基于视觉的太阳方位检测装置的研究

研究一种基于视觉的太阳方位检测装置,该装置根据小孔成像原理将太阳投影为光斑图像,供图像传感器采集;采集的图像经2000系列DSP芯片处理,定位光斑的位置进而得到太阳光偏离的高度角和方位角,获取太阳位置信息,达到对太阳定位的目的;根据计算得到的太阳位置,可调整太阳能接收装置以最大限度的获取太阳能;通过求图像最大灰度值方法完成太阳定位,简化了定位算法;实验表明,该低速采集系统可实现太阳方位的检测;通过改进也可实现对各种点光源的检测,有一定的实用价值。     

Observations of typhoon eye using SAR and IR sensors

In this review, recent studies on the observations of typhoon eyes by images acquired by multiple sensors, including synthetic aperture radar (SAR), and infrared (IR) radiometer, are first summarized. Large horizontal distances between typhoon eyes on the ocean surface by SAR and those on the cloud top by IR sensors have been demonstrated; these have previously been ignored but should not be ignored in typhoon forecasts and numerical simulations. Then, based on nine published typhoon cases, the horizontal shifts and vertical tilt angles from the cloud-top typhoon eye locations by IR sensors on board the Feng-Yun 2 (FY-2) and Multi Functional Transport Satellite (MTSAT) to those at sea surface by SAR are further estimated. This shift difference between different sensors raises an issue on project distortion and navigation system errors for FY-2 and MTSAT satellites, which are of concern to both space agencies and data users. Finally, issues for current ongoing study and future research related to typhoon eyes are discussed, including rainband tracking between sensors for local wind speeds.     

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.     

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。     

Assessing the sun glint effect on the data bias of CrIS shortwave surface channels near 3.7 μm

The Suomi National Polar-orbiting Partnership (NPP) satellite was successfully launched on 28 October 2011. The on-board Cross-track Infrared Sounder (CrIS) provides the hyperspectral infrared radiance covering a spectral range of 3.92–15.4 μm, inheriting the task to improve numerical weather prediction (NWP) from previous hyperspectral sounders. The so-called sun glint effect results in large biases in CrIS shortwave surface channels near 3.7 μm and therefore impedes the usage of those channels in the operational data assimilation, because the data biases are required to be evaluated appropriately by any data assimilation system. This work assesses the sun glint effect on bias characteristics of those shortwave surface channels near 3.7 μm, with the help of a sun glint model developed in the community radiative transfer model (CRTM). It is demonstrated that the daytime biases of those shortwave surface channels are decreased markedly after applying sun glint correction with values close to 0 K. The dependence of daytime biases on sensor zenith angles is also eliminated by using the sun glint model. It is seen that the differences between daytime and night-time biases can reach 0.6 K near mid-latitudes in the southern hemisphere after including the sun glint effect, which implies that the sun glint model needs further enhancement. Overall, the direct assimilation of CrIS shortwave surface channels near 3.7 μm is possibly accomplished by utilizing the sun glint model implemented in CRTM during both daytime and night-time.     

Simulations for optimal payload tilt to avoid sunglint in IRS-P4 Ocean Colour Monitor (OCM ) data around the Indian subcontinent

Ocean Colour Monitor (OCM) payload, onboard Indian Remote Sensing Satellite (IRS)-P4, detects the water constituents from the spectrum of solar radiation backscattered from the ocean waters. The radiation received by the sensor is contaminated by the specularly reflected solar radiation from the water surface. This specularly reflected radiation, called sunglint, contains no information on the water constituents, as it has not entered into the seawater and interacted with it. The intensity and spread of sunglint is determined by the solar illumination and sensor viewing directions and the sea surface roughness caused by the wind. For the accurate estimation of oceanic constituents, it is essential to minimize the sunglint in the detected radiances (preferably below ～2-3%). In this work, sunglint simulations were computed for the instrument specifications of OCM and the optimal sensor viewing tilt angle identified for each month for the oceans around India.     

建筑物目标高分辨率SAR图像模拟方法

随着TerraSAR-X,Cosmo-SkyMed和Radarsat-2等高分辨率合成孔径雷达(SAR)卫星系统的升空,星载SAR图像空间分辨率达到了米级。在这些高空间分辨率SAR图像中,单个建筑物结构的散射特征能够得到明显的展现,推动了SAR在城市监测中的应用。而城市环境的复杂性给SAR图像的解译和应用带来了巨大的挑战,由于SAR图像模拟有助于图像的解译和应用,因此城市目标高分辨率SAR图像模拟成为一个研究热点。提出了一种基于射线追踪法的图像模拟方法,它能够模拟城市目标SAR图像上叠掩、阴影和多次散射等主要特征,非常有利于SAR图像的解译。该模拟方法首先构建虚拟SAR传感器,发射电磁波射线与场景中三维模型相互作用,并接收回波信号成像,电磁波的传播以及与场景的多次散射在整个过程中都能够被追踪。为了评价该模拟方法的有效性,首先通过模拟平顶屋、尖顶屋模型的SAR图像,然后选择国家体育馆和大场景小区三维模型作为输入,将生成的模拟图像与真实TerraSAR-X聚束模式图像进行比较。结果表明:该模拟器能够模拟城市目标的散射特征并应用于图像的理解和变化检测。     

Sun-view angle effects on reflectance factors of corn canopies

The bidirectional reflectance characteristics of vegetation canopies vary with time of day and through the growing season. In this study the effects of sun and view angles on bidirectional reflectance factors from corn (Zea mays L.) canopies ranging in development from the six leaf stage to harvest maturity were examined. For nadir-acquired reflectance factors there was a strong solar angle dependence in all spectral bands for canopies with low leaf area index (LAI). A decrease in contrast between bare soil and vegetation due to shadows at larger solar zenith angles appeared to be the cause of this dependence. Sun angle dependence was least for well-developed canopies with higher LAI. However, for higher LAI canopies a moderate increase in reflectance factor was observed at larger solar zenith angles and was attributed to the presence of specular reflectance. Trends of off-nadir reflectance factors with respect to sun angle at different view azimuth angles indicated that the position of the sensor relative to the sun was an important factor for determining the angular reflectance characteristics of corn canopies. Reflectance factors were maximized for coincident sun and view angles and minimized when the sensor view direction was towards the sun. View direction relative to row orientation also contributed to the variation in reflectance factors.     

Speed ambiguity in hurricane wind retrieval from SAR imagery

Under strong ocean surface wind conditions, the normalized radar cross section of synthetic aperture radar (SAR) is dampened at certain incident angles, compared with the signals under moderate winds. This causes a wind speed ambiguity problem in wind speed retrievals from SAR, because two solutions may exist for each backscattered signal. This study shows that the problem is ubiquitous in the images acquired by operational space‐borne SAR sensors. Moreover, the problem is more severe for the near range and range travelling winds. To remove this ambiguity, a method was developed based on characteristics of the hurricane wind structure. A SAR image of Hurricane Rita (2005) was analysed to demonstrate the wind speed ambiguity problem and the method to improve the wind speed retrievals. Our conclusions suggest that a speed ambiguity removal algorithm must be used for wind retrievals from SAR in intense storms and hurricanes.     

聚光系统中太阳位置的鱼眼图像识别

提出一种采用图像目标特征识别法跟踪太阳的方法,对图像进行仿人眼光学滤色镜效果的(H,S,I)变换域处理,凸现图像中高亮度区域;其次对高亮度区域进行鱼眼图像与平面图像的变换;然后根据各区域的轮廓、偏移量、面积等特征动态地识别出图像中的太阳斑区;最后通过鱼眼图像中太阳斑区中心点计算出太阳与视点之间的高度角与方位角.实验结果证明,该方法不仅能准确地实现太阳方位的跟踪,而且在物体遮挡、图像失真等情况下定位依然有效.     

基于PSD的太阳方位对准系统研究

针对现有太阳方位跟踪系统中对准精度较低、易受环境制约等问题,设计了一种太阳方位对准系统。该系统一方面接收GPS输出数据信息,利用视日运动轨迹跟踪方法计算初始方位;另一方面利用斜入射平凸透镜的太阳光线在二维四边形PSD器件上形成光斑,计算光斑中心与PSD器件几何中心之间偏差,结合二者信息对太阳方位进行准确定位,对安装倾斜误差进行补偿。实验结果表明,该太阳方位对准系统能够实现全天候、大范围内准确定位,设计方法合理、可行。     

Application of a new algorithm using Doppler information to retrieve complex wind fields over the Black Sea from ENVISAT SAR images

Several algorithms have been proposed to retrieve near-surface wind fields from C-band synthetic aperture radar (SAR) images acquired over the ocean. They mainly differ in the way they retrieve the wind direction. Conventionally, the wind direction is taken from atmospheric models or is extracted from the linear features sometimes visible in SAR images. Recently, a new wind retrieval algorithm has been proposed, which also includes the Doppler shift induced by motions of the sea surface. In this article, we apply three wind retrieval algorithms, including the one using Doppler information, to three complex wind events encountered over the Black Sea and compare the SAR-derived wind fields with model wind fields calculated using the high-resolution weather research and forecasting (WRF) model. It is shown that the new algorithm is very efficient in resolving the 180° ambiguity in the wind direction, which is often a problem in the streak-based wind retrieval algorithms. However, the Doppler-based algorithm only yields good results for wind directions that have a significant component in the look direction of the SAR antenna. Furthermore, it is dependent on good separation of the contributions to the Doppler shift induced by surface currents and wind-related effects (wind drift and wind-sea components of the ocean wave spectrum). We conclude that an optimum wind retrieval algorithm should consist of a combination of the algorithms based on linear features and Doppler information.     

Comparison of annual changes in winter ERS-1 SAR images and glacier mass balance of Slakbreen,Svalbard

The Arctic glaciers are sensitive to climate change, and glacier mass balance is used as an indicator of climate change. However, few mass balance observations are available from the Arctic region. Winter ERS-1 SAR images of the Arctic glacier Slakbreen (78degreesN, 16degrees 30'E) on Svalbard were analysed to investigate a possible relation between SAR backscatter and temporal variations in glacier mass balance. A winter ERS-1 SAR image acquired in 1993 after a summer with large ablation was compared with a winter ERS-1 SAR image acquired after the following summer with low ablation. Changes in mass balance from one year to another were difficult to detect using SAR backscatter data only. Comparison of ground-penetrating radar and SAR data showed that the SAR data contain a signal of the near-surface glacier properties. SAR data were interpreted to reflect variations in accumulation and ablation integrated over several years.     

A new method for extracting ship motion parameters in Radarsat-2 SAR imagery

Traditionally, moving ship detection by Synthetic Aperture Radar (SAR) image is primarily based on the ship wake feature. However, many ship wakes cannot be imaged by SAR owing to changes in imaging conditions, such as the SAR band, polarisation, incident angle, and sea state. In this study, we discovered a unique phenomenon called ‘azimuth tail’ from Radarsat-2 SAR images. Following research and analysis, we determined that the azimuth tail is not manifested as a visible disturbance on sea surface waves. Instead, it is an observation enabled by certain SAR imaging principles. Consequently, we propose a new method for extracting information on moving vessels after a preliminary analysis of the principle of the azimuth tail. The results of experimental analysis of the correctness of the method indicate that the error of the vessel’s velocity from its azimuth tail is less than 20%, and the azimuth tail can be applied to the detection of moving vessels in oceans using Radarsat-2 SAR imagery.     

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…     

Topographic information of sand dunes as extracted from shading effects using Landsat images

Topographic variations affect the reflectance properties of the Earth's surface and are often removed in remote sensing studies, especially when significant terrain variations exist. In this study, however, we show that shading effects assessed by Landsat can be treated as a signal that stores important topographic information, especially when the spectral characteristics of a surface are homogenous. The coastal transverse dunes of the Ashdod area, and the desert linear dunes of Nizzana (both located in Israel), were selected to investigate the abovementioned idea. The dune heights in these areas are 10 m on average (relative to their surroundings) and have maximum slopes of 33°. An innovative method for extracting slope, aspect, and height data for sand dunes using Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) images was developed, based on the regularity and periodicity of dunes' landscapes. Using two Landsat images representing different sun zenith and azimuth angles, reflectance values of each image were converted to cos(i) values (i=incident angle between the surface normal and the solar beam radiation), applying histogram matching methods. The slope and aspect of each pixel were determined as those that give the best prediction of the observed value of cos(i). Height profiles were then extracted, using simple trigonometric relationships. The accuracies of heights and slopes along selected profile lines were to the order of 1 m and 3°, respectively (at a spatial resolution of 15 m). Best results were obtained when the images included one from the summer and the other from the winter, corresponding to maximum difference in solar zenith and azimuth angles. Errors in heights were attributed to surface heterogeneity (e.g., presence of biogenic soil crusts in the rainy season), geometric correction errors, cast shadows, and Bidirectional Reflectance Distribution Function (BRDF) effects. Comparison to Advanced Thermal Emission and Reflection Radiometer (ASTER) 3D information showed that the proposed method is better in representing the topographic variation of the area than the digital elevation model (DEM) produced by ASTER.     

多视SAR图像交叉谱虚部反演海浪信息

提出利用多视处理子视图交叉谱虚部反演海浪信息的方法。子视图中心频率差分别为400Hz,600Hz和800Hz。所用四幅SAR图像均由JERS-1不同时间采集的。地点为东京附近海域,并有对应的测量所得海浪信息,包括波向、波高和周期。反演结果表明,中心频率差为400Hz的处理方法所得结果精度最高,对应的波向和周期信息都与实测数据相差无几。     

应用Radon变换方法检测窄V形船舶航迹

ＳＡＲ航天遥感图像中的舰船航迹影象包含有舰船的进行速度、行进行方向、船体长度等重要信息,而充分开发利用与运动舰船航迹相关的信息资源,能够加强对上舰船航行的监测能力。通过对舰船航迹在卫星遥感ＳＡＲ图象中不同模式及Ｒａｄｏｎ变换原理和特性的分析,提出了应用Ｒａｄｏｎ变换对ＳＡＲ图象中的窄Ｖ形舰船航迹进行了检测的方法,通过实例测试,效果比较理想。     

Estimation of Arctic glacier motion with satellite L-band SAR data

Offset fields between pairs of JERS-1 satellite SAR data acquired in winter with 44 days time interval were employed for the estimation of Arctic glacier motion over Svalbard, Novaya Zemlya and Franz-Josef Land. The displacement maps show that the ice caps are divided into a number of clearly defined fast-flowing units with displacement larger than about 6 m in 44 days (i.e. 50 m/year). The estimated error of the JERS-1 offset tracking derived displacement is on the order of 20 m/year. Occasionally, azimuth streaks related to auroral zone ionospheric disturbances were detected and dedicated processing steps were applied to minimize their influence on the estimated motion pattern. Our analysis demonstrated that offset tracking of L-band SAR images is a robust and direct estimation technique of glacier motion. The method is particularly useful when differential SAR interferometry is limited by loss of coherence, i.e. for rapid and incoherent flow and large acquisition time intervals between the two SAR images. The JERS-1 results, obtained using SAR data acquired by a satellite operated until 1998, raise expectations of L-band SAR data from the ALOS satellite launched in early 2006.     

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.