卫星导航服务的全球电离层时变特性分析

针对电离层对无线电波应用技术的影响,该文利用IGS提供的1998年—2012年的全球电离层TEC数据,结合相应的太阳活动数据,采用时间序列分析、相关性分析以及等值线图等数理统计的方法,分析了全球电离层的时变特性;分析了电离层TEC、F10.7和太阳黑子数的相关性,发现3者之间的相关系数高度线性相关。通过电离层日变化规律的研究发现:电离层TEC日极大值出现的时刻集中在当地时间12时至16时,其中14时占38.47%,12时占26.58%,16时占19.05%;夜间TEC值与太阳活动强度密切相关,在太阳活动低峰年,夜间全球电离层TEC平均值在5TECU左右,在太阳活动高峰年,夜间全球电离层TEC平均值在17TECU左右,最大值可达24TECU。最后,从日地距离和太阳活动强度两个方面,讨论了全球电离层TEC季节变化规律以及成因。     

基于NOAA卫星的陕西省太阳总辐射反演研究

本文以2007年3-9月陕西省地面总日射实测小时累计值及NOAA卫星影像为信息源,采用半物理模式的统计经验反演得出地面太阳总辐射,进而分析了陕西省地面总日射的分布状况.对反演结果与地面辐射站实测结果进行比较,平均相对误差为8.43%.     

太阳活动高峰年山东区域电离层时空变化研究

2012年为太阳活动高峰年份,为了研究太阳活动高年区域电离层的变化特征,该文选取了山东区域内的SDCORS站点,构建了山东区域垂直电子含量(VTEC)球谐格网模型,对该年山东区域电离层时空变化规律进行分析。实验研究表明,在空间变化上山东区域电离层表现出较强的纬度相关性,出现了明显的分层现象。同时给出了山东电离层在时间上呈现出的时段变化、日变化、月变化、季节变化,发现VTEC受太阳活动影响较大,除了存在明显的单峰和双峰结构外,该年还发生了半年度异常现象。     

Contribution of Starlette,Stella, and AJISAI to the SLR-derived global reference frame

The contribution of Starlette, Stella, and AJISAI is currently neglected when defining the International Terrestrial Reference Frame, despite a long time series of precise SLR observations and a huge amount of available data. The inferior accuracy of the orbits of low orbiting geodetic satellites is the main reason for this neglect. The Analysis Centers of the International Laser Ranging Service (ILRS ACs) do, however, consider including low orbiting geodetic satellites for deriving the standard ILRS products based on LAGEOS and Etalon satellites, instead of the sparsely observed, and thus, virtually negligible Etalons. We process ten years of SLR observations to Starlette, Stella, AJISAI, and LAGEOS and we assess the impact of these Low Earth Orbiting (LEO) SLR satellites on the SLR-derived parameters. We study different orbit parameterizations, in particular different arc lengths and the impact of pseudo-stochastic pulses and dynamical orbit parameters on the quality of the solutions. We found that the repeatability of the East and North components of station coordinates, the quality of polar coordinates, and the scale estimates of the reference are improved when combining LAGEOS with low orbiting SLR satellites. In the multi-SLR solutions, the scale and the \(Z\) component of geocenter coordinates are less affected by deficiencies in solar radiation pressure modeling than in the LAGEOS-1/2 solutions, due to substantially reduced correlations between the \(Z\) geocenter coordinate and empirical orbit parameters. Eventually, we found that the standard values of Center-of-mass corrections (CoM) for geodetic LEO satellites are not valid for the currently operating SLR systems. The variations of station-dependent differential range biases reach 52 and 25 mm for AJISAI and Starlette/Stella, respectively, which is why estimating station-dependent range biases or using station-dependent CoM, instead of one value for all SLR stations, is strongly recommended. This clearly indicates that the ILRS effort to produce CoM corrections for each satellite, which are site-specific and depend on the system characteristics at the time of tracking, is very important and needs to be implemented in the SLR data analysis.     

The potential of the greenness and radiation (GR) model to interpret 8-day gross primary production of vegetation

Remote sensing of vegetation gross primary production (GPP) is an important step to analyze terrestrial carbon (C) cycles in response to changing climate. The availability of global networks of C flux measurements provides a valuable opportunity to develop remote sensing based GPP algorithms and test their performances across diverse regions and plant functional types (PFTs). Using 70 global C flux measurements including 24 non-forest (NF), 17 deciduous forest (DF) and 29 evergreen forest (EF), we present the evaluation of an upscaled remote sensing based greenness and radiation (GR) model for GPP estimation. This model is developed using enhanced vegetation index (EVI) and land surface temperature (LST) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and global course resolution radiation data from the National Center for Environmental Prediction (NCEP). Model calibration was achieved using statistical parameters of both EVI and LST fitted for different PFTs. Our results indicate that compared to the standard MODIS GPP product, the calibrated GR model improved the GPP accuracy by reducing the root mean square errors (RMSE) by 16%, 30% and 11% for the NF, DF and EF sites, respectively. The standard MODIS and GR model intercomparisons at individual sites for GPP estimation also showed that GR model performs better in terms of model accuracy and stability. This evaluation demonstrates the potential use of the GR model in capturing short-term GPP variations in areas lacking ground measurements for most of vegetated ecosystems globally.     

Contribution to the Investigation of Structure and Origin of the East African Graben by Gravimetry

IntroductionGeophysics as its nameindicates has to do withthe physics of the earth and its surrounding at-mosphere . Gilbert’s discovery that the earth be-haves as a great and rather irregular magnet andNewton’s theory of gravitation may be said toconstitute the beginning of geophysics . Miningand the search of metals date from the earliestti mes , but the scientific record began with thepublicationin 1956 of the famous treatise De remetallica by Georgius Agricola , whichfor manyyears was …     

Contribution to the investigation of structure and origin of the east African graben by gravimetry

For our investigation we have 235 measurements done in the east part of Democratic Republic of Congo by P. Herrinck during a magnetic survey including the graben region from the parallel joining Goma city and Mahagi city, the region between Albert and Aka lakes, and the route from Aba to Kinsagani. During the surveys the density of recording points has been selected according to the importance of anomalies. In this way, the offset was 1 km where the disturbance was high in Goma city and 20 km have been sufficient along the route from Aba to Kinsagani. For the topographic and isostatic reductions only one cartographic document has been chosen that was the international map of the scale 1/1 000 000 which presents a certain characteristic of homogeneity.     

Retrieval of remotely sensed LAI using Landsat ETM+ data and ground measurements of solar radiation and vegetation structure: Implication of leaf inclination angle

A time series of leaf area index (LAI) of a managed birch forest in Germany (near Dresden) has been developed based on 16-day normalized difference vegetation index (NDVI) data from the Landsat ETM+ sensor at 30 m resolution. The Landsat ETM+ LAI was retrieved using a modified physical radiative transfer (RTM) model which establishes a relationship between LAI, fractional vegetation cover (fC), and given patterns of surface reflectance, view-illumination conditions and optical properties of vegetation. In situ measurements of photosynthetically active radiation (PAR) and vegetation structure parameters using hemispherical photography (HSP) served for calibration of model parameters, while data from litter collection at the study site provided the ground-based estimates of LAI for validation of modelling results. Influence of view-illumination conditions on optical properties of canopy was simulated by a view angle geometry model incorporating the solar zenith angle and the sensor viewing angle. Effects of intra-annual and inter-annual variability of structural properties of the canopy on the light extinction coefficient were simulated by implementing variability of the leaf inclination angle (LIA), which was confirmed in the study site. The results revealed good compatibility of the produced Landsat ETM+ LAI data set with the litter-estimated LAI. The results also showed high sensitivity of the LAI retrieval algorithm to variability of structural properties of the canopy: the implementation of LIA dynamics into the LAI retrieval algorithm significantly improved the model accuracy.     

Temporal and spatial variations of global ionospheric total electron content under various solar conditions

By utilizing the numerical technique of principal component analysis (PCA), this work analyses temporal and spatial variations of the ionosphere under various solar conditions during the period 1999–2013. Applying the PCA technique to the time series of the global ionospheric total electron content (TEC) maps provides an efficient method for analyzing the main ionospheric variability on a global scale that is able to decompose periodic variations (e.g., annual and semiannual oscillations) while retaining the asymmetry in the temporal and spatial domains (e.g., seasonal and equator anomalies). The TEC series of different local times are processed separately at two time scales: (1) the whole 15 years of the period of study and (2) the individual years. In contrast with previous studies, the analysis of the dataset of the 15 years shows that dawn (e.g., LT4–6) and late morning (LT10–12) are the more remarkable characteristic times for ionospheric variability. This study also reveals a cyclic trend of the variability with respect to local times. The first two modes, which contain 80–90% of the total variance, represent spatial distributions and temporal variations with respect to the different stages of the solar cycle and local times. Annual and semiannual variations are demodulated from the first two modes, and the results show that these variations evidently have distinct trends for daytime and nighttime. An exception is that, under active solar conditions, extremely strong solar irradiance during the daytime has a residual effect on the variability of the nighttime.     

北斗三号卫星经验型太阳光压模型分析与精化

太阳光压摄动作为在轨导航卫星受到的最大的非保守力,是卫星精密定轨的重要误差源。ECOM模型、ECOM2模型,这两种经验型光压模型被广泛应用于导航卫星定轨。然而,ECOM模型和ECOM2模型分别是针对GPS和GLONASS卫星设计的,并不完全适用于我国北斗三号(BDS-3)卫星。针对五参数ECOM模型在BDS-3卫星低太阳高度角时期轨道不连续性增大的问题,本文提出在 D方向引入一阶周期项来吸收未被模型化光压加速度。结果表明,引入一阶余弦周期项 D_c,能将低太阳高度角时期CAST卫星的切向、法向、径向重叠轨道误差分别减小约60%、52%、29%。针对ECOM2模型中 D_2c和 D₀、D_2s和 B_s之间存在的强相关性,本文提出了不估计 D_2c参数的八参数ECOM2模型和不估计 D_2c与 D_2s的七参数ECOM2模型。结果表明,相较九参数ECOM2模型,不估计 D_2c参数的八参数ECOM2模型能够将CAST卫星和SECM卫星径向重叠轨道误差分别减少约18%和27%。在此基础上,继续移除 D_2s后(七参数ECOM2),径向重叠轨道误差可进一步减小5.2%~8.5%。综合考察重叠轨道精度和SLR检核精度,不顾及 D_2c和 D_2s的七参数ECOM2模型表现最佳。CAST卫星和SECM卫星重叠轨道切向、法向、径向精度分别为5.0、3.4、1.4 cm和5.4、3.5、1.5 cm;SLR检核残差标准差分别为3.1~3.2 cm、4.4~4.7 cm。     

全球热带森林分布区NPP变化及其气候响应分析

本文基于MODIS遥感数据,利用掩膜分析提取了研究区的NNP数据;并结合气象因子数据,利用地理信息技术及数学统计方法,对比分析了2001—2013年全球3个主要热带森林分布区森林NPP变化及其气候响应的异同。结果表明,研究区NPP总量整体呈减少趋势。对比3个区域NPP与温度、降水和光合有效辐射(PAR)的相关关系,亚马孙流域研究区和刚果河流域研究区NPP对PAR变化更为敏感,东南亚研究区NPP对降水变化更为敏感;东南亚研究区NPP对气候变化响应的敏感性较高,刚果河流域研究区次之,亚马孙流域研究区最低。该研究对于进一步了解全球热带森林变化与气候的相关关系具有一定的参考价值。     

Effects of the partitioning of diffuse and direct solar radiation on satellite-based modeling of crop gross primary production

Modeling crop gross primary production (GPP) is critical to understanding the carbon dynamics of agro-ecosystems. Satellite-based studies have widely used production efficiency models (PEM) to estimate cropland GPP, wherein light use efficiency (LUE) is a key model parameter. One factor that has not been well considered in many PEMs is that canopy LUE could vary with illumination conditions. This study investigates how the partitioning of diffuse and direct solar radiation influences cropland GPP using both flux tower and satellite data. The field-measured hourly LUE under cloudy conditions was 1.50 and 1.70 times higher than that under near clear-sky conditions for irrigated corn and soybean, respectively. We applied a two-leaf model to simulate the canopy radiative transfer process, where modeled photosynthetically active radiation (PAR) absorbed by canopy agreed with tower measurements (R² = 0.959 and 0.914 for corn and soybean, respectively). Derived canopy LUE became similar after accounting for the impact of light saturation on leaf photosynthetic capacity under varied illumination conditions. The impacts of solar radiation partitioning on satellite-based modeling of crop GPP was examined using vegetation indices (VI) derived from MODIS data. Consistent with the field modeling results, the relationship between daily GPP and PAR × VI under varied illumination conditions showed different patterns in terms of regression slope and intercept. We proposed a function to correct the influences of direct and diffuse radiation partitioning and the explained variance of flux tower GPP increased in all experiments. Our results suggest that the non-linear response of leaf photosynthesis to light absorption contributes to higher canopy LUE on cloudy days than on clear days. We conclude that accounting for the impacts of solar radiation partitioning is necessary for modeling crop GPP on a daily or shorter basis.     

测绘情结(下)——记中国科学院资深院士陈述彭先生

3 超越门户 1954年,他当时在中科院地理研究所,正值参加黄河流域规划,需要大比例尺地图.但当时国家测绘局还没有成立,没有可利用的地图,于是他就主持编绘我国的1∶25万地形图.他编的这套地形图与众不同,怎么不同呢?即山区的等高线是20米,而平原的等高线是2.5米.这在现在看来也都是难以理解的,其难度是很难想像的.因为他觉得,如果平原的等高线是20米的话,黄河口海拔是0米,郑州海拔为95米,整个华北平原也就三条等高线就够了,对经济建设和老百姓来说,根本不够用,于是他将平原的等高线改为2.5米.现在看来这是很古怪的一套图.     

Estimation of daily average net radiation and its variation over West Bengal,India using MODIS products

A simple and exclusively satellite-based instantaneous net radiation (INR) and its components have been estimated and it is based on the method proposed. The method has been used to check its suitability over West Bengal, India on clear sky days which is eliminating the need for ground-based observations. The method explicitly recognizes the need for spatially varied input parameters to estimate net radiation using remote sensing information and provides a distributed daily average net radiation map with finer spatial resolution. The satellite-derived radiative flux components were compared with the Eddy Flux Tower measurements located at Bonnie camp within mangrove forest. The bias, root-mean square error and R² between modelled and measured INR are 52.07, 49.5 W m^?2 and 0.74, respectively which are within the acceptable limit. The errors in the downward short wave and long wave as well as upward short-wave and long-wave radiative flux are 13.78, 3.54, 12.41 and 3.54%, respectively. Further, variations of all the radiative flux components were analysed over different eco-systems.     

基于DTM的黄土丘陵沟壑区太阳辐射值计算模型及应用研究

以地形特征及周围地形遮蔽状况、经纬度位置等作为太阳辐射空间分布差异的变量，在地理信息系统（GIS）支持下，建立黄土丘陵沟壑区数字高程模型（DTM）和太阳辐射值计算模型，并实现理论太阳辐射值空间分布的可视化表达。     

Spectral anisotropy of subtropical deciduous forest using MISR and MODIS data acquired under large seasonal variation in solar zenith angle

Recent studies in Amazonian tropical evergreen forests using the Multi-angle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) have highlighted the importance of considering the view-illumination geometry in satellite data analysis. However, contrary to the observed for evergreen forests, bidirectional effects have not been evaluated in Brazilian subtropical deciduous forests. In this study, we used MISR data to characterize the reflectance and vegetation index anisotropies in subtropical deciduous forest from south Brazil under large seasonal solar zenith angle (SZA) variation and decreasing leaf area index (LAI) from the summer to winter. MODIS data were used to observe seasonal changes in the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI). Topographic effects on their determination were inspected by dividing data from the summer to winter and projecting results over a digital elevation model (DEM). By using the PROSAIL, we investigated the relative contribution of LAI and SZA to vegetation indices (VI) of deciduous forest. We also simulated and compared the MISR NDVI and EVI response of subtropical deciduous and tropical evergreen forests as a function of the large seasonal SZA amplitude of 33°. Results showed that the MODIS-MISR NDVI and EVI presented higher values in the summer and lower ones in the winter with decreasing LAI and increasing SZA or greater amounts of canopy shadows viewed by the sensors. In the winter, NDVI reduced local topographic effects due to the red-near infrared (NIR) band normalization. However, the contrary was observed for the three-band EVI that enhanced local variations in shaded and sunlit surfaces due to its strong dependence on the NIR band response. The reflectance anisotropy of the MISR bands increased from the summer to winter and was stronger in the backscattering direction at large view zenith angles (VZA). EVI was much more anisotropic than NDVI and the anisotropy increased from the summer to winter. It also increased from the forward scatter to the backscattering direction with the predominance of sunlit canopy components viewed by MISR, especially at large VZA. Modeling PROSAIL results confirmed the stronger anisotropy of EVI than NDVI for the subtropical deciduous and tropical evergreen forests. PROSAIL showed that LAI and SZA are coupled factors to decrease seasonally the VIs of deciduous forest with the first one having greater importance than the latter. However, PROSAIL seasonal variations in VIs were much smaller than those observed with MODIS data probably because the effects of shadows in heterogeneous canopy structures or/and cast by emergent trees and from local topography were not modeled.     

Ionospheric super-bubble effects on the GPS positioning relative to the orientation of signal path and geomagnetic field direction

Using GPS data of the Japanese network GEONET, we analyze occurrence of GPS-phase slips and positioning errors during the geomagnetic storm of February 12, 2000. Although the storm was not intensive, registering a minimum Dst excursion of −133 nT and a maximum Kp = 6.7 value, it attracted the attention of researchers because of the appearance of a super-bubble at mid-latitudes. We identified numerous GPS-phase slips in the area of the super-bubble. By the time of the bubble’s appearance, a total of 33% of GPS receivers experienced positioning errors of more than 500 m. Around 13:00 UT, the positioning quality was worse than 100 m almost all of Japan. We also found that the occurrence of phase slips of the satellite signals depends on the angle γ between the receiver-satellite line of sight and geomagnetic field lines. The maximum value of GPS-phase slips corresponds to γ = 0° and 90°. For the satellites positioned close to the magnetic zenith region, the density of phase slips reached 32%. In addition to carrier-phase slips, the super-bubble caused sharp increases in positioning errors of several hundred meters at receiver locations below 38°N latitude. As a result, precise positioning was not possible for about 2 h.     

Comparison of polar motion with oceanic and atmospheric angular momentum time series for 2-day to Chandler periods

During a 4-year period starting in July 1996 and using intervals ranging from 3 days to 4 years, four precise polar motion (PM) series have been compared to excitation by atmospheric angular momentum (AAM) augmented with oceanic angular momentum (OAM) data. The first three series (C03, C04 and Bulletin A) are multi-technique combinations generated by the International Earth Rotation and Reference Systems Service (IERS) and the fourth combined series (IGS00P02) is produced by the International GPS Service (IGS) using only GPS data. The IGS PM compared the best with the combined excitations of atmosphere and oceans (AAM+OAM) at all intervals, showing high overall correlation of 0.8–0.9. Even for the interval of only three days, the IGS PM gave a significant correlation of about 0.6. Moreover, during the interval of February 1999 – July 2000, which should be representative of the current precision of the IGS PM, a significant correlation (>0.4) extended to periods as short as 2.2 days and 2.5 days for the x_p and y_p PM components, respectively. When using the IERS Bulletin B (C04) PM and an interval of almost 6 years, starting in November 1994, the combined OAM+AAM accounted for practically all the annual, semi-annual and Chandler wobble (CW) PM signals. When only AAM was used, either the US National Centers for Environment Prediction reanalysis data, which were used throughout this study, or the Japanese Meteorological Agency data, two large and well-resolved amplitude peaks of about 0.1 mas/day, remained at the retrograde annual and CW periods.