ra-2 m. roca 1, r. francis 1, c. zelli 2, s. laxon 3, h. jackson 1 and all the absolute calibration...
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RA-2
M. Roca1, R. Francis1, C. Zelli2, S. Laxon3, H. Jackson1 and all the Absolute Calibration Team
RA-2 Absolute Range and Sigma-0 Calibration&
In-flight Verification
(1) ESTEC/ESA, Keplerlaan 1, 2200 AG Noordwijk, Nederlands (2) ALENIA SPAZIO S.p.A. via Saccomuro, 24-00131 Roma, Italia (3) UCL, 17 Gordon St., London WC1H 0AH, UK
RA-2
Presentation Overview
• Introduction• Absolute Calibration organisation approach• RA-2 Abs Range Calibration• RA-2 Abs Sigma-0 Calibration• RA-2 In-flight Instrument Calibration &
Level1b Verification• Summary and Conclusions
RA-2
Introduction
RA-2 represents a new generation of radar altimeters:• low height noise (higher PRF)
• robust on-board tracking and on-ground estimation
• autonomous resolution control (0.5 m, 2m, 8m)
• dual frequency (Ku and S-band) for ionospheric corrections
• individual echo sampling (full rate non-averaged)
• near real time (NRT) geophysical data products
To fully exploit the measurements: – relate these measurement to a reference system;
– knowledge of their stability;
– before: optimisation of the parameters from instrument and processors.
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RA-2 Abs Cal Overall Organisation Approach
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Absolute Range Calibration Requirements• High confidence range calibration requires many observations.
• 35-day repeat orbit + 6 month Commissioning Phase: Single reference point (e.g. Venice Tower) has 5 hits.
• Murphy et al (1996): regional calibration in English Channel for ERS-1 and TOPEX/Poseidon. Reuse of this concept for EnviSat.
• Many reference points regional calibration.
• Spatial + temporal averaging reduce some systematic errors.
• Extended calibration region increases the numbers of measurements.
Selected region: NW Mediterranean Sea
Supplemented by some dedicated sites.
RA-2 Abs Range Cal
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Abs Range Cal Concept - Regional
RA-2 Abs Range Cal
Objectives: 1cm in bias error 1mm/yr in bias drift
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Cal RegionNW Mediterranean
RA-2 Abs Range Cal
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RMS radial orbit error, in cm, for ascending (night) tracks.
1. The Orbit - POD
RA-2 Abs Range Cal
Local trajectory by: – local geometrical fit to
laser measurements;– constrained by
restituted global orbit.
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RA-2
The RA-2 measurements will be retracked and corrected by:
• Troposphere – Wet (water vapour): Upward-looking radiometers and Dual-
frequency GPS receivers at each dedicated site;– Dry (~ oxygen): Surface pressure measurement at dedicated sites.
• Ionosphere:– DORIS beacon at Toulouse and Corsica;– Dual-frequency GPS;– Interpretation via tomographic modelling.
• SSB: – Avoid data with significant SSB effect (60% of data SWH< 1m);– Evaluate the residual errors of used data by theoretical and
empirical models
2. The RA-2 Range - Corrections
RA-2 Abs Range Cal
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3. The Sea Surface Height - Measurements
RA-2 Abs Range Cal
At coast and dedicated sites (platforms and small islands):– Tide Gauges (existing or newly installed).– Associated meteo station;
In open sea:– Large, moored
GPS buoys;– Light, attended
GPS buoys;– Bottom pressure
recorders.
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3. The Sea Surface Height - The Modelling
RA-2 Abs Range Cal
• A model is being developed, which combines: - the Mean Sea Surface (from ERS-1, ERS-2 and other satellites),
- a tidal model, and
- a model of atmospheric forcing (driven by pressure and wind),
to provide instantaneous sea-surface height above the ellipsoid as a function of time and location, in the calibration region.
• During the calibration campaign the results of the model will be combined with the in-situ measurements.
• The model will be used to propagate these measurements to other points along the nearby tracks.
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Evaluation of the Bias
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RA-2 Absolute Sigma-0 Calibration
RA-2 Abs Sigma-0 Cal
• Absolute Sigma-0 Calibration has never been attempted.
• Scientific needs:– New Parameters: exploitation of altimeter data to derive new
parameters, specifically Wave Period.
– New models: dual frequency models exploiting the different frequency’s relative sensitivity to short (capillary) and long waves.
– Sea-State Bias: depends on probability density function of scatterers, and thus related to the factors determining absolute sigma-0.
RA-2
RA-2 Absolute Sigma-0 Calibration
RA-2 Abs Sigma-0 Cal
• Absolute Sigma-0 Calibration has never been attempted.
• Scientific needs:– New Parameters: exploitation of altimeter data to derive new
parameters, specifically Wave Period.
– New models: dual frequency models exploiting the different frequency’s relative sensitivity to short (capillary) and long waves.
– Sea-State Bias: depends on probability density function of scatterers, and thus related to the factors determining absolute sigma-0.
Requirement: » ±0.2 dB
» stability over 3 years of ±0.1 dB
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• Necessary to provide (relative) calibration corrections to all Ku-band altimeters, ranging from tenths to several dB’s.
• Absolute calibration has never been attempted.
• Absolute values are in error by several dB’s.
• RA-2 confidence in absolute calibration of about 0.4 dB before considering antenna gain errors.
Current Situation - Sigma-0 Corrections
RA-2 Abs Sigma-0 Cal tekton
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• The capability of using a Point Target (Transponder) to calibrate a Distributed Target is demonstrated, and meets the requirement.
• Dedicated transponder (under development at ESTEC)
• Sigma-0 depends on mean squared slope of the (low-pass filtered) sea surface and the form of the slope probability density function.
RA-2 Absolute Sigma-0 Calibration
RA-2 Abs Sigma-0 Cal
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General radar equation: Pr Pt G2
2
4 3 h4 L [W]
The Principle
RA-2 Abs Sigma-0 Cal
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General radar equation: Pr Pt G2
2
4 3 h4 L [W]
Pr Pt G2
2
4 3 h4 Lhcc [W]Altimeter equation over
distributed target:
The Principle
RA-2 Abs Sigma-0 Cal
RA-2
General radar equation: Pr Pt G2
2
4 3 h4 L [W]
Pr Pt G2
2
4 3 h4 Lhcc [W]Altimeter equation over
distributed target:
Altimeter equation over transponder:
Pr Pt G2
2
4 3 h4 L2
4GR Gelec GT [W]
The Principle
RA-2 Abs Sigma-0 Cal
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Bias Data Processing
RA-2 Abs Sigma-0 Cal
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RA-2 In-flight Instrument Calibration &Level 1b Verification
RA-2 In-flight Instr Cal & Level 1b Verif
Objectives:
1 Instrument verification of main capabilities and operations in all its modes - Switch-on Phase (SODAP)
2 Instrument parameter tuning and optimisation. Optimisation and verification of auxiliary data retrieval approach - Phase 1
3 Algorithm parameter optimisation, and verification of the use of auxiliary data in these algorithms - Phase 2
4 Instrument routine verification - Routine Phase
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RA-2 Abs Cal Overall Organisation Approach
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ParameterOptimisation
Example
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0 20 40 60 80Acquisition Mode Occurence (%)
After Seymour Laxon, MSSL/UCL
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• Interactions among all the calibration activities have been identified.
• Absolute range cal will be performed in NW Mediterranean by a combination of regional and dedicated sites;
• many specific measurement systems will be deployed;
• specific processing of measurements (by IECF in ESTEC/ESA);
• activities are harmonised with CNES (Jason-1), e.g. the POD.
Summary (1)
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• Need for absolute Sigma-0 Calibration is soundly based, and historically unsatisfied - First time Absolute Sigma-0 calibration;
• the capability of using a Point Target (Transponder) to calibrate a Distributed Target is demonstrated, and meets the requirement;
• specific processing of measurements (by IECF).
• Previous In-flight cal and verification activities to – optimise on-board and processing parameters,– Auxiliary data retrieval,– and prepare data for cal and val;
through dedicated algorithms in IECF.
Summary (2)
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