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

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Chen-Quegan Approach

• Good results for compact polarisation

• System model: transmit in right-circular polarised, receive in linear H,V.

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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