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RA-2 Absolution Range and Sigma-0 Calibration and In-flight Verification
M. Roca, R. Francis, C. Zelli, S. Laxon, H. Jacksonand the Absolute Calibration Team
ESTEC/ESA, Keplerlaan 1, 2200 AG Noordwijk, Nederlands
ALENIA SPAZIO S.p.A. via Saccomuro, 24-00131 Roma, Italia
UCL, 17 Gordon St., London WC1H 0AH, UK
Presentation Overview
- Introduction
- Absolute Calibration organisation approach
- RA-2 Abs Range Calibration
- RA-2 Abs Sigma-0 Calibration
- RA-2 In-flight Instrument Calibration and Level1b Verification
- Summary and Conclusions
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.
Overall Organisation Approach
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.
Absolute Range Calibration Concept - Regional
Objectives: 1cm in bias error
1mm/yr in bias drift
Calibration Region - NW Mediterranean
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.
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
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.
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.
Evaluation of the Bias
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 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
- 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
- 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
The Principle
General radar equation:
Altimeter equation over distributed target:
Altimeter equation over transponder:
Bias Data Processing
RA-2 In-flight Instrument Calibration & Level 1b Verification
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
Overall Organisation Approach
Parameter Optimisation Example
Acquisition Mode Occurence (%)
After Seymour Laxon, MSSL/UCL
Summary (1)
- 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 (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.
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