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

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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

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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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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.

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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.

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

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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.

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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

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4 3 h4 L [W]

Pr Pt G2

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4 3 h4 Lhcc [W]Altimeter equation over

distributed target:

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:

Altimeter equation over transponder:

Pr Pt G2

2

4 3 h4 L2

4GR Gelec GT [W]

The Principle

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Bias Data Processing

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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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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)