基于分形的大气湍流随机相位屏数值模拟

激光大气传输湍流效应是制约合成孔径激光雷达高质量成像的重要因素之一,对大气湍流统计特性进行了研究,建立了大气湍流随机相位屏分形模型,使用随机中点位移法进行模拟,分别产生了符合Kolmogorov及non-Kolmogorov统计规律的湍流相位屏.通过计算模拟相位屏的相位结构函数,与理论值比对,验证模拟相位屏的准确性,并与现有的谱反演法和结构函数法进行了对比,分析了各自的优缺点.结果表明,基于分形方法产生的相位屏与理论值更加接近,效果良好.     

部分相干径向偏振拉盖尔-高斯脉冲光束在大气湍流中传输的光谱移动和光谱开关

研究了部分相干径向偏振拉盖尔-高斯(partially coherent radially polarized Laguerre-Gaussian, PCRPLG)脉冲光束在大气湍流中传输时的光谱移动和光谱开关。研究发现,在湍流大气传输时,光束轴上点光谱因受湍流影响产生的双谱峰现象会发生跃变,湍流越强,光谱在跃变后相对谱移变化越稳定,而轴外点光谱在越强的湍流中相对谱移变化范围越小。研究还发现,适当增加脉冲宽度会明显减小大气湍流对光束轴上点和轴外点光谱移动的影响。在不同脉宽下,轴上和轴外点光谱均会出现光谱开关,且轴外点光谱会产生两次光谱跃变的独特现象。在固定的传输距离上,拓扑荷越大,轴上点光谱移动越小,近轴范围内光谱相对谱移的变化范围越小,此外,不同拓扑荷下光谱均会出现谱开关。相干度越大时,光谱的变化越稳定,轴外点光谱的光谱开关发生位置越远。本文的研究结果为多普勒激光雷达的应用提供理 论依据,PCRPLG脉冲光束本身具有较强抗湍流能力,且通过调控光束参数可以减小光束光谱移动。     

Fluctuations of a focused beam wave for atmospheric turbulence probing

This paper demonstrates that a focused beam wave may be effectively used to probe the atmospheric turbulence. Unlike plane and spherical waves, the focused beam has two parameters, i.e., the beam size and the radius of curvature of phase front at the aperture which may be easily varied experimentally. General formulations for the amplitude and phase correlation functions and structure functions of a focused beam wave are given including the effects of wind velocity and time delay. The effects of the beam size and the focal length on the spectral and spatial filter functions and temporal frequency spectrum are examined in detail including the effects of separation, wind velocity, and time delay. Using these functions, procedures are shown to probe the form of the spectral density of the index of refraction, the structure constant, and the wind velocity along the path.     

基于武汉MST雷达的降雨物理过程观测与研究

利用武汉MST(Mesosphere-Stratosphere-Troposphere)雷达对降雨事件的观测, 建立了二阶高斯谱模型, 基于最小二乘法拟合得到相应的大气湍流谱与降雨谱信息.从大气湍流谱和降雨谱中可以分别提取大气及降雨粒子的回波能量及径向运动状态, 从而反演大气背景三维风场和讨论降雨形成机制.同时, 基于雨滴粒径伽马分布特征, 根据降雨回波谱反演得到不同高度的雨滴粒径分布, 从而研究对流层区域降雨的微物理过程.     

无线光自适应副载波MDPSK调制系统特性分析

无线光通信中的自适应传输技术可有效抑制大气湍流引起的信道衰落.根据接收信噪比和预定目标误比特率要求,本文对通过瞬时湍流特性改变调制阶数的无线光自适应传输的通信系统进行了研究.给出采用Malaga分布湍流模型分别描述Gamma-Gamma、对数正态以及K分布湍流信道的相关参数设置;推导了Malaga湍流信道下无线光自适应多进制差分相移键控（MDPSK,M-ary Differential Phase Shift Keying）调制系统的平均误比特率、频谱效率以及中断概率表达式,以及基于MeijerG函数的渐近表达式.MDPSK自适应和非自适应两种调制系统特性分析结果表明：自适应调制系统不但在相同电信噪比下可获得更低的平均误比特率,还可以大大提高频谱效率,而不需要增加额外传输功率或牺牲误比特率.     

上海大气闪烁的频谱

大气闪烁的频谱是闪烁现象中一个有意义的统计特征。本文以上海为例,着重研究了当平均风场与湍流场非均匀分布时对频谱的作用,由此得出了一些新的认识与定量结果。     

1.06 μm多普勒激光雷达的低对流层风场测量

介绍了一套基于双边缘检测技术的1 064 nm Mie 多普勒测风激光雷达系统。采用Farbry?蛳Perot标准具作为频率检测器,分析了风场中大气气溶胶运动造成的多普勒频移。利用转盘硬目标的速度校准系统对接收机校准,在±40 m/s的径向速度范围内,校准精度小于1％。给出了从2006年4月23日起连续八天的风场测量结果。     

Temporal frequency spectra of multifrequency waves in turbulent atmosphere

General formulations for temporal frequency spectra of the fluctuations of plane, spherical, and beam waves operating at two frequencies are given based on weak turbulence and frozen-in assumptions. The cross spectra and the coherence are obtained for the amplitude at two frequencies, the phase at two frequencies, and the amplitude at one frequency and the phase at another frequency. The results are examined in detail for plane and spherical waves. For the spectrum of the index of refractionkappa^{-n}in the inertial subrange, the amplitude spectrum behaves ask^{(5-n)/2}foromega rightarrow 0andk^{2}omega^{1-n}foromega rightarrow infty. The phase spectrum foromegarightarrow 0and foromegarightarrowinftybehaves ask^{2}omega^{1-n}with different constants. These results agree well with the experimental work of Janes et al. [11] at 9.6 and 34.5 GHz, and explains the ratio of the spectra at two frequencies. Also noted is the experimental slope of -2.6 as and foromega rightarrow inftywhich may be compared with1-n = -2.66using the Kolmogorov spectrum ofn = 11/3. The amplitude and phase coherence are calculated, and the results agree well with the experimental data. This agreement is indicative of the general validity of the theory for frequencies as low as10sim30GHz and the path length as long as 60 km. It is also shown that using the preceding theory, the wind velocity and the structure constantC_{n}can be deduced from the experimental data. Theoretical wind velocity of 15.6 knots obtained from the propagation data compares favorably with the meteorologically measured value of 14 knots, and two values ofC_{n}obtained independently from the amplitude and phase measurements closely agree with each other.     

Effects of phase noise on subcarrier modulation performance over Malaga turbulence channel

The performance of wireless optical system employing subcarrier phase-shift keying modulation was researched,which was influenced by phase noise of system demodulator under on Malaga atmospheric turbulence channel.Combined Fourier series and MeijerG function properties,the probability density function of instantaneous signal to noise ratio in the Malaga turbulence model was got.The Fourier series of receiving signal phase and the closed form expression of symbol error probability were derived,which were in the wireless optical subcarrier shift keying modulation system.Under different modulation order and light intensity fluctuation variance,the influence of phase noise on the system symbol error rate was analyzed.The numeric results present that the system symbol error rate increases with the light intensity fluctuation variance when the phase noise is lower.When the phase noise is larger,the influence of modulation order is greater than of turbulence intensity to the error-rate floor appearing.And with the increasing of modulation order,the corresponding average signal to noise is lower when symbol error-rate floor appears.     

基于相干多普勒激光雷达的斜程湍流参数反演方法研究

大气湍流广泛存在于大气中, 其中斜程湍流对航空航天、天文观测等会产生不可忽视的影响。一方面利用相 干多普勒测风激光雷达获得的高时空分辨风场探测数据, 另一方面基于Kolmogorov 局地均匀各项同性理论, 即在惯 性子区内湍流的特征只与湍能耗散率 ε 有关, 将速度结构函数法应用到飞机着陆阶段下滑道扫描模式中, 通过实测 数据的速度结构函数与模型速度结构函数最小二乘法拟合, 从而估算激光雷达扫描范围内的大气湍流参数 (径向风速 方差、湍流积分尺度和 ε 等)。根据下滑道扫描区域数据的时空分布特征, 给出大气湍流参数的斜程空间分布图, 并与 同步观测的飞机下滑道风切变强度数据进行了对比分析, 发现两者具有较好的一致性, 证实了斜程湍流参数反演方 法的可靠性。     

Estimating wind speed from HF skywave radar sea backscatter

Linear expressions relating ocean surface wind speed to a theoretical estimate of the - 10 dB width of the high frequency (HF) sea echo Doppler spectrum (0.078 Hz Doppler shift frequency resolution) are derived for different radar frequencies. The -10 dB width, determined from theoretical model estimates of the Doppler spectrum as a function of radar frequency for a wide range of ocean wave conditions, changes in a complex way with the continuum of second-order echoes surrounding the stronger first-order echo. Because the amplitude of the second-order echoes is directly related to changes in the directional ocean wave spectrum, the wind speed estimates derived from these expressions are highly dependent on the wave field (both the total energy and its distribution with direction); significant differences in these expressions are also found, for example, between fetch-limited and fully developed seas, and for wind seas and swell. These expressions are extremely difficult to use experimentally to obtain reliable estimates of wind speed because a priori knowledge of the wave field is required to apply the correct expression, and yet this knowledge cannot be determined from the unresolved second-order structure at this spectral resolution. Several Doppler spectra recorded under known ocean wind and wave conditions illustrate the difficulty in applying these theoretical expressions to estimate wind speed. We conclude that the -10 dB width is not a good estimator of wind speed. Wind speed estimates are better obtained from HF radar spectra by first estimating the directional wind-wave spectrum from the second-order echoes (0.01 Hz Doppler shift frequency resolution) and then computing wind speed from a wind-wave prediction model.     

An Atmospheric Turbulence Probe

In this paper, a complete low-cost atmospheric turbulence probe, capable of measuring temperature, differential temperature, humidity, horizontal wind speed and direction, and vertical wind component is described. Special care has been taken to minimize the probe's weight so that it may be carried easily by a balloon. Its various sensors, accuracy and frequency response, determined from power spectra measured during daytime convective conditions, are given and the minimization of noice is confirmed by equivalent noise spectra measurements.     

联合时频分析在相干测风激光雷达中的应用

相干测风激光雷达具有风场测量精度高、高时空分辨率、探测范围广等突出优点,已广泛应用于风切变探测、飞机尾流探测、风力发电和大气湍流探测等方面。如何从大气回波信号中提取微弱的多普勒频移信息是激光雷达信号处理的难点。基于大气分层模型仿真生成相干激光雷达大气回波信号,对模拟回波信号应用不同的时频分布进行时频分析。随后对比了时频分析的效果,自适应最优核时频分布具有运算量小,交叉项抑制效果好,时频聚集度高等优点。最后,使用1.5m相干多普勒激光雷达于2017年3月份在安徽合肥进行实地观测,将自适应最优核时频分布应用于实测数据,与传统的快速傅里叶方法对比风速反演结果。结果表明:自适应最优核时频分布能更好地反映出风速细节信息,3 km内距离分辨率为1.2 m,3 km后经平滑保持了对远场弱信号风速估计的连续性,时间分辨率为1 s时其最远水平探测范围约在6 km。     

基于激光雷达脉冲特性的湍流风速估计方法

提出了一种基于激光脉冲距离权重函数的湍流风场速度估计的方法,可以解决多普勒信息探测晴空湍流风场中精细化的风速测量问题。算法以划分的距离门为单位对速度值进行空间平均,将各距离单元速度与激光脉冲距离权重函数进行卷积运算得到风速的局部估计值。并考虑高斯激光脉冲在湍流风场中的有效空间展宽传输特性,根据直接选取距离门中心位置的速度估计方法和快速的线性平均近似方法的处理过程,引入激光脉冲的传输特性来表达湍流径向风速的统计平均值,以实现湍流风速的测量以及激光雷达在探测湍流上的应用。实验结果表明,在有明显湍流条件的风场环境中,脉冲距离权重方法比线性平均方法在保留真实风场属性的前提下具有相对更小的速度标准差,显示出较好的风速修正效果,提高了激光雷达对湍流风场的测速性能。     

大气对外差探测激光雷达影响分析

对于光外差体制的激光雷达而言,为减弱大气扰动的影响,需要研究大气衰减及大气湍流对其探测性能的影响.从光场的复振幅和探测面上光斑的爱里斑模型出发,给出了信噪比这一表征外差探测系统关键性能指标.分析了大气衰减和大气湍流对信号光复振幅的影响,研究了大气衰减和弱湍流相位起伏下的光外差探测效率.研究表明,大气衰减不会造成外差效率...     

大气Gamma-Gamma湍流信道中采用圆偏振位移键控的信道容量分析

在自由空间光通信(FSO)中,由系统设计以及大气湍流所引起的光强闪烁和相位起伏会造成圆偏振控制误差,这严重影响圆偏振位移键控(CPolSK)通信系统的性能。因此,基于Gamma-Gamma大气信道模型,在较全面考虑光强起伏、相位噪声和圆偏振控制误差三个参数的基础上,建立了CPolSK系统信道的数学统计模型,推导了平均信道容量的闭合表达式,仿真研究了误码率在连续相位噪声下的性能以及三个参数对信道容量的影响。结果表明,光强起伏的增强和相位噪声的增大均会导致CPolSK系统性能的劣化,而提高平均信噪比在一定程度上能够改善系统性能。特别地,当圆偏振控制误差大于8时,CPolSK系统的平均信道容量显著下降。     

基于正交多项式拟合的风廓线雷达风谱识别

风廓线雷达是一种以大气湍流为主要探测对象的晴空测风雷达,由于大气湍流对电磁波的散射非常微弱,湍流回波常常被淹没,特别是在降水条件下更是如此。为了能准确识别湍流回波提高谱矩估计精度,文中提出了基于正交多项式拟合的风谱识别方法。结果表明,该方法在降水天气下能很好地保留湍流回波信息,在风廓线雷达谱数据处理中取得了良好的效果,具有很大的实用价值。     

Numerical study on the along-track interferometric radar imagingmechanism of oceanic surface currents

The phase information in along-track interferometric synthetic aperture radar (along-track INSAR, ATI) images is a measure of the Doppler shift of the backscattered signal and thus of the line-of-sight velocity of the scatterers. It can be exploited for oceanic surface current measurements from aircraft or spacecraft. However, as already discussed in previous publications, the mean Doppler frequency of the radar backscatter from the ocean is not exclusively determined by the mean surface current, but it includes contributions associated with surface wave motion. The authors present an efficient new model for the simulation of Doppler spectra and ATI signatures. The model is based on Bragg scattering theory in a composite surface model approach. They show that resulting Doppler spectra are consistent with predictions of an established model based on fundamental electrodynamic expressions, while computation times are reduced by more than one order of magnitude. This can be a key advantage with regard to operational applications of ATI. Based on model calculations for two simple current fields and various wind conditions and radar configurations, they study theoretical possibilities and limitations of oceanic current measurements by ATI. They find that best results can be expected from ATI systems operated at high microwave frequencies like 10 GHz (X band), high incidence angles like 60°, low platform altitude/speed ratios, and vertical (VV) polarization. The ATI time lag should be chosen long enough to obtain measurable phase differences, but much shorter than the decorrelation time of the backscattered field     

Chirp强度调制与近红外激光合成孔径雷达距离向处理

合成孔径激光雷达是利用同一孔径(望远镜)与目标作相对运动并采用信号处理方法来模拟孔径阵列,获得高方位(横向)分辨率的相干成像雷达.采用窄线宽的单频光纤激光光源和相干探测方法;发射光波被啁啾信号作幅度调制,接收后进行距离向脉冲压缩;激光雷达和目标相对运动引起的回波相位变化,由相关运算实现相位补偿和累加(孔径合成),以提高方位向分辨率.提出了新的工作体制:距离向啁啾信号调制在发射光波振幅(强度)上,方位向多普勒频移反映在回波光波相位(频率)上,便于解决距离向与方位向之间的耦合模糊问题.报道了中期实验成果:Chirp 射频信号调制激光强度,实现合成孔径激光雷达距离向的压缩.     

基于模糊逻辑的风廓线雷达目标检测技术

作为风廓线雷达探测目标的大气湍流信号,极易受地杂波、海杂波、降水、无线电信号、飞机、鸟、昆虫等一系列外界环境和信号的干扰,从而形成一种具有多峰特征的功率谱数据。为了有效抑制和去除杂波,提高风廓线雷达输出的数据质量,必须从功率谱数据中将大气湍流目标检测出来。通过分析大气湍流在风廓线雷达回波功率谱中体现出的特征,将其作为风廓线雷达目标检测的准则。文中提出了一种基于模糊逻辑的风廓线雷达目标检测方法,并进行了实际数据的处理和分析,验证了该方法的可行性和有效性。