SAMI2物理模型与东亚扇区GPS TEC数据 比较研究

SAMI2 (Sami2 is Another Model of the Ionosphere)是美国海军实验室开发的电离层物理模型.利用该物理模型,模拟了东亚扇区不同太阳活动强度、不同纬度地区三个站的电离层电子浓度总含量 (TEC). 通过模拟结果与GPS观测站 TEC 数据的比较,检验 SAMI2 在此扇区的电离层 TEC 计算精度. 结果表明,物理模型输出的电离层 TEC 具备与观测数据一致的周日变化、季节变化,太阳活动变化. 周日分布上,上午时段SAMI2? TEC 与观测数据吻合度优于午后时段;季节分布上,SAMI2 TEC 在冬季与观测值偏差小于其他季节;SAMI2? TEC 与GPS TEC 相关系数各站均达到0.87以上,与赤道地区Guam站相关性最好;太阳活动低年计算结果优于太阳活动高年;多数情况下,SAMI2 TEC 相对GPS TEC 偏大. 本文结果为基于SAMI2模型构建背景误差分布特征,开展该区域电离层数值预报研究可行性提供了理论支持.      

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。     

利用掩星技术研究南极地区顶部电离层特性

掩星观测能够提供地面到低轨卫星轨道高度处的整个电离层电子密度剖面,对于顶部电离层的研究有重要的作用。本文利用COSMIC(constellation observing system for meteorology ionosphere and climate)掩星数据反演了电子密度剖面,提取了F₂层峰值高度(hmF2)、F₂层峰值密度(NmF2)、垂直标尺高(vertical scale height,VSH)等电离层参数,研究了南极地区的F₂层在太阳活动周期内的变化、年际变化、周日变化等,并且重点分析了南极地区的顶部电离层的垂直结构特征,尤其是威德尔海异常在垂直方向上的变化。结果表明,整个南极的hmF2每日均值在250~300 km左右,NmF2每日均值在1~8×1011 el/m3之间,VSH每日均值在100~250 km,威德尔海异常主要表现在顶部电子密度的增强和底部电子密度的减少。     

电离层活动对GNSS的影响和抑制方法

一、引言从2011年开始,"太阳风暴"这个词在各大媒体出现的频率越来越高。剧烈的太阳黑子活动会扰动电离层环境,直接影响GNSS(全球导航卫星系统)的性能,轻则影响卫星定位精度,重则导致卫星通信失灵。因此,有必要了解电离层活动的现状,并探讨如何抑制其对GNSS的影响。二、太阳活动和电离层电离层随着纬度、经度的变化有着复杂的空间变化,并且具有昼夜、季节、年、太阳黑子等周期变化。电离层中的电子密度主要受太阳活动的影响。1.太阳黑子与电离层长期变化     

海口地区扩展F和GPS电离层闪烁发生时间相关性分析

对2012年8月至2013年7月太阳活动高年海口地区扩展F和GPS L1频段电离层闪烁进行了发生时间相关性分析。结果表明,该地区的GPS电离层闪烁与扩展F具有较好的相关性,二者发生的相关系数为095.受太阳活动高年低纬电离层不均匀体发展演化特性和设备观测方式的影响,观测到的电离层闪烁起始时间稍早于扩展F。扩展F结束时间比电离层闪烁结束时间有所滞后,分析认为,在低纬电离层不均匀体的消亡阶段,1 km以下中小尺度的不均匀体首先消失。      

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GNSS的川藏高原电离层活动特性分析

研究川藏高原地区的电离层活动特性，能够为本地区提供高精度导航定位和授时服务。本文基于欧洲轨道确定中心(CODE)2014—2019年的全球电离层格网(GIM)总电子含量(TEC)数据和太阳参数F_10.7的修正指数F_10.7p,利用时间序列分析、相关性分析以及快速傅里叶分析的方法，对川藏高原地区的电离层时空特性进行分析。实验结果表明：(1)川藏高原不同季节的TEC存在差异，尤其是在太阳活动高年时，体现了TEC的年度异常、半年度异常和冬季异常。(2)川藏高原的TEC主要周期表现为26.4、121.7、182.6、219.1、365.0 d,体现了TEC的27 d周期变化，半年变化，年变化。(3)受赤道异常影响，低纬度地区(30°N以下)的TEC较大。TEC在08:00:00—20:00:00相对较大，在20:00:00—08:00:00较小，这一特征在各经度处相同。     

第24太阳活动周武汉电离层VTEC变化特性分析

利用2008-2015年(第24太阳活动周)中国地壳运动监测网络（CMONOC）武汉站(30.5°N, 114.4°E)GPS双频接收机监测的垂直总电子含量（VTEC）数据,分析电离层VTEC变化特性,并讨论VTEC与太阳活动的相关性．结果表明,VTEC估值周日变化在14:00-16:00LT左右达到最大值;同时,电离层VTEC也表现出明显的逐日变化特性,特别是在VTEC峰值处呈现较大差异．在太阳活动高年(2011-2015),VTEC变化呈现明显的双峰结构,即所谓的“半年异常”现象,春季峰值大于秋季峰值,“冬季异常”也更为明显．在各个季节(除春季外),VTEC与F_10.7p的相关性最好,与F_10.7的相关性次之,与太阳黑子数（SSN）的相关性稍弱;秋季VTEC与太阳活动的相关性最好(Max_F10.7p=0.92232, Max_SSN=0.85575),冬季VTEC与太阳活动的相关性最差(Min_F10.7p=0.79028, Min_SSN=0.72703)．      

磁暴期全球电离层电子含量变化分析研究

电离层电子含量(TEC)受太阳活动影响较大,磁暴发生时,TEC变化在全球范围内变化不一,研究该时期的TEC扰动变化情况对电离层的研究至关重要．本文以2015年3月特大磁暴为研究对象,利用包括北斗系统在内的全球卫星导航系统（GNSS）TEC数据和中国区域的电离层测高仪f _oF2数据,对此次电离层磁暴的扰动特性进行研究并讨论其可能的物理机制．      

广西及周边地区电离层时空特性分析北大核心CSCD

针对电离层延迟对导航定位精度的影响,该文利用IGS提供的2012—2015年的电离层VTEC数据,基于时序分析、快速傅里叶分析和线性回归分析方法,研究了广西及周边地区电离层的时空变化,结合相应的太阳、地磁活动数据分析太阳活动、地磁活动与电离层的相关性。结果显示:广西及周边地区电离层VTEC值在纬向上随纬度升高而降低,在经向上几乎不变;探测得到VTEC具有365.7天的年周期和182.9天的半年周期;无论在太阳活动高、低年,VTEC春季达最大值,2014年和2015年出现冬季异常现象;VTEC变化与太阳长期活动存在较好相关性,相关系数在0.57左右;VTEC在地磁活动高发期时与Dst有较好的相关性,相关系数达0.52。     

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.     

Determination of the ionospheric foF2 using a stand-alone GPS receiver

The critical frequency of ionospheric F2 layer (foF2) is a measure of the highest frequency of radio signal that may be reflected back by the F2 layer, and it is associated with ionospheric peak electron density in the F2 layer. Accurate long-term foF2 variations are usually derived from ionosonde observations. In this paper, we propose a new method to observe foF2 using a stand-alone global positioning system (GPS) receiver. The proposed method relies on the mathematical equation that relates foF2 to GPS observations. The equation is then implemented in the Kalman filter algorithm to estimate foF2 at every epoch of the observation (30-s rate). Unlike existing methods, the proposed method does not require any additional information from ionosonde observations and does not require any network of GPS receivers. It only requires as inputs the ionospheric scale height and the modeled plasmaspheric electron content, which practically can be derived from any existing ionospheric/plasmaspheric model. We applied the proposed method to estimate long-term variations of foF2 at three GPS stations located at the northern hemisphere (NICO, Cyprus), the southern hemisphere (STR1, Australia) and the south pole (SYOG, Antarctic). To assess the performance of the proposed method, we then compared the results against those derived by ionosonde observations and the International Reference Ionosphere (IRI) 2012 model. We found that, during the period of high solar activity (2011–2012), the values of absolute mean bias between foF2 derived by the proposed method and ionosonde observations are in the range of 0.2–0.5 MHz, while those during the period of low solar activity (2009–2010) are in the range of 0.05–0.15 MHz. Furthermore, the root-mean-square-error (RMSE) values during high and low solar activities are in the range of 0.8–0.9 MHz and of 0.6–0.7 MHz, respectively. We also noticed that the values of absolute mean bias and RMSE between foF2 derived by the proposed method and the IRI-2012 model are slightly larger than those between the proposed method and ionosonde observations. These results demonstrate that the proposed method can estimate foF2 with a comparable accuracy. Since the proposed method can estimate foF2 at every epoch of the observation, it therefore has promising applications for investigating various scales (from small to large) of foF2 irregularities.     

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.     

The ionospheric impact of the October 2003 storm event on Wide Area Augmentation System

The United States Federal Aviation Administrations (FAA) Wide-Area Augmentation System (WAAS) for civil aircraft navigation is focused primarily on the Conterminous United States (CONUS). Other Satellite-Based Augmentation Systems (SBAS) include the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-transport Satellite-based Augmentation System (MSAS). Navigation using WAAS requires accurate calibration of ionospheric delays. To provide delay corrections for single frequency global positioning system (GPS) users, the wide-area differential GPS systems depend upon accurate determination of ionospheric total electron content (TEC) along radio links. Dual-frequency transmissions from GPS satellites have been used for many years to measure and map ionospheric TEC on regional and global scales. The October 2003 solar-terrestrial events are significant not only for their dramatic scale, but also for their unique phasing of solar irradiance and geomagnetic events. During 28 October, the solar X-ray and EUV irradiances were exceptionally high while the geomagnetic activity was relatively normal. Conversely, 29–31 October was geomagnetically active while solar irradiances were relatively low. These events had the most severe impact in recent history on the CONUS region and therefore had a significant effect on the WAAS performance. To help better understand the event and its impact on WAAS, we examine in detail the WAAS reference site (WRS) data consisting of triple redundant dual-frequency GPS receivers at 25 different locations within the US. To provide ground-truth, we take advantage of the three co-located GPS receivers at each WAAS reference site. To generate ground-truth and calibrate GPS receiver and transmitter inter-frequency biases, we process the GPS data using the Global Ionospheric Mapping (GIM) software developed at the Jet Propulsion Laboratory. This software allows us to compute calibrated high resolution observations of TEC. We found ionospheric range delays up to 35 m for the day-time CONUS during quiet conditions and up to 100 m during storm time conditions. For a quiet day, we obtained WAAS planar fit slant residuals less than 2 m (0.4 m root mean square (RMS)) and less than 25 m (3.4 m RMS) for the storm day. We also investigated ionospheric gradients, averaged over distances of a few hundred kilometers. The gradients were no larger than 0.5 m over 100 km for a quiet day. For the storm day, we found gradients at the 4 m level over 100 km. Similar level gradients are typically observed in the low-latitude region for quiet or storm conditions.     

Climatological study of ionospheric irregularities over the European mid-latitude sector with GPS

High-frequency variability of the ionosphere, or irregularities, constitutes the main threat for real-time precise positioning techniques based on Global Navigation Satellite Systems (GNSS) measurements. Indeed, during periods of enhanced ionospheric variability, GNSS users in the field—who cannot verify the integrity of their measurements—will experience positioning errors that can reach several decimeters, while the nominal accuracy of the technique is cm-level. In the frame of this paper, a climatological analysis of irregularities over the European mid-latitude region is presented. Based on a 10 years GPS dataset over Belgium, the work analyzes the occurrence rate (as a function of the solar cycle, season and local time) as well as the amplitude of ionospheric irregularities observed at a single GPS station. The study covers irregularities either due to space weather events (solar origin) or of terrestrial origin. If space weather irregularities are responsible for the largest effects in terms of ionospheric error, their occurrence rate highly depends on solar activity. Indeed, the occurrence rate of ionospheric irregularities is about 9 % during solar maximum, whereas it drops to about 0 % during medium or low solar activity periods. Medium-scale ionospheric disturbances (MSTIDs) occurring during daytime in autumn/winter are the most recurrent pattern of the time series, with yearly proportions slightly varying with the solar cycle and an amplitude of about 10 % of the TEC background. Another recurrent irregularity type, though less frequent than MSTIDs, is the noise-like variability in TEC observed during summer nighttime, under quiet geomagnetic conditions. These summer nighttime irregularities exhibit amplitudes ranging between 8 and 15 % of the TEC background.     

Simulation study of the influence of the ionospheric layer height in the thin layer ionospheric model

This work aims to contribute to the understanding of the influence of the ionospheric layer height (ILH) on the thin layer ionospheric model (TLIM) used to retrieve ionospheric information from the GNSS observations. Particular attention is paid to the errors caused on the estimation of the vertical total electron content (vTEC) and the GNSS satellites and receivers inter-frequency biases (IFB), by the use of an inappropriate ILH. The work relies upon numerical simulations performed with an empirical model of the Earth’s ionosphere: the model is used to create realistic but controlled ionospheric scenarios and the errors are evaluated after recovering those scenarios with the TLIM. The error assessment is performed in the Central and the northern part of the South American continents, a region where large errors are expected due to the combined actions of the Appleton Anomaly of the ionosphere and the South-Atlantic anomaly of the geomagnetic field. According to this study, there does not exist a unique ILH that cancels the vTEC error for the whole region under consideration. The ILH that cancels the regional mean vTEC error varies with the solar activity and season. The latitude-dependent conversion error propagates to the parameters of the model used to represent the latitudinal variation on the vTEC on the ionospheric layer, and to the IFB, when these values are simultaneously estimated from the observed sTEC. Besides, the ILH that cancels the regional mean vTEC error is different from the one that cancels the IFB error and the difference between both ILH varies with the solar activity and season.     

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.