前斜视SAR成像几何形变校正方法研究

针对弹载合成孔径雷达（SAR）成像存在运动参数抖动的问题,分析了不规则运动造成图像几何失真的机理,提出了一种基于多项式逼近的弹载SAR线性调频（LFM）信号前斜视成像几何形变校正方法。挂飞试验证明,该方法能从雷达回波数据中准确消除几何形变,提高成像质量。     

A simple method for MSK modulation and demodulation

In this letter, a simple method for modulation and demodulation of minimum shift keying (MSK) is proposed. It has a serial structure, and is easier to implement than the conventional parallel type. The MSK signal is generated by a digital synthesis method and the demodulation process is done by a simple sampling scheme.     

A method for accurate geometric correction of NOAA AVHRR HRPT data

A method for the geometric correction of NOAA Advanced Very High Resolution Radiometer (AVHRR) high-resolution picture transmission (HRPT) data is presented. After precise determination of nominal attitude angles for each time instant, geometric correction is done for ground control points (GCPs), and residual errors are interpreted as attitude angle variation effects and in this way corrected. All attitude angle deviations (in pitch, roll, and yaw) are simultaneously corrected by applying to two reference vectors (the vector normal to the scanning plane and the vector that defines the instantaneous viewing direction of the first pixel of each line) a three-axis rotation. A separate program performs the geometric correction, applying the orbital model to each point of the desired output geographical area. An application of this method is presented, in which AVHRR data are registered over a 1:25000 topographic map with subpixel accuracy, allowing the use of AVHRR data in a nearly local scale in combination with other high-resolution data, such as Landsat TM or SPOT     

A sparsity-driven approach for joint SAR imaging and phase error correction

Image formation algorithms in a variety of applications have explicit or implicit dependence on a mathematical model of the observation process. Inaccuracies in the observation model may cause various degradations and artifacts in the reconstructed images. The application of interest in this paper is synthetic aperture radar (SAR) imaging, which particularly suffers from motion-induced model errors. These types of errors result in phase errors in SAR data, which cause defocusing of the reconstructed images. Particularly focusing on imaging of fields that admit a sparse representation, we propose a sparsity-driven method for joint SAR imaging and phase error correction. Phase error correction is performed during the image formation process. The problem is set up as an optimization problem in a nonquadratic regularization-based framework. The method involves an iterative algorithm, where each iteration of which consists of consecutive steps of image formation and model error correction. Experimental results show the effectiveness of the approach for various types of phase errors, as well as the improvements that it provides over existing techniques for model error compensation in SAR.     

A new phase correction method in NMR imaging based on autocorrelation and histogram analysis

A new statistical approach to phase correction in NMR imaging is proposed. The proposed scheme consists of first-and zero-order phase corrections each by the inverse multiplication of estimated phase error. The first-order error is estimated by the phase of autocorrelation calculated from the complex valued phase distorted image while the zero-order correction factor is extracted from the histogram of phase distribution of the first-order corrected image. Since all the correction procedures are performed on the spatial domain after completion of data acquisition, no prior adjustments or additional measurements are required. The algorithm can be applicable to most of the phase-involved NMR imaging techniques including inversion recovery imaging, quadrature modulated imaging, spectroscopic imaging, and flow imaging, etc. Some experimental results with inversion recovery imaging as well as quadrature spectroscopic imaging are shown to demonstrate the usefulness of the algorithm.     

一种扫描图像几何畸变的数字校正方法

由于胶订书籍不能完全平铺于扫描仪的玻璃板上,从而使得扫描图像产生非线性几何畸变,为此,提出一种对此类畸变图像进行横向畸变数字校正的方法,并详细介绍了实现该数字校正的算法。实验结果表明,此算法能够大大减小横边误差,使畸变图像得到很好的复原。     

A new method for gain correction in ultrasonic imaging

A system is proposed in which the depth for each ultrasonic image pixel, which is needed for scan conversion, is simultaneously given with the pixel coming out from the digital scan processor. The gain memory is only a line memory with less circuitry and is updated faster when the gain function is modified by the user. The use of the system in routine clinical work shows that post-gain correction capability is an improvement in terms of ease-of-use and image quality optimization     

A simple exact method for generating continuous phase modulatedsignals

A new, relatively simple architecture for generating continuous phase modulation (CPM) signals is presented. It can produce arbitrary frequency pulses for binary full-response signals with arbitrary modulation indexes. It can also produce partial-response signals or M -ary signals with reasonable accuracy. The method filters and then hard limits the switched and summed outputs of a 90° hybrid     

A new time-gain correction method for standard B-mode ultrasound imaging

A method used to provide gain variation with received echo range to compensate for tissue attenuation in standard B-mode ultrasound imaging systems achieves signal multiplication by first logarithmically compressing the complete dynamic range of the received echo. The time-gain correction method described here is a modification of this method. It performs the time-gain correction operation digitally between the digital scan converter and the display. The overall system transfer function can be specified by means of a software interface without changing the characteristics of the analog front end. This interface can be automated to almost any degree, and it can include preset settings for typical imaging requirements. This method also allows the gain function to be changed even when new images are not being generated.     

分布式卫星混合基线SAR-ATI相位校正方法

本文提出了一种分布式卫星混合基线SAR-ATI相位校正方法.分布式卫星在SAR-ATI工作模式下,由于编队构型、任务需求、地球自转、轨道摄动和卫星姿态变化等诸多因素的影响,卫星之间存在沿航向和切航向的混合基线.卫星间的切航向基线加剧了基线去相关引起的相位随机噪声,使得ATI相位中运动目标相位和地面高程相位混合在一起,造成了大量的虚警,导致动目标检测失败.本文提出了一种新的SAR-ATI混合相位校正方法,较好地抑制了切轨迹干涉相位,改善了SAR-ATI的检测性能.本文基于分布式卫星SAR仿真系统进行了仿真实验,实验结果证明了该方法的良好性能.     

A homogeneity correction method for magnetic resonance imaging with time-varying gradients

When time-varying gradients are used for imaging, the off-resonance behavior does not just cause geometric distortion as is the case with spin-warp imaging, but changes the shape of the impulse response and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. The authors introduce a reconstruction and homogeneity correction method to correct for the zeroth order effects of inhomogeneity using prior knowledge of the inhomogeneity. In this method, the data are segmented according to collection time, reconstructed using some fast, linear algorithm, correlated for inhomogeneity, and then superimposed to yield a homogeneity corrected image. This segmented method is compared to a conjugate phase reconstruction in terms of degree of correction and execution time. The authors apply this method to in vivo images using projection-reconstruction and spiral-scan sequences.     

光电信号的自动增益控制和相位校正的方法

在高精度光电测量中对光电信号的正交性和稳定性的要求很高。本文介绍了一种基于AD605的自动增益控制电路,以实现信号的幅值稳定,利用硬件向量校正法对光电信号进行相位校正,它是基于两路信号幅值相等且稳定的基础上实现,而且电路简单,精度高,响应速度快,得到正交性高、幅值稳定的光电信号,提高了软件细分的精度。     

A simple method for determining the modulation transfer function in digital radiography

The authors developed a simple method for determining the presampling modulation transfer function (MTF). which includes the unsharpness of the detector and the effect of the sampling aperture, in digital radiographic (DR) systems. With this method, the presampling MTF is determined by the Fourier transform of a ;finely sampled' line spread function (LSF) obtained with a slightly angulated slit in a single exposure. Since the effective sampling distance becomes much smaller than the original sampling distance of the DR system, the effect of aliasing on the MTF calculations can be eliminated. The authors applied this method to the measurement of the presampling MTF of a compound radiographic system and examined the directional dependence, the effect of exponential extrapolation, and the effect of different sampling distances. It is shown that the technique of multiple slit exposure and exponential extrapolation of the LSF tail, which has been commonly used in analog seven-film systems, can be employed in DR systems. The authors determined the glare fraction in order to estimate the component of low-frequency drop mainly due to ;glare'     

Accelerating near-field 3D imaging approach for joint high-resolution imaging and phase error correction

The computational complexity and memory requirements of large-scale data seriously affect the application of compressed sensing (CS) in near-field three-dimensional (3-D) imaging system. In addition, as influenced by the measurement environment, the error in echo phase results in imaging defocusing. This paper proposes a CS near-field 3-D imaging approach based on nonuniform fast Fourier transform and phase error correction. It applies the fast Gaussian gridding nonuniform fast Fourier transform technique and Separable Surrogate Functionals with only matrix and vector multiplied to accelerate imaging speed and reduce memory requirements; it adopts the phase error correction technique to realize highly-focused imaging; in addition, a sparse observation approach based on Logistic sequence is proposed in this paper for easy availability of engineering realization for CS imaging. As indicated by numerical analysis and actual measurement in anechoic chamber, the approach proposed in this paper, compared with traditional imaging approaches, has the following advantages: accurate high resolution 3-D image of target can be obtained by applying small amount of observation data (10%); the computational complexity falls from O(LN) to O(3N) and memory occupation quantity drops from O(LN) to O(N); it can effectively perform highly-focused imaging for echo signal with phase error; the measurement matrix designed has better non-coherence and easy availability for engineering realization.

     

A simple numerical method based simultaneous stochastic perturbation for estimation of high dimensional matrices

Multidimensional Systems and Signal Processing - We describe a simple algorithm for estimating the elements of a matrix as well as its decomposition under the condition that only the product of...     

基于OFDM的WLAN相位校正算法研究

相位校正对基于OFDM的高速数据传输系统至关重要.文章针对802.11a协议提出了一种使用导频进行相位校正的算法.该算法对接收导频样本进行最小二乘估计以获得相偏参数,算法综合考虑了抽样时钟偏移(SCO)和载波频偏(CFO)所引起的相位偏移.文章通过仿真比较了该算法与另一种相位校正算法在不同参数下的性能,结果证明在相偏主要由SCO和CFO引起的情况下该算法性能较好.     

A quasi-optimum receiver for continuous phase modulation

A simple receiver structure for any continuous phase modulation (CPM) scheme is introduced. Its front end is just the ordinary linear receiver followed by a subinterval sampler, eliminating the need for the standard analog matched filters. Its design is based on the decomposition of the CPM signal in the Walsh signal space. This brings the hardware requirement to a minimum, and near-optimum performance can be easily obtained     

Scatter correction method for X-ray CT using primary modulation: theory and preliminary results

An X-ray system with a large area detector has high scatter-to-primary ratios (SPRs), which result in severe artifacts in reconstructed computed tomography (CT) images. A scatter correction algorithm is introduced that provides effective scatter correction but does not require additional patient exposure. The key hypothesis of the algorithm is that the high-frequency components of the X-ray spatial distribution do not result in strong high-frequency signals in the scatter. A calibration sheet with a checkerboard pattern of semitransparent blockers (a "primary modulator") is inserted between the X-ray source and the object. The primary distribution is partially modulated by a high-frequency function, while the scatter distribution still has dominant low-frequency components, based on the hypothesis. Filtering and demodulation techniques suffice to extract the low-frequency components of the primary and hence obtain the scatter estimation. The hypothesis was validated using Monte Carlo (MC) simulation, and the algorithm was evaluated by both MC simulations and physical experiments. Reconstructions of a software humanoid phantom suggested system parameters in the physical implementation and showed that the proposed method reduced the relative mean square error of the reconstructed image in the central region of interest from 74.2% to below 1%. In preliminary physical experiments on the standard evaluation phantom, this error was reduced from 31.8% to 2.3%, and it was also demonstrated that the algorithm has no noticeable impact on the resolution of the reconstructed image in spite of the filter-based approach. Although the proposed scatter correction technique was implemented for X-ray CT, it can also be used in other X-ray imaging applications, as long as a primary modulator can be inserted between the X-ray source and the imaged object.     

多项式建模解电离层相位污染阶数选择新方法

电离层相位污染使天波超视距雷达杂波频谱展宽,影响舰船等低速目标检测.针对这一问题,近年来发展了基于分段多项式相位建模和高阶模糊函数的(HAF)解电离层污染算法,如何合理地选取模型阶数是该算法的核心.给出了两种改进的自适应多项式建模阶数选择方法.并利用仿真方法进行了验证,结果表明:基于高阶模糊函数变换性质的频域和时域阶数选择方法的计算量较现有算法有较大提高,且频域法可在较低信噪比下准确判断多项式阶数.     

电离层空时相位特性对天波海态雷达影响研究

电离层空时相位随机变化特性使天波海态雷达方位波束宽度和杂波频谱展宽,降低雷达方位分辨率和频谱分辨率,影响海态参数反演精度。针对这一问题,近年来发展了诸多解电离层相位污染信号处理算法,但均受一定的适用条件限制。首次引入相延时和群延时,深入分析了电离层空时相位变化对天波海态雷达方位波束形成、杂波频谱影响,并利用仿真方法进行了验证。最后,分别采用空、时信号处理对上述展宽进行了抑制,提高了方位分辨率,谱峰明显锐化。