RA-2 M. Roca 1, R. Francis 1, C. Zelli 2, S. Laxon 3, H. Jackson 1 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, 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.

RA-2 RA-2 Abs Cal Overall Organisation Approach tektonTeckton bold tekton

RA-2 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

RA-2 Abs Range Cal Concept - Regional RA-2 Abs Range Cal Objectives: 1cm in bias error 1mm/yr in bias drift tekto n Tekton bold

RA-2 Cal Region NW Mediterranean RA-2 Abs Range Cal

RA-2 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. tekton

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

RA-2 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.

RA-2 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.

RA-2 Evaluation of the Bias RA-2 Abs Range Cal tekton

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.

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

RA-2 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

RA-2 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

RA-2 General radar equation: The Principle RA-2 Abs Sigma-0 Cal

RA-2 General radar equation: Altimeter equation over distributed target: The Principle RA-2 Abs Sigma-0 Cal

RA-2 General radar equation: Altimeter equation over distributed target: Altimeter equation over transponder: The Principle RA-2 Abs Sigma-0 Cal

RA-2 Bias Data Processing RA-2 Abs Sigma-0 Cal tekton

RA-2 RA-2 In-flight Instrument Calibration & Level 1b Verification RA-2 In-flight Instr Cal & Level 1b Verif Objectives: 1Instrument verification of main capabilities and operations in all its modes - Switch-on Phase (SODAP) 2Instrument parameter tuning and optimisation. Optimisation and verification of auxiliary data retrieval approach - Phase 1 3Algorithm parameter optimisation, and verification of the use of auxiliary data in these algorithms - Phase 2 4Instrument routine verification - Routine Phase

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RA-2 Parameter Optimisation Example RA-2 In-flight Instr Cal & Level 1b Verif After Seymour Laxon, MSSL/UCL tekton

RA-2 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)

RA-2 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)