BDS格网点电离层信息性能评估

电离层延迟是造成卫星导航系统误差的重要来源之一,因此,电离层延迟的修正精度直接影响用户定位精度．随着北斗卫星导航系统（BDS）全面服务亚太地区,用户对BDS高精度定位导航服务的需求日益迫切．同时BDS将基本导航服务和广域差分服务进行了一体化设计,为用户发布了高更新频率的格网点电离层信息,有效提升了用户的定位精度．本文利用2017年1月—2018年10月的数据对BDS格网点电离层信息的服务范围和服务精度进行评估,结果表明:格网点电离层信息有效覆盖区域基本覆盖中国区域,修正偏差约1.62 TECU,修正率约为86.7％;格网点电离层信息修正精度具有季节变化,冬季修正精度较低且波动较大,修正率约为82％,其他季节修正率均优于87％;修正偏差、修正率白天均高于夜间;格网点电离层信息具有较强的抗拢动能力．      

局部电离层异常对局域增强系统的影响及其监测方法

传统的局域增强系统认为影响用户和基准站伪距的观测误差是基本相同的,因而可通过伪距差分技术将这部分误差消除,但为发生电离层风暴时,叠加在基准站和用户伪距上的电离层延迟误差将会不同,因此会影响定位结果.本文利用CCDMA算法和DSCMA算法对电离层异常进行探测,并对伪距差分技术进行改进.仿真结果显示,这种方法可有效减小局部电离层异常对CATⅢ飞行阶段的威胁.     

基于原始观测值的单频精密单点定位算法

研究了一种基于GPS原始观测值的单频PPP算法。该算法通过增加电离层延迟先验信息、空间和时间约束的虚拟观测方程,将电离层延迟当作未知参数与其他定位参数一并进行估计来高效修正电离层延迟误差。通过使用全球178个IGS站1d的实测数据对本算法的收敛速度、定位精度和电离层VTEC的精度进行检验与分析。结果表明,该算法的收敛速度和稳定性均得到了改善,其静态单频单天PPP解的精度可达2~3cm、模拟动态单频单天PPP解的精度可达2~3dm,并且单频PPP与双频PPP提取的电离层总电子含量平均偏差小于5个TECU,可作为一种附属定位产品使用。     

三种电离层延迟多频修正算法的比较

电离层延迟多频修正算法在修正电离层延迟的同时会放大观测噪声等伪距误差的影响。分析了电离层延迟双频修正、三频一阶修正和三频二阶修正三种修正算法及观测噪声的影响。以Galileo系统的四个载波频率为例,对不同频率组合下三种修正算法进行了比较。结果表明电离层延迟多频修正中,并不是观测量的频点数越多或修正掉的高阶项越多修正精度越高,还与观测伪距噪声的大小以及采用的修正方法有关。为多频测距系统的电离层延迟修正算法和最佳频率组合设计提供了一种可行的分析方法。     

星地时间比对大气误差修正及其影响因素分析

由于空间环境较为复杂，大气对无线电微波信号的影响是星地高精度时间比对的主要误差源之一.针对星地高精度时间比对的需求，研究了双向测量体制下的电离层误差修正方法及对流层色散延迟修正方法，对影响大气误差修正的主要因素展开了讨论，并对不同场景下大气误差修正情况及星地时间比对结果进行了仿真分析.仿真结果表明：当卫星姿态误差控制在100 as以内、相位中心标定误差控制在5 mm以内、精密定轨误差控制在30 cm以内时，通过相应的误差修正算法修正后，电离层误差残差的RMS值小于0.006 ps，对流层误差残差的RMS值小于0.06 ps，星地时间比对精度优于皮秒量级.     

GNSS大气掩星电离层修正方法研究

对四种主要的电离层修正方法的修正效果进行了对比分析,理论及模拟分析结果表明：由于对伪距的测量精度较低,简单的单频电离层修正法误差较大,一般只对单频接收机或者载波L2的观测数据缺失或质量较差时应用;在双频电离层修正方法中,相位线性组合法由于没有考虑电离层色散引起的电波路径差别,修正效果不如另外几种双频修正方法,多在地面导航中进行高仰角观测时应用。弯曲角线性组合法的修正效果最好,优于其他电离层修正方法。     

两种GPS测定电离层电子密度模型的探讨

详细推导出利用GPS双频码距及载波相位观测值进行电离层子密度求定的Klobuchar模型以及Georgiadou模型,给出了求定这两种模型参数的数学表达,从理论上指出该两种模型适合单历元解算的基本条件,通过实例进行了两种模型的电离层修正误差与误差频率的分析和比较。结果表明,与Klobuchar模型相比,Georgiadou模型具有表达形式简单,对模型参数的初值精度要求较低,数据利用率较高,参数估计精度较高等优点。     

一种基于三角分区的广域电离层改正新方法

根据中国地形分布难以建立格网模型的特点,为了解决我国区域电离层精确改正的问题,提出了广域电离层改正三角分区的方法。选择中国地震电离层监测实验网中纬度地区的5个监测站,建立覆盖我国中纬度整个网络服务区域的三角分区电离层模型,并利用8个基准站的数据对该方法的修正精度进行评估,结果表明,对于三角分区内部区域,该方法可以修正到90%左右;对于三角分区外部几百公里以内的区域该方法也能达到80%以上的修正精度,同时利用原始GNSS数据对美国、加拿大等4个IGS跟踪站进行补充实验也验证了该方法的可行性,在保证模型精度的同时较格网法更加简单、有效,对广域电离层延迟误差的修正具有重要的参考价值。     

电离层延迟修正方法评述

电离层延迟是卫星导航定位的重要误差源之一,为了有效消除该误差的影响,需要选择适当的电离层延迟改正方法。对电离层延迟修正精度和实时性要求不同,选用的改正方法也不尽相同。本文在分析各修正方法原理的基础上,论述了各方法的优缺点、存在问题、以及适用范围,该研究对于选用电离层修正方法具有指导意义。     

非差非组合精密单点定位提取区域电离层延迟及精度评定

电离层延迟是GNSS定位中最难处理,也是很重要的的误差来源之一,目前常用线性组合的方式处理电离层延迟,这些方法都会引入多余噪声,在不同程度上影响了模糊度的整数特性,同时也造成了某些有用数据丢失。本文提出了一种基于非差非组合精密单点定位的方式提取区域参考站电离层延迟的方法,再将提取得到的区域电离层延迟内插至仿用户站,在仿用户站实施单频PPP,最后检验得到定位的精度。实验结果表明:仿用户站单频PPP的定位精度平面方向约为4—5 cm,在高程方向低于1 dm,与全球电离层格网模型和半和改正等模型相比,采用非差非组合的方法提取电离层延迟后的定位精度更高。     

Ionospheric F-layer global scintillation index variation using COSMIC during the period of 2007–2013

The three-dimensional global morphology and seasonal characteristics of the ionospheric scintillation index of the F-layer between 150 and 550 km altitudes are analyzed using the GPS radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate during the 7-year period of low and high sunspot activity from 2007 to 2013. The results show that the prominent scintillation intensity, which is confined within ±30° geomagnetic latitude, starts at post-sunset, reaches a maximum at around pre-midnight, and often persists until postmidnight. Moderate scintillation activity can be observed in the high-latitude region almost at any time, whereas weak scintillation prevails in the midlatitude region. The noticeable scintillation peak near midnight occurs at an altitude of approximately 250 km in most cases. However, the peak of the scintillation activity during the solar maximum extends to higher altitudes than observed during the solar minimum. Additionally, the local variation in time and altitude of the scintillation intensity is closely correlated with ionospheric HmF2. Statistical analysis indicates that an increase in solar activity or geomagnetic activity enhances the occurrence rate of scintillation and results in intense scintillation. The current research is beneficial for directly studying global ionospheric irregularities at GHz frequency based on high-rate L1 data and constructing a global scintillation model.     

NeQuick模型算法研究及性能比较

研究了NeQuick2算法改进及其实现方法,从不同角度分析了NeQuick2模型在全球区域和中国区域内的性能优势。一个太阳活动周期内,中国区域NeQuick2模型计算的电子总含量（total electron content,TEC）比NeQuick1模型精度有显著提升,改正精度与太阳活动水平具有较强的相关性,低年比高年的改善效果更为显著。以全球电离层数据（global ionosphere maps,GIM）为参考标准,中国中高纬区域太阳活动低年NeQuick2模型TEC的系统年平均偏差减少了76%,年平均均方根（root mean square,RMS）值减少了约72%。太阳活动高年NeQuick2模型TEC的系统年平均偏差减少了38%,平均RMS减少了13%左右,且中高纬区域改正精度优于低纬区域11%~13%。全球区域太阳活动峰值期间NeQuick2模型TEC比NeQuick1模型日平均偏差改善了25%,日平均RMS改善了30%左右。分别用NeQuick1和NeQuick2模型得出F₂层顶部区域在太阳活动峰值期电子密度随高度剖面分布,顶部电子密度剖面精度改善近40%。最后分别得出了两个模型中国区域中高纬地区E和F₁层区域在100 km、150 km和200 km高度的电子密度分布图,结果显示NeQuick2模型改善了电子密度分布状况,有效避免了NeQuick1在底部区域电子密度梯度不连续以及电离层异常结构的情况。     

全球电离层时空变化特性分析

采用谱分析和小波分解的方法对全球电离层VTEC量的时空变化特性进行了分析。使用IGS中心发布的全球电离层网格图数据,分别从高中低纬度全年变化特性、南北半球全年变化特性、全球范围内随经纬度的变化特性对电离层VTEC进行了分析。结果表明,高中低纬度地区VTEC量具有周日和半周年变化现象,在二分点处存在峰值,南半球电离层较北半球电离层活跃,经度变化对VTEC值的影响较纬度变化大,总体上,太阳辐射是电离层活动的主要影响因素。     

Effects of ionospheric disturbances on GPS observation in low latitude area

In this paper, ionospheric disturbance data from a local GPS network in Hong Kong (low latitude region) are studied in the solar maximum period (2001–2003). The spatial and temporal distributions of the disturbances in Hong Kong are investigated. It is found that strong ionospheric disturbances occur frequently during the solar maximum period, particularly around March and September, and concentrate at the region around geographic latitude 22°N (geomagnetic latitude 12°N). The effects of the disturbances on GPS geodetic receivers, such as loss of lock and measurement noise level, are also analyzed for the 3-year period. It shows that the measurement noise level and the number of losses of lock in GPS data increase dramatically during ionospheric disturbance periods. The behaviors of different types of GPS receivers during the disturbances are also compared.     

等角圆锥投影基准纬度非迭代算法

为简化传统正轴等角圆锥投影求解基准纬度时繁琐的迭代算法,引入平均纬度和平均纬差的概念,借助计算机代数系统Mathematica,在平均纬差处级数展开,导出了基于球体模型的正轴等角圆锥投影求解基准纬度的非迭代算法。以全国和不同纬差的省区为例,将其与传统椭球迭代算法进行对比分析。结果表明,推导的基于球体模型的非迭代公式计算基准纬度B0、B1、B2的相对误差最大值为2.011%,长度变形的相对误差小于1×10-6,基本可满足全国以及各省区地图制图的精度要求,从而验证了所研究算法的精确性与实用性。     

GPS slant total electron content accuracy using the single layer model under different geomagnetic regions and ionospheric conditions

The use of observations from the Global Positioning System (GPS) has significantly impacted the study of the ionosphere. As it is widely known, dual-frequency GPS observations can provide very precise estimation of the slant Total Electron Content (sTEC—the linear integral of the electron density along a ray-path) and that the precision level is bounded by the carrier-phase noise and multi-path effects on both frequencies. Despite its precision, GPS sTEC estimations can be systematically affected by errors in the estimation of the satellites and receivers by Inter-Frequency Biases (IFB) that are simultaneously determined with the sTEC. Thus, the ultimate accuracy of the GPS sTEC estimation is determined by the errors with which the IFBs are estimated. This contribution attempts to assess the accuracy of IFBs estimation techniques based on the single layer model for different ionospheric regions (low, mid and high magnetic latitude); different seasons (summer and winter solstices and spring and autumn equinoxes); different solar activity levels (high and low); and different geomagnetic conditions (quiet and very disturbed). The followed strategy relies upon the generation of a synthetic data set free of IFB, multi-path, measurement noise and of any other error source. Therefore, when a data set with such properties is used as the input of the IFB estimation algorithms, any deviation from zero on the estimated IFBs should be taken as indications of the errors introduced by the estimation technique. The truthfulness of this assessment work is warranted by the fact that the synthetic data sets resemble, as realistically as possible, the different conditions that may happen in the real ionosphere. The results of this work show that during the high solar activity period the accuracy for the estimated sTEC is approximately of ±10 TECu for the low geomagnetic region and of ±2.2 TECu for the mid-latitude. During low solar activity the accuracy can be assumed to be in the order of ±2 TECu. For the geomagnetic high-disturbed period, the results show that the accuracy is degraded for those stations located over the region where the storm has the strongest impact, but for those stations over regions where the storm has a moderate effect, the accuracy is comparable to that obtained in the quiet period.     

Analysis of inversion errors of ionospheric radio occultation

The retrieved electron density profile of ionospheric radio occultation (RO) simulation data can be compared with the background model value during the simulation and the inversion error can be obtained exactly. This paper studies the inversion error of ionospheric RO through simulation. The sources of the inversion errors are analyzed. The impacts of measurement errors, such as the errors in phase measurements and satellite orbits, are very small and can be neglected. The approximation of straight-line propagation introduces errors at the height of the F1 layer under solar maximum condition. The spherical symmetry approximation of the electron density distribution is found to be the main source of the inversion error. The statistical results reveal some characteristics of the inversion errors. (1) The relative error increases with enhanced solar activity. (2) It is larger in winter than in equinox season, and it is smallest in summer. (3) For all seasons, it is smaller at middle latitude than at other latitudes. (4) For all seasons and geomagnetic latitudes, it is smaller at daytime than at other times. The NmF2 of the ROs from COSMIC are compared with the measurements of ionosondes, and the relative differences show the same dependencies on season, geomagnetic latitude and local time, as the relative errors of the simulated ionospheric ROs.

Error assessment of grid‐based terrain shading algorithms for solar radiation modeling over complex terrain

In mountainous regions, solar radiation exhibits a strong spatial heterogeneity due to terrain shading effects. Terrain shading algorithms based on digital elevation models can be categorized into two types: area‐based and point‐specific. In this article, we evaluated two shading algorithms using designed mathematic surfaces. Theoretical shading effects over four Gauss synthetic surfaces were calculated and used to evaluate the terrain shading algorithms. We evaluated the area‐based terrain shading algorithm, Hillshade tool of ArcGIS, and the point‐specific shading algorithm from Solar Analyst (SA) in ArcGIS. Both algorithms showed shading overestimation, and Hillshade showed more accuracy with a mean absolute error (MAE) of 1.20%, as compared to the MAE of 1.66% of SA. The MAE of Hillshade increases exponentially as the spatial extent of the study area increases because the solar position for all locations on the surface is the same in Hillshade. Consequently, we suggest that the surface should be divided into more tiles in Hillshade when the discrepancy in the latitude of the whole surface is greater than 4°. Skyshed, which represents the horizon angle distribution in SA, is error‐prone over more complex terrain because horizon angle interpolation is problematic for such areas. We also propose a new terrain shading algorithm, with solar positions calculated using local latitude for each cell and the horizon angle calculated for every specific time interval, but without projections. The new model performs better than Hillshade and SA with an MAE of 0.55%.     

Ionospheric correction using NTCM driven by GPS Klobuchar coefficients for GNSS applications

Global Navigation Satellite Systems (GNSS) require mitigation of ionospheric propagation errors because the ionospheric range errors might be larger than tens of meters at the zenith direction. Taking advantage of the frequency-dispersive property of ionospheric refractivity, the ionospheric range errors can be mitigated in dual-frequency applications to a great extent by a linear combination of carrier phases or pseudoranges. However, single-frequency GNSS operations require additional ionospheric information to apply signal delay or range error corrections. To aid single-frequency operations, the global positioning system (GPS) broadcasts 8 coefficients as part of the navigation message to drive the ionospheric correction algorithm (ICA) also known as Klobuchar model. We presented here an ionospheric correction algorithm called Neustrelitz TEC model (NTCM) which can be used as complementary to the GPS ICA. Our investigation shows that the NTCM can be driven by Klobuchar model parameters to achieve a significantly better performance than obtained by the mother ICA algorithm. Our research, using post-processed reference total electron content (TEC) data from more than one solar cycle, shows that on average the RMS modeled TEC errors are up to 40% less for the proposed NTCM model compared to the Klobuchar model during high solar activity period, and about 10% less during low solar activity period. Such an approach does not require major technology changes for GPS users rather requires only introducing the NTCM approach a complement to the existing ICA algorithm while maintaining the simplicity of ionospheric range error mitigation with an improved model performance.     

Global morphology of ionospheric F-layer scintillations using FS3/COSMIC GPS radio occultation data

We report on the FormoSat-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) limb-viewing observations of GPS L-band scintillations since mid-2006 and propose to study global F-layer irregularity morphology. The FS3/COSMIC has generally performed more than 1000 ionospheric radio occultation (RO) observations per day. We reprocess 1-Hz amplitude data and obtain complete limb-viewing profiles of the undersampling (sampling frequency lower than Fresnel frequency) S4 scintillation index from about 80% of the RO observations. There are a few percent of FS3/COSMIC RO observations having greater than 0.09 undersampling S4max values on average. However, seven identified areas, Central Pacific Area (?20° to 20° dip latitude, 160°E–130°W), South American Area (?20° to 20° dip latitude, 100°W–30°W), African Area (?20° to 20° dip latitude, 30°W–50°E), European Area (30°–55°N, 0°–55°E), Japan Sea Area (35°–55°N, 120°–150°E), Arctic Area (>65° dip latitude), and Antarctic Area (<?65° dip latitude), have been designated to have a much higher percentage of strong limb-viewing L-band scintillations. During the years in most of the last sunspot cycle from mid-2006 to the end 2014, the scintillation climatology, namely, its variations with each identified area, season, local time, magnetic activity, and solar activity, have been documented.