Eye on the Ionosphere: GPS after SA

Selective Availability (SA), the intentional degradation of the accuracy of the single-frequency GPS position by the DoD, was ended on May 2, 2000. This major policy decision, promised to occur some time in the next 5 years, had an instantaneous and dramatic impact on the users of single-frequency GPS. Figure 1 shows the “before and after” of the turn-off of SA, and illustrates how significant and error source SA was. SA was turned off at 0400 UTC, the point on the graph at which the horizontal and vertical errors were markedly reduced. The magnitudes of the Standard Positioning Service (SPS) circular error probable (CEP), and sperical error probable (SEP) – the uncertainty in a fix – were reduced to less than 5 meters. ? 2001 John Wiley & Sons, Inc.     

Eye on the Ionosphere: Measuring Ionospheric Scintillation Effects from GPS Signals

From time to time, this column will include short contributions from invited guest contributors on specialized subjects pertaining to inonospheric effects on GPS signals. In this issue, Dr. A. J. Van Dierendonck discusses the required specifications of a civilian GPS receiver specially designed to make quantitative measurements of both ionospheric amplitude and carrier phase scintillation effects from GPS signals. ? 1999 John Wiley & Sons, Inc.     

Eye on the Ionosphere: The Spatial Variability of Ionospheric Range Delay

The ionospheric range delay is highly variable both in space and in time. The ionospheric correlation distance is defined as the distance over which a measurement of the difference from monthly mean ionospheric range delay at one location can be used to determine the difference from mean conditions at a second location. The percentage improvement required is dependent upon the actual correlation coefficient, which must be relatively large to produce a significant improvement over differences from average conditions at two stations. Results of studies of the correlation distance show that, in order to obtain even a 28% improvement over monthly median conditions, the measurement location must be within approximately 3,000 km in longitude or 1,800 km in latitude from the location where the update from monthly mean conditions is required, at least in the mid-latitude region. Another study showed that, in order to be able to determine an improvement of 1 m in ionospheric range delay from average ionospheric conditions at a location remote from where the actual measurement of range delay is made, the measurement must be within approximately 500 km of that location. ? 2000 John Wiley & Sons, Inc.     

Eye on the Ionosphere: Potential Solar Radio Burst Effects on GPS Signal to Noise

A. J. VanDierendonck joins the regular contributors to this column to discuss potential solar radio burst effects on GPS. While this topic does not spedivically involve ionospheric effects on GPS, it falls under the general area of environmental concerns that are of importance to the GPS community. ? 1999 John Wiley & Sons, Inc.     

Eye on the Ionosphere: The Correlation between Solar 10.7 cm Radio Flux and Ionospheric Range Delay

Patricia Doherty joins the regular contributors of this column to discuss the correlation between measurements of solar 10.7 cm radio flux and ionospheric range delay effects on GPS. Mrs. Doherty has extensive experience in the analysis of ionospheric range delays from worldwide systems and in the utilization and development of analytical and theoretical models of the Earth's ionosphere. Ionospheric range delay effects on GPS and other satellite ranging systems are directly proportional to the Total Electron Content (TEC) encountered along slant paths from a satellite to a ground location. TEC is a highly variable and complex parameer that is a function of geographic location, local time, season, geomagnetic activity, and solar activity. When insufficiently accounted for, ionospheric TEC can seriously limit the performance of satellite ranging applications. Since the ionosphere is a dispersive medium, dual-frequency Global Positoning System (GPS) users can make automatic corrections for ionospheric range delay by computing the apparent difference in the time delays between the two signals. Single-frequency GPS users must depend on alternate methods to account for the ionospheric range delay. Various models of the ionosphere have been used to provide estimates of ionospheric range delay. These models range from the GPS system's simple eight-coefficient algorithm designed to correct for approximately 50% rms of the TEC, to state-of-the-art models derived from physical first principles, which can correct for up to 70 to 80% rms of the TEC but at a much greater computational cost. In an effort to improve corrections for the day-to-day variability of the ionosphere, some attempts have been made to predict the TEC by using the daily values of solar 10.7 cm radio flux (F_10,7). The purpose of this article is to show that this type of prediction is not useful due to irregular, and sometimes very poor, correlation between daily values of TEC and F_10.7. Long-term measurements of solar radio flux, however, have been shown to be well correlated with monthly mean TEC, as well as with the critical frequency of the inonospheric F2 region (foF2), which is proportional to the electron density at the peak of the ionospheric F2 region. ? 2000 John Wiley & Sons, Inc.     

一种基于卫星穿刺点位置的区域电离层增强方法

提出了一种基于卫星穿刺点位置的区域电离层增强方法,采用非差非组合精密单点定位模型提取得到测站上空各个卫星斜向的电离层延迟值,结合IGS发布的DCB文件,考虑电离层延迟薄层假设理论,利用穿刺点空间三维坐标进行区域内插。试验结果表明,本方法能利用较为稀疏的参考站点进行区域建模,能为流动站提供与实际值偏差0.1 m以内的电离层延迟先验值;同时,流动站利用电离层增强信息,在保证定位结果精度的同时,相较于非差非组合模型,N、E、U 3个方向收敛到10 cm以内的时间均在30 min内,提升50%以上。     

电离层内插模型的分析

常见的电离层延迟内插模型有:线性组合法LCM(Linear Combination Model)、距离相关线性内插法DIM(Distance Based Linear Interpolation Model)、线性内插法LIM(Linear Interpolation Model)和低次曲面模型法LSM(Low-order Surface Model)等。文中对4种模型的基本原理和方法进行了详细的介绍,并利用河北省CORS网的实测数据进行计算分析,从而对模型之间的内插精度进行比较。结果表明,LSM模型的内插精度无论是流动站在网内还是在网外都要稍高于其他3种模型;其他3种模型的内插精度相当。当流动站分别位于网内和网外时,LSM模型的精度变化最小;LIM变化次之;LCM变化最大。     

VLBI观测的电离层延迟改正模型研究

电离层是大气层中的一个电离区域,高度范围大约在60～1 000 km.电磁波信号穿越电离层时其传播速度会发生变化,传播路径也会略微发生弯曲,从而使信号的传播时间乘以在真空中的光速不等于信号源至测站的几何距离. 对VLBI观测来讲,电离层引起的差异可达近百米.文中从电磁波的传播原理出发,讨论了信号传播速度和传播路径变化引起的VLBI观测延迟;对目前采用的各种电离层延迟模型进行了分析总结;并指出单频率VLBI观测应顾及高阶项和路径弯曲的影响或使用区域性电离层延迟改正模型.     

VLBI观测的电离层延迟改正模型研究

电离层是大气层中的一个电离区域，高度范围大约在60－1000km。电磁波信号穿越电离层时其传播速度会发生变化，传播路径也会略微发生弯曲，从而使信号的传播时间乘以在真空中的光速不等于信号源至测站的几何距离。对VLBI观测来讲，电离层引起的差异可达近百米百米。文中从电磁波的传播原理出发，讨论了信号传播速度和传播路径变化引起的VLBI观测延迟；对目前采用的各种电离层延迟模型进行了分析总结；并指出单频率VLBI观测应顾及高阶项和路径弯曲的影响或使用区域性电离层延迟改正模型。     

基于EOF的实时三维电离层模型精度分析

讨论了建立基于EOF(empirical orthogonal runction)的实时三维电离层模型的方法,利用欧洲区域14个IGS跟踪站全天的观测数据模拟实时数据流进行了实验,将这种方法与二维球谐函数模型的结果进行了比较,结果显示基于EOF的实时三维电离层模型全天的模型残差以及与实测值偏差的均方根都在0.2 m以内,优于二维球谐函数模型。     

利用GPS观测值建立格网电离层模型的探讨

介绍了建立格网电离层延迟模型的基本原理,着重阐述了多面函数算法、距离加权算法的原理以及用这2种算法建立区域格网电离层延迟模型的方法,并利用地壳网络观测数据对这2种算法进行检核。结果表明这2种模型均可达到±0.5 m 的精度。