New field of application of the IRI modeling – Determination of ionosphere transfer characteristic for radio astronomical signals

We suggest a new field of application of IRI modeling – determination of ionosphere transfer characteristic (ITC) for radio astronomical signals (RAS). VHF and HF RAS are widely used for observations of the Sun and pulsars. It is necessary to take into account possible distortions of RAS in the Earth ionosphere. However, in contrast to modern navigation systems (GPS, GLONASS, GALILEO), where very accurate reconstruction of ionosphere parameters is a built-in function, in present-day radio astronomy a retrieve of ITC has not been appropriately worked out yet. It collides with increasing requirements to accuracy of the analysis of RAS amplitude profile and to the angular and polarizing resolution of radio telescopes of new generation. We have developed a method and software for calculation of the ionosphere measure of rotation (RM) and the measure of dispersion (DM). We used the ionosphere model IRI-2001, magnetic-field model IGRF-10 and values of ionosphere total electron content as deduced from GPS measurements. The obtained values of the ionosphere DM and RM were recalculated into characteristics of phase delay, Faraday amplitude modulation and polarization changes. We made calculations for different levels of geomagnetic activity and for different angular position of radio sources as well.     

Use of varying shell heights derived from ionosonde data in calculating vertical total electron content (TEC) using GPS – New method

The dispersive nature of the ionosphere makes it possible to measure its total electron content (TEC). Thus Global Positioning System, which uses dual-frequency radio signals, is an ideal system to measure TEC. When data from an ionosonde situated in polar region was observed, the height of an approximated thin shell of electrons (shell height) used in GPS studies was seen not to be fixed but rather changing with time. Here we introduce a new method in which we included the varying shell heights derived from the ionosonde to map the slant total electron content from GPS to obtain a more precise vertical total electron content of the ionosphere contrary to some previous methods which used fixed shell heights. In this paper we also compared the ionosonde derived TEC with the GPS derived vertical TEC (vTEC) values. These GPS vTEC values were obtained from GPS slant TEC (sTEC) measurements using both fixed shell height and varying shell heights (from ionosonde measurements). For the polar regions, the varying shell height approach produced better results than the fixed shell height and compared to exponential function, Chapman function seems to be a better function to model the topside ionosphere.     

基于GPS信号的电离层S4指数计算方法研究

电波穿越电离层时,由于受到电离层不均匀结构的影响,电波的幅度、相位、时延等有时会发生快速抖动,这就是所谓的电磁波电离层闪烁现象.电离层闪烁会影响卫星通信系统的质量和导航系统的精度.本文分析了GPS信号研究电离层闪烁的基本原理,讨论了电离层闪烁监测中S4指数的计算方法及其修正方法.通过数据模拟,评估了原始S4指数计算方法及其修正方法的性能特点.针对原始S4指数计算方法及修正方法的不足,提出了一种新的修正方法,并采用实测GPS数据对上述方法进行了检验.结果表明,上述方法是有效的和可靠的.      

Computerized ionospheric tomography with the IRI model

Computerized ionospheric tomography (CIT) is a method to estimate ionospheric electron density distribution by using the global positioning system (GPS) signals recorded by the GPS receivers. Ionospheric electron density is a function of latitude, longitude, height and time. A general approach in CIT is to represent the ionosphere as a linear combination of basis functions. In this study, the model of the ionosphere is obtained from the IRI in latitude and height only. The goal is to determine the best representing basis function from the set of Squeezed Legendre polynomials, truncated Legendre polynomials, Haar Wavelets and singular value decomposition (SVD). The reconstruction algorithms used in this study can be listed as total least squares (TLS), regularized least squares, algebraic reconstruction technique (ART) and a hybrid algorithm where the reconstruction from the TLS algorithm is used as the initial estimate for the ART. The error performance of the reconstruction algorithms are compared with respect to the electron density generated by the IRI-2001 model. In the investigated scenario, the measurements are obtained from the IRI-2001 as the line integral of the electron density profiles, imitating the total electron content estimated from GPS measurements. It has been observed that the minimum error between the reconstructed and model ionospheres depends on both the reconstruction algorithm and the basis functions where the best results have been obtained for the basis functions from the model itself through SVD.     

南极GPS定位中的电离层延迟分析

基于南极地区国际GNSS服务组织(IGS,International GNSS Service)跟踪站的全球定位系统(GPS,Global Position System)双频实测数据,分析了南极地区电离层延迟的变化情况及其二阶项延迟对南极GPS定位结果的影响.结果表明:南极地区的总电子含量(TEC,Total Electron Content)日间波动频繁,其日间TEC最大值变化较中纬度地区剧烈;在南极地区夏季,电离层二阶项延迟对GPS定位结果的影响可达cm级.同时,由于欧洲定轨中心(CODE,The European Center for Orbit Determination)提供的全球电离层模型(GIM,Global Ionosphere Maps)在南极区域应用的局限性,通过选取南极地区6个IGS跟踪站作为基准站建立了区域电离层TEC模型(RIM,Regional Ionosphere Model).经实测数据计算证明,对于南极地区,RIM的定位精度在一定程度上优于全球电离层模型GIM.     

Analysis of a topside ionospheric model using GPS and ionosonde observables

Ground-based vertical incidence soundings are well suited to model the bottom-side ionosphere but are not so good for dependably modelling the topside ionosphere. This study aims to combine vertical incidence sounding and dual-frequency GPS measurements to reconstruct the topside profile. The reconstruction technique relays on the use of the so-called vary-Chap approach that use an α-Chapman function with a continuously varying scale height.     

Rapid local ionosphere modeling based on Precise Point Positioning over Iran: A case study under 2014 solar maximum

Presently, the ionosphere effect is the main source of the error in the Global Positioning System (GPS) observations. This effect can largely be removed by using the two-frequency measurements, while to obtain the reasonable results in the single-frequency applications, an accurate ionosphere model is required. Since the global ionosphere models do not meet our needs everywhere, the local ionosphere models are developed. In this paper, a rapid local ionosphere model over Iran is presented. For this purpose, the GPS observations obtained from 40 GPS stations of the Iranian Permanent GPS Network (IPGN) and 16 other GPS stations around Iran have been used. The observations have been selected under 2014 solar maximum, from the days 058, 107, 188 and 271 of the year 2014 with different geomagnetic activities. Moreover, ionospheric observables based on the precise point positioning (PPP) have been applied to model the ionosphere. To represent our ionosphere model, the B-spline basis functions have been employed and the variance component estimation (VCE) method has been used to regularize the problem.To show the efficiency our PPP-derived local ionosphere model with respect to the International GNSS Service (IGS) global models, these models are applied on the single point positioning using single-frequency observations and their results are compared with the precise coordinates obtained from the double-differenced solution using dual-frequency observations. The results show that the 95th percentile of horizontal and vertical positioning errors of the single-frequency point positioning are about 3.1 and 13.6?m, respectively, when any ionosphere model are not applied. These values significantly improve when the ionosphere models are applied in the solutions. Applying CODE’s Rapid Global ionosphere map (CORG), improvements of 59% and 81% in horizontal and vertical components are observed. These values for the IGS Global ionosphere map (IGSG) are 70% and 82%, respectively. The best results are obtained from our local ionosphere model, where 84% and 87% improvements in horizontal and vertical components are observed. These results confirm the efficiency of our local ionosphere model over Iran with respect to the global models. As a by-product, the Differential Code Biases (DCBs) of the receivers are also estimated. In this line, we found that the intra-day variations of the receiver DCBs could be significant. Therefore, these variations must be taken into account for the precise ionosphere modeling.     

The spatio-spectral localization approach to modeling VTEC over the western part of the USA using GPS observations

The precise ionosphere modeling is crucial and remains a challenge for GPS positioning and navigation, as well as many other Earth observation systems. In this research, two approaches have been proposed to model the vertical total electron content (VTEC) of the ionosphere using spherical slepian function. For the two-dimensional case, VTEC has been modeled in a sun-fixed reference frame and the three-dimensional approach based on the system of the three-dimensional base functions has been defined as the tensor product of the spherical slepian function for the longitude and latitude in an Earth-fixed reference frame, and the polynomial B-spline function for time. Rather than the spherical harmonics, the spherical slepian functions can be employed to produce the locally and globally orthogonal bases to optimally represent the data in any arbitrary region up to a given degree. The spherical slepian functions have been applied to the real data obtained from the ground-based GPS observation across the western part of the USA.     

Impact of geomagnetic storm of September 7–8, 2017 on ionosphere and HF propagation: A multi-instrument study

The present study reveals the features of ionospheric parameters variations during the geomagnetic storm of September 7–8, 2017. In particular, parameters of vertical (foF2, foEs) and oblique ionospheric sounding (MOF, modes), absorption level, Total Electron Content (TEC) and particle fluxes at high altitudes were under analysis. The storm was characterized by two Dst-index mimima and can be considered as a sequence of two storms: first - with Dstmin?=??142?nT at 02 UT on September 8th and second - with Dstmin?=??122?nT and at 15 UT on September 8th. It was found that these two storms had different impacts on the ionosphere and HF propagation at mid- and high-latitudes of Northern Hemisphere. The signals of vertical and oblique ionospheric sounding were present in all ionograms before the first storm. Further, at the maximum of the first storm these signals were totally absorbed. Then, before the second storm and during its maximum the signals were detected again in the ionograms due to the low absorption. GOES satellite data showed the significant burst of electrons and protons only during the first storm and small particle fluxes - during the second storm. This feature was also confirmed with GPS data: TEC increased during the first storm and decreased during the second storm.     

Numerical study of traveling ionosphere disturbances with vertical incidence data

High frequency ionosphere vertical sounding, as an important and representative application for detecting the ionosphere and studying the characteristics of radio propagation, can be utilized to monitor the ionosphere continuously variation and to acquire the ionosphere asymmetrical features of diverse scale above the ionosphere vertical sounding stations. This is a first article on real time application of numerical methods to obtain the parameters of traveling ionosphere disturbances (TIDs) using vertical incident ionograms. In this paper, the distribution of ionosphere electron density with TIDs is constructed using a background ionosphere model superimposed a perturbation theory model. The background ionosphere electron density is modelled by the inversion of vertical incident ionograms which are observed before the appearance of the disturbance. Based on the fourth order Adams-Bashforth-Moulton (the so-called ABM) predictor corrector method, instead of Runge-Kutta method, the fast digital ray tracing method is established. According to process of the disturbed trace simulation and parameters inversion, the characteristic parameters of ionosphere disturbance at different detection time can be obtained in real time. The numerical analysis of TIDs is then captured completely.     

Application of small-size antennas for estimation of angles of arrival of HF signals scattered by ionospheric irregularities

A modification of the Doppler Interferometry Technique is suggested to enable estimating angles of arrival of comparatively broadband HF signals scattered by random irregularities of the ionospheric plasma with the use of small-size weakly directional antennas. The technique is based on the measurements of cross-spectra phases of the probe radiation recorded at least in three spatially separated points. The developed algorithm has been used to investigate the angular and frequency-time characteristics of HF signals propagating at frequencies above the maximum usable one (MUF) for the direct radio path Moscow-Kharkiv. The received signal spectra show presence of three families of spatial components attributed, respectively, to scattering by plasma irregularities near the middle point of the radio path, ground backscatter signals and scattering of the sounding signals by the intense plasma turbulence associated with auroral activations. It has been shown that the regions responsible for the formation of the third family components are located well inside the auroral oval. The drift velocity and direction of the auroral ionosphere plasma have been determined. The obtained estimates are consistent with the classical conception of the ionospheric plasma convection at high latitudes and do not contradict the results of investigations of the auroral ionosphere dynamics using the SuperDARN network.     

基于空间统计方法的电离层折射修正技术

针对中国上空电离层所具有的特殊性和GPS观测站在中国西部分布相对稀疏的特点, 尝试探索中国卫星增强系统电离层时延信息修正技术, 为卫星导航定位以及遥感、遥测等空间应用工程的电波修正提供数据. 利用中国地壳形变监测网提供的双频GPS数据, 以空间统计方法为主要工具, 给出了普通Kriging电离层估计算法, 构建了平静期和磁暴期电离层理论变异模型, 详细分析了电离层折射修正的精度. 结果表明, 将空间统计方法应用于卫星增强系统中的电离层时延改正问题, 有利于提高增强系统的电离层折射修正精度, 特别是在观测样点相对较少的情况下, 有利于系统完整性的实现.      

Adaptation of the bottomside electron density profile of the IRI model to data of topside radiosounding

A method is proposed for reconstructing the electron density profiles N(h) of the IRI model from ionograms of topside satellite sounding of the ionosphere. An ionograms feature is the presence of traces of signal reflection from the Earth's surface. The profile reconstruction is carried out in two stages. At the first stage, the N(h) –profile is calculated from the lower boundary of the ionosphere to the satellite height (total profile) by the method presented in this paper using the ionogram. In this case, the monotonic profile of the topside ionosphere is calculated by the classical method. The profile of the inner ionosphere is represented by analytical functions, the parameters of which are calculated by optimization methods using traces of signal reflection, both from the topside ionosphere and from the Earth. At the second stage, the profile calculated from the ionogram is used to obtain the key parameters: the height of the maximum hmF2 of the F2 layer, the critical frequency foF2, the values of B0 and B1, which determine the profile shape in the F region in the IRI model. The input of key parameters, time of observation, and coordinates of sounding into the IRI model allows obtaining the IRI-profile corrected to real experimental conditions. The results of using the data of the ISIS-2 satellite show that the profiles calculated from the ionograms and the IRI profiles corrected from them are close to each other in the inner ionosphere and can differ significantly in the topside ionosphere. This indicates the possibility of obtaining a profile in the inner ionosphere close to the real distribution, which can significantly expand the information database useful for the IRTAM (IRI Realmax Assimilative Modeling) model. The calculated profiles can be used independently for local ionospheric research.     

VLF电波渗透到卫星高度电离层传播的全波计算

考虑斜向地磁场的影响将电离层设为多层水平分层各向异性有耗介质, 利用传播矩阵法求解全波方程, 进而研究分析VLF频段电离层反射系数随电波频率的变化, 电离层中两种特征极化波的折射和极化特性, 两特征波的电磁场水平分量以及坡印廷能流密度随传播高度的变化. 数值计算结果表明, 地—电离层波导中的垂直极化波比平行极化波易渗透进入电离层; 电离层中两种特征极化波可分为左旋和右旋圆极化波, 左旋分支由于D层强吸收作用表现为速衰减模, 而右旋分支表现为可传播模, 在传播过程中电磁波的能量主要存储在磁场中; 电波频率越低, 其在电离层中的传播损耗越小. 由数值模拟结果发现, 卫星监测VLF频段的低频部分及更低频段的水平磁场变化对于发现地震电离层电磁前兆异常可能更为有效.      

赤道异常区GPS-TEC与系统硬件偏差的反演

提出了一种基于最小二乘的赤道异常区GPS-TEC与系统硬件偏差的反演方法.利用设置在福州、厦门、广州和南宁4个台站的观测数据,可以得到GPS卫星和接收机的硬件偏差以及(20°～28°N,105°～123°E)区域中48个3°×1°网格的TEC(时间分辨率为15 min).应用于2006年观测数据,得到了较稳定的系统硬件偏差,其中卫星硬件偏差值与欧洲定轨中心公布的结果接近,得到的TEC具有合理的日变化和季节变化特征.该反演方法可以应用于赤道异常区电离层的研究.      

新型高空探测系统

研发了一种新型全自动高空探测系统,其利用GPS定位,宽波瓣天线捕捉和低噪声接收的方法替代雷达或无线电经纬仪对高空探测仪进行跟踪定位和获取信息,具有不受低仰角的限制,目标不易丢失,性能价格比高等特点,成功将GPS应用于1680 MHz遥测数据传输体制.利用该系统进行了空中大气电场的探测实验,获得了满意的探测结果.由于系统轻便、操作简单、成本较低以及良好的机动性和可靠性,在近地空间环境探测和高空大气科学探测试验及常规的气象探测中有广阔的应用前景.     

电离层多普勒接收机的设计与实验

利用软件无线电技术设计了一种电离层多普勒接收机. 该接收机采用DSP, FPGA等数字芯片与PXI总线进行架构, 使用GPS作为接收机的时间和频率同 步模块, 能够灵活设置系统参数. 实验接收来自中国蒲城陕西天文台的高 频时间信号, 实时获取由于电离层扰动所产生的多普勒频率偏移信息. 使用通过MATLAB语言实现的信号处理软件平台, 对接收到的高频信号进行处理. 观测结果表明, 接收机能够分析电离层回波信号的多普勒频移随时间的变化, 是获取不同空间尺度电离层扰动信息的一种有效手段.      

General aspect of GPS data use for atmospheric science

The radio link between a GPS satellite and a GPS receiver is appropriate for bistatic radar sounding of the Earth's atmosphere, ionosphere, and ocean surface (latter in case of GPS reflection). Measurements of GPS phases and amplitudes are currently performed by spaceborne, airborne, mountain- and ground-based GPS receivers. In the present paper, an uniform approach based on geometrical optics and spherical symmetry of the atmosphere is applied to various GPS observation configurations. Atmospheric mapping function, influence and retrieval of ionospheric layers/disturbances, tropospheric water vapor, and possible measurement of vertical winds and wave velocities are investigated by use of simulation data of GPS phase path excess and bending angle.     

Validation of FORMOSAT-3/COSMIC radio occultation electron density profiles by incoherent scatter radar data

Low Earth Orbiting satellites carrying a dual frequency GPS receiver onboard offer a unique opportunity to remote sensing of the global ionosphere on a continuous basis. No other profiling technique unifies profiling through the entire F2-layer with global coverage. The FORMOSAT-3/COSMIC data can make a positive impact on the global ionosphere study providing essential information about the height electron density distribution and particularly over regions that are not accessible with ground-based measuring instruments such as ionosondes and GPS dual frequency receivers. Therefore, it is important to verify occultation profiles with other techniques and to obtain experience in the reliability of their derivation. In the given study we present results of comparison of the electron density profiles derived from radio occultation measurements on-board FS-3/COSMIC and from the Kharkov incoherent scatter radar sounding.     

The lower ionosphere response to its disturbances by powerful radio waves

The paper presents data from some campaigns at Sura heating facility in 2011–1016. The experiments on probing of the artificial disturbed region of the lower ionosphere were carried out at two observation sites. One of them was located near Vasil’sursk 1 km from Sura facility (56.1°N; 46.1°E) and the other site was located at the Observatory (55.85°N; 48.8°E) of Kazan State University, 170 km to the East. Investigation of the features of the disturbed region of the lower ionosphere based on its diagnostics by the methods of the vertical sounding and oblique backscattering is the main goal of this paper. Ionosphere disturbance was fulfilled by the effect of the powerful radio wave of the ordinary or extraordinary polarization emitted by transmitters of the Sura facility with effective radiated power ERP = 50–120 MW at the frequency of 4.3, 4.7 and 5.6 MHz. Pumping waves were emitted with period from 30 s to 15 min. The disturbed region of the ionosphere in Vasil’sursk was probed by the vertical sounding technique using the partial reflexion radar at the frequency of 2.95 and 4.7 MHz. For the oblique sounding of the disturbed region the modified ionosonde Cyclon-M, operating at ten frequencies from 2.01 to 6.51 MHz was used at the Observatory site. On many heating sessions simultaneous variations of the probing partial reflection signals in Vasil’sursk and backscattered signals in Observatory were observed at the height at 40–100 km below the reflection height of the pumping wave. These observations were correlated with the pumping periods of the Sura facility. Possible mechanisms of the appearance of the disturbance in the lower ionosphere and its effect on the probing radio waves are discussed.