Half-sine and triangular despreading chip waveforms for coherentdelay-locked tracking in DS/SS systems

The performance of a coherent delay-locked tracking scheme for direct-sequence/spread-spectrum systems using half-sine or triangular chip waveforms for early and late despreading sequences is analyzed. The effect of band-limiting on the received signals is considered. Mean time to lose lock (MTLL) and root mean square (rms) tracking error of the delay-locked loop (DLL) are compared with that of a conventional DLL which uses rectangular chip despreading waveforms. Linear and nonlinear (based on the renewal process approach) analyses are employed to evaluate the performance of the DLL. Results show that the use of either the half-sine or triangular chip waveform reduces the rms tracking error and increases the MTLL considerably when the early-late spacing is approximately between 0.7-1.3 chip times. The results apply in particular to the commonly used DLL using one chip early-late spacing. Computer simulation for band-limited signals confirms the analytical results. The use of these despreading chip waveforms also reduces tracking offset in multipath environments     

Target localization and tracking in noisy binary sensor networks with known spatial topology

Target localization and tracking are two of the critical tasks of sensor networks in many applications. Conventional localization and tracking techniques developed for wireless systems that rely on direction‐of‐arrival (DOA) or time‐of‐arrival (TOA) information are not suitable for low‐power sensors with limited computation and communication capabilities. In this paper, we propose a low‐complexity and energy‐efficient method for target localization and tracking in noisy binary sensor networks, where the sensors can only perform binary detection, and the physical links are characterized by additive white Gaussian noise (AWGN) channels. The proposed method is based on known spatial topology. An efficient wake‐up strategy is used to activate a particular group of sensors for cooperative localization and tracking. We analyze the localization error probability and tracking miss probability in the presence of prediction errors. Simulation results validate the theoretic analysis and demonstrate the effectiveness of the proposed approach. Copyright © 2008 John Wiley & Sons, Ltd.     

基于立体方向图和十字跟踪扫描法校准天线指向

针对明安图射电频谱日像仪（Mingantu spectral radioheliograph，MUSER）天线高指向精度动态跟踪要求，同时为降低非理想因素对复杂系统天线指向测量影响，提出校准天线指向的组合方法.先用网格法测量天线的双圆极化多频率通道立体方向图定性评估系统测量链路状态，后对太阳运行轨道进行十字跟踪扫描测量天线指向偏差，再用最小二乘法拟合获得天线指向模型的8个参数.国家天文台明安图观测基地的2台20 m地平式天线将在MUSER系统观测和校准中起重要作用.对20 m天线跟踪指向进行了校准，指向偏差均方根从12'改善到4.4'，验证了方法的可行性，为类似复杂系统的相关工作提供了借鉴和参考.     

Large-scale spatial angle measurement and the pointing error analysis

A large-scale spatial angle measurement method is proposed based on inertial reference. Common measurement reference is established in inertial space, and the spatial vector coordinates of each measured axis in inertial space are measured by using autocollimation tracking and inertial measurement technology. According to the spatial coordinates of each test vector axis, the measurement of large-scale spatial angle is easily realized. The pointing error of tracking device based on the two mirrors in the measurement system is studied, and the influence of different installation errors to the pointing error is analyzed. This research can lay a foundation for error allocation, calibration and compensation for the measurement system.     

Frequency tracking of atrial fibrillation using hidden Markov models.

A hidden Markov model (HMM) is employed to improve noise robustness when tracking the dominant frequency of atrial fibrillation (AF) in the electrocardiogram (ECG). Following QRST cancellation, a sequence of observed frequency states is obtained from the residual ECG, using the short-time Fourier transform. Based on the observed state sequence, the Viterbi algorithm retrieves the optimal state sequence by exploiting the state transition matrix, incorporating knowledge on AF characteristics, and the observation matrix, incorporating knowledge of the frequency estimation method and signal-to-noise ratio (SNR). The tracking method is evaluated with simulated AF signals to which noise, obtained from ECG recordings, has been added at different SNRs. The results show that the use of HMM improves performance considerably by reducing the rms error associated with frequency tracking: at 4-dB SNR, the rms error drops from 0.2 to 0.04 Hz.     

Adaptive Fuzzy Output-Feedback Tracking Control for a Class of Switched Stochastic Nonlinear Time-Delay Systems

In this paper, a design scheme for an adaptive fuzzy tracking controller is proposed for a class of switched stochastic nonlinear time-delay systems via dynamic output-feedback. First, a reduced-order observer is introduced to estimate the unmeasurable states of the switched system. During the adaptive controller design procedure, an appropriate stochastic Lyapunov–Krasovskii functional deals with the time-delay terms, and fuzzy logic systems are employed to approximate the unknown nonlinearities. Based on the designed controller, the semi-globally uniform ultimate boundedness of all the closed-loop signals is guaranteed and the tracking error converges to a small neighborhood of the origin. Finally, a simulation example is given to illustrate the validity of the proposed approach.     

Enhanced MMSE channel estimation using timing error statistics for wireless OFDM systems

Estimation and tracking of the frequency-selective time-varying channel response is a challenging task for wireless communication systems incorporating coherent OFDM. In pilot-symbol-assisted (PSA) OFDM systems, the minimum mean-square-error (MMSE) estimator provides the optimum performance based on the channel statistics (channel correlation function and SNR). In OFDM systems, FFT-block timing error introduces a linear phase rotation to data modulated on individual subcarriers. An MMSE channel estimator designed only using the wireless channel statistics performs only sub-optimally when subcarrier phase rotations due to block timing errors are present. In this paper, we show that by using the block timing error statistics of the OFDM time-synchronizer the performance of the MMSE channel estimation can be significantly improved. Numerical results show that the bit-error-probability (BEP) performance degradation due to timing errors can be almost completely recovered by the proposed technique.     

A 0.7-2-GHz self-calibrated multiphase delay-locked loop

A 0.7-2-GHz precise multiphase delay-locked loop (DLL) using a digital calibration circuit is presented. Incorporating with the proposed digital calibration circuit, the mismatch-induced timing error among multiphase clocks in the proposed DLL can be self-calibrated. When the calibration procedure is finished, the digital calibration circuit can be turned off automatically to save power dissipations and reduce noise generations. A start controlled circuit is proposed to enlarge the operating frequency range of the DLL. Both the start-controlled circuit and the calibration circuit require an external reset signal to ensure the correctness of the calibration after temperature,operating frequency, and power supply voltage are settled. This DLL with the digital calibration circuit has been fabricated in a 0.18-/spl mu/m CMOS process. The measured results show the DLL exhibits a lock range of 0.7-2 GHz while the peak-to-peak jitter and rms jitter is 18.9ps and 2.5 ps at 2 GHz, respectively. When the calibration procedure is completed and the DLL operates at 1 GHz, the maximum mismatch-induced timing error among multiphase clocks is reduced from 20.4 ps (7.34 degree) to 3.5 ps (1.26 degree).     

Three-dimensional motion tracking of coronary arteries in biplane cineangiograms

A three-dimensional (3-D) method for tracking the coronary arteries through a temporal sequence of biplane X-ray angiography images is presented. A 3-D centerline model of the coronary vasculature is reconstructed from a biplane image pair at one time frame, and its motion is tracked using a coarse-to-fine hierarchy of motion models. Three-dimensional constraints on the length of the arteries and on the spatial regularity of the motion field are used to overcome limitations of classical two-dimensional vessel tracking methods, such as tracking vessels through projective occlusions. This algorithm was clinically validated in five patients by tracking the motion of the left coronary tree over one cardiac cycle. The root mean square reprojection errors were found to be submillimeter in 93% (54/58) of the image pairs. The performance of the tracking algorithm was quantified in three dimensions using a deforming vascular phantom. RMS 3-D distance errors were computed between centerline models tracked in the X-ray images and gold-standard centerline models of the phantom generated from a gated 3-D magnetic resonance image acquisition. The mean error was 0.69 (+/- 0.06) mm over eight temporal phases and four different biplane orientations.     

Precise, fault-tolerant pointing using a Stewart platform

Presents a precision pointing strategy. The principal contribution is the development of a fault-tolerant control which allows active pointing to continue despite multiple failures. A six-axes active platform is utilized to reject disturbances from a vibrating base to a precision payload. A decentralized controller is proposed which converts desired rotations into corresponding strut lengths via a decoupling transformation. The decoupling approach allows for simple single-input-single-output compensator design and for the incorporation of fault-tolerant strategies. The proposed strategy was evaluating on the microprecision interferometer testbed (a full-scale model of a future spaceborne optical interferometer) at the Jet Propulsion Laboratory, Pasadena, CA. Experimental pointing results demonstrate 50 dB of disturbance rejection at low frequency. In the laboratory ambient disturbance environment, this corresponds to a 1-μrad rms pointing error     

Multi-camera parameter tracking

The quality of 3-D reconstructions with multi-camera acquisition systems is strongly influenced by the accuracy of the camera calibration procedure. In fact, when acquiring a long sequence of views, mechanical shocks, vibrations and thermal gradients could cause a significant drift of the camera parameters. The authors propose a method for tracking the camera parameters and, whenever possible, correcting them accordingly. This technique does not need any a priori knowledge or test objects to be positioned in the scene, as it exploits natural scene features. The approach is based on accurate detection, matching and back-projection of luminance corners and spots in the scene space. Such features are then tracked over time to detect unexpected parameter changes or drifts, and to apply corrections to them. Experimental results on real sequences are reported in order to prove the effectiveness of the proposed technique. It is shown that changes in the calibration parameter are correctly detected and when this happens, the camera system can be re-calibrated with an accuracy that increases with the number of tracked feature points     

An implementation of a direction-finding antenna for mobilecommunications using a neural network

Direction-finding systems for radio signals are mostly used in mobile communications and avionics applications for antenna tracking or navigation purposes. In general, such systems require accurate calibration and may be sensitive to noise and external interference. In this paper, we investigate the performance of a neural network-based direction-finding system under such conditions. The proposed topology is a hybrid one, combining a simple RF signal beamformer with a neural network. The training of the neural network is accomplished experimentally with a three-element antenna array by varying the beam's direction and the carrier frequency. The error on the estimated direction of arrival caused by the environment and training limitations are investigated     

基于标校经纬仪的测量船坞内标校新方法

光轴漂移和惯性导航设备惯性元件更新维护,是船载无线电测量设备测量精度逐渐下降的主要原因。为此,提出了一种标校新方法,以标校经纬仪测量数据为基准,在甲板系中标定无线电测量设备零位和轴系误差,同时通过坐标系取齐达到消除惯导姿态测量误差的目的。分析表明,该方法完全满足无线电测量设备标校精度要求,具有简便、经济、高效等优点,是保持和提高测量船测量精度的有效途径。     

Frequency acquisition and tracking for optical heterodyne communication systems

A frequency acquisition and tracking system for use in optical heterodyne communication systems employing semiconductor lasers is described. Analytical models for acquisition and tracking are used to predict performance; probability of incorrect acquisition is predicted for the acquisition system, and rms error is predicted for the tracker. Experimental data is presented and compared to the analytic results.     

3-D-Skeleton-Based Head Detection and Tracking Using Range Images

Vision-based 3-D head detection and tracking systems have been studied in several applications like video surveillance, face-detection systems, and occupant posture analysis. In this paper, we present the development of a topology-based framework using a 3-D skeletal model for the robust detection and tracking of a vehicle occupant's head position from low-resolution range image data for a passive safety system. Unlike previous approaches to head detection, the proposed approach explores the topology information of a scene to detect the position of the head. Among the different available topology representations, the Reeb graph technique is chosen and is adapted to low-resolution 3-D range images. Invariance of the graph under rotations is achieved by using a Morse radial distance function. To cope with the particular challenges such as the noise and the large variations in the density of the data, a voxel neighborhood connectivity notion is proposed. A multiple-hypothesis tracker (MHT) with nearest-neighbor data association and Kalman filter prediction is applied on the endpoints of the Reeb graph to select and filter the correct head candidate out of Reeb graph endpoints. A systematic evaluation of the head detection framework is carried out on full-scale experimental 3-D range images and compared with the ground truth. It is shown that the Reeb graph topology algorithm developed herein allows the correct detection of the head of the occupant with only two head candidates as input to the MHT. Results of the experiments demonstrate that the proposed framework is robust under the large variations of the scene. The processing requirements of the proposed approach are discussed. It is shown that the number of operations is rather low and that real-time processing requirements can be met with the proposed method.     

结构光场光束直线跟踪的两步标定方法

为了实现结构光系统的三维重建,针对传统标定方 法结构复杂、标定过程繁琐等问题,提出一种基于光束直线跟踪 的两步标定方法。本文方法首先在前后两个平面上分别对投射侧模式上的每个检测点进行 标定,建立投射侧光线方程;然后在 前后两个平面上分别对平面上的每个目标点建立接收侧CCD的光线方程。在三维测量时,光 场中的一个点分别映射为投射 侧和接收侧前后平面上的两个点,连接前后平面上的两个点各得到一条空间直线,通过求解 两条空间直线的交点即可求得三 维空间点的坐标。分析了系统结构在标定时可能产生的误差,系统误差主要来自角度偏移和 水平偏移,并提出了误差解决方 案。实验结果表明,与传统的基于针孔模型的标定方法相比,本文方法无须考虑系统的成像 模型及其参数求解,简洁、高效。标定产生的景深相对误差小于0.2% ,能够改善结构光系统的几何标定精度。     

大型跟踪测量雷达的卫星标定方法研究

卫星标定是跟踪测量雷达保证测量精度的重要手段。首先阐述了大型跟踪测量雷达误差的标定方法,然后详细介绍了卫星标定的优点及原理和方法。结合某试验雷达的工作模式和特点,给出了符合该雷达的卫星标定方法和误差模型,并对标定效果进行了仿真验证。结果表明,对于大型跟踪测量雷达,利用卫星标定的方法进行误差标定合理有效,且可行性好,避免了常规标定方法所涉及的大量的人工参与,比常规标定方法方便、快捷,为今后跟踪测量雷达的误差标定提供了一定的参考,具有广阔的工程应用前景。     

A Low-Jitter Polyphase-Filter-Based Frequency Multiplier With Phase Error Calibration

A new low-jitter polyphase-filter-based frequency multiplier incorporating a phase error calibration circuit to reduce the phase errors is presented. Designing with a multiplication ratio of eight, it has been fabricated in a 0.13-mum CMOS process. For input frequency of 25 MHz, the measured jitter is 2.46 ps (rms) and plusmn9.33 ps (pk-pk) at 200-MHz output frequency, while achievable maximum static phase error of the calibration circuit is 2.4 ps. The calibration leads to the normalized rms jitter of 0.049%.     

Magneto-Optical Tracking of Flexible Laparoscopic Ultrasound: Model-Based Online Detection and Correction of Magnetic Tracking Errors

Electromagnetic tracking is currently one of the most promising means of localizing flexible endoscopic instruments such as flexible laparoscopic ultrasound transducers. However, electromagnetic tracking is also susceptible to interference from ferromagnetic material, which distorts the magnetic field and leads to tracking errors. This paper presents new methods for real-time online detection and reduction of dynamic electromagnetic tracking errors when localizing a flexible laparoscopic ultrasound transducer. We use a hybrid tracking setup to combine optical tracking of the transducer shaft and electromagnetic tracking of the flexible transducer tip. A novel approach of modeling the poses of the transducer tip in relation to the transducer shaft allows us to reliably detect and significantly reduce electromagnetic tracking errors. For detecting errors of more than 5 mm, we achieved a sensitivity and specificity of 91% and 93%, respectively. Initial 3-D rms error of 6.91 mm were reduced to 3.15 mm.