them is the products
More work needs to be done in the Wave community:
» Calibration seems to depend on sea age.
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Results from ERS-2/ERS-1 Cross-Calibration
(3)
· Microwave Radiometer
Gains were stable 50 days after switch-on
Water vapor was calibrated with radio soundings
Path delay well cross-calibrated with ERS-1 MWR (within 2 mm).
Verified with TMR
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Validation and Cross-Calibration output
· URA, QLOPR and OPR were verified and used by the RA CWG. This
was an enhancement to the validation work
An OPR validation report was sent to all RA & MWR product
users, to others on request
· RA Cross-Calibration
The range bias is not significantly different from zero
» USO and SPTR correction files were created and made available to
users (on www)
Sigma
0
(wind) calibration value is applied in URA/QLOPR, OPR
Fine tuning of Waveheight calibration values
» Consensus: LUT unchanged, calibration to be applied by users
ERS-ENVISAT Symposium
ENVISAT RA-2/MWR Cross-Cal & Val Team
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Conclusion
· Solid experience of relative calibration and validation has been acquired
with the novel ERS-2/ERS-1 RA & MWR cross-calibration activities.
· The ENVISAT RA-2/MWR cross-calibration and validation activities are
based on proven methodology. Experience to apply the Cal/Val methods
exist in Europe in various institutes, each of which have their own
algorithms and methodology, experience, and areas of interest.
· The RA-2/MWR Cross-Calibration and Validation Team has been
constituted with AO PIs, ESLs, ESA staff, and is at work.
· The RA-2/MWR Cross-Calibration and Validation Plan is drafted and is
being peer-reviewed and refined by the Team.
Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry