A stable realization of apodization filtering applied to noise SAR and SAR range sidelobe suppression
 Xin Wu^{1, 2}Email author,
 Yanfei Wang^{1},
 Shu Li^{1, 2} and
 Chang Liu^{1}
DOI: 10.1186/168761802012112
© Wu et al; licensee Springer. 2012
Received: 23 November 2011
Accepted: 19 May 2012
Published: 19 May 2012
Abstract
Pulse response of radar system always suffers from high sidelobe level resulting in resolution degradation. Investigated here is a sidelobe suppression method based on apodization filtering technique for range responses of synthetic aperture radar (SAR) and noise SAR systems. The core of apodization filtering is finding an appropriate filtering vector in time domain. Compared with original apodization filtering, the proposed method could be realized stably because it could get correct filtering vector efficiently. This method contains three important steps: constructing coefficient matrix and desired response vector; performing illposed analysis; and solving equation to find filtering vector. In these steps, convolution kernel method is adopted to construct coefficient matrix; spectral condition is introduced as an indicating function for illposed analysis; and total variation method is used to resolve illposed equation for getting filtering vector. Elaborate theoretical derivation is presented to demonstrate the feasibility of this method. In order to test its effect, simulation experiments are implemented. Simulation results show that there is a great suppression of range sidelobes after processed by this method. With increasing filter length, the performance of filtered output is improved but time cost is increasing correspondingly. Furthermore, the proposed method is also effective with noise disturbance.
Keywords
apodization filtering sidelobe suppression stability noise synthetic aperture radar1. Introduction
Synthetic aperture radar (SAR) is a technique in which backscattered microwave pulses are collected at different positions to synthesize a long antenna. It is able to get radar images with high resolution [1]. To obtain radar images of the detected regions, SAR first transmits linear frequency modulation signals, and then performs pulse compression along range and azimuth directions for echo signals. The pulse responses of SAR along range and azimuth are both sinc functions with high levels of sidelobes [2], thus, it is important to reduce sidelobes for highresolution SAR images.
Noise SAR system combines random noise radar with SAR technique holding both advantages of the two individuals. Random noise radar refers to radar whose transmitted signal is a microwave noise or is modulated by noise source. It has excellent electronic counter countermeasure capability, very low probability of intercept, high electromagnetic compatibility, good counter electronic support measure capability, and ideal 'thumbtack' ambiguity function [3]. In [4–7], basic principles and typical noise radar systems are presented with further analysis. Noise radar was originally developed with homodyne receivers for shortrange applications [5]. In current, it has been used in diverse areas, such as SAR and inverse synthetic aperture radar [8–10], collision warning, altimetry, ground penetration detection of buried objects, and Doppler estimation [11–13]. Recently, depending on random noise radar system, throughwall detection technique is developing fast for antiterrorism and earthquake rescue [14, 15].
Noise SAR makes random noise radar acquire the capability to imaging, meanwhile it improves the performance of antijamming for SAR system due to its truly random transmission signal [16]. It has become a useful remote sensor for military and civil missions, such as highresolution land mapping, wave height monitoring, forest inventory, etc. Noise SAR imaging has been studied a lot in recent years. Experimental researches focusing on noise SAR imaging algorithms have been implemented in practice [8, 9, 17]. In noise SAR imaging, the system also suffers from high sidelobes which leads to resolution degradation for images. Target detection is severely deteriorated by high sidelobe level, especially in subsurface profiling application. Therefore, sidelobe reduction is meaningful to obtain correct output responses and highquality noise SAR images.
Amplitude weighting is usually applied to depress sidelobes by multiplying a window function to the response in frequency domain [18]. However, this method always results in mainlobe broadening. Some image domain deconvolution methods such as CLEAN technique are investigated to suppress sidelobes [19]. These methods require some special conditions, for instance, the brightest spot in image must be a true scatterer, so that their applications are limited. Spatially variant apodization (SVA) is another method for sidelobe suppression. It is able to control sidelobes of SAR images effectively by applying sequential nonlinear operations to complexvalued SAR imagery [20]. This method is originally proposed with requirement that the data should be sampled at an integer multiple of Nyquist frequency, but now it can be used to suppress sidelobes in any case of Nyquist sampling rates [21].
Signal processing of noise SAR is different from general SAR because random transmitted signal has no specific analytic expression. As an alternative to matched filtering, correlation operation is implemented directly in time domain to perform pulse compression along range direction for noise SAR imaging. The range profiles are formed depending upon delay line which provides changed time delays to correlate with echoed signals by directly sampling in time domain. Amplitude weighting cannot be applied to noise SAR imaging because it should be operated in frequency domain. Image domain deconvolution methods such as CLEAN technique are not good choices either because of the required conditions. Using a set of different short filters optimized for every pixel, SVA is indeed an effective method. Applying SVA to noise radar could be an interesting research, nevertheless, this article studies about another method. To control the output sidelobes of random noise radar, apodization filtering technique is proposed as a special algorithm [22]. Its effect has been proved by the application to ground penetration of random noise radar. As a new technique, some problems still to be resolved. For instance, realization process is not stable enough, mathematic model is not accurate adequately, applications are not extensive, restrictive conditions are excessive, etc. Therefore, further investigations are required to perfect it.
In this article, a stable realization of apodization filtering (SRAF) is proposed, which is a modified method to original realization. It is able to depress sidelobes significantly and improve the stability of apodization filtering. SRAF is not only carried out on range sidelobe control of correlation output for random noise SAR, but also implemented in matched filtering output for general SAR. This article is organized as follows: the basic principle of apodization filtering technique is first reviewed in Section 2. With elaborate theoretical derivation and specific mathematical representations, three important steps of SRAF are presented in Section 3. Applications of SRAF in range sidelobe suppression for SAR and noise SAR are demonstrated in Section 4. In this section, simulation results and some important data are further analyzed. And finally a summary and some conclusions are provided in Section 5.
2. Principle of apodization filtering
Apodization filtering technique is originally proposed to control sidelobes for random noise radar. Its basic principle is simply introduced in this section. After getting a filtering function in time domain, output response should be filtered by this function to achieve sidelobe suppression.
3. Sidelobe suppression algorithm based on SRAF
From the previous section, finding filtering vector F is the key of apodization filtering. As can be seen from Equations (1) and (2), getting F is essentially to solve a first kind integral equation which generally represents illposed system, so that the stability of adopted method is as important as its efficiency. Original realization of apodization filtering (ORAF) is introduced in [22]. It has successfully been implemented for random noise radar in order to suppress the range sidelobes by applying projection method to resolve the illposed problem. However, its stability is not enough because inappropriate solution may be obtained. Improper filtering can lead to obvious distortion of output response. Therefore, a modified method SRAF is proposed in this article to acquire correct filtering vector with following processing details.
3.1. Coefficient matrix and desired response vector
As shown in Figure 1, coefficient matrix K and desired response vector G should be determined before resolving Equation (2). Approaches to formations of K and G are presented in this section.
Elements not represented in Equation (5) are zero. As shown in Equation (5), matrix K is determined by original response and filter length together. Formulated by Equation (5), coefficient matrix K is guaranteed to accomplish the convolution operation without approximation. Compared with ORAF, it has adequate accuracy of matrix model with no restriction on filter length.
where ${a}_{{m}_{i}}\left(i=1,2,\dots ,k\right)$ should be located at the center of G.
3.2. Illposed analysis
After matrix K and vector G have been determined, a direct solution for Equation (2) could be achieved by least squares method. However, if Equation (2) represents an illposed system, direct solution is practically impossible. Therefore, illposed analysis is necessary to decide which method is feasible to obtain correct solution. Whether an equation is ill posed or not depends on the coefficient matrix, so that matrix K is further analyzed starting with singular value decomposition (SVD) involved.
where σ_{ i }(i = 1, 2,...,n) denote the nonzero singular values of matrix K with degressive order from σ_{1} to σ_{ n }.
where u_{ i } and v_{ i } denote the column vectors of matrices U and V, respectively.
Determined by the quotient of the maximum singular value to the minimum one, κ(K) is called spectral condition of matrix K. If the value of κ(K) is very large with the form of 10^{ n } (n > 6), Equation (2) is confirmed to be ill posed.
3.3. Approach to getting filtering vector
Based on illposed analysis result, a suitable method for getting filtering vector should be adopted. In essence, this problem is to solve a first kind integral equation which generally represents illposed system. In this section, total variation (TV) method is introduced to resolve the problem.
Depending on the peculiarity of equation, illposed problem is generally resolved via constructing an additional regularizing function to resume stability [23, 24]. In this article, a solution vector with finite length is utilized to approach the true solution which makes plentiful nonzero elements of original response become zero, so that the solution is likely to have several discontinuous points. TV is a specific regularization method with advantage of nonrestricting solution to be smooth [25]. It is capable of resolving the illposed problem in this article because it is able to retain the discontinuous boundary of solution.
where U is the support domain of F(τ). α and β are the regularization and regulated parameters with positive values. The two parameters have influences on the stability of solving illposed equation.
Equation (19) is easy to solve by traditional conjugate gradient method.
In Equation (21), G(i) is composed of the m middlemost elements after Fourier transforming to desired response G, and A(i) is the Fourier transform of original response A. Through one dimension searching, it is easy to find an optimal λ that makes φ(λ) be zero because φ(λ) is a single descending function. Then, the optimal α can be estimated by α = q/λ, where q is the mean of Q_{ m }. In [26, 28], the methods about selecting regularization parameter are provided. With small and positive value, parameter β is regulated to ensure that regularizing function J_{ β }(F) is differentiable at F = 0. In practice, it is estimated by experience and iterative tests.
4. Experimental results and performance analysis
In order to test the effect of SRAF, simulations are performed and the results are provided in this section. SRAF is not only applied to range sidelobes suppression of correlation output for noise SAR, moreover, it is implemented to control the sidelobes of matched filtering output for general SAR.
4.1. Implementation of sidelobe suppression based on SRAF
System parameters
Parameter  Value  

Range correlation (noise SAR)  Matched filtering (general SAR)  
Carrier frequency  1 GHz  1 GHz 
transmitted signal bandwidth  51.2 MHz  15 MHz 
Pulse duration  10 μs  2 μs 
Sampling time interval  19.5 ns  6.67 ns 
4.2. Analysis of experimental data
To evaluate the performance and efficiency of proposed method, some important simulation data are presented for further analysis. All data in this section are relevant to the simulations in Section 4.1 (SNR = ∞). In the implementation of SRAF, threshold of iterative convergence is 10^{5}; parameters α and β are 2 × 10^{6} and 0.01.
Singular values and spectrum conditions of K
Noise SAR  Filter length  300  600  900  1200  1500  1800  2100  2400  2700  3000 

σ _{max}  14.02  17.46  18.19  20.03  19.58  20.26  20.21  20.77  20.35  20.86  
σ_{min} (10^{10})  3.78  1.73  1.55  0.92  0.69  0.65  0.48  0.43  0.37  0.36  
κ(K) (10^{10})  3.71  10.09  11.73  21.77  28.38  31.17  42.10  48.30  54.86  57.93  
General SAR  Filter length  300  400  500  600  700  800  900  1000  1100  1200 
σ _{max}  22.57  23.19  23.55  23.76  23.91  23.99  24.07  24.12  24.16  24.19  
σ_{min} (10^{10})  1.87  1.73  1.65  1.54  1.38  1.31  1.26  1.20  1.11  1.07  
κ(K) (10^{10})  12.07  13.40  14.27  15.42  17.33  18.31  19.10  20.10  21.76  22.61 
Iterative steps and running time of SRAF
Noise SAR  Filter length  300  600  900  1200  1500  1800  2100  2400  2700  3000 

Iterative steps  2  3  3  3  3  3  3  4  3  3  
Running time (s)  0.16  1.58  4.31  12.83  23.22  43.95  74.93  146.79  177.05  208.06  
General SAR  Filter length  300  400  500  600  700  800  900  1000  1100  1200 
Iterative steps  5  4  5  8  8  5  10  8  7  7  
Running time (s)  2.35  5.01  14.73  46.57  74.51  70.29  200.73  216.53  245.39  324.28 
5. Conclusion
The range resolution of SAR system is generally degraded due to high sidelobe level. Amplitude weighting is usually used to suppress sidelobes at the expense of mainlobe broadening and not applicable to noise SAR system. As a novel technique of sidelobe suppression, apodization filtering was originally proposed to control sidelobes for range response of noise radar. To improve the stability of this technique, a modified apodization filtering algorithm is suggested in this article. It can be applied to the range responses both of noise SAR and general SAR systems. The proposed method is SRAF with noticeable advantage of adequate stability for getting appropriate filtering vector.
There are three key steps of SRAF: getting coefficient matrix and desired response vector; performing illposed analysis; and solving equation for filtering vector. Convolution kernel method is adopted to construct coefficient matrix; it makes the product between matrix K and vector F be equivalent to the convolution of Equation (1). Spectral condition is introduced to estimate whether Equation (2) represents an illposed system; its value is large means that the equation is ill posed. TV method is implemented to solve Equation (2) for correct filtering vector because the least square method is not feasible to resolve illposed equation. With no restriction of solution to be smooth, TV ensures that solving equation process is stable enough to obtain appropriate filtering vector.
SRAF is executed to suppress range sidelobes of correlation output and matched filtering output for noise SAR and general SAR, respectively. Indeed, correct filtering vector can be obtained within several iterative steps. Sidelobe reductions of 12 dB and greater can be achieved by this method; moreover, mainlobe energy is preserved well, which is contrast to obvious mainlobe broadening after amplitude weighting. With increasing filter length, the performance of sidelobe suppression is improved at the cost of increasing running time, so that it is important to select a balanced filter length. Noise disturbance is also considered in simulations. As SNR degenerates from infinite to 20 dB, the performance of filtered outputs have no obvious degradation. In the cases of SNR = 0.5 and 10 dB, SRAF is still able to depress sidelobes effectively.
Abbreviations
 ISLR:

integrated sidelobe level ratio
 LSS:

least square solution
 ORAF:

original realization of apodization filtering
 PSLR:

peak sidelobe level ratio
 SAR:

synthetic aperture radar
 SRAF:

stable realization of apodization filtering
 SVA:

spatially variant apodization
 SVD:

singular value decomposition
 TV:

total variation.
Declarations
Acknowledgements
This study was supported by the Department of Airborne Microwave Remote Sensing System of Institute of Electronics. We appreciate the assistance of Research Fellow GaoXin who provided many helpful comments.
Authors’ Affiliations
References
 Davidson GW, Cumming IG, Ito MR: A chirp scaling approach for processing squint mode SAR data. IEEE Trans Aerosp Electron Syst 1996, 32(1):121133.View Article
 Cumming IG, Wong FH: Digital Processing of Synthetic Aperture Radar Data: Algorithms and Implementation. Artech House, Norwood 2005, 36168.
 Liu GS, Gu H, Su WM: Development of random signal radars. IEEE Trans Aerosp Electron Syst 1999, 35(3):770777. 10.1109/7.784050View Article
 Narayanan RM, Xu XJ: Principles and applications of coherent random noise radar technology. Paper presented at the Proc. SPIE Conference on Noise in Devices and Circuits, Santa Fe, NM 2003.
 Axelsson SRJ: Noise radar using random phase and frequency modulation. IEEE Trans Geosci Remote Sens 2004, 42(11):23702384.View Article
 Narayanan RM, Xu Y, Homffmeyer PD, Curtis JO: Design, performance, and applications of a coherent ultrawide and random noise radar. Opt Eng 1998, 37(6):18551869. 10.1117/1.601699View Article
 Theron IP, Walton EK, Gunawan S, Cai L: UltrawideBand noise radar in the VHF/UHF band. IEEE Trans Antennas Propagat 1999, 47(6):10801084. 10.1109/8.777135View Article
 Garmatyuk D, Narayanan RM: Ultrawideband noise synthetic aperture radar: theory and experiment. Paper presented at the IEEE Antennas Propagat Soc Int Symp 1999, Orlando, FL 1999.
 Tarchi D, Lukin K, FortunyGuasch J: SAR imaging with noise radar. IEEE Trans Aerosp Electron Syst 2010, 46(3):12141225.View Article
 Bell DC, Narayanan RM: ISAR imaging using a coherent ultrawideband random noise radar system. Opt Eng 2001, 40(11):26122620. 10.1117/1.1413212View Article
 Xu X, Narayanan RM: FOPEN SAR imaging using UWB stepfrequency and random noise waveforms. IEEE Trans Aerosp Electron Syst 2001, 37(4):12871300. 10.1109/7.976965View Article
 Narayanan RM, Xu X, Henning JA: Radar penetration imaging using ultrawideband (UWB) random noise waveforms. IEE Proc Radar Sonar Navigat 2004, 151(3):143148. 10.1049/iprsn:20040418View Article
 Narayanan RM, Dawood M: Doppler estimation using a coherent ultrawideband random noise radar. IEEE Trans Antennas Propagat 2000, 48(6):868878. 10.1109/8.865218View Article
 Wang H, Narayanan RM, Zhou ZO: ThroughWall imaging of moving targets using UWB random noise radar. IEEE Antennas Wirel Propagat Lett 2009, 8: 802805.View Article
 Lai CP, Narayanan RM: Ultrawideband random noise radar design for throughwall surveillance. IEEE Trans Aerosp Electron Syst 2010, 46(4):17161730.View Article
 Garmatyuk DS, Narayanan RM: ECCM capabilities of an ultrawideband bandlimited random noise imaging radar. IEEE Trans Aerosp Electron Syst 2002, 38(4):12431255. 10.1109/TAES.2002.1145747View Article
 Jiang H, Zhang BC, Lin YG, Hong W, Wu YR: Random noise SAR based on compressed sensing. Paper presented at the IEEE Geosic Remote Sensing Int Symp, Honolulu, HI 2010.
 Nuttall AH: Some windows with very good sidelobe behavior. IEEE Trans Acoust Speech Signal Process 1981, 29(1):8491. 10.1109/TASSP.1981.1163506View Article
 Fry RD, Gray DA: CLEAN deconvolution for sidelobe suppression in random noise radar. Paper presented at the Proc IEEE 2008 Radar Conf, Adelaide, SA5 2008.
 Stankwitz HC, Dallaire RJ, Fienup JR: Nonlinear apodization for sidelobe control in SAR imagery. IEEE Trans Aerosp Electron Syst 1995, 31(1):267279.View Article
 CastilloRubio C, LlorenteRomano S, BurgosGarcia M: Robust SVA method for every sampling rate condition. IEEE Trans Aerosp Electron Syst 2007, 43(2):571580.View Article
 Xu X, Narayanan RM: Range sidelobe suppression technique for coherent ultra wideband random noise radar imaging. IEEE Trans Antennas Propagat 2001, 49(12):18361842. 10.1109/8.982467View Article
 Hanke M, Hansen PC: Regularization methods for largescale problems. Surv Math Ind 1993, 3: 253315.MathSciNet
 Morozov VA: On regularization of illposed problems and selection of regularization parameter. Comput Math Phys 1996, 6(1):170175.
 Blomgren P, Chan TF, Mulet P, Wong CK: Total variation image restoration: numerical methods and extension. Paper presented at the IEEE Int Cof on Image Processing, Washington, DC 1997.
 Zou MY: Deconvolution and Signal Recovery. National Defence Industry Press, Beijing; 2001:242251.
 Zou MY, Zou X: Global optimization: an auxiliary cost function approach. IEEE Trans Syst Man Cybern A: Syst Humans 2000, 30(3):347354. 10.1109/3468.844358View Article
 Strong DM, Chan TF: Relation of regularization parameter and scale in total variation based image denoising.1996. [http://www.math.ucla.edu/~imagers/htmls/reports.html] Accessed 6 Feb 2012
Copyright
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.