Watershed scale temporal and spatial stability of soil moisture and its role in validating satellite estimates

Watershed scale soil moisture estimates are necessary to validate current remote sensing products, such as those from the Advanced Microwave Scanning Radiometer (AMSR). Unfortunately, remote sensing technology does not currently resolve the land surface at a scale that is easily observed with ground measurements. One approach to validation is to use existing soil moisture measurement networks and scale these point observations up to the resolution of remote sensing footprints. As part of the Soil Moisture Experiment 2002 (SMEX02), one such soil moisture gaging system in the Walnut Creek Watershed, Iowa, provided robust estimates of the soil moisture average for a watershed throughout the summer of 2002. Twelve in situ soil moisture probes were installed across the watershed. These probes recorded soil moisture at a depth of 5 cm from June 29, 2002 to August 19, 2002. The sampling sites were analyzed for temporal and spatial stability by several measures including mean relative difference, Spearman rank, and correlation coefficient analysis. Representative point measurements were used to estimate the watershed scale (∼25 km) soil moisture average and shown to be accurate indicators with low variance and bias of the watershed scale soil moisture distribution. This work establishes the validity of this approach to provide watershed scale soil moisture estimates in this study region for the purposes of satellite validation with estimation errors as small as 3%. Also, the potential sources of error in this type of analysis are explored. This study is a first step in the implementation of large-scale soil moisture validation using existing networks such as the Soil Climate Analysis Network (SCAN) and several Agricultural Research Service watersheds as a basis for calibrating satellite soil moisture products, for networks design, and designing field experiments.     

星载微波辐射计的应用与发展

描述了星载微波辐射计基本原理与设计,介绍了星载微波辐射计应用、技术现状和发展趋势,提出了关于我国星载微波辐射计发展思考的建议。     

被动毫米波制导下辐射特性控制的考虑

为提高地面目标在未来战场的生存能力,分析了被动毫米波探测与制导的作用原理与实际威胁,指出了目标的毫米波辐射特性控制的实际需要。在对毫米波辐射特性测量机理的深入分析,与辐射计探测距离公式的推导、讨论基础上,指出毫米波辐射特性控制的主要方向为目标的形状和材料特性的控制,并从这两方面分析了辐射特性控制的实际措施。     

Development of a daily long term record of NOAA-14 AVHRR land surface temperature over Africa

We developed a new 6-year daily, daytime and nighttime, NOAA-14 AVHRR based land surface temperature (LST) dataset over continental Africa for the period 1995 through 2000. The processing chain was developed within the Global Inventory Modeling and Mapping System (GIMMS) at NASA's Goddard Space Flight Center. This paper describes the processing methodology used to convert the Global Area Coverage Level-1b data into LST and collateral data layers, such as sun and view geometries, cloud mask, local time of observation, and latitude and longitude. We used the Ulivieri et al. [Ulivieri, C., M.M. Castronuovo, R. Francioni, and A. Cardillo (1994), A split window algorithm for estimating land surface temperature from satellites, Adv. Space Research, 14(3):59-65.] split window algorithm to determine LST values. This algorithm requires as input values of surface emissivity in AVHRR channels 4 and 5. Thus, we developed continental maps of emissivity using an ensemble approach that combines laboratory emissivity spectra, MODIS-derived maps of herbaceous and woody fractional cover, and the UNESCO FAO soil map. A preliminary evaluation of the resulting LST product over a savanna woodland in South Africa showed a bias of < 0.3 K and an uncertainty of < 1.3 K for daytime retrievals (< 2.5 K for night). More extensive validation is required before statistically significant uncertainties can be determined. The LST production chain described here could be adapted for any wide field of view sensor (e.g., MODIS, VIIRS), and the LST product may be suitable for monitoring spatial and temporal temperature trends, or as input to many process models (e.g., hydrological, ecosystem).     

基于支持向量机预测偏振模式

基于多光谱多角度偏振辐射探测研究了大气偏振模式预测方法。首先,介绍了航空多角度偏振辐射计的探测原理与支持向量机回归算法;然后,从矢量传输模型出发,说明大气状况不变时,偏振模式主要取决于地表特征与观测几何,并介绍了观测几何与姿态之间的关系以及地表特征的表达形式;最后,在考虑平台姿态与地表特征的情况下,利用支持向量机回归算法预测了航空多角度偏振辐射计的探测偏振度,并对预测与实际试验探测的偏振度进行了比较。结果显示:偏振度预测误差小于1%,影响模型精度的主要因素不是姿态变化本身,而是姿态改变造成的观测地表特性变化。     

周期定标毫米波辐射计的性能分析

辐射计的性能主要取决于灵敏度、准确度和稳定度.文中给出了周期定标辐射计灵敏度、准确度和长期稳定度的分析方法,并研制了周期定标毫米波辐射计.该辐射计采用直放式接收机体制,工作频率为33～38GHz,利用有源噪声源的开和关作为高温和低温定标源.实验结果表明:该毫米波辐射计稳定度可达1.28K,准确度为1.71K.当场景测量的积分时间与高、低温定标的积分时间都为7.7ms时,灵敏度优于0.50K.     

红外光谱辐射计的研制

介绍了红外光谱辐射计的组成及工作原理,分析了其信号处理方法。红外光谱辐射计的核心是分光元件、光学系统及信号处理部分。该设备采用容易调整的高像质反射光学系统和波长在２５～１４μｍ连续可变的分光系统,并采用高集成度、高精度的数据采集、数字锁相和数据处理、存储系统。观察瞄准、探测接收的光学系统以及数据采集、处理系统都被放到一个体积很小的测量头中,具有体积小、质量轻牢固可靠、抗干扰性能强和使用方便等优点。入瞳的最小可探测率为１０－１２Ｗ／ｃｍ２,光谱分辨率为２％,测量速度为１０条谱线／ｓ。经实际检测和使用,该设备的性能达到国内外先进水平,并具有野外工作的特点。     

太阳辐照度绝对辐射计的光电不等效性修正

针对绝对辐射计光电不等效性来源复杂、实验测量难度大的特点,提出了修正太阳辐照度绝对辐射计(SIAR)光电不等效性的有限元单元法。结合SIAR的测量方法,对真空中辐射计的腔温响应进行了实验测试。基于有限元单元法,建立了与实验腔温度响应相对误差仅为0.14%的有限元模型,对接收腔的温度响应进行了实验测试。测试结果显示:入射光功率为73.8mW时,接收腔与热沉之间的温度差异约为0.85K,响应的时间常数为29.8s。运用建立的有限元模型对SIAR的光电不等效性进行了评估和修正。结果表明:太阳辐照度绝对辐射计的光电不等效性来源主要为不同加热途径和不同加热区域引起的偏差,SIAR的光电不等效性因子N为0.999 621±0.000 004。该修正模型完善了仪器的修正体系,提高了测量精度,为绝对辐射计的发展提供了可靠的数据来源。     

海洋二号卫星微波辐射计的动平衡设计仿真与试验

为了实现卫星的高精度姿态控制,需要对以辐射计为代表的大型回转载荷进行严格的动平衡设计仿真与试验。结合海洋二号卫星研制需求,在产品设计初期开展了针对微波辐射计的动平衡设计仿真,以提供优化产品结构和布局的依据。为了评估空气环境对动平衡配平的影响,进行了辐射计动平衡配平的风阻影响分析。考虑了重力因素、在轨热变形和无重力下轴承径向游隙变化对辐射计动平衡的影响。最终在真空环境下开展了针对辐射计的动平衡试验,以很小的配重质量,实现了微波辐射计的配平。     

Detection of foliage conditions and disturbance from multi-angular high spectral resolution remote sensing

Disturbance of forest ecosystems, an important component of the terrestrial carbon cycle, has become a focus of research over recent years, as global warming is about to increase the frequency and severity of natural disturbance events. Remote sensing offers unique opportunities for detection of forest disturbance at multiple scales; however, spatially and temporally continuous mapping of non-stand replacing disturbance remains challenging. First, most high spatial resolution satellite sensors have relatively broad spectral ranges with bandwidths unsuitable for detection of subtle, stress induced, features in canopy reflectance. Second, directional and background reflectance effects, induced by the interactions between the sun-sensor geometry and the observed canopy surface, make up-scaling of empirically derived relationships between changes in spectral reflectance and vegetation conditions difficult. Using an automated tower based spectroradiometer, we analyse the interactions between canopy level reflectance and different stages of disturbance occurring in a mountain pine beetle infested lodgepole pine stand in northern interior British Columbia, Canada, during the 2007 growing season. Directional reflectance effects were modelled using a bidirectional reflectance distribution function (BRDF) acquired from high frequency multi-angular spectral observations. Key wavebands for observing changes in directionally corrected canopy spectra were identified using discriminant analysis and highly significant correlations between canopy reflectance and field measured disturbance levels were found for several broad and narrow waveband vegetation indices (for instance, r²_NDVI = 0.90; r²_CHL3 = 0.85; p < 0.05). Results indicate that multi-angular observations are useful for extraction of disturbance related changes in canopy reflectance, in particular the temporally and spectrally dense data detected changes in chlorophyll content well. This study will help guide and inform future efforts to map forest health conditions at landscape and over increasingly coarse scales.