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Journal ArticleDOI

Calibration of Spaceborne CTLR Compact Polarimetric Low-Frequency SAR Using Mixed Radar Calibrators

14 Mar 2011-IEEE Transactions on Geoscience and Remote Sensing (Institute of Electrical and Electronics Engineers)-Vol. 49, Iss: 7, pp 2712-2723

TL;DR: A novel algorithm for calibrating the circular-transmit-and-linear-receive mode spaceborne compact polarimetric SAR using mixed calibrators is proposed, which is able to correct precisely both FR and radar system errors.
Abstract: Spaceborne synthetic aperture radar (SAR) systems operating at lower frequencies, such as P-band, are significantly affected by Faraday rotation (FR) effects. A novel algorithm for calibrating the circular-transmit-and-linear-receive (CTLR) mode spaceborne compact polarimetric SAR using mixed calibrators is proposed, which is able to correct precisely both FR and radar system errors (i.e., channel imbalance and crosstalk). Six sets of mixed calibrators, consisting of both passive calibrators and polarimetric active radar calibrators (PARCs), are investigated. Theoretical analysis and simulations demonstrate that the optimal calibration scheme combines four polarimetric selective mixed calibrators, including two gridded trihedrals and two PARCs, together with total-electron-content measurements by the Global Navigation Satellite System system.
Topics: Synthetic aperture radar (57%), Radar (53%)

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Published paper
Chen, J., Quegan, S., Calibration of spaceborne CTLR compact polarimetric low-
frequency SAR using mixed radar calibrators, IEEE Transactions on Geoscience
and Remote Sensing, 49 (7), pp. 2712-2723
http://dx.doi.org/10.1109/TGRS.2011.2109065

1
Calibration of Spaceborne CTLR Compact
Polarimetric Low Frequency SAR Using Mixed
Radar Calibrators
Jie Chen
(1)
, Member, IEEE, Shaun Quegan
(2)
, Member, IEEE
(1) School of Electronic and Information Engineering, Beihang University (BUAA), Beijing, 100191, China
e-mail: chenjie@buaa.edu.cn; Jie.Chen@sheffield.ac.uk
(2) School of Mathematics and Statistics, University of Sheffield, Sheffield, S37RH, UK
e-mail: s.quegan@sheffield.ac.uk
Abstract—Spaceborne synthetic aperture radar (SAR) systems operating at lower
frequencies, such as P-band, are significantly affected by Faraday rotation (FR) effects. A novel
algorithm for calibrating the circular transmit and linear receive mode spaceborne compact
polarimetric SAR using mixed calibrators is proposed, which is able to correct precisely both FR
and radar system errors (i.e. channel imbalance and cross-talk). Six sets of mixed calibrators,
consisting of both passive calibrators and polarimetric active radar calibrators (PARCs), are
investigated. Theoretical analysis and simulations demonstrate that the optimal calibration
scheme combines four polarimetric selective mixed calibrators, including two gridded trihedrals
and two PARCs, together with total electron content measurements by the GNSS system.
Index Terms— Calibration, Faraday rotation, Ionosphere, Compact polarimetry, Synthetic
aperture radar (SAR).
I. INTRODUCTION
There is growing interest in deploying lower frequency spaceborne Synthetic Aperture Radars (SARs)
for monitoring of the Earth, such as the P-band BIOMASS mission to measure forest biomass [1],
which is currently under Phase-A study by the European Space Agency. However, the ionosphere can

2
significantly affect such systems; in particular, L- and P-band spaceborne SAR measurements will
suffer from Faraday rotation (FR) [1]–[4]. Furthermore, two conflicting factors often affect the design
of such systems, namely the need for frequent global coverage and the need to maximize information
content, which often requires polarimetric information. Full polarimetry (FP) suffers from reduced
swath width compared to SAR systems transmitting on a single polarization, thus increasing the time
needed for global coverage. As a result, there has been growing interest in the compact polarimetric (CP)
SAR mode [5]-[14], because, for a given swath width, it operates with reduced data rate, system power
and pulse repetition frequency compared to a FP system, while still allowing estimates of some of the
key polarimetric quantities.
The first system of this type, proposed by Souyris et al. [5], [6], used the /4 CP mode, which
transmits H+V (45
o
linearly polarized) and receives echoes in the H and V polarizations. However, such
a system could also be severely affected by FR [11]–[14]. A way to reduce the effects of FR was
suggested by Raney [8] when he introduced the hybrid mode (also called the /2 mode [11] or CTLR
mode [12]) which transmits on circular polarization and receives on the two linear (H, V) polarizations.
This is a promising approach, since circular polarizations are preserved under FR [6], [10], [12] and
[13]; hence the polarization of the incident wave would be undistorted and only FR effects on the return
signal would need to be corrected.
Freeman [14] developed a system model for CTLR mode compact polarimetry with FR. On the basis
of this model, this paper proposes a novel algorithm for calibrating the CTLR mode using both passive
and active calibration targets. After an introduction to the system model in Section II, the mathematical
analysis in Section III leads to a set of new calibration algorithms and an optimized set of calibrators.
Computer simulations presented in Section IV verify the effectiveness of the approach; these include
simulations accounting just for radar system errors and FR, and simulations that also take calibrator

3
errors into account.
II. SYSTEM MODEL FOR CTLR COMPACT POLARIMETRY
A. Faraday Rotation
When a polarized electromagnetic wave traverses the ionosphere, its interaction with free electrons
and the Earth’s magnetic field leads to rotation of the polarization vector [4], [15]. This phenomenon is
known as Faraday rotation. The one-way FR for a SAR signal can be approximated as [15]

TECseccos
400
2
0
B
f
K
(1)
where f
0
is the carrier frequency in Hz, K is a constant of value 2.36510
4
[Am
2
/kg], B is the magnetic
flux density in Wb/m
2
, and
and
are the angles the wave-normal makes with the Earth’s magnetic
field and the downward vertical, respectively. TEC is the total electron content in TEC units (1 TECU =
10
16
electrons m
-2
). The “magnetic field factor”,
400
seccos
B
, is calculated at a height of 400 km.
B. System Model
We assume a CTLR mode SAR system that transmits right-circular polarization chirps and receives
linear (H, V) polarization echoes. In the presence of cross-talk, the transmitted electric field will include
a component from the orthogonal left-circular polarization, so has the form [14]

c
c
c
V
H
j
jj
T
T
1
1
2
1
11
2
1
where
c
is a cross-talk parameter.
With Faraday rotation, , the electric field incident on the Earth’s surface will be [14]

j
c
j
j
c
j
V
H
i
V
i
H
eej
ee
T
T
E
E
2
1
cossin
sincos
(2)
Freeman [14] introduced a system model for this CTLR mode, in which the measured scattering vectors
are given by

4


2
1
1
2
cossin
sincos
1
2
1
,
N
N
eej
ee
SS
SS
f
erA
M
M
j
c
j
j
c
j
VVVH
HVHH
j
RV
RH
(3)
where S
HH
, S
HV
, S
VH
and S
VV
are the components of the true scattering matrix, M
RH
and M
RV
are the
components of the measured scattering vector, f denotes channel imbalance,
i
, i = 1-2, are crosstalk
terms in the receiving channel, and N
i
, i = 1-2, are additive noise terms present in each measurement.
III. CALIBRATION ALGORITHM VIA MIXED CALIBRATORS
A. Signatures of Mixed Calibrators
By mixed calibrators we refer to a set of passive and active radar calibrators operating in combination.
Their use for calibrating spaceborne FP SAR systems is discussed in [16]-[22]. Passive radar calibrators
usually consist of the dihedral, trihedral and gridded trihedral (the classical trihedral with gridded base
wires or thin plates [22], see Fig.1(c)), while the polarimetric active radar calibrators (PARCs) include
three types [16], denoted as PARC
X
, PARC
Y
and PARC
P
, respectively, having signature matrices:
01
00
X
S
00
10
Y
S
11
11
P
S
(4)
where PARC
X
and PARC
Y
are polarimetric selective active calibrators.
For the passive calibrators illustrated in Fig.1, the scattering matrixes can be written as [22]



10
01
,
,
Tri
j
TriTri
eAS
(5)



2cos2sin
2sin2cos
,
,
Di
j
DiDi
eAS
(6)


22
2
222
,
sincossincossin
sincossinsin
sincossin
,
Gt
j
Gt
Gt
eA
S
(7)
where A
Tri
, A
Di
and A
Gt
are gain factors,
Tri
,
Di
and
Gt
are phase factors,
and
are the azimuth and
elevation angles, and
is the rotation angle of the dihedral.
Without loss of generality, we assume that the gain and phase factors in the ideal responses of (4)-(7)
are known, and can be normalized for simplicity. Then we have
10
01
Tri
S
10
01
Di
S
00
01
1Gt
S
10
00
2Gt
S
(8)
where S
Tri
, S
Di
, S
Gt1
and S
Gt2
denote the signature matrices of the trihedral, dihedral (
= 0) and gridded

Citations
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Hongsheng Zhang1, Hui Lin1, Yu Li1, Yuanzhi Zhang2  +1 moreInstitutions (3)
Abstract: Synthetic aperture radar (SAR) data can provide complementary information to improve the mapping of urban impervious surfaces. However, most studies have focused on using only single polarization SAR data. This paper presents a comparative study on the combined use of multispectral optical data and dual polarization SAR data to identify urban impervious surfaces. The experimental results using SPOT-5, TerraSAR-X and ALOS PALSAR data were consistent compared with our previous results using single polarization SAR data. The two-fold result showed that polarimetric SAR images were generally superior to single polarization SAR data for extracting impervious surface areas, although not every individual polarimetric feature could provide a positive result for impervious surfaces mapping. Compared with using only optical and SAR data, the separate HH and HV polarization data improved the accuracy of the results. The incorporation of both Entropy and Alpha features also improved the accuracy. However, the HH/HV ratio and the separate use of coherence did not provide positive results. Noticeably, a combination of all of the dual-polarimetric SAR features was capable of obtaining the best accuracy, with an improvement of approximately 3.5% compared with that of only using SPOT-5 images. This result indicates the superiority of dual-polarimetric SAR data over single polarimetric SAR data for the mapping of urban impervious surfaces.

40 citations


Journal ArticleDOI
Yu Li1, Yuanzhi Zhang1, Jie Chen2, Hongsheng Zhang1Institutions (2)
TL;DR: An improved reconstruction algorithm is proposed for compact polarimetric (CP) synthetic aperture radar (SAR) on oil spill detection that can iteratively reconstruct quad-pol SAR images from CP SAR data.
Abstract: An improved reconstruction algorithm is proposed for compact polarimetric (CP) synthetic aperture radar (SAR) on oil spill detection. Based on the differences in statistical behavior between open and oil-covered sea surfaces, the proposed algorithm can iteratively reconstruct quad-pol SAR images from CP SAR data. During the experiment, it outperformed two existing compact SAR reconstruction algorithms in terms of both statistical and information theoretical analysis.

37 citations


Cites background from "Calibration of Spaceborne CTLR Comp..."

  • ...However fully polarimetric SAR systems suffer from the disadvantage of doubled pulse repetition frequency, which halves swath width [8] and challenges the power budget....

    [...]

  • ...and low susceptibility to cross-channel errors and noise [8], transmits circularly polarized signal and receives linearly horizontal and vertical polarizations simultaneously....

    [...]


Proceedings ArticleDOI
Ridha Touzi1, François Charbonneau1Institutions (1)
13 Jul 2014
TL;DR: The state of the art in compact SAR calibration is reconsidered and it is shown that all the existing Compact calibration methods are built on the assumption that the Compact transmits a perfect circular polarization (CP).
Abstract: The state of the art in compact SAR calibration is reconsidered. It is shown that all the existing Compact calibration methods are built on the assumption that the Compact transmits a perfect circular polarization (CP). Unfortunately, the actual technology does not permit the generation of a perfect CP wave. The impact of non circularity of the transmitted polarization on Compact polarization information is assessed. The variations with incidence angles of the helicity and ellipticity of the transmitted polarization are simulated for C-band and L-band Compact SAR using Radarsat-2 and Alos data. This permits quantification of the resulting radiometric error on RH and RV dual-pol measurements. The impact of the non circularity of the transmitted polarization on the degree of polarization (DoP) and Compact polarization synthesis is also quantified. The requirements on the calibration of compact SAR for the extraction of a meaningful polarimetric SAR information from compact, are then setup.

22 citations


Cites background or methods from "Calibration of Spaceborne CTLR Comp..."

  • ...Very limited work has been published on Compact SAR calibration [7], [8]....

    [...]

  • ...However, both Freeman and Quegan calibration methods [7], [8] assume that the Compact transmits a perfect circular polarization (CP)....

    [...]

  • ...Freeman’s model has been adopted in [8] and a calibration method was suggested using the Amazonian forest and corner reflectors....

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Journal ArticleDOI
Abstract: In this paper, we present a comparison between several algorithms for oil spill classifications using fully and compact polarimetric SAR images. Oil spill is considered as one of the most significant sources of marine pollution. As a major difficulty of SAR-based oil spill detection algorithms is the classification between mineral and biogenic oil, we focus on quantitatively analyzing and comparing fully and compact polarimetric satellite synthetic aperture radar (SAR) modes to detect hydrocarbon slicks over the sea surface, discriminating them from weak-damping surfactants, such as biogenic slicks. The experiment was conducted on quad-pol SAR data acquired during the Norwegian oil-on-water experiment in 2011. A universal procedure was used to extract the features from quad-, dual- and compact polarimetric SAR modes to rank different polarimetric SAR modes and common supervised classifiers. Among all the dual- and compact polarimetric SAR modes, the π/2 mode has the best performance. The best supervised classifiers vary and depended on whether sufficient polarimetric information can be obtained in each polarimetric mode. We also analyzed the influence of the number of polarimetric parameters considered as inputs for the supervised classifiers, onto the detection/discrimination performance. We discovered that a feature set with four features is sufficient for most polarimetric feature-based oil spill classifications. Moreover, dimension reduction algorithms, including principle component analysis (PCA) and the local linear embedding (LLE) algorithm, were employed to learn low dimensional and distinctive information from quad-polarimetric SAR features. The performance of the new feature sets has comparable performance in oil spill classification.

22 citations


Cites background from "Calibration of Spaceborne CTLR Comp..."

  • ...Since the 2000s, CP SAR has become a new research trend [16,18,19]....

    [...]


Journal ArticleDOI
Hai Liu1, Huang Xiaoyun2, Feng Han2, Jie Cui1  +2 moreInstitutions (4)
TL;DR: It is concluded that radar polarimetry can provide not only richer information than single-polarization GPR, but also a reliable approach for orientation estimation of a subsurface elongated object.
Abstract: Ground penetrating radar (GPR) has been widely applied to the detection of subsurface elongated targets, such as underground pipes, concrete rebars, and subsurface fractures. The orientation angle of a subsurface elongated target can hardly be delineated by a commercial single-polarization GPR system. In this paper, a hybrid dual-polarimetric GPR system, which consists of a circularly polarized transmitting antenna and two linearly polarized receiving antenna, is employed to detect buried elongated objects. A polarimetric calibration experiment using a gridded trihedral is carried out to correct the imbalances and cross talk between the two receiving channels. A full-polarimetric scattering matrix is extracted from the double-channel GPR signals reflected from a buried elongated object. An improved Alford rotation method is proposed to estimate the orientation angle of the elongated object from the extracted scattering matrix, and its accuracy is validated by a numerical test. A laboratory experiment was further conducted to detect five metal rebar buried in dry sand at different orientation angle relative to the GPR scan direction. The maximum relative error of the estimated angles of the buried rebars in the migrated GPR images is less than 5%. It is concluded that radar polarimetry can provide not only richer information than single-polarization GPR, but also a reliable approach for orientation estimation of a subsurface elongated object.

21 citations


Cites background or methods from "Calibration of Spaceborne CTLR Comp..."

  • ...A polarimetric calibration method using a gridded trihedral has been successfully applied to a hybrid polarimetric SAR system [20]....

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  • ...Ignoring noise, the measured radar signals in the two linear polarization channels can be expressed as [20] [ MRH...

    [...]

  • ...Various methods have been proposed for calibrating full-polarimetric radar systems [18], [19], as well as hybrid polarimetric synthetic aperture radar (SAR) systems in the circular transmitting and linear receiving mode [20], [21], which have a similar architecture with our hybrid GPR system....

    [...]


References
More filters

Journal ArticleDOI
R.K. Raney1Institutions (1)
01 Jul 2006
TL;DR: Data from a CL-pol SAR yield to decomposition strategies such as the m-delta method introduced in this paper, which is the architecture of choice for two lunar radars scheduled for launch in 2008.
Abstract: A synthetic aperture radar (SAR) often is constrained to transmit only one polarization. Within this constraint, two aggressive measurement objectives are 1) full characterization and exploitation of the backscattered field, and 2) invariance to geometrical orientations of features in the scene. Full characterization implies coherent dual-polarization to support the four Stokes parameters. These are rotationally invariant with respect backscatterer orientation if and only if the transmission is circularly polarized. Given that the data products are the Stokes parameters, the receivers can use any orthogonal polarization basis. A SAR in hybrid-polarity architecture (CL-pol) transmits circular polarization and receives two orthogonal mutually coherent linear polarizations, which is one manifestation of compact polarimetry. The resulting radar is relatively simple to implement, and has unique self-calibration features and low susceptibility to noise and cross-channel errors. It is the architecture of choice for two lunar radars scheduled for launch in 2008. Data from a CL-pol SAR yield to decomposition strategies such as the m-delta method introduced in this paper.

409 citations


"Calibration of Spaceborne CTLR Comp..." refers methods in this paper

  • ...[8] R. K. Raney, “Hybrid-polarity SAR architecture,” IEEE Trans....

    [...]

  • ...[7] R. K. Raney, “Dual-polarized SAR and Stokes parameters,” IEEE Geosci....

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  • ...A way to reduce the effects of FR was suggested by Raney [8] when he introduced the hybrid mode (also called the π/2 mode [11] or circular transmit and linear receive (CTLR) mode [12]) which transmits on circular polarization and receives on the two linear (H and V) polarizations....

    [...]


Journal ArticleDOI
Jean-Claude Souyris1, P. Imbo2, Roger Fjortoft1, Sandra Mingot1  +1 moreInstitutions (3)
TL;DR: The polarization uniqueness in transmission of this mixed basis mode, hereafter referred to as the /spl pi//4 mode, maintains the standard lower pulse repetition frequency operation and hence maximizes the coverage of the sensor.
Abstract: We assess the performance of synthetic aperture radar (SAR) compact polarimetry architectures based on mixed basis measurements, where the transmitter polarization is either circular or orientated at 45/spl deg/(/spl pi//4), and the receivers are at horizontal and vertical polarizations with respect to the radar line of sight. An original algorithm is proposed to reconstruct the full polarimetric (FP) information from this architecture. The performance assessment is twofold: it first concerns the level of information preserved in comparison with FP, both for point target analysis and crop fields classification, using L-band SIRC/XSAR images acquired over Landes forest and Jet Propulsion Laboratory AIRSAR images acquired over Flevoland. Then, it addresses the space implementation complexity, in terms of processed swath, downloading features, power budget, calibration, and ionospheric effects. The polarization uniqueness in transmission of this mixed basis mode, hereafter referred to as the /spl pi//4 mode, maintains the standard lower pulse repetition frequency operation and hence maximizes the coverage of the sensor. Because of the mismatch between transmitter and receiver basis, the power budget is deteriorated by a factor of 3 dB, but it can partly be compensated.

302 citations


"Calibration of Spaceborne CTLR Comp..." refers background or methods in this paper

  • ...As a result, there has been growing interest in the compact polarimetric (CP) SAR mode [5]–[14], because for a given swath width, it operates with reduced data rate, system power, and pulse...

    [...]

  • ...[5], [6], used the π/4 CP mode, which transmits H+V (45◦ linearly polarized) and receives echoes in the H and V polarizations....

    [...]


Journal ArticleDOI
TL;DR: Modifications of the original reconstruction algorithm proposed by Souyris are discussed, which show potential to better reconstruct fully polarimetric data.
Abstract: Compact polarimetry is a technique that allows construction of pseudo quad-pol information from dual-polarization synthetic aperture radar (SAR) systems. Compact polarimetry showed promise of being able to reduce the complexity, cost, mass, and data rate of a SAR system while attempting to maintain many capabilities of a fully polarimetric system. In this paper, we study different transmit/receive configurations to determine which polarimetric configurations allow for superior reconstruction of the fully polarimetric data. We discuss modifications of the original reconstruction algorithm proposed by Souyris , which show potential to better reconstruct fully polarimetric data.

171 citations


"Calibration of Spaceborne CTLR Comp..." refers background or methods in this paper

  • ...This is a promising approach since circular polarizations are preserved under FR [6], [10], [12], [13]; hence, the polarization of the incident wave would be undistorted, and only FR effects on the return signal would need to be corrected....

    [...]

  • ...A way to reduce the effects of FR was suggested by Raney [8] when he introduced the hybrid mode (also called the π/2 mode [11] or circular transmit and linear receive (CTLR) mode [12]) which transmits on circular polarization and receives on the two linear (H and V) polarizations....

    [...]


Journal ArticleDOI
Zheng-Wen Xu1, Jian Wu, Zhensen Wu1Institutions (1)
Abstract: In this survey, we fully review almost all potential ionospheric effects on the performance of space-based radar systems (SBRs), which operate in the ambient ionosphere environment; in particular, we review the use of space-based synthetic aperture radar systems (SARs) for imaging. There are two families of effects involved. One is the effects of the background ionosphere (non-turbulent ionosphere), such as dispersion, group delay, refraction, Faraday rotation, and phase shift. The other is the effects due to ionospheric irregularities, such as refractive index fluctuation, phase perturbation, angle-of-arrival fluctuation, pulse broadening, clutter, and amplitude scintillation. These effects adversely affect SAR imaging in several respects, such as by causing image shift in the range, and degradations of the range resolution, azimuthal resolution, and/or the resolution in height (elevation). We also review ionospheric irregularity characteristics and descriptions, propagation channel statistics, ...

147 citations


Journal ArticleDOI
TL;DR: Both simple analytic approximations and numerical models are used to derive likely values of Faraday rotation and determine their impact on polarimetric imagery and derived products and methods for implementing corrections based on estimates of the Faraday rotated signal.
Abstract: Several proposed near-future spaceborne radar missions, such as the Advanced Land Observing Satellite (ALOS) and TerraSAR, will include an L-band instrument. At such low frequencies, the Faraday rotation in the ionosphere, which rotates the polarization plane of the radar signal, becomes an important consideration in instrument design. In this paper, both simple analytic approximations and numerical models are used to derive likely values of Faraday rotation and determine their impact on polarimetric imagery and derived products. One-way rotations exceeding 5/spl deg/ are likely to significantly reduce the accuracy of geophysical parameter recovery, such as forest biomass. On average, Faraday rotation can be neglected at solar minimum, but correction methods are needed at other times of the solar cycle and under disturbed conditions. Methods for implementing such corrections based on estimates of the Faraday rotation and prerotation of the transmitted signal are described.

139 citations


"Calibration of Spaceborne CTLR Comp..." refers background or methods in this paper

  • ...When a polarized electromagnetic wave traverses the ionosphere, its interaction with free electrons and the Earth’s magnetic field leads to rotation of the polarization vector [4], [15]....

    [...]

  • ...The one-way FR for a SAR signal can be approximated as [15]...

    [...]


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