无源雷达目标信号时延与多普勒频率联合估计

提出一种基于STFRFT的无源雷达目标时延与多普勒联合估计新方法。首先,给出了一种STFRFT时频分辨率量化方法及其数学表达式;其次,利用STFRFT的投影包络对回波信号时延与Doppler频移进行了联合估计;最后,讨论分析了脉内信噪比、高斯窗参数等对所提方法估计性能的影响。仿真结果表明,与CAF方法相比,所提方法更容易获得高精度的时延与Doppler频移的联合估计,具有更高的可靠性。     

瑞利多普勒激光雷达频率响应函数的研究

概述了瑞利多普勒测风激光雷达的基本工作原理,讨论和分析了基于F-P标准具双边缘鉴频技术的测量精度,在窄带实验光源和宽带发射光源入射下,分别测量了鉴频器的透过率曲线,并比较了两种光源入射下由于频率响应函数引起的系统误差。结果表明:瑞利测风激光雷达的频率响应函数满足设计要求,在-100~100 m/s的径向风速动态范围内,系统误差随径向风速的增大而增大,宽带光源测量时,由频率响应函数引起的系统误差在0.74~1.38 m/s之间;窄带光源测量时,由频率响应函数引起的系统误差在0.75~0.84m/s之间。窄带光源较宽带光源测量时,系统误差降低了0~0.48m/s。     

Spectral estimation using combined time and lag weighting

This paper presents a five-step method for spectral estimation that combines time and lag weighting and leads to a procedure that requires less than one-half the computations of standard methods. The Blackman-Tukey method and the Weighted Overlapped Segment Averaging (WOSA) method (widely used in sonar and other applications) are shown to be special cases of the combined method. An analysis of the mean value of the spectral estimate leads to an unusual lag weighting that corrects for poor (for example, rectangular) time weighting and affords effective windows with very good sidelobe behavior. Several examples of achievable windows are provided. Finally, the variance of the spectral estimate of the combined method is presented and evaluated for the case of rectangular time weighting, no segment overlap, and lag reshaping. It is found that the variance is virtually as low as any technique that realizes the same frequency resolution with the same finite data record length. Furthermore, the computational costs are less than one-half those of the WOSA method, primarily because no time weighting multiplications or overlap are required.     

Simultaneous measuring of Doppler frequency shift and group delay time by means of amplitude-modulated LFM-Signal

New method of the simultaneous measurement of the frequency dependencies of Doppler shift and group delay time of separate ionosphere modes by means of amplitude modulated chirp signal is presented in this paper. The algorithms for data processing are presented.     

A novel frequency estimator for sampled Doppler signals

In multigate pulsed Doppler systems with serial processing the Doppler signals are retained in their sampled form. The time-discrete nature of the Doppler signals affects the procedure to estimate the average frequency of the Doppler signal. Based on computer simulation studies it is demonstrated that a time-discrete frequency estimator based on either the density of zero-crossings or the time average of the instantaneous frequency exhibit a large relative error under poor signal-to-noise conditions or in the case of relatively wide band signals. Especially, the frequency estimator based on the instantaneous frequency functions poorly for average Doppler frequencies close to the Nyquist frequency. However, restricting the detected instantaneous frequency to a specific interval around its average improves the estimator performance considerably, while it allows tracking of center frequencies beyond the Nyquist frequency. A hardware realization of this modified estimator as incorporated in a high-resolution multigate pulsed Doppler system is described. In vitro and in vivo registrations as assessed with this system demonstrate the ability of the system to track frequencies close to and beyond the Nyquist frequency.     

A step in time Changes in standard frequency and time signal broadcasts

An improved Coordinated Universal Time (UTC) system has been adopted by the International Radio Consultative Committee (CCIR).* It was implemented internationally by the standard frequency and time broadcast stations on January 1, 1972. The new UTC system eliminates the frequency offset of 300 parts in 1010 between the old UTC and Atomic Time, thus making the broadcast time interval, the UTC second, constant and defined by the resonant frequency of cesium atoms. The new time scale will be kept in synchronism with the rotation of the earth within ±0.7 second by step-time adjustments of exactly 1 second, when needed. A time code will be added to the disseminated time signals that will permit Universal Time to be obtained from the broadcasts to the nearest 0.1 second for users requiring such precision.     

Cramer-Rao bounds of the frequency estimation in the time selective radio channels with Doppler spread

The mobile channel is slow fading and time selective, thus the multiplicative and additive noise of the channel will smear the spectral line, or arouse Doppler spread. This spread will make the parameters estimation accuracy degrade. The goal of this paper is to analytically assess this degradation when Carrier Frequency Offset （CFO） and Doppler shift exist jointly. Then the finite-sample Cramer-Rao Lower Bound （CRLB） is derived and close-form asymptotical expression is given for large-sample CRLB. These expressions give insights into the performance room for frequency estimation. Also the variance of Doppler shift estimator is simulated to illustrate the theoretical results.     

Absolute stability of nonlinear control systems with time lag

A nonlinear control system with time lag is transformed into a nonlinear control system with no time lag by means of certain types of transformations. If absolute stability for this transformed nonlinear control system is obtained by Popov's method, absolute stability in the nonlinear control system with time lag is realized.     

原子法拉第效应器件稳频与鉴频测速的多普勒激光雷达

利用FADOF透射频谱曲线稳定而又可调谐的特点,设计了一种测速多普勒激光雷达,一方面用其对发射激光稳频,同时用同种原子的FADOF对回波光鉴频,使发射激光频率和鉴频曲线建立了相对稳定的关系,可有利于提高测速精度。试验系统采用Cs原子FADOF,在-40~+40m/s的测速范围内,测速误差为±0.74m/s。     

A time and frequency synchronization scheme for multiuser OFDM

We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on the universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system     

The past, present, and future of atomic time and frequency

The early history of atomic time and frequency standards is reviewed. The most accurate and stable present standards are described. Prospective future improvements, particularly with trapped ions and atoms, are discussed. A historical perspective on this topic is provided     

Doppler frequency estimation and the Cramer-Rao bound

Addresses the problem of Doppler frequency estimation in the presence of speckle and receiver noise. An ultimate accuracy bound for Doppler frequency estimation is derived from the Cramer-Rao inequality. It is shown that estimates based on the correlation of the signal power spectra with an arbitrary weighting function are approximately Gaussian-distributed. Their variance is derived in terms of the weighting function. It is shown that a special case of a correlation-based estimator is a maximum-likelihood estimator that reaches the Cramer-Rao bound. These general results are applied to the problem of Doppler centroid estimation from SAR (synthetic aperture radar) data     

Standard time and frequency generation

The basic properties of atomic primary frequency standards are reviewed. A continuously running frequency source combined with counting, storage, and display devices results in a clock. Time scales are obtained by setting clocks with respect to a convenient origin. The accuracy of a primary frequency standard is a combined theoretical and experimental assessment of the uncertainties of all known possible sources of bias from the idealized definition. Recommended standard measures for the stability are the spectral density of fractional frequency fluctuations (frequency domain) and the two-sample no-dead time Allan variance (time domain). The operation of atomic frequency standards is based on various methods of particle interrogation to observe the transitions, particle confinement to obtain sufficient interaction time, and particle preparation to obtain the desired energy level populations. Passive resonator and active oscillator (maser) modes of operation are discussed; the former has some fundamental advantages. A review of the state of the art and current practice shows that cesium beam resonator standards have the best documented accuracy capability (5 × 10^-13) and, in their commercial versions, are presently the best available clocks. There are three concepts related to time: time interval, date, and synchronization. In order to assign dates to events, time scales have to be established. The steadily growing need for a very precise and uniform time scale has resulted in a new internationally coordinated time scale, IAT. Frequency metrology will have increasing impact on both length and voltage metrology in the near future.     

International time and frequency coordination

The significant advances in the development of international coordination in time determination and dissemination are briefly reviewed. Before 1955, time was determined exclusively by astronomical observations and international cooperation was primarily the concern of the International Astronomical Union (IAU). In the past 15 years, an independent time scale, based on atomic phenomena, has been realized. Astronomical and atomic time each have their field of application, and both require international cooperation in order to achieve the standards of accuracy now demanded. Responsibility for coordination has been assigned to the Bureau International de l'Heure (BIH). The adoption by the General Conference of Weights and Measures (CGPM) of an atomic definition of the second as the basic time interval in the international system of units has led to the establishment of an international scale of atomic time (IAT). In the dissemination of time by radio, a compromise system designed to meet the needs for both astronomical and atomic time was introduced experimentally and later recommended for general adoption by the International Radio Consultative Committee (CCIR). Following the recommendations of the international working party, an improved compromise system was inaugurated on January 1, 1972 and is now being followed by all major time services. Developments of new observational techniques now supplement conventional astronomical methods and offer a further useful field for international cooperation.     

基于GPS的时间和频率解决方案

随着对规格更为敏感的应用需求在市场上的重要性不断增加,精度更高的低成本设备应运而生     

A comparison of time versus frequency domain antenna patterns

This paper compares the performance of antenna arrays that use sinusoidal and ultra-short pulse waveforms. There are two parts to the paper: first, a comparison of narrowband versus ultra-wideband linear arrays using an analytic approach and second, a series of computer simulations used to extend the analytic results and to show the characteristics of nonstandard array configurations. Analysis shows that antennas using ultra-short waveforms have highly desirable pattern properties that do not appreciably degrade under conditions of high array sparsity, failed elements, or antenna element position errors. Patterns are given that show these properties directly. These results are of particular importance to the emerging technology of base-band or time-domain communications and remote sensing. It addresses the practical problem of designing extremely large aperture space-based arrays without populating those arrays densely with antenna elements and also reducing the traditional fabrication tolerances.     

近场动目标多普勒频率、距离及方位估计算法

针对近场多动目标的定位问题,提出一种近场动目标多普勒频率、距离及方位三维参数估计算法。采用多重信号分类（MUSIC）算法估计回波信号频率,以各个频率估计为参考对信号进行时域滤波,从而实现各目标回波信号的分离。使用二维MUSIC方法由每个目标的回波信号估计其距离与方位。该算法能精确估计近场动目标多普勒频率、距离及方位参数,计算机仿真结果证明了其有效性。     

Evaluation of a simplified Doppler frequency shifter

Mechanical Doppler frequency shifting of a millimeter or submillimeter wave signal provides a means to implement a sensitive and highly coherent heterodyne receiver in a laboratory system. The rotary shifter, which we have previously described, is a precision-machined device suitable for use down to submillimeter wavelengths. We found at 140GHz, however, that the coherence of such a system is not affected by mechanical imprecision of the frequency shifter, and that the system dynamic range is only moderately affected by mechanical imprecision and by certain design simplifications. We have therefore built several versions of simpler and less precise Doppler frequency shifters and report here on their performance at 140GHz.     

低信噪比条件下快变多普勒频偏捕获算法

针对卫星移动通信低信噪比条件下快变多普勒频偏捕获提出了一种新的算法即基于搜索空间压缩的谱线循环平移算法。首先,介绍了现有的载波频偏捕获技术,分析其不能适应快变多普勒频偏条件的原因;其次,分析了多普勒变化率对载波频偏捕获的影响,主要考虑一次变化率与二次变化率将严重限制低信噪比情况下对载波频偏估计时非相干累加的有效性;最后,在此基础上,提出了低信噪比条件下快变多普勒频偏捕获的基于搜索空间压缩的谱线循环平移算法。对算法进行的Matlab仿真结果表明,其具有2~3 dB的改善增益,与最大似然算法相比,在性能仅有0.2 dB损失的情况下运算量大大减少。