29 January 2015 Marine target detection in quad-pol synthetic aperture radar imagery based on the relative phase of cross-polarized channels
Author Affiliations +
J. of Applied Remote Sensing, 9(1), 096092 (2015). doi:10.1117/1.JRS.9.096092
Abstract
A focus on marine target detection in noise corrupted fully polarimetric synthetic aperture radar (SAR) is presented. The property of the relative phase between two cross-polarized channels reveals that the relative phases evaluated within sea surface area or noise corrupted area are widely spread phase angle region [−π,π] due to decorrelation effect; however, the relative phases are concentrated to zero and ±π for real target and its first-order azimuth ambiguities (FOAAs), respectively. Exploiting this physical behavior, the reciprocal of the mean square value of the relative phase (RMSRP) is defined as a new parameter for target detection, and the experiments based on fully polarimetric Radarsat-2 SAR images show that the strong noise and the FOAAs can be effectively suppressed in RMSRP image. Meanwhile, validity of the new parameter for target detection is also verified by two typical Radarsat-2 SAR images, in which targets’ ambiguities and strong noise are present.
Wang, Li, Zhang, and Guo: Marine target detection in quad-pol synthetic aperture radar imagery based on the relative phase of cross-polarized channels

1.

Introduction

Target detection based on synthetic aperture radar (SAR) images has been a hot topic because of its important application in maritime environment monitoring. Since the scattering fields from man-made targets, such as ships and oil platforms, are generally stronger than those from surrounding sea surface, the intensity-based constant false alarm rate (CFAR) algorithms have been widely established to distinguish targets from sea surface.12.3.4.5.6.7.8.9.10.11.12 However, the detection algorithms based on single-channel SAR data are sometimes ineffective due to various factors, such as speckle noise, the ambiguities of targets, and other strong noise induced by some uncertain reasons. Recently, with the availability of polarimetric SAR sensors, the polarimetric scattering behavior of targets can be described by the scattering matrix, through which features relating to both the intensity and the relative phase can be extracted. Recently, many polarimetric detection algorithms using polarimetric information provided by fully polarimetric SAR are successfully exploited for detecting targets, especially marine targets.78.9.10.11.12 In Ref. 12, Novak proposed the polarimetric whitening filter method for target detection. On the other hand, the coherent target decomposition (CTD) theory can also be used to observe ships because the scattering mechanisms of targets are generally different from that of surrounding sea surface. The results in Ref. 13 showed that even in low resolution, SAR imageries marine targets can also be distinguished from sea clutter by CTD theory. Generally, the backscattered field from sea surface is dominated by Bragg scattering with low entropy value. However, owing to the complex metallic structures of man-made targets, the scattering field from a man-made target always contains complex scattering mechanisms with higher entropy value. In Refs. 14 and 15, based on CTD theory, polarimetric SAR images were used to detect ships exploring entropy values. Note that the effectiveness of the entropy-based detector decreases with increasing wind speed because the scattering field from breaking waves also contains complex scattering mechanisms with higher entropy value. Although the methods mentioned above have been widely used to detect targets in SAR images, the strong noises induced by ambiguities and so on, which are often mistaken as real targets and cause false alarms, cannot be effectively removed.

In Ref. 16, the eigenvalues of the coherency scattering matrix are used to distinguish ambiguities from real targets. However, the method in Ref. 16 will no longer perform well in the case of strong depolarized scattering. In Ref. 17, Liu and Gierull found that in HV+VH image, the scattering fields of azimuth ambiguities can be suppressed effectively because the HV channel data of the first-order azimuth ambiguities (FOAAs) are approximately equal to VH channel in magnitude but shifted about ±π in phase. In Ref. 18, based on the intensity of HV+VH image (it is called as ambiguity free image HVfree by Velotto), a new parameter γ, which is a simple combination of slope and departure from the uniform distribution of the phase parameters in GK distribution, is proposed for target detection (refer to Ref. 18 for detail). Velotto et al. pointed out that γ is always smaller than 10 for real targets but larger than 10 for sea surface and ambiguities. The methods proposed in Refs. 17 and 18 are applicable to detect targets effectively in high-quality SAR images, by this I mean the SAR imageries without exotic noises. Nevertheless, besides ambiguities, many SAR images have also been corrupted by other strong noise induced by uncertain reasons. For the case of the poor-quality SAR images, methods of Liu and Velotto will be no longer in validity.

This paper mainly focuses on detection of azimuth ambiguities in noise severely corrupted fully polarimetric SAR images. We define reciprocal of the mean square value of the relative phase (RMSRP) as a new indicator of marine targets considering the different behaviors of phase difference over real targets and FOAAs. Based on the central limit theorem, we find that the mean value of RMSRP estimated through a 11×11 spatially moving window satisfies the Gaussian distribution well. The theoretical distribution of RMSRP is derived and in terms of this, a RMSRP-based CFAR algorithm is established for marine target detection. The experiment results using Radarsat-2 imageries acquired in fine quad-polarization mode are promising, demonstrating that our detection algorithm is not only robust for removal of azimuth ambiguities in SAR images but also is able to suppress noise induced by unknown reasons. This paper is organized as follows. In Sec. 2, the statistic properties of the relative phase and RMSRP are discussed, and the probability density function (PDF) of RMSRP for surrounding sea surface is derived. Section 3 addresses the RMSRP-based CFAR algorithm. Experimental results are presented in Sec. 4, whereas conclusions are given in Sec. 5.

2.

Definition of the New Parameter RMSRP

In this work, the relative phase is estimated by

(1)

φ=Arg(1Lk=1LSHVkSVHk*),
where SHV and SVH denote the single-look complex (SLC), HV and VH scattering amplitudes, and L represents the look-number.

For convenience, the new parameter RMSRP is represented by symbol Θ, and it is defined as

(2)

Θ=1ψ,
where ψ=φ2¯ denotes the mean value of φ2 within a 11×11 spatial window. From Eq. (2), one can easily conclude that (1) the narrower the PDF of φ, the higher the value of Θ is, and vice versa, and (2) the closer is the mean value of the relative phase from zero, the higher is the value of Θ, and vice versa.

2.1.

For Real Targets and Its FOAAs

Azimuth ambiguities in SAR images are caused by the discrete sampling of the Doppler signal at finite intervals of the pulse repetition frequency (PRF). The Doppler frequencies higher than PRF would appear as aliased, folded into the azimuth spectrum. Azimuth ambiguities occur at fixed along-track repeat positions with respect to the position of the actual target, i.e., Ref. 19:

(3)

x=x0+nλR02VPRF,
where x0 denotes the position of actual target, n=0,±1,±2, is the ambiguity index, λ and R0 denote the radar wavelength and the slant range, respectively. V is the relative speed between radar and target. For RS-2 data applied in this work, the theoretical distance between the FOAAs and the target is about 4.1 to 4.36km depending on the range position.

For fully polarimetric SAR systems, the horizontal (H) and vertical (V) pulses are transmitted alternately. Thus, the time interval between HV and VH signals is 1/2PRF. If the HV ambiguous response occurs at position x=x0+n(λR0/2V)PRF, then the corresponding VH ambiguous would occur at

(4)

x=x0+nλR02VPRF+V2PRF.

The azimuth phase history for a target positioned at x0 can be represented by

(5)

s(x)=exp(j4πR0λ)exp{j2πλR0(xx0)2}.

Substituting Eqs. (3) and (4) into Eq. (5), the relative phase between the ambiguous HV and VH returns is obtained as

(6)

φ=nπ+2πλR0V24PRF2.

For real targets, the reciprocity condition, i.e., SVH=SHV, should be satisfied. However, there exists an additional phase shift (2π/λR0)(V2/4PRF2) in Eq. (6), which can be removed by multiplying a complex number during the resampling processes so that Eq. (6) is simplified to φ=nπ. Then the relative phase between two cross-polarized channels is

(7)

φ={0for real target±πforFOAAs.

2.2.

For Sea Surface and the Strong Noise

In SAR images, for sea surface and the strong noise, the relative phase between two cross-polarized channels has been widely spread in angle region [π,π] due to decorrelation effect. The histograms of φ evaluated by SLC quad-pol RS-2 SAR data are illustrated in Fig. 1. It is clearly shown that the relative phase is equal to 0 rad for real target, and the PDF is very narrow. Just as discussed in Sec. 2.1, the PDF for FOAAs is distributed around ±π, but it has been spread because the field intensities of ambiguities are generally much lower than real target and can be easily affected by sea clutter. Figure 1 also shows that the relative phases for sea surface and strong noise are widely spread in angle region [π,π] due to decorrelation effect.

Fig. 1

Phase differences between cross-polarized channels.

JARS_9_1_096092_f001.png

In order to detect target by RMSRP-based CFAR algorithm, the “prior probability” of Θ should be obtained first. In Refs. 20 and 21, the PDF of the relative phase φ has been derived by Lee as

(8)

P(φ)=Γ(L+1/2)(1|2)Lβ2πΓ(L)(1β2)+(1|ρ|2)L2πF(L,1;1/2;β2),
where F(L,1;1/2;β2) is a Gauss hypergeometric function, β=|ρ|cos(φφ), the correlation coefficient |ρ| and the mean phase φ are related to the measured data by

(9)

|ρ|exp(jφ)=SHVSVH*|SHV|2|SVH|2.

Using Eq. (8), the PDF of φ2 is derived as

(10)

P(φ2)=Γ(L+1/2)(1|ρ|2)βL4φπΓ(L)(1β2)+(1|ρ|2)L2πF(L,1;1/2;β2).

Based on the central limit theorem, if the dimension number of a moving window is large enough, the PDF of ψ (i.e., the mean value of φ2 within the moving window) can be described approximately by a Gaussian distribution

(11)

P(ψ)=12πσψ2exp[(ψμψ)22σψ2],
where μψ and σψ2 denote the mean value and the mean square deviation of ψ, respectively. Then, using the identity equation P(ψ)|dψ|=P(Θ)|dΘ|, the PDF of Θ is obtained

(12)

P(Θ)=1Θ22πσψ2exp[(1/Θμψ)22σψ2].

Using Eqs. (10) and (12), the mean values of Θ for different look numbers are evaluated, and the curves are plot in Fig. 2. Just as shown in this figure, the value of Θ increases with the increase of look number and correlation coefficient. And the effect of look number is more remarkable when the correlation coefficient is larger. This property implies that multilook processing would improve the ability of Θ for target detection because the correlation coefficients for real targets are always larger than those for surrounding sea surface. However, it should be pointed out that the multilook processing would reduce the spatial resolution. On the other hand, to ensure the applicability of the central limit theorem, the dimension of the moving window, in which the mean value of φ2 is evaluated, is set to be 11×11pixels. In this work, a data set that contains six Radarsat-2 SAR images acquired in fine quad-polarization mode are processed to show the effectiveness of the theoretical PDF. The detailed descriptions of the images are given in Table 1. The comparisons in Fig. 3 illustrate that the theoretical PDFs match the histograms well.

Fig. 2

Plots of Θ versus the correlation coefficient for different look numbers.

JARS_9_1_096092_f002.png

Table 1

The data set used in this work.

Image’s nameAcquired time (UTC)Center locationsInc. angle Nr-Fr (deg)
Image RS_IJuly 30, 2008 09:17:22129°07′16′′E 34°50′51′′N28.04 to 29.91
Image RS_IISeptember 18, 2009 10:49:30109°24′03′′E 18°06′16′′N32.35 to 34.01
Image RS_IIIMay 15, 2010 11:56:3788°18′34′′W 28°33′06′′N29.17 to 30.92
Image RS_IVAugust 19, 2011 10:52:36120°09′25′′E 38°20′09′′N36.47 to 38.00
Image RS_VFebruary 15, 2013 10:13:39118°14′41′′E 39°01′04′′N46.02 to 47.25
Image RS_VIJuly 13, 2010 09:39:37122°34′54′′E 31°11′43′′N48.31 to 49.47
Image RS_VIIAugust 18, 2012 10:12:01116°43′27′′E 20°42′55′′N29.19 to 30.94

Fig. 3

Comparisons of the theoretical probability density functions (PDFs) with the histograms of Θ.

JARS_9_1_096092_f003.png

3.

Θ-based CFAR Target Detector

Here, a Θ-based CFAR algorithm is established for marine target detection. The detection threshold ξ is determined by the following integration:

(13)

η=F(ξ)=ξP(Θ)dΘ=ξ1Θ22πσψ2exp[(1/Θμψ)22σψ2]dΘ=12[erf(μψ2σψ2)erf(μψ1/ξ2σψ2)],
where η is the given false alarm. And erf(·) denotes the error function. Using Eq. (13), the relation between the false alarm rate η and the detection threshold ξ is derived as

(14)

ξ=1μψ2σψ2erfinv[erf(μψ2σψ2)2η],
where erfinv(·) denotes the inverse error function.

Based on the analysis above, the following rule is used to detect targets, i.e.,

(15)

Θ>ξtargetΘξsea background.

4.

Experimental Results

In this section, three meaningful Radarsat-2 SAR imageries (namely Image RS_V, Image RS_VI and Image RS_VII in Table 1), in which targets, azimuth ambiguities, and strong noise are simultaneously present, have been selected to demonstrate the effectiveness of the Θ-based target detector. In the following discussions, the false alarm rate η is set to be 105.

First, the Θ-based target detector is tested to remove azimuth ambiguities. Figure 4 shows the SAR image of the Dongsha Island acquired at 10:12 (UTC) on August 8, 2012 (namley Image RS_VII in Table 1). From Fig. 4, we can find that the first-order azimuth ambiguity of the Dongsha Island can be seen in |HH| and |HV| images. In |HV+VH| image (i.e., the so called HVfree image in Ref. 18), the intensity of the azimuth ambiguity has been suppressed due to its special phase difference. From Fig. 4(d), we can find that the value of Θ (i.e., RMSRP) for target/azimuth ambiguity is higher/lower than the surrounding sea surface. Thus, the parameter Θ can also be used to remove the azimuth ambiguities. Figure 4(e) shows the detection results by the Θ-based target detector by using threshold 0.77, and it is obvious that this proposed algorithm is good to detect real targets.

Fig. 4

An example of SAR azimuth ambiguity removal using RMSRP. The bright target is Dongsha Island in East China Sea. (a) |HH|, (b) |HV|, (c) |HV+VH|, (d) RMSRP, (e) Detection result.

JARS_9_1_096092_f004.png

By comparison in Fig. 4, it is evident that the azimuth ambiguities can be suppressed effectively within HVfree or RMSRP image. However, in the case of strong noise corrupted SAR images, the HVfree-based algorithm will no longer perform well. This behavior is shown in Fig. 5(a), and the SAR image was acquired at 10:13 (UTC) on February 15, 2013 (named Image RS_ V in Table 1) in the Bohai sea. The center longitude and latitude coordinates are about 118°14′41′′E and 39°01′04′′N. The yellow rectangle in Fig. 5(a) indicates the blown-up area, as shown in Figs. 5(b)5(f). The red circles in Fig. 5(c) highlight the areas with strong noise induced by unknown reasons. Unlike azimuth ambiguities, the strong noise cannot be suppressed effectively in HVfree image [see Fig. 5(d)], while reduced over RMSRP image as given in Fig. 5(e). The reason is attributed to the fact that PDF of the relative phase in the noise-polluted region is always broader than that for real targets, making RMSRP over noise-region much smaller than that of real targets. Figure 5(f) shows the detection results by Θ-based target detector with threshold 1.49, as we expected that this algorithm is able to suppress both azimuth ambiguities and strong noise. To validate the detection results, the detailed information of targets achieved by Electronic Navigation Chart (ENC) and Automatic Identification System (AIS) are given in Table 2. This ground truth is consistent with the detection results, further supporting our proposed detector. The largest ship in Fig. 5 is T18, with 294 m in length and 32 m in width. There do exist two odd targets circled by red rectangle in Fig. 5(f), and no AIS information indicates these targets.

Fig. 5

An example of target detection in noise-corrupted SAR image (RS_ V in Table 1). (a) |HH|, (b) |HH|, (c) |HV|, (d) |HV+VH|, (e) RMSRP, (f) Detection result.

JARS_9_1_096092_f005.png

Table 2

The information of the targets in Fig. 5.

TargetsTarget’s namePositions (°)
T1Ship AALBORG118.121776E38.964581N
T2Ship TIAN SHAN HAI118.103622E38.956699N
T3Ship MAPLE RUBY118.129448E38.949818N
T4Ship DA QING XIA118.112557E38.947428N
T5Ship KAVALA SEAS118.111329E38.937426N
T6Ship GREAT HAPPY118.159423E38.952945N
T7Ship DA FU118.148537E38.943874N
T8Ship M.V.SANGTHAI IRIS118.150238E38.931187N
T9Ship ANSHENG16118.166473E38.926121N
T10No AIS information118.176674E38.928608N
T11Ship FENG KANG SHAN118.201885E38.932927N
T12Ship JIA SHENG SHAN118.178040E38.912934N
T13Ship SSI PRIDE118.197892E38.916819N
T14Ship ALEXANDRIA BRIDGE118.218254E38.917630N
T15Ship KING MERRY118.201452E38.904839N
T16Ship DA XIN HUA YAN TAI118.218908E38.905806N
T17Ship APL OAKLAND118.237244E38.912748N
T18Ship TIAN AN HE118.214931E38.892525N
T19Ship TRITON LARK118.234558E38.891247N
T20Ship MU MIAN SONG118.229302E38.880814N
T21Ship BAO YING118.286148E38.913395N
T22Ship FU SHENG 17118.304709E38.904203N
T23Ship XIN HAI ZHOU 28118.314102E38.912476N
T24Ship HUA HAO 8118.332726E38.904437N
T25Ship HENG RUI 5118.333906E38.914114N
T26No AIS information118.199974E38.972668N
T27Oil platform Mo(U)W6m5M118.257286E38.984062N
T28Oil platform Mo(C)Y12s8.4m6M118.267586E38.982758N
T29Ship CHANG QI 6118.127552E38.904323N
T30Ship JIN PING118.115250E38.893744N
T31Ship JUPITER118.111580E38.885360N
T32Ship E.R. BASEL118.122007E38.875521N

Note: The information for T27 and T28 are achieved from ENC (electronic navigation chart) C1311700. No AIS information are available for T10 and T26. The information for the other targets is all obtained by AIS (automatic identification system).

Another typical example of target detection in a noise-corrupted SAR imagery is presented in Fig. 6. The SAR image in Fig. 6 was acquired at 09:39 (UTC) on July 13, 2010, in the East China Sea. And the center longitude and latitude coordinates are 122°34′54′′E and 31°11′43′′N, respectively. Besides real targets, some filaments could be seen in Figs. 6(a)6(c), and we have no idea about the origin of these odd noise. Figure 6(f) demonstrates promising detection result with threshold of 0.67. It is exciting to find that the strong noise has been suppressed in the RMSRP image, highlighting the visually inspected ships. This imagery provides further evidence to the applicability of our proposed algorithm.

Fig. 6

Another example of target detection in noise-corrupted SAR image (named RS_ VI in Table 1). (a) |HH|, (b) |HH|, (c) |HV|, (d) |HV+VH|, (e) RMSRP, (f) Detection result.

JARS_9_1_096092_f006.png

To present a quantitative description about the reduction degree of noise, we introduce the standard deviation of |HH|, |HV|, |HV+VH|, and RMSRP over the noise-corrupted areas to indicate this improvement. We select two areas in Fig. 5(d) marked as AR1, AR2, and one area AR3 in Fig. 6(b) to demonstrate the improvement of RMSRP in suppressing noise. The detailed estimates for different parameters are given in Table 3, and it is evident that the noise is primarily reduced by RMSRP while others do not exhibit such capability. It also should be noted that the standard deviation of |HV+VH| is always the largest value, by this I mean that noise presents the highest variability in |HV+VH| images.

Table 3

Standard deviation over noise-corrupted areas.

Std|HH||HV||HV+VH|RMSRP
AR115.81067.950020.70664.9899
AR24.264237.142086.07390.9620
AR37.788712.673933.18460.0053

5.

Conclusions

In this paper, the RMSRP between cross-polarized channels is proposed as a new promising indicator for SAR target detection at sea. Meanwhile, an analytical form of the PDF for RMSRP was derived, and the comparisons between the histograms of the sea clutter and theoretical PDF show the correctness of the statistic model. Based on the statistic model, a RMSRP-based CFAR algorithm is developed to detect targets in noise corrupted SAR images. From the tests over quad-pol RS-2 SAR images, we find that not only the azimuth ambiguities but also strong noise can be reduced successfully by the RMSRP-based algorithm. Standard deviation is introduced to indicate the quantitative improvement in suppressing noise. The experiment results certify that RMSRP is a promising indicator for Pol-SAR target detection purpose, particularly when pol-SAR images have been corrupted by strong noise and azimuth ambiguities. This proposed algorithm would be helpful in detecting targets in SAR imageries and further applying to coastal management.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (Grant Nos. 41376179, 41106153) and the National Science Foundation for Distinguished Young Scholars of China (Grant No. 61225002).

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Biography

Yunhua Wang received his MS and PhD degrees in radio science from Xidian University, Xi’an, China, in 2005 and 2006, respectively. He is currently with the Ocean Remote Sensing Institute, Ocean University of China, Qingdao, China. His research interests include asymptotic and numerical simulations of electromagnetic wave scattering from random sea surfaces and SAR remote sensing of the ocean.

Huimin Li is currently studying for an MS degree in ocean information sensing and processing from the Ocean University of China. She received a Bachelor of Science in marine technology from the Ocean University of China, Qingdao, China, in 2013. Her main research of interest focuses on the utilization of polarimetric SAR data for target detection and dynamic element retrieval.

Yanmin Zhang was born in Shandong, China, in 1979. She received the PhD degree in optics from Huazhong University of Science and Technology, Wuhan, China, in 2007. She is currently with the College of Information Science and Engineering, Ocean University of China, Qingdao, China. Her current research interests include electromagnetic scattering from sea surface and applications for remote sensing.

Lixin Guo received the MS degree in radio science from Xidian University, Xi’an, China, and the PhD degree in astrometry and celestial mechanics from the Chinese Academy of Sciences, Beijing, China, in 1993 and 1999, respectively. He was a visiting scholar with the School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu, Korea, from 2001 to 2002. His current research interests include electromagnetic wave propagation and scattering in random media, computational electromagnetic.

Yunhua Wang, Huimin Li, Yanmin Zhang, Lixin Guo, "Marine target detection in quad-pol synthetic aperture radar imagery based on the relative phase of cross-polarized channels," Journal of Applied Remote Sensing 9(1), 096092 (29 January 2015). http://dx.doi.org/10.1117/1.JRS.9.096092
Submission: Received ; Accepted
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KEYWORDS
Target detection

Synthetic aperture radar

Ocean optics

Detection and tracking algorithms

Scattering

Polarimetry

Sensors

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