Atmospheric multiple scattering effects on GLAS altimetry. II. Analysis of expected errors in Antarctic altitude measurements

For pt.I see ibid., vol.39, no.1, p.92-101 (2001). The altimetry bias in the Geoscience Laser Altimeter System (GLAS) or other laser altimeters resulting from atmospheric multiple scattering is studied in relationship to current knowledge of cloud properties over the Antarctic Plateau. Estimates of seasonal and interannual changes in the bias are presented. Results show the bias in altitude from multiple scattering in clouds would be a significant error source without correction. The selective use of low-optical-depth clouds or cloud-free observations, as well as improved analysis of the return pulse such as by the Gaussian method used here, is necessary to minimize the surface altitude errors. The magnitude of the bias is affected by variations in cloud height, cloud effective particle size, and optical depth. Interannual variations in these properties as well as in cloud cover fraction could lead to significant year-to-year variations in the altitude bias. Although cloud-free observations reduce biases in surface elevation measurements from space, over Antarctica these may often include near-surface blowing snow, also a source of scattering-induced delay. With careful selection and analysis of data, laser altimetry specifications can be met.     

大气多次散射效应对星载激光测高仪测距偏差值的影响

大气多次散射效应会使得星载激光测高仪脉冲回波信号发生拖尾现象,导致激光测高仪的测距值出现偏差。根据激光测高仪的工作原理并利用半解析型蒙特卡罗方法,建立了大气多次散射效应条件下的脉冲回波信号及距离偏差的数学模型,并基于GLAS系统参数,仿真分析了云和雾的多次散射效应对距离偏差值的影响规律。结果表明,随着大气参数的变化,距离偏差会呈现不规则的起伏变化。当云层粒子尺度大于150μm或雾的消光系数小于1.68 km-1时,大气多次散射效应对测距偏差值的影响小于1 cm。所得的结论为星载激光测高仪测量天气的选取以及系统参数的优化提供了理论依据。     

单次大气散射效应对星载激光测高仪接收脉冲回波的影响

大气散射效应是影响星载激光测高仪接收脉冲回波的重要因素。根据星载激光测高仪接收脉冲回波信号与大气响应函数之间的关系式,在忽略大气多次散射效应的条件下,通过分析散射激光束的几何轨迹和散射概率,推导出单次大气散射激光脉冲和接收脉冲回波的特征参数的数学解析式。以地球科学激光测高仪系统参数为输入,采用数值仿真分析的方法,模拟了大气散射介质分布、激光指向角和目标倾斜角对接收脉冲回波信号特征参数的影响。结果表明,若散射介质的高度和粒子半径范围分别为0.2~6 km和0~120 m,则其对接收脉冲回波的能量、重心和均方根脉宽的影响最大值分别超过15%、250 cm和800 cm。随着激光指向角或目标倾斜角的增加,接收脉冲回波的能量基本不产生影响,但是其重心和均方根脉宽近似呈线性增加趋势。同时,采用高斯拟合方法可以减小大气散射效应对接收脉冲回波的影响。所得结论对于接收脉冲回波的数据处理与分析以及激光测距精度的评估具有一定的指导意义。     

Electromagnetic scattering by multiple three-dimensional scatterersburied under multilayered media. I. Theory

A general procedure is developed for the analysis of electromagnetic (EM) scattering by multiple three-dimensional (3D) dielectric and/or conducting objects buried under one-dimensional (1D) multilayered media. In this first part of a two-part paper, general closed-form formulations for the electric fields excited by an arbitrarily oriented electric dipole under the layered media are first presented, from which electric-field integral equations for the buried dielectric objects, pure conducting objects, and their combinations are then obtained, and the scattered electric fields in the upper space are formulated. Finally, the physical significance of the above formulations is discussed. In the second part, numerical implementations for these integral equations and the scattered fields are investigated     

Optical UWB doublet pulse generation using multiple nonlinearities of single SOA

All-optical ultra-wideband (UWB) doublet pulse generation based on the multiple nonlinear effects of a single semiconductor optical amplifier (SOA) is demonstrated. First, a monocycle pulse is obtained by means of cross-gain modulation and self gain-saturation in the SOA, and a Gaussian pulse at conjugated light is obtained using four-wave mixing of the SOA. Secondly, the Gaussian pulse at the conjugated light is combined with the monocycle pulse by proper time delay to form a doublet pulse. The central frequency and 10 dB bandwidth of the doublet radio frequency are 5 and 6.25 GHz, respectively. The scheme is simple since only a single SOA is required.     

Multiport scattering analysis of general multilayered printedantennas fed by multiple feed ports. I. Theory

A general solution is given for a class of printed antenna geometries composed of multiple dielectric layers or ground planes, radiating patches, dipoles, or slots, and an arbitrary configuration of multiple transmission lines proximity-coupled or aperture-coupled to the radiating elements. The solution uses a full-wave spectral-domain moment method approach, and a new generalized multiport scattering formulation to model the excitation from the multiple feed lines. This method treats infinite phased arrays as well as isolated elements. The general theory using the new multiport scattering formulation is elaborated, with details of the key analytical and numerical aspects. Considering the unified nature of the multiport scattering analysis, and its simplicity, this analysis is appropriate for computer simulation of a large variety of multilayered microstrip antennas involving radome layers, dual polarized feeds, proximity-coupled or aperture-coupled elements, multifeed stacked or parasitic patches, and several related configurations for integrated phased array applications     

Experimental study on the single event effects in pulse width modulators by laser testing

This paper presents single event effect (SEE) characteristics of UC1845AJ pulse width modulators (PWMs) by laser testing. In combination with analysis to map PWM circuitry in the microchip dies, the typical SEE response waveforms for laser pulses located in different circuit blocks of UC1845AJ are obtained and the SEE mechanisms are analyzed. The laser SEE test results show that there are some differences in the SEE mechanisms of different circuit blocks, and phase shifts or changes in the duty cycles of few output pulses are the main SEE behaviors for UC1845AJ. In addition, a new SEE behavior which manifests as changes in the duty cycles of many output pulses is revealed. This means that an SEE hardened design should be considered.     

Effects of multiple scattering and Fresnel diffraction on random volume scattering

A wave incident on a volume of random fluctuations of medium parameters will be scattered in all directions. Two kinds of effects on random volume scattering are considered: 1) multiple scattering effect, and 2) Fresnel diffraction effect. The cumulative forward scattering is known to be responsible for the scintillation phenomenon. When strong, such cumulative or multiple scattering can drive the signal statistics into the saturated regime in which the complex amplitudes behave as a complex Gaussian process. These cumulative scattering effects on signals of a bistatic radar are investigated. Additionally, to allow for the possibility that the size of the irregularity is comparable or larger than the first Fresnel zone, Fresnel diffraction instead of Fraunhofer diffraction is considered. A generalized formula for calculating average scattered power, taking both multiple scattering and Fresnel diffraction into account, is derived. It shows that the Fresnel diffraction pattern appearing around the forward direction tends to be smeared out by multiple scattering. Also, both multiple scattering and Fresnel diffraction effects weaken and broaden the forward-scattered beam, and reduce the backscattered power when compared with the single scattering and Fraunhofer diffraction cases respectively. Under certain conditions, the enhancement effect of the backward scattered power is confirmed.     

Effect of multiple scattering on line-of-sight propagation

This paper deals with the effect of small random variations in the atmosphere's refractive index upon electromagnetic waves propagated along a line-of-sight path. These fluctuations are considered to be due only to the turbulent character of the medium, and the effect of the earth and other bodies is taken to be negligible. Previous analyses of this problem have utilized the first Born or single-scattering approximation. This method, however, assumes that each part of the scattering region is illuminated by the incident wave, i.e., that any incident wave is scattered at most once. The applicability of this approximation in the microwave region is questionable since the mean free path for scattering at high frequencies decreases as the square of the frequency. If at microwave frequencies the transmission distance is large compared to the mean free path length multiple-scattering effects are of importance. It is these effects which are considered here. Multiple-scattering analysis is used to determine expressions for the mean value, mean-square value, and variance of the field at a single receiver. Also determined is the correlation function for zero time difference for the fields at two spaced receivers, as well as the mean value, mean square, and variance of the difference of the fields at two spaced receivers.     

静电场中高斯激光束与电子作用产生单个阿秒脉冲

提出了利用线极化的高斯光束穿过固体表面与其电 离的一个电子在Thomson散射作用下产生单个阿秒脉冲辐射的方法。发现了上述环境中有合 适大小均匀静电场的情况下可以产生单个阿秒 脉冲。分别研究了4种不同强度的静电场大小对处在光场半腰束位置的电子产生脉冲的影响 。结果显 示在相同参数,束腰半径r₀=5,脉冲宽度w₀=3,持续周期时间为200ms,P=1的情况下,四种不 同大小静电场环境中均可以产生阿秒脉冲。但是只有当高斯光速强度为5,静电场大小为归 一化的 E_s=5×10－4时,可以产生单个阿秒脉冲,脉冲宽度为16as。并且比无其他静电场产生脉冲的强度更 强,方向单一化。因此得出高斯强度E_s=5×10 －4的静电场与静止的单个电子作用可以得出单个阿秒脉冲辐射。     

Local spectral analysis of short-pulse excited scattering fromweakly inhomogeneous media. I. Forward scattering

In this two-part sequence, we extend a previously formulated pulsed plane wave (PPW)-based time-domain (TD) diffraction tomography for forward and inverse scattering from weakly inhomogeneous lossless nondispersive media to a more highly localized pulsed beam (PB) wavepacket-based diffraction tomography. In the PPW version, the incident and scattered fields have been parameterized in the space-time wavenumber domain in terms of slant-stacked TD plane waves whose wavefronts move through the scattering medium at the ambient propagation speed, thereby accumulating information along time-resolved laterally extended planar cuts. The PB parameterized localization confines the laterally sampled regions to the spatial domains of influence transverse to the relevant beam axes. These localizations are performed in two stages. The present paper implements the PB parameterization by PB post processing of the forward scattered fields excited by an incident PPW; the companion paper (Part 2, see ibid., vol.47, no.7, p.1218-27, July 1999) deals with the inverse problem by back propagation of the PB parameterized data. An “ultimate” localization of a space-time resolved scattering cell is also briefly summarized in Part 2     

Nonlinear optical pulse propagation in a semiconductor medium inthe transient regime. I. Temporal and spectral effects

The propagation of optical pulses in a semiconductor medium is modeled in the limit where the optical pulse width is assumed to be shorter than the interband recombination time of the semiconductor. Intense pulses cause saturation of the optical transitions, leading to transient changes in absorption and refractive index of the semiconductor. The changes in absorption and refractive index lead to distortions in the spectra of the pulses     

大气后向散射对弹道导弹激光引信的影响分析

由于作用距离的不同,与空空导弹及反坦克导弹等激光引信相比,大气后向散射对弹道导弹激光引信的具体影响也不同.本文利用Mie散射理论计算了大气后向散射系数,通过数值计算分析了大气后向散射对弹道导弹激光引信的影响,探讨了消除大气后向散射影响的技术措施,以供相关课题的工程实施参考.     

Terminals for a high-speed optical pulse code modulation communication system: I. 224-Mbit/s single channel

The design of an experimental single-channel 224-Mbit/s (megabits per second) optical pulse code modulation terminal is described and data are presented which have been obtained with such a terminal using a helium-neon laser operating at 6328 Å. The basic elemets is an optical gate using lithium tantalate whose design and operating characteristics are described.     

Noise of the stretched pulse fiber laser. I. Theory

A perturbation formalism is developed for the passively mode-locked stretched pulse fiber ring laser analogous to that of the fiber ring soliton laser. It is applied to determine the amplitude fluctuations, carrier frequency noise, and the pulse to pulse jitter due to the amplifier spontaneous emission noise     

脉冲光布里渊散射信号的拉曼放大研究

实验研究了前向拉曼泵浦方式下脉冲信号光产生的自发布里渊散射信号和受激布里渊散射(SBS)信号的拉曼放大规律。拉曼泵浦放大自发布里渊散射信号时,随泵浦功率增大会出现SBS现象,对散射信号的放大由拉曼放大和布里渊放大两部分引起,因此增益较大,当拉曼泵浦功率为1 000 mW时Stokes光增益可达54 dB。拉曼泵浦放大SBS信号时,放大过程中只存在拉曼放大。且当泵浦功率增大至600 mW时,会引起多级布里渊散射,致使一级Stokes和泵浦能量会转移到下一级布里渊散射,一级Stokes光增益饱和并下降。     

The effect of pulse walkoff on stimulated Raman scattering in fibers

We have experimentally investigated stimulated Raman scattering in single-mode fibers in the regime of large Raman Stokes pulse walkoff from the pump pulse by the effect of group-velocity dispersion. Measurements are made with 36 ps duration pulses at 532 nm produced from a frequency-doubled and harmonically mode-locked Nd:YAG laser. We find that for 20 percent conversion, the Raman output is produced about two walkoff lengths into the fiber as a pulse of approximately the same duration as the input pulse. We also find that the Raman pulse is produced with a strong frequency chirp.     

脉冲激光对单晶硅打孔的研究

采用波长为1064 nm的重复脉冲激光对单晶硅进行打孔实验,观测了小孔烧蚀深度以及表面孔径大小随脉冲个数的变化规律,并对激光辐照单晶硅的热力学过程进行了理论分析。研究结果表明:入射激光在穿过等离子体到达单晶硅的表面时,光斑尺寸会有所增大,小孔孔径会大于聚焦光束尺寸。小孔内的等离子体本身具有很高的温度,高温等离子体在膨胀过程中会通过热辐射和热传导等过程向小孔周围传递热量,这也会对小孔孔径起到一定的拓展作用。当脉冲个数低于6个时,孔深随入射脉冲个数的增加近似线性增长,而后开始缓慢增长直至保持不变,这主要是由激光等离子体屏蔽效应决定的。     

A novel measurement method of a single dust particle permittivity at a microwave frequency band as I/P to accurate scattering computations. Part I

In electromagnetic scattering computation the scattering properties of a single particle can be evaluated if the solid permittivity of its constituent material is known. If the material is not homogeneous (e.g. water layer on the particle surface) both the structure and the permittivity of the different parts must be known. However, it is possible (at least at microwave frequencies) to define an equivalent homogeneous permittivity $\left( {\bar \varepsilon } \right)$ which would produce the same scattering properties for a homogeneous particle of the same outer shape as the real one. In this work a novel measurement method of a single particle scattering for evaluating the dielectric properties of dust is derived. Results are compared with another work on bulk/scaled measurement (Part II) of dust permittivity at the same frequency band (X-band) gave matched results.     

Atmospheric correction of Landsat ETM+ land surface imagery. I.Methods

To extract quantitative information from the Enhanced Thematic Mapper-Plus (ETM+) imagery accurately, atmospheric correction is a necessary step. After reviewing historical development of atmospheric correction of Landsat Thematic Mapper (TM) imagery, the authors present a new algorithm that can effectively estimate the spatial distribution of atmospheric aerosols and retrieve surface reflectance from ETM+ imagery under general atmospheric and surface conditions. This algorithm is therefore suitable for operational applications. A new formula that accounts for adjacency effects is also presented. Several examples are given to demonstrate that this new algorithm works very well under a variety of atmospheric and surface conditions