Ionospheric mitigation schemes and their consequences for BIOMASS product quality O. French & S....
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Transcript of Ionospheric mitigation schemes and their consequences for BIOMASS product quality O. French & S....
Ionospheric mitigation schemes and their consequences for BIOMASS product quality
O. French & S. Quegan, University of Sheffield, UK
J. Chen, Beihang University, China
ESA, Holland, 20th May 2010
Task 200: Definition of Ionospheric Correction Schemes
Ionospheric Scintillation - Outline
• Summary of previous results
• Effect of 9m antenna
• GPS TEC data
• SAR simulator
• Correction strategies
Ionospheric Scintillation: Local Time
Summary
• Scintillation effects severe for Boreal latitudes at all local times
• For temperate and equatorial zones, much reduced levels of scintillation in general...
• ...BUT: severe post-dusk scintillations in the equatorial zone for orbits with local time later than 20:00.
Effect of 9m Antenna
• Shorter antenna of 9m does not change the previous conclusions, in fact very little apparent effect on PSF.
• However, increased resolution drastically increases required computing power to simulate a given area.
-Increased synthetic aperture;-Increased resolution on aperture;-Number of data points for 2D screen increases as d-4.
• Developed 1D slice simulator for which required number of datapoints scales as d-2.
GPS TEC data
• Spatial resolution at 2.5° latitude, 5 ° in longitude.• Temporal resolution of 2 hours.
GPS TEC data
GPS TEC data
• Still to do: perform comparison of IRI and GPS TEC data.
• Expect IRI to be “smoother” than GPS data- Compare IRI with window averaged GPS data
• GPS likely to be better data source - GPS data accurate within 3-5 TECU- IRI comprises a model fitted to experimental data
Correction Strategies
• 1D SAR simulator built that images point target • Incorporates arbitrary phase perturbation across synthetic aperture
- Polynomial, degree n;- 1D Ionospheric phase simulations.
• Simple multi-aperture mapdrift (MAM) correction strategy employed.
Correction Strategies: MAM
• Phase error modelled as
• Require N sub-apertures to estimate the ak’s
• Decrease in SNR with increasing number of sub-apertures limits N
• Simulations performed for 2π phase error per term in polynomial, i.e. ak = 2π/Lk
Correction Strategies: Mapdrift
Uncorrected and corrected images for N=4
Correction Strategies: Mapdrift
Uncorrected and corrected images for N=8
Correction Strategies: Mapdrift
Uncorrected and corrected images for N=4 and with simulated ionospheric phase
Correction Strategies: Mapdrift
• Mapdrift ill-equipped to correct for ionospheric scintillation
• Probably due to high order of taylor expansion required to capture phase perturbations
• Alternative is PGA which does not rely on model for phase error
Ionospheric mitigation schemes and their consequences for BIOMASS product quality
O. French & S. Quegan, University of Sheffield, UK
J. Chen, Beihang University, China
ESA, Holland, 20th May 2010
Task 500: Definition of Calibration Scheme
Calibration - Outline
• Fujita three target approach
• Chen-Quegan compact polarisation approach
• Preliminary results for HV full polarisation approach
• Further work
Fujita Three Target ApproachAssumptions
• crosstalk same for transmit and receive channel;• discount all quadratic terms;• zero-mean complex Gaussian noise.
Five parameters (4 complex, 1 real): • transmit and receive channel imbalance, Fr and Ft;
• hv and vh crosstalk, C1 and C2;• Faraday rotation, Ω.
Masaharu Fujita, ‘Polarimetric Calibration of Space SAR Data Subject to Faraday Rotation – A Three-Target Approach’, IEEE 2005
Fujita Three Target Approach
System model: M = RTFSFT + N
where
are transmit and receive distortion matrices,
is Faraday rotation matrix. N is additive noise.
Fujita Three Target Approach
Fujita Three Target Approach
Simulation parameters used (taken from original paper)• Fr = 0.9;• Ft = 0.9;• C1 = 0.1;• C2 = -0.1;• Ω from 0 to 360°;• Additive noise -20dB in power;• No error in scattering matrices.
Two estimators for each quantity, denoted a and b
Fujita Three Target Approach
Faraday Rotation, Ω
Fujita Three Target Approach
Receive channel imbalance, FR
Fujita Three Target Approach
Transmit channel imbalance, FT
Fujita Three Target Approach
vh crosstalk, C1
Fujita Three Target Approach
hv crosstalk, C2
Fujita Three Target Approach
vh crosstalk, C1 hv crosstalk, C2
Fujita Three Target Approach
Introduce phase into single quantity, C1
• Fr = 0.9;• Ft = 0.9;• C1 = 0.1 exp (iπ/12);• C2 = -0.1;• Ω from 0 to 360°;• Additive noise -20dB in power;• No error in scattering matrices.
Fujita Three Target Approach
Amplitude Phasevh crosstalk, C1
Fujita Three Target Approach
Amplitude PhaseTransmit channel imbalance, Ft
Fujita Three Target Approach
Limitations of Fujita approach:• Assumes equivalence of transmit and receive crosstalk• Poor phase results• Discounts all quadratic terms• Does not consider large TEC/FR
Incorporate GNSS TEC estimator
22
round
GNSSF
Chen-Quegan Approach
• Good results for compact polarisation
• System model: transmit in right-circular polarised, receive in linear H,V.
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eej
ee
SS
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VVVH
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RV
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Chen-Quegan Approach
• Five parameters:- Circular crosstalk on transmit, δc;- Crosstalk on receive, δ2 (hv) and δ1 (vh);- Channel imbalance on receive, f;- Faraday rotation, Ω.
• δc and f calculated first, ignoring quadratic terms
• All other quantities derived from these including quadratic terms
• Estimators optimised
Chen-Quegan Approach
• Uses at most 4 calibrators from- PARCx- PARCy Active- PARCp- Dihedral- Trihedral Passive- Gridded trihedral (x 2)
Chen-Quegan Approach
Amplitude PhaseChannel imbalance, f
|1| = |2 |= 0.1, |c| =0.32, arg{1}= arg{2}= arg{c}=0, = /4
arg{f} = /3 |f | = 1.5
Chen-Quegan Approach
Amplitude PhaseCrosstalk, 1
|f | = 1.5, arg{f} = /3, |2 |= 0.1, |c| =0.32, arg{2}= arg{c}=0, = /4
arg{}=0 |=0.1
Chen-Quegan Approach
• Excellent results for compact polarisation - Amplitude- Phase
• Reduced set of assumptions- Quadratic terms only initially discarded
• Need to extend to full HV polarimetric case
Full Polarisation Scheme
System model: M = RTFSFT + N
Where now
are transmit and receive distortion matrices,
is Faraday rotation matrix. N is additive noise.
Full Polarisation Scheme
Reduced assumptions• discount some quadratic terms;• zero-mean complex Gaussian noise.
Seven parameters (6 complex, 1 real): • transmit and receive channel imbalance, Fr and Ft;
• receive crosstalk, Cr1 and Cr2;• transmit crosstalk, Ct1 and Ct2; • Faraday rotation, Ω.
Preliminary results with same values as previously, using PARCx, PARCy, GT1 and GT2 calibrators.
Full Polarisation Scheme
Faraday rotation, Ω
Full Polarisation Scheme
Amplitude PhaseTransmit channel imbalance, Ft
Full Polarisation Scheme
Amplitude PhaseReceive channel imbalance, Fr
Full Polarisation Scheme
• Preliminary results show better (in)sensitivity to phase in crosstalk
Future work• Full analysis needs to be performed using method of Chen & Quegan.• Incorporate GNSS FR estimator• Consider suitability of calibrators