Radar altimeter mean return waveforms from near-normal-incidence ocean surface scattering

Under assumptions common in radar altimetry, the mean backscattered return power for a short-pulse radar and near-normal-incidence scattering from a rough ocean surface is given by the convolution of several terms. For a nearly Gaussian transmitted pulse shape scattered from a nearly Gaussian distributed sea surface, a small-argument series expansion of one of the terms within the convolution leads to a several-term power series expansion for the mean return waveform. Specific expressions are given for the first four terms. These results, which require much less computer time than would the otherwise necessary numerical convolution, are useful for data analysis from current or past radar altimeters and for design studies of future systems. Several representative results are presented for an idealized SEASAT radar altimeter.     

地形地物对星载激光测高仪回波信号的影响

地形地物的回波信号特性影响着星载激光测高仪测量精度。对星载激光测高仪回波脉冲信号进行了理论分析,建立了地形地物数学物理模型,对不同地形地物的回波信号进行了数值模拟研究;在采用不同回波信号检测的情况下,分析了不同地形地物造成回波信号的变化所引起的测量误差。研究结果对保证星载激光测高仪测量精度具有重要的意义。     

基于complex-EOF的高频地波雷达海流数据的空时特性

利用布设在高频地波雷达福建示范区的东山、龙海2个雷达站的约3天的海流数据进行矢量正交经验函数分析。分析结果表明前3个模态的方差贡献率分别为91%、4%和1.4%,累计方差贡献率达到了96%,是主要模态。模态1的空时分布特性表明模态1主要反映了半日潮流的信息,并且在一定程度上反映了水深分布的信息;模态2的空时分布特性表明模态2主要反映了风生流以及流场旋转特性的信息;模态3的空时分布特性表明模态3主要反映了上升流的信息。     

GF-7星载激光测高仪全波形自适应高斯滤波

全波形星载激光测高仪通过向地表发射激光,获得目标表面的后向散射完整波形,可用于地表剖面高程信息及目标表面几何物理参数的反演。GF-7星载激光测高仪部分原始波形存在噪声显著、波峰左偏/右偏和非饱和平峰等情况,对提取有效信息造成干扰。针对GF-7星载激光测高仪全波形数据,首先提出了一种波形背景噪声迭代去除方法,然后对波形噪声特点及几何结构进行分析,并进行定量化描述,最终设计了一种顾及波形噪声与结构异构的自适应高斯滤波器。在实验中,将文中方法与已有经典波形滤波算法进行比较,最终验证了文中方法在噪声去除、有效信号保留及非饱和平峰波形处理上的有效性。使用文中方法进行波形滤波后,波形信噪比更高,同时波形幅值下降量均在3倍的噪声标准差以内,非饱和平峰波形滤波后波形高斯分解参数振幅、均值和标准差的分解精度分别为(0.69 ± 2.34) mV, (0.007 ± 0.024) ns和(0.026 ± 0.069) ns。     

高分七号卫星激光测高仪光斑质心位置变化分析

高分七号卫星搭载了我国首个具备全波形记录能力的对地观测激光测高仪,可以大范围地获取高精度三维坐标,其定位精度高度依赖于激光指向角的测量精度.针对数据特点,提出了一种阈值约束的椭圆拟合光斑质心提取算法,并建立了长周期指向角稳定性监测与分析系统.首先,用阈值法确定激光光斑轮廓的边缘;其次,通过腐蚀操作消除孔隙、噪声的影响;...     

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。     

星载激光测高仪多模式回波参数提取方法（特邀）

全波形星载激光测高仪的接收波形特征参数可以用于反演目标的形貌信息,传统的波形处理算法不能用于混叠严重以及偏离高斯形态的多模式波形特征参数提取。针对混叠严重的多模式回波,提出一种基于偏正态拟合模型,使用激励Richardson-Lucy反卷积算法、逐层分解算法、梯度下降法和非线性最小二乘拟合算法相组合的波形特征参数提取方法。采用已知参数的波形数据集、机载仿真波形数据集和全球生态系统动态调查（GEDI）激光雷达波形数据,基于波形相关系数与均方根误差（RMSE）、波形特征参数相对误差、波形分量个数提取正确率等评价指标开展波形处理试验,并将处理结果与传统的高斯分解结果进行比较分析。已知参数波形数据集处理结果的平均波形相关系数提升了约2%,RMSE降低了约47%,波形特征参数相对误差平均降低了约5%,分量个数提取正确率提升了约34%;机载仿真数据和GEDI波形数据处理结果的平均波形相关系数分别提升了约1%和2%,RMSE分别降低了约56%和54%。同时,开展了陡坡区域植被高度解算的仿真试验,得到的植被高度准确程度明显高于传统方法。所有处理结果均表明该方法更有利于多模式回波特征参数的提取以及目标参数的反演。     

Measurement of river surface currents with coherent microwave systems

River surface currents have been measured using coherent microwave systems from a bridge, a cableway, several riverbanks, a helicopter, and an airplane. In most cases, the microwave measurements have been compared with conventional measurements of near-surface currents and found to be accurate to within about 10 cm/s. In all cases, the basis for the microwave measurement of surface current is the Doppler shift induced in the signal backscattered from the rough water surface. In this paper, we outline the principles of the measurements and the various implementations that have been used to make microwave measurements of surface currents. Continuous-wave (CW) microwave systems have been used from a bridge to make long-term measurements of surface currents; these are compared with current-meter measurements and with time series of stage. A compact CW system has been developed and used on a cableway to measure surface currents at various distances across a river; these measurements have been compared with acoustic ones. Pulsed Doppler radars have been used to measure river surface currents from a riverbank, a helicopter, and an airplane. In the first two cases, comparisons with both current-meter and acoustic measurements have been made. We suggest that the CW system would be preferable to the pulsed Doppler radar to make such measurements from helicopters in the future. Finally, we consider the implications of our experiments for the measurement of surface currents from aircraft or satellites using interferometric synthetic aperture radars (INSARs). We find that a combination along-track, cross-track INSAR is necessary but that significant limitations are inherent in the technique.     

Radar reflectivity of the ocean surface for circular polarization

If one transmits an electromagnetic wave, an electric field is set up in space. The earth's surface modifies the field and an interference pattern results. To calculate such a pattern one must know the reflection characteristics of the surface for the polarization of the incident wave. These are known for linear polarizations. Since a circularly polarized wave is a combination of the two linear polarizations, we can compute the reflection coefficient for circular polarizations by proper combination of the two linear reflection coefficients. This reflection coefficient is known for smooth sea conditions. In this paper the vector model of reflectivity for a rough sea is extended to include circular polarization. Reflection coefficients for the rough sea case are derived. Analytical expressions are given for the total received signal including the coherent and incoherent components. Receiver antenna polarizations of the same and opposite senses to that of the transmitter are considered.     

Models of radar imaging of the ocean surface waves

A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation:tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed.     

On airborne measurement of the sea surface wind vector by a scatterometer (altimeter) with a nadir-looking wide-beam antenna

A pilot needs operational information about wind over sea as well as wave height to provide safety for a hydroplane landing on water. Near-surface wind speed and direction can be obtained with an airborne microwave scatterometer, a radar designed for measuring the scatter characteristics of a surface. Mostly narrow-beam antennas are applied for such wind measurement. Unfortunately, a microwave narrow-beam antenna has considerable size that hampers its placement on flying apparatus. In this connection, a possibility to apply a conventional airborne radar altimeter as a scatterometer with a nadir-looking wide-beam antenna in conjunction with Doppler filtering for recovering the wind vector over sea is discussed, and measuring algorithms of sea surface wind speed and direction are proposed. The obtained result can be interesting for many studies in oceanography, meteorology, air-sea interaction, and climate change and for creation of an airborne radar system for amphibious airplane safe landing on the sea surface, in particular for search and rescue operations in coastal areas.     

新型多波束陆-海激光雷达探测卫星技术发展研究

从国家未来战略需求出发,对发展陆海激光雷达需求进行了分析,介绍了陆海激光雷达的特点和国内外星载激光雷达卫星发展的现状,提出了未来星载陆海激光雷达卫星的发展方向,以及在轨预期数据的应用产品,给出了星载海洋激光雷达关键技术及解决途径,阐述了“十四五期间”陆海激光雷达的应用前景。     

Estimating terrestrial snow depth with the TOPEX-Poseidon altimeter and radiometer

Active and passive microwave measurements obtained by the dual-frequency TOPEX-Poseidon radar altimeter from the Northern Great Plains of the United States are used to develop a snow pack radar backscatter model. The model results are compared with daily time series of surface snow observations made by the U.S. National Weather Service. The model results show that Ku-band provides more accurate snow depth determinations than does C-band. Comparing the snow depth determinations derived from the TOPEX-Poseidon nadir-looking passive microwave radiometers with the oblique-looking Satellite Sensor Microwave Imager (SSM/I) passive microwave observations and surface observations shows that both instruments accurately portray the temporal characteristics of the snow depth time series. While both retrievals consistently underestimate the actual snow depths, the TOPEX-Poseidon results are more accurate.     

Retrieving ocean surface wind speed from the TRMM Precipitation Radar measurements

Spaceborne scatterometery has been used for many years now to retrieve the ocean surface wind field from normalized radar cross-section measurements of the ocean surface. Though designed specifically for the measurement of precipitation profiles in the atmosphere, the Precipitation Radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) also acquires surface backscattering measurements of the global oceans. As such, this instrument provides an interesting opportunity to explore the benefits and pitfalls of alternative radar configurations in the satellite remote sensing of ocean winds. In this paper, a technique was developed for retrieving ocean surface winds using surface backscattering measurements from the TRMM PR. The wind retrieval algorithm developed for TRMM PR makes use of a maximum-likelihood estimation technique to compensate for the low backscattering associated with the PR configuration. The high vertical resolution of the PR serves to filter-out rain-contaminated cells normally integrated into Ku-band scatterometer measurements. The algorithm was validated through comparisons of ocean surface wind speeds derived from PR with remotely measured winds from TMI and QuikSCAT, as well as in situ observations from oceanographic buoys, revealing good agreements in wind speed estimations.     

Effects of surface waves on the currents of truncated periodic arrays

The behavior of surface waves in truncated periodic arrays is examined through analysis of the currents. The surface waves to be studied are guided by the perfectly conducting elements of the array itself and are to be distinguished from the dielectric slab-guided surface waves encountered elsewhere in the literature. The conditions under which surface waves may arise are given. The surface wave currents are extracted from the method of moments solution for the finite by an infinite array using a least squares algorithm. Surface wave excitation and reflection coefficients are then be determined from the data and compared with the semi-infinite array solution.     

Azimuthal harmonic coefficients of the microwave backscattering from a non-Gaussian ocean surface with the first-order SSA model

In this paper, the first-order small slope approximation is applied to a rough sea surface with non-Gaussian statistics, for which the third- and the fourth-order statistics are taken into account in the calculation of the radar cross section. From the Cox and Munk slope distribution, the higher order statistic moments are derived, and behaviors of the corresponding correlation functions are assumed. We show that the fourth order (related to the peakedness or kurtosis) is isotropic, whereas the third order (related to the skewness) has a behavior as cos(/spl psi/), where /spl psi/ is the wave direction along the wind direction. Thus, using the Elfouhaily et al. sea height spectrum, related to the second-order statistics, we show that the normalized radar backscattering cross section (NRBCS) can be expanded as an even Fourier series in cos(n/spl phi/) (where n is a positive integer), for which the harmonic coefficients require only a single integration over the radial distance. This result is consistent with experimental data done for microwave frequencies. In addition, we show for microwave frequencies (like C- and Ku-bands) that the Fourier series can be truncated up to the second order, since the higher order harmonic coefficients vanish. The NRBCS is also compared with empirical backscattering models CMOD2-I3 and SASS-II, valid in C- and Ku-bands, according to the scattering angle and the wind direction. The first-order harmonic coefficient predicts the surface asymmetry along the upwind and downwind directions, whereas the second-order harmonic coefficient describes the surface asymmetry along the upwind and crosswind directions.     

硅基底表面上多种形态碳纳米管的生长

以Fe/Ru双金属纳米颗粒为催化剂,在含有一层自然氧化层的硅基底上制备单分散碳纳米管,并用原子力显微镜进行表征。实验结果表明在同一基底上碳纳米管具有多种形态,如环形、树枝状分叉形、锥形,单根和成束等。这种多种形态碳纳米管的出现,为更进一步了解碳纳米管的生长机制以及对控制碳纳米管制备有着重要意义。     

Tutorial review of synthetic-aperture radar (SAR) with applications to imaging of the ocean surface

A synthetic aperture radar (SAR) can produce high-resolution two-dimensional images of mapped areas. The SAR comprises a pulsed transmitter, an antenna, and a phase-coherent receiver. The SAR is borne by a constant velocity vehicle such as an aircraft or satellite, with the antenna beam axis oriented obliquely to the velocity vector. The image plane is defined by the velocity vector and antenna beam axis. The image orthogonal coordinates are range and cross range (azimuth). The amplitude and phase of the received signals are collected for the duration of an integration time after which the signal is processed. High range resolution is achieved by the use of wide bandwidth transmitted pulses. High azimuth resolution is achieved by focusing, with a signal processing technique, an extremely long antenna that is synthesized from the coherent phase history. The pulse repetition frequency of the SAR is constrained within bounds established by the geometry and signal ambiguity limits. SAR operation requires relative motion between radar and target. Nominal velocity values are assumed for signal processing and measurable deviations are used for error compensation. Residual uncertainties and high-order derivatives of the velocity which are difficult to compensate may cause image smearing, defocusing, and increased image sidelobes. The SAR transforms the ocean surface into numerous small cells, each with dimensions of range and azimuth resolution. An image of a cell can be produced provided the radar cross section of the cell is sufficiently large and the cell phase history is deterministic. Ocean waves evidently move sufficiently uniformly to produce SAR images which correlate well with optical photographs and visual observations. The relationship between SAR images and oceanic physical features is not completely understood, and more analyses and investigations are desired.     

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     

On the edge reflections of high-frequency surface currents inducedon a cylindrical strip

High-frequency surface currents induced on a surface with edges can be interpreted as being composed of components attributable to the reflections and edge or surface diffractions of the incident field as well as to the edge reflections of these components themselves. The explicit expressions of the former components can be obtained from a set of properly formulated canonical problems if the surface consists of a cylindrically or spherically curved sheet. These types of canonical problems are not suitable for the derivation of the edge-reflected components, a fact that should be considered in dealing with currents induced on sheets of finite extent. A conjecture is made about the edge-reflected currents induced on a cylindrically curved sheet, and some of its consequences are shown. The normal and oblique incidence cases are considered separately