GNSS receiver autonomous integrity monitoring (RAIM) performance analysis

The availability of GPS signals is a major concern for many existing and potential applications. Fortunately, with the development of Galileo by the European Commission (EC) and European Space Agency (ESA) and new funding for the restoration of the Russian GLONASS announced by the Russian Federation (Revnivykh et al., in European Navigation Conference 2005, Munich, Germany, July 19–22), the future for satellite-based positioning and navigation applications is extremely promising. With the complete cooperation from all these global navigation satellite systems (GNSS), greater levels of satellite visibility and therefore integrity can be expected. In this paper, a receiver autonomous integrity monitoring (RAIM) scheme along with reliability and separability measures are used to assess integrity performance levels of standalone GPS and integrated GPS/GLONASS, GPS/Galileo and GPS/GLONASS/Galileo systems where the clock offsets for each of the additional systems are estimated. It is shown, herein, that a minimum of three satellites must be visible in an additional system in order to provide a full integrity contribution when the system’s clock offset is to be estimated within the adjustment. A comparison of the integrity results obtained via system clock offsets estimated in the adjustment versus the case where the offsets are known and the measurements are corrected prior to the adjustment is also made for a high elevation mask scenario. Global simulation results for combined GPS/GLONASS/Galileo show that, theoretically, for the time of simulation and for any point on the globe, an outlier of 20 m can be detected with 80% probability at the 0.5% significance level and then separated from any other measurement with 90% probability. Corresponding values for the GPS only and combined GPS/GLONASS and GPS/Galileo systems, respectively, are approximately 435, 110 and 28 m, respectively, for the maximum MSBs and 312, 50 and 26 m, respectively, for the maximum MDBs. Temporal 24 h simulations for the GPS/GLONASS/Galileo scenario delivered agreeable results with the global snapshots for a 15° elevation mask. For the case where system clock offsets are estimated within the adjustment, it was shown that only the reliability measure was available for 100% of the time, with horizontal external reliability values of no more than about 12 m when a 30° masking angle was used. By assuming the clock offsets were determined and corrected for prior to the adjustment, the separability measure was markedly improved and was also available 100% of the time.     

大气科学类专业毕业生就业意向对气象部门人才招聘工作的启示

大气科学类专业毕业生是全国气象部门补充人才资源、建设气象人才梯队的主要来源。然而,随着气象事业发展的需求以及外部竞争的加剧,气象部门招聘大气科学类专业毕业生面临的压力持续增长。调查研究大气科学类专业毕业生的就业意向,对气象部门提高招聘工作效率和招聘质量有重要意义。     

气象部门事业单位公开招聘工作现状及问题分析

毕业生是气象部门进人的主要来源,气象部门事业单位公开招聘主要是针对应届毕业生招聘。通过对气象部门各直属事业单位、各省市区气象部门的招聘工作开展的调研分析,总结气象部门招聘工作特点和不足,为各级气象部门有针对性地开展招聘,提高招聘工作效率和人才质量提供有价值的参考信息。     

An alternative approach to calculate the posterior probability of GNSS integer ambiguity resolution

When precise positioning is carried out via GNSS carrier phases, it is important to make use of the property that every ambiguity should be an integer. With the known float solution, any integer vector, which has the same degree of freedom as the ambiguity vector, is the ambiguity vector in probability. For both integer aperture estimation and integer equivariant estimation, it is of great significance to know the posterior probabilities. However, to calculate the posterior probability, we have to face the thorny problem that the equation involves an infinite number of integer vectors. In this paper, using the float solution of ambiguity and its variance matrix, a new approach to rapidly and accurately calculate the posterior probability is proposed. The proposed approach consists of four steps. First, the ambiguity vector is transformed via decorrelation. Second, the range of the adopted integer of every component is directly obtained via formulas, and a finite number of integer vectors are obtained via combination. Third, using the integer vectors, the principal value of posterior probability and the correction factor are worked out. Finally, the posterior probability of every integer vector and its error upper bound can be obtained. In the paper, the detailed process to calculate the posterior probability and the derivations of the formulas are presented. The theory and numerical examples indicate that the proposed approach has the advantages of small amount of computations, high calculation accuracy and strong adaptability.     

Analysis of the upper bounds for the integer ambiguity validation statistics

Integer ambiguity validation is an essential quality control step for high-precision positioning and navigation with global navigation satellite systems (GNSS). In order to validate the resolved integer ambiguities, statistical tests, such as the R-ratio test, F-ratio test, W-ratio test, difference test, and projector test, have been favored. In practice, the critical values for these statistical tests are determined either empirically or from the assumed distributions. However, previous research has revealed that some of these statistics have upper bounds, which can be obtained from simulations. In this contribution, we find that under the framework of the integer aperture estimation, the upper bounds for these ambiguity validation statistical tests can be derived without actual measurements or simulation. As a result, the assumed distributions for these statistical tests are inappropriate. According to the derivation, it has been concluded that the upper bounds of these ambiguity validation tests depend only on the ambiguity geometry (e.g., the float ambiguity variance–covariance matrix) and can be obtained at the design stage of GNSS positioning. Thus, the critical value for these ambiguity validation statistics has a rigorous range, and it should be chosen to be smaller than a priori derived upper bound. Otherwise, no integer ambiguities can be obtained.     

Modeling and quality control for reliable precise point positioning integer ambiguity resolution with GNSS modernization

Recent research has demonstrated that the undifferenced integer ambiguities can be recovered using products from a network solution. The standard dual-frequency PPP integer ambiguity resolution consists of two aspects: Hatch-Melbourne-Wübbena wide-lane (WL) and ionosphere-free narrow-lane (NL) integer ambiguity resolution. A major issue affecting the performance of dual-frequency PPP applications is the time it takes to fix these two types of integer ambiguities, especially if the WL integer ambiguity resolution suffers from the noisy pseudorange measurements and strong multipath effects. With modernized Global Navigation Satellite Systems, triple-frequency measurements will be available to global users and an extra WL (EWL) model with very long wavelength can be formulated. Then, the easily resolved EWL integer ambiguities can be used to construct linear combinations to accelerate the PPP WL integer ambiguity resolution. Therefore, we propose a new reliable procedure for the modeling and quality control of triple-frequency PPP WL and NL integer ambiguity resolution. First, we analyze a WL integer ambiguity resolution model based on triple-frequency measurements. Then, an optimal pseudorange linear combination which is ionosphere-free and has minimum measurement noise is developed and used as constraint in the WL and the NL integer ambiguity resolution. Based on simulations, we have investigated the inefficiency of dual-frequency WL integer ambiguity resolution and the performance of EWL integer ambiguity resolution. Using almanacs of GPS, Galileo and BeiDou, the performances of the proposed triple-frequency WL and NL models have been evaluated in terms of success rate. Comparing with dual-frequency PPP, numerical results indicate that the proposed triple-frequency models can outperform the dual-frequency PPP WL and NL integer ambiguity resolution. With 1 s sampling rate, generally, only several minutes of data are required for reliable triple-frequency PPP WL and NL integer ambiguity resolution. Under benign observation situations and good geometries, the integer ambiguity can be reliably resolved even within 10 s.     

南京城市土壤某些元素的富集特征及其对浅层地下水的影响

本研究以南京市的公园、校园绿地、道路绿地和郊区菜地等不同功能区的土壤为对象,对土壤的磷素和重金属以及一些土壤剖面点位的地下水组成进行了分析.研究发现城市土壤构成了区别于自然背景的某些元素(如磷和重金属)的地球化学垒,形成了明显的元素富集区域,其中磷素含量是背景土壤的数倍甚至十数倍,Pb含量也远高出背景土壤,60%以上样品达到中等污染程度以上.从磷素在剖面中的积累模式可以看出,城市土壤中磷含量最高的层次出现在表层以下的不同深度,说明土壤经历了不同的扰动过程,这意味着在城市发展过程中,土壤的性质遭受强烈的人为活动影响,元素在土壤中的分布形态可以指示不同的利用历史.质量改变后的土壤对地下水等环境产生影响,使地下水的组成发生改变,其中磷素含量直接受到土壤的影响,效区菜地地下水中硝态氮含量极高,而本研究尚未发现地下水中的重金属含量与土壤有显著相关.     

GPS无线电掩星技术监测地球大气

ＧＰＳ无线电掩星技术是从行星掩星（飞行器）探测行星大气^{［１～３］}的遥感技术发展而来的。１９９５年４月３日小型卫星ＭｉｃｒｏＬａｂ１的发射升空，标志着以ＧＰＳ无线电掩星建议为基础ＧＰＳ／ＭＥＴ计划纳入了正式实验阶段。实验结果证实了ＧＰＳ无线电掩星技术在探测地球大气方面潜在的科学价值与实用意义。简要系统地介绍了利用ＧＰＳ无线电掩星技术监测地球大气折射率、温度、气压和水蒸汽丰度等物理参量廓线分布的基本原理、主要误差源和空间分辨率，以及数字模拟结果和初步实验结果，并分析了ＧＰＳ无线电掩星技术在监测全球气候变化和提高数值天气预报精度等方面的潜在贡献。     

Distribution of Hg in mangrove trees and its implication for Hg enrichment in the mangrove ecosystem

The aim of this study was to evaluate Hg distribution in mangrove plants and changes of Hg content during leaf aging; the contribution of litterfall to Hg enrichment in mangrove ecosystems is also discussed. Contents of total Hg (THg) and methylmercury (MeHg) in mangrove plants and sediments were determined. Contents of THg and MeHg in the sediments were 225 ± 157 ng/g and 0.800 ± 0.600 ng/g. Concentrations of THg and MeHg in the mangrove plants were 1760 ± 1885 ng/g and 0.721 ± 0.470 ng/g (dry weight), respectively, which were much higher than those in terrestrial plants. Enrichment of THg in mangrove plants was different, following the order Rhizophra apiculata > Rhizophora stylosa > Kandelia candel > Aegiceras corniculatum > Avicennia marina; while MeHg contents in mangrove plants decreased in the order of R. stylosa > K. candel > A. corniculatum > R. apiculata > A. marina. There were obvious interspecies differences, regional differences, individual differences and tissue differences between THg and MeHg contents of mangrove plants, all of which were closely related to the environmental and the physiological characteristics of mangrove plants. In juvenile leaves, mature leaves and leaf litter, THg contents ranged 55.3-1760 ng/g, 204-1800 ng/g, and 385-2130 ng/g (dry weight), respectively; MeHg contents ranged 0.17-2.39 ng/g, 0.01-1.28 ng/g, and 0.13-1.47 ng/g (dry weight), respectively. Except for A. corniculatum and Bruguier gymnorrhiza, THg content of mature leaves was always higher than that in juvenile leaves, but MeHg showed a contrasting trend. THg content of litter leaves was between that of juvenile leaves and mature leaves, while MeHg content was generally lower than that of juvenile leaves and mature leaves. In the mangrove ecosystem, Hg enrichment contributed by the litterfall decreased in the order of K. candel > A. corniculatum > A. marina.     

Use of Landsat thermal imagery for dynamically monitoring spontaneous combustion of Datong Jurassic coalfields in China

It is highly helpful and necessary to investigate and monitor the status of coal seam. Fortunately, remote sensing has facilitated the identification and dynamical monitoring of spontaneous combustion for a large area coal mining area, especially using the time series remotely-sensed datasets. In this paper, Datong Jurassic coal mining area is used as the study area, China, and an exclusion method and a multiple-factor analysis method are jointly used to identify the spontaneous combustion, including land surface temperature (LST), burnt rocks, and land use and land cover change (LUCC). The LST is firstly retrieved using a single-window algorithm due to a thermal infrared band of Landsat-5 TM (Thematic Mapper). Burnt rocks is then extracted using a decision-tree classification method based on a high-resolution SPOT-5 image. The thermal anomaly areas are identified and refined by the spatial overlay analysis of the above affecting factors. Three-period maps of coal fire areas are obtained and dynamically analyzed in 2007, 2009 and 2010. The results show that a total of 12 coal fire areas have been identified, which account for more than 1% of the total area of the study area. In general, there is an increasing trend yearly and a total of 771,970 m\(^{2}\) is increased. The average annual increase is 257,320 m\(^{2}\), the average annual growth rate is 3.78%, and the dynamic degree is 11.29%.