Inferring hemispherical reflectance of the earth's surface for global energy budgets from remotely sensed nadir or directional radiance values

The importance of the hemispherical reflectance (albedo) of terrestrial surfaces to biospheric and atmospheric processes is briefly reviewed. It is proposed that satellite-borne instruments represent the only practical means of obtaining global estimates of surface albedo data at reasonable time resolution, the problem being how to relate the nadir or directional reflectance observations obtained from such sensors to the integrated hemispherical reflectance. This paper discusses results measured at ground level in which NOAA satellite 7/8 AVHRR data, Bands 1 (0.58–0.68 μm) and 2 (0.73–1.1 μm), were used to investigate 1) the relationships between directional reflectances (spanning the entire reflecting hemisphere) and hemispherical reflectance (albedo) and 2) the effect of solar zenith angle and cover type on these relationships. Eleven natural vegetation surfaces ranging from bare soils to dense vegetation canopies were considered in the study. The results show that errors in inferring hemispherical reflectance from nadir reflectance can be as high as 45% for all cover types and solar zenith angles. By choosing a time of observation such that the solar zenith angle is between 30 and 40° the same error is reduced to less than 20% in both bands. For both bands a view angle of 60° off-nadir and ±90° from the solar azimuth reduces this error to less than 11% for all sun angles and cover types. A technique using two specific view angles reduces this error to less than 6% for both bands and for all sun angles and cover types. These techniques may yield considerable dividends in terms of more reliable estimation of hemispherical reflectance of natural surfaces.     

Optimal directional view angles for remote-sensing missions

Directional reflectance measurements spanning the entire exitance hemisphere for a number of vegetation and bare soil sites were measured from the ground in NOAA-7 AVHRR bands 1 (0-58-0-68 μm) and 2 (0-73-1-1 μm) for various solar zenith angles. The normalized difference and ratio transformations of these bands were calculated. The effects of the atmosphere on the directional radiance above the atmosphere was explored by applying the average mid-latitude atmospheric model of Dave to a series of ground measurements. These data and simulations were analysed for the optimal directional view angles with respect to two strategies. The first strategy views the utility of off-nadir measurements as extending spatial and temporal coverage while the second strategy views the utility of off-nadir measurements as increasing information content about the physical characteristics of the target(s). The optimum view angles for the first strategy are view angles which minimize the change in sensor response of target(s) relative to the sensor response to the nadir direction. The optimum view angles for the second strategy are view angles which, relative to a nadir view, provide superior and/or additional information about the physical characteristics of the target(s).

For the first strategy the results indicate that, from the ground level, the optimum view angles are in the azimuth direction perpendicular to the principal plane of the Sun for all targets, bands and Sun angles. The differences in reflectance between the nadir and off-nadir view angles in this azimuth plane decreased significantly with decreasing solar zenith angle for both bands and all targets. The normalized difference transformation of bands 1 and 2 is significantly less sensitive to variations of view angles than the individual bands for all Sun angles and targets. The results for the second strategy from the ground level showed that off-nadir view angles with an aximuth in the principal plane of the Sun were optimum for all targets and both bands. Finally, the results of the atmospheric simulation study indicated that a mid-latitude summer atmosphere does not significantly change the optimum directional view angles for either the first strategy or the second strategy. However, the atmosphere increased off-nadir sensor responses relative to the ground-level responses particularly in the forwardscattering direction. The normalized difference transformation of bands 1 and 2 was a good transformation for the first strategy for off-nadir viewing angles of less than 45°.

The study provides valuable information for interpreting present remotely sensed off-nadir data and in designing future systems with off-nadir capabilities.     

小型卫星电视地面无线转播系统

目前,农村和山区卫星电视的接收大多采用小功率无线转播。本文分析了无线转播方案、卫星接收天线和转播发射天线的选择以及天线的架设、站址的确定等,并给出了我国部分城市接收中星五号、亚洲一号、东方红二号甲卫星电视时,天线的方位角和仰角,具有一定的参考价值。     

卫星数据广播系统CMACast接收天线安装调试

随着新一代卫星数据广播系统CMAcast即将投入使用,全省气象部门都要对卫星接收天线进行安装调试。该文简单介绍了卫星接收天线选址的注意事项及天线架设的正确方法,详细介绍了卫星接收天线方位角、仰角和极化角正确的计算方法及对其进行调整的具体方法步骤。     

Airborne experimental measurements of the angular variations in surface temperature over urban areas: case study of Marseille (France)

The directional surface temperatures over urban areas are measured using an airborne TIR camera equipped with wide-angle lenses. The experimental design is described and the possible sources of errors related to the instruments and the atmosphere are analysed and corrections proposed. The data acquisition protocol is adapted to provide directional TIR measurements in a −60° to +60° range for zenith and 0 to 360° for azimuth view angles. Two parts of the city of Marseille, the densely built old city centre and a built/vegetated mixture of individual houses and gardens in a suburb, have been studied during summer 2001 in the framework of the UBL-ESCOMPTE project. The results obtained reveal important hot spot effects and differences of surface brightness temperatures varying between −5 and 7 K between nadir and off-nadir measurements, according to azimuth view angles. Acquisitions performed over the two areas also illustrate the impact of the surface structure and presence of vegetation on the hot spot. An example application for the correction of two temporal series of NOAA 14 and NOAA 16 data is given.     

Extraction of spectral hemispherical reflectance (albedo) of surfaces from nadir and directional reflectance data

Abstract

A radiative transfer model was used to explore how the error in inferring spectral hemispherical reflectance (p_λ) from nadir reflectance values varies as a function of wavelength, solar zenith angle, leaf area index and leaf orientation distribution. Secondly, a technique using multiple spectral nadir reflectance values to infer p_λ for a single wavelength was tested using field data. In addition, several techniques that use multiple off-nadir view angles taken in azimuth planes (called strings of data) were tested using field data. These latter techniques were very accurate (with errors less than 4 percent of the true value)and are ideally suited to present and future sensor systems that scan in a known azimuth plane (e.g. Advanced Very High Resolution Radiometer (AVHRR) and other scanning radiometers) or view fore and aft in a known azimuth plane (e.g. Advanced Solid-State Array Sensor (ASAS)Moderate Resolution Imaging Spectrometer (MODIS)High Resolution Imaging Spectrometer (HIRIS)), a brief analysis was performed to explore the effects of errors in hemispherical reflectance on terrestrial energy budget and productivity calculations.     

Estimating albedo from limited spectral and angular data

A database of synthetic albedo and directional reflectance values for vegetated surfaces was constructed utilizing mathematical models. This database enables the comparison of albedo with reflectances measured in narrow spectral bands in particular viewing directions for specified vegetation canopy and solar conditions. The analysis reported here is for spectral bands and angular regimes corresponding to the Along-Track Scanning Radiometer (ATSR-2) sensor on ERS-2. In the analysis multiple linear regression is used to calculate the best fit between modelled reflectance and modelled albedo. A primary estimate of albedo is calculated using reflectance data from the nadir direction only. Data from the forward view of the ATSR sensor are then used to provide additional information to correct the nadir estimate. The relationship between the regressed coefficients and the illumination conditions was investigated in order to provide a universal albedo estimation. Preliminary results for representative solar zenith and azimuth angles show an extremely good fit between modelled albedo and that estimated using the modelled ATSR-2 reflectance.     

Doppler properties of radars in circular orbits

Abstract

Expressions are presented for Doppler frequency shift, Doppler bandwidth, zero Doppler offset angle in spacecraft yaw and, in the SAR mode, the rate of Doppler frequency modulation, azimuth time-bandwidth product, resolution, available integration time and the location of principal azimuth ambiguities. The equations are simply expressed with virtually no approximations based on angles referenced to the satellite. Earth rotation is included, as is the geosynchronous case. (These results differ from expressions to be found elsewhere in the literature, most of which have been derived using flat-Earth approximations.).     

基于Fuzzy ART聚类的卫星在轨姿态监测系统设计

针对传统卫星在轨姿态监测系统无法确定在轨姿态向量与正常向量匹配度,导致姿态角度监测结果不精准的问题,提出基于Fuzzy ART聚类的卫星在轨姿态监测系统设计;系统硬件根据GPS解算方位角和俯仰角,隔离载体扰动;使用MSP430数字/模拟信号转换器,统一CPU指令和寻址模式;使用AS5145B磁角位置编码器输出脉冲信号;通过双轴磁传感器HMC6352的电磁罗盘电路得出卫星在轨姿态大致方向;软件部分利用Fuzzy ART聚类卫星在轨姿态监测数据,确定聚类中心,判定卫星在轨姿态向量与正常向量匹配度,根据匹配结果,开辟新的记忆节点,给出报警信息,完成姿态监测;实验结果表明,所设计系统在轨道圈数为1圈时施加控制力矩,能够迅速使姿态达到稳定;轨道圈数为2圈时,z轴方向的俯仰角与实际三维角度相差1°,表明该系统对卫星在轨姿态监测准确率较高.     

Canopy bidirectional reflectance dependence on leaf orientation

Abstract

The bidirectional reflectance patterns of a complete (dense) canopy are examined as functions of canopy architecture, as specified by azimuth angle δ_e and zenith angle ψ for a leaf normal. The leaves are assumed to be opaque Lambertian reflectors, all with identical orientation and reflectance properties throughout the canopy, and randomly distributed with respect to the irradiation field and the viewing direction. Multiple reflections are not considered and irradiation is by direct beam only. Simple analytical expressions for the bidirectional reflectance factor are presented and analysed. The nadir reflectance (expressed as a fraction of the leaf reflectance) for canopies whose leaves face the sun, δ_e = 0, is bounded by cos ψ and 1/2; cos ψ. The nadir reflectance initially increases with increasing ψ, but then decreases when ψ reaches moderate to large values. For a δ_e = π canopy, on the other hand, the much lower nadir reflectance is bounded by ½ cos ψ and 0, and decreases with increasing ψ throughout the entire range of ψ (0 to ½π). The maximum bidirectional reflectance occurs at large viewing zenith angles (i.e. close to the horizon). The maximum reflectance is always higher for a δ_e = 0 canopy than for a δ_e = π canopy, but the differences become small when ψ approaches ½π. The bidirectional reflectance thus depends on the leaf azimuth as well as the zenith angle. Leaf-area azimuthal distributions should be considered when conducting model inversions to infer canopy characteristics and architecture.     

Design and experimental validation of a sliding-mode stabilizer for a ship-carried satellite antenna

This study designs a sliding-mode controller to stabilize the angular orientation of a ship-carried satellite antenna. The design process starts with calculating the pointing angle of the considered satellite antenna arm as opposed to given ship vibrations due to certain sea waves. This calculation is carried out by the method of Denavit–Hartenberg (D–H) transformation, which is followed by establishing a dynamic model of the satellite antenna system and platform using conventional kinematics modeling techniques. The resulted kinematics relationships are next used as the basis for designing a sliding-mode controller to maintain the antenna in a specified orientation as the ship pitches and rolls under the combined effects of wind and the sea’s waves. The effectiveness of the designed controller is investigated both numerically and experimentally. Both sets of results confirm the feasibility of the proposed control scheme. It is shown that the antenna converges to the required azimuth and elevation angles within 2?s and maintains the specified orientation as the ship continues to pitch and roll.     

Diurnal patterns of bidirectional vegetation indices for wheat canopies

The perpendicular vegetation index (PVI) and normalized difference vegetation index (NDVI) were calculated from Mark II radiometer RED (0.63-0.69 μm) and NIR (0.76–0.90 μ) bidirectional radiance observations for wheat canopies. Measurements were taken over the plant development interval flag leaf expansion to watery ripeness of the kernels during which the leaf area index (LAI) decreased from 40 to 2-5. Spectral data were taken on four cloudless days five times (09.30, 11.00, 12.30, 14.00 and 15.30 hours (central standard time, C.S.T.) at five view zenith, Zv (0, 15, 30,45 and 60°) and eight view azimuth angles relative to the Sun, A_v (0, 45, 90, 135, 180, 225, 270 and 315°). The PVI was corrected to a common solar irradiance (PVIC) based on simultaneously observed insolation readings.

The PVIC at nadir view (?=0°) increased as (l/cosZ_s) increased on all the measurement days whereas the NDVI changed little as solar zenith angle (Z_s) changed. Thus, the PVIC responded to increasing path length through the canopy, or the number of leaves encountered, as solar zenith angle changed whereas the NDVI, which has saturated by the time an LAI of 2 was achieved, was nonresponsive.

Off-nadir PVIC ratioed to nadir PVIC increased as the view zenith and solar zenith angles increased (reciprocity in Sun and view angles), and as the view azimuth, A angle approached the Sun position (back scattering stronger that forwardscattering). In contrast, the DNVI was very stable for all view and solar angles consistent with saturation in its response. Even though the PVI is subject to bidirectional effects, it contains more useful information about wheat canopies at LAI > 2 than does the NDVI. The NDVI of the plant canopies changed rapidly at low vegetative cover but its bidirectional sensitivity at low LAI was not investigated.     

A broadband radar cross section reduction metasurface based on integrated polarization conversion and scattering cancelation

In this paper, we proposed and demonstrated a broadband radar cross section (RCS) reduction metasurface by integrating polarization conversion and scattering cancelation. A broadband polarization conversion structure was designed based on split concentric ring. We used a 4 × 4 array of split ring structure and its mirror‐symmetric structure as the super‐cell units to construct a metasurface with chessboard structure. The metasurface can efficiently reduce the co‐polarization RCS for all azimuth angles of incident wave. The metasurface sample with optimized structure parameters was fabricated and measured. The measured 10 dB RCS reduction band of the metasurface is from 8.6 to 17.7 GHz.     

未知噪声背景下多声源方位角的估计方法

以平面波、窄带声源的阵列测向模型为基础,利用阵列信号处理的理论和方法,在假设理想的白噪声背景下,对均匀线阵接收的信号数据,采用多信号分类法（ＭＵＳＩＣ法）,利用其正交性的谱估计来判断声源的方位角。在此基础上,对不少实际环境中的噪声模型是非白和不确定的情况下,提出一种辅助阵元的方法,以确定在未知噪声背景下利用ＭＵＳＩＣ法确定多声源的方位角。声学实验证实,在实际环境中,该方法能很好地分辨多声源的方位角,且标准差很小。通过与假想的白噪声环境进行对比,本文方法能有效地消除实际环境带来的测向误差,大大提高测向分辨率,由此证实了该方法的有效性。     

Inversion of the Baseline Incline Angle for Tiangong-2 Interferometric Imaging Radar Altimeter

Tiangong-2 Interferometric Imaging Radar Altimeter(InIRA) is the first wide-swath radar altimeter adopting small incidences angles with short baseline.The knowledge of the roll angle of the baseline is crucial for measuring the wide-swath Sea Surface Height(SSH) with centimeter-level accuracy.In this work,we aim to validate the technique of baseline angle determination from spaceborne nadir interferometric echoes which has been tested by airborne experiment.According to the observation geometry,the interferometric phases of nadir echoes acquired by Tiangong-2 Imaging Radar Altimeter(InIRA) are related to the roll angle of the baseline,so it should be possible for us to retrieve the incline angle from the interferometric phases under some ocean conditions not so high.In order to do so,the Tiangong-2 was tilted about 5° so as to realize a 0° baseline and in this way,a lot of data was collected.In this paper,we present the retrieved roll angles and compare them with the measured angles by the platform.Due to the Earth is an ellipsoid,and the control of the Tiangong-2 is referring to the Earth center,while the retrieved roll angle is referring to the nadir point on the Earth surface,there is a systematic error related to the orbit,and after calibration of which,the expected results are obtained：two measurements agree with each other very well not only for the measured trend but also for the standard bias between them.     

星载SAR影像上船舶方位向模糊去除算法

目的 掌握海上船舶分布状态对于海上交通流分析和通航安全管理具有重要作用。遥感技术,特别是星载合成孔径雷达(SAR)技术的发展,为大范围海上船舶检测提供了有效的手段,但受SAR成像机制影响,海上船舶目标在星载SAR影像上通常存在着不同程度的方位向模糊噪声,这些噪声易被误判为船舶,导致船舶识别中虚警率提高。方法 本文简述了方位向模糊噪声的产生原因,提出了一种新的星载SAR影像上船舶方位向模糊去除算法,该算法的核心是构建目标方位向角度一致性、方位向位置偏移距离和方位向模糊能量衰减3个判别规则,对潜在SAR影像亮斑目标进行逐层筛选,实现船舶真实目标和方位向模糊目标的判别。结果 选取中国渤海海域和黄海海域的30 m分辨率的Radarsat-2数据进行案例分析,并与船舶自动识别系统(AIS)实测数据进行比对校验,结果表明,传统的双参数恒虚警率(CFAR)算法和基于K分布的CFAR等算法对于船舶难以剔除方位向模糊,容易造成虚警,而本文算法对实验影像的船舶方位向模糊去除准确率优于95.8%,能够有效剔除船舶方位向模糊。结论 该算法为星载SAR影像上船舶方位向模糊去除提供了新的手段,有助于提高SAR影像上船舶目标检测的准确性。     

Terrain topographic inversion using single-pass polarimetric SAR image data

1IntroductionFullypolarimetricSARimagerytechnologyisoneofthemostimportantadvance-mentsforspace-borneremotesensing.Ithasbeenextensivelyappliedtoterrainsurfaceclassification.The22-D(Dimensional)complexscatteringamplitudefunctionsFpq(p,q=v,h),and44-DrealMuellermatrixMij(i,j=1,…,4)canbemeasured[1].Co-polarizedorcross-polarizedbackscatteringsignatureisthefunctionoftheincidencewavewiththeellipticityanglecandorientationangley.Recently,twoflightsofpo-larimetricSARimagedatahavebeenutilizedtogene…     

Nadir and off‐nadir hyperspectral field data: strengths and limitations in estimating grassland biophysical characteristics

In recent years, hyperspectral and multi‐angular approaches for quantifying biophysical characteristics of vegetation have become more widely used. In fact, as both hyperspectral and multi‐angle reflectance decrease the level of noise on retrieved geophysical parameter values, they increase their reliability by also reducing the saturation problem of the relationships between vegetation indices and biophysical characteristics. To test which is the best methodology in estimating some important biophysical grassland parameters (biomass, total and percent biomass nitrogen content, phytomass and its total and percent nitrogen content), nadir and off‐nadir measurements were carried out, three times during the vegetative period of 2004, in a permanent flat meadow located in the experimental farm of the University of Padua, Italy. The two approaches and the broad band vegetation indices calculated using Landsat bands were compared considering both the best determination coefficients of five vegetation indices, calculated with the two analysis, and through a partial least squares regression using different spectral regions measured at different angles as predictive variables. Using nadir data the red edge region was the most useful for the prediction of biophysical variables, especially phytomass, but also nitrogen content. The off‐nadir data did not provide any significance differences in results to that of data obtained in nadir view but both methods seem to be better adapted to describe biophysical parameters of vegetation than the use of broad band vegetation indices.     

Effect of terrain orientation and solar position on satellite-level luminance observations

Changes in the luminance of a terrain along the local nadir direction are studied as a function of terrain slope, and the solar azimuth and elevation angles. The digital terrain data for the Santa Clara valley-Mt. Hamilton region of California, available from the U.S. National Cartographic Information Center, are used for this purpose. The elevation above mean sea-level of this selected region varies between 24 and 1,282 m. Luminance computations are performed by assuming the Lommel-Seeliger law of reflection, and the terrain reflectance to be a constant and independent of its geographic location. The atmospheric effects and contributions due to multiple reflections are neglected. It is shown that the ratio of the actual terrain luminance to that for the corresponding flat surface for 64 equal-size (～ 4 km²) finite subregions, changes by a very significant amount over the selected region, and also with changes in the solar position. Selected Landsat MSS band 7 data of the same region are presented for validation purposes.     

Fast self-localization method for mobile robots using multiple omnidirectional vision sensors

This paper presents a method for estimating position and orientation of multiple robots from a set of azimuth angles of landmarks and other robots which are observed by multiple omnidirectional vision sensors. Our method simultaneously performs self-localization by each robot and reconstruction of a relative configuration between robots. Even if it is impossible to identify correspondence between each index of the observed azimuth angles and those of the robots, our method can reconstruct not only a relative configuration between robots using `triangle and enumeration constraints' but also an absolute one using the knowledge of landmarks in the environment. In order to show the validity of our method, this method is applied to multiple mobile robots each of which has an omnidirectional vision sensor in simulation and the real environment. The experimental results show that the result of our method is more precise and stabler than that of self-localization by each robot and our method can handle the combinatorial explosion problem. Correspondence to:T. Nakamura (e-mail: ntakayuk@sys.wakayama-u.ac.jp)