An experiment to study solar flare effects on radio-communication signals

The occurrence of radio signal fading events caused by ionospheric absorption plays an important role in the performance of radio-communication systems. It is necessary to know the magnitude and time-scale of such events in order to specify technical parameters of the communication system to be used. Generally, fading events are associated with solar flares, which are characterized by sudden increase in the solar X-ray flux that causes an increase in the ionization in the lower ionosphere. The abrupt increase of ionization causes the absorption of radio waves propagating in the Earth–ionosphere wave-guide and is reported as radio signal fading events. A simple experiment to monitor the behavior of lower ionosphere has been carried out at the Southern Space Observatory-SSO/INPE (29.43°S, 53.8°W), located in southern Brazil. The experiment is basically a computer controlled radio receiver that records the received signal strength of Amplitude Modulated (AM) radio signals in the HF (High Frequencies) range. We analyzed data of the 6 MHz beacon signal that has been transmitted by a broadcasting radio station located about 400 km from the observation site. In this work we present initial results of daily variation of the received signal strength and fading events associated with solar flares observed in the 6 MHz signal monitored by the experiment during 2001. X-ray solar flux data from the GOES-8 satellite were used to identify X-ray solar bursts associated with solar flares. Based on the one-year data collected by the experiment, a statistical summary of fading occurrences and their correlation with solar flares, as well as the distributions of time-scales and magnitudes of such events are presented.     

Fading in the HF ionospheric channel and the role of irregularities

It is well known that the ionosphere affects radio wave propagation especially in the high frequency (HF) range. HF radio waves reflected by the ionosphere can reach considerable distances, often with changes in amplitude, phase, and frequency. The ionosphere is a dispersive in frequency and time, bi-refractive, absorbing medium, in which multipath propagation due to traveling irregularities is very frequent. The traveling irregularities undulate the reflecting ionospheric layer, introducing variations in signal amplitude (fading). In this multipath time variant channel fading is mainly considered, even though it is not the sole effect. Echo signals from a single reflection, as in ionospheric vertical sounding (VIS) techniques, are affected by a certain degree of variability even in quiet ionospheric conditions. In this work the behavior of the ionospheric channel is studied and characterized by observing the power variation of received echoes using the VIS technique. Multipath fading was analyzed quantifying the power variation of the signal echo due to irregularities on a temporal scale from 0.5 to 256 s. An experimental set-up derived from an ionosonde was implemented and the analysis was performed employing a special numerical algorithm operating off-line on the acquired time sequence of the signal. The gain-loss of the irregularity shapes are determined in some special cases.     

Klobuchar模型的实用分析与改进

电离层延迟误差是无线电信号传播中不可忽视的误差源.GPS特许用户利用双频接收机的双频观测值直接对电离层延迟进行实时测定,其所得结果精度很高.多数普通用户所使用的单频接收机依靠电离层模型对其进行误差修正,效果不很理想.本文通过WUHN观测站双频接收机10天的实测数据对GPS广播星历采用的Klobuchar模型进行了验证,其结果与前人论述相一致.此外,经由反映太阳活动强度的太阳相对黑子数对Klobuchar模型提出了新的改进方法.实验数据结果表明,该方法对此模型修正效果有大幅提升,对原模型修正效果>50%的修正率由60%左右提升至85%以上,>80%的修正率由10%左右提升至40%以上.在实际应用中单频接收机用户可以参考本文改进方法对Klobuchar模型进行修正.      

Physical and model interpretation of HF radio propagation on the St. Petersburg–Longyearbyen (Svalbard) path

HF radio wave observations have been carried out with an oblique ionospheric sounding (OIS) method on the radio path from St. Petersburg to Longyearbyen (Svalbard), and experimental ionograms were obtained for December 2001. These ionograms have been analysed to investigate the impact of the main ionospheric trough (MIT) and magnetic disturbances on the signals on this path. The observations during weakly disturbed (K_р = 2) magnetic conditions on 14–15 December 2001 were compared with predictions from ray-tracing through a numerical model of the ionosphere. The ray-tracing computer program synthesizes the OIS ionograms by means of the “shooting method”. This method calculates trajectories of HF radio waves for different values of elevation angle and transmission frequency. There was a variety of calculated trajectories, from which we choose those which reach the receiver, and the selected paths provide a synthesis of the oblique ionograms. To simulate HF radio wave propagation, we apply a three-dimensional distribution of the electron density calculated with the mathematical model of the high-latitude ionosphere developed in the Polar Geophysical Institute (PGI). These numerical simulations permit us to interpret specific peculiarities of the OIS data such as abnormal propagation modes, increased delays of signals, enhanced MOF (maximum observed frequency) values etc. New results of the study are summarised as follows. (1) An unusual feature of the propagation along the path is the change of propagation mechanism during substorms on entering a path midpoint (or 1-hop reflection point) to the MIT. (2) Even weak substorms, having the distinguished intensities, lead to the appearance of different types of irregularities observed by the CUTLASS radar and therefore to the different propagation modes and F2MOF values. (3) The PGI model of the ionosphere was first used for ray-tracing at high latitudes. The model results are basically in a good qualitative agreement with experimental observations. This model provides the satisfactory agreement between the calculated and experimental F2MOF values while not correctly representing the fine structure of the experimental OIS ionograms at night. An agreement between the calculated and experimental data is better for day and evening hours than at night.     

Electron density retrieval from occulting GNSS signals using a gradient-aided inversion technique

In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere.     

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.     

Method of the HF wave absorption evaluation based on GIRO network data

High frequency (HF) communication is strongly dependent on the state of the ionosphere, which specifies the mode structure of the radio wave propagating in ionosphere. Another core factor defining the strength of the HF signal at the receiving site is the ionospheric absorption. Accurate modelling the effect of absorption is an essential part of many studies of the HF propagation in the ionosphere.This paper proposes a method for estimating the absorption. The method is based on analysis of vertical sounding ionograms. The main idea of the approach is to compare the main parameters retrieved from measured and simulated ionograms. The combination of Global Ionospheric Radio Observatory (GIRO, http://giro.uml.edu) data and ionograms modelling allows for developing the empiric absorption model available at near real-time. The ionogram simulation taking into account absorption utilizes the NIM-RT (North Ionospheric Model and Ray Tracing) software. As a result, the proposed technique provides more reliable and accurate evaluation of minimum frequency at which echoes are observed in vertical incidence ionosonde soundings. The values of these frequencies should be used in the following simulation to optimize parameters in the empirical formulae for defining absorption HF wave in ionosphere.The ultimate objective of this work is the designing the method, which allows the simulating of HF radio channel accounting for regular absorption due to UV radiation of the Sun. Eventually it could be considered as some kind of the HF propagation forecasting.     

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.     

Observation of wave structures in the upper atmosphere by means of radio holographic analysis of the radio occultation data

A radio holographic approach, developed by Pavelyev (1998), Hocke (1999), Igarashi (2000), is applied to observation of wave phenomena in the upper atmosphere using Global Positioning System — “Microlab-1” satellite (GPS/MET) radio occultation data. In the current state the radio holography approach uses the radar focused synthetic aperture principle to obtain high spatial resolution, and to remove the interference part corresponding to scattering from the upper ionosphere. High spatial resolution and accuracy of the radio halographic method is validated by means of revealing the weak signal reflected from the sea in the GPS/MET radio occultation data. The radio holographic method gives a new possibility to measure directly the vertical gradient of the electron density altitude profile in the D-layer using the radio occultation signal. The results of the application of radio holographic analysis to two GPS/MET occultation events (07 February 1997, No. 0447, 0158), in the D-region of the ionosphere, are discussed. Wave structures in the electron density concentration with a vertical spatial period of 1.4–6 km, and variations in the electron density gradient from ±5·10⁹ to ±8·10⁹ [1/(m³km)], have been retrieved from the D-layer data. The features observed in the vertical electron density profiles may be connected with breaking of gravity waves in the D-layer of the ionosphere.     

Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

Anthropogenic trigger of substorms and energetic particles precipitations

The high-frequency (HF) emission in near-Earth space from various powerful transmitters (radio communications, radars, broadcasting, universal time and navigation stations, etc.) form an integral part of the modern world that it cannot do without. In particular, special-purpose research facilities equipped with powerful HF transmitters are used successfully for plasma experiments and local modification of the ionosphere. In this work, we are using the results of a complex space-ground experiment to show that exposure of the subauroral region to HF emission can not only cause local changes in the ionosphere, but can also trigger processes in the magnetosphere–ionosphere system that result in intensive substorm activity (precipitations of high-energy particles, aurorae, significant variations in the ionospheric parameters and, as a consequence, in radio propagation conditions).     

Effects of a “day-time” substorm on the ionosphere and radio propagation

Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation.     

Quantitative analyses of complexity and nonlinear trend of radio refractivity gradient in the troposphere

In this paper, the complexity and nonlinear trends of Radio Refractivity Gradient (RRG) in the troposphere over selected locations in Nigeria are analyzed and discussed extensively. The RRG is an important parameter in estimating path clearance and propagation effects such as ducting, surface reflection and multi-path on terrestrial line of-sights links. Also, radio wave signal propagating in the troposphere is affected by unpredictability of a weather condition which includes the variations of meteorological parameters such as temperature, pressure and relative humidity. The complex state of the atmosphere, which is the medium for the transmission of radio signals tend to have very strong influence such as scintillation and ducting on the quality of the radio signal, amplitude and phase. Variations in the meteorological parameters also induce variations in the refractive index of the atmosphere which in-turn results in the effect known as radio refractivity. For effective prediction and modeling of radio signal propagation, one should be able to characterize the nature and predictability of the computed RRG information. Chaotic Quantifiers (CQ) such as Phase Plot Reconstruction (PPR), Average Mutual Information (AMI), False Nearest Neighbor (FNN), Recurrence Plot (RP) and Recurrence Quantification Analyses (RQA) are used to assess the RRG. The information reveal, however, is based on the prediction techniques, design and frequency planning of microwave networks which may be useful for optimum performances during atmospheric turbulence.     

Case study of inclined sporadic E layers in the Earth’s ionosphere observed by CHAMP/GPS radio occultations: Coupling between the tilted plasma layers and internal waves

We have used the radio occultation (RO) satellite data CHAMP/GPS (Challenging Minisatellite Payload/Global Positioning System) for studying the ionosphere of the Earth. A method for deriving the parameters of ionospheric structures is based upon an analysis of the RO signal variations in the phase path and intensity. This method allows one to estimate the spatial displacement of a plasma layer with respect to the ray perigee, and to determine the layer inclination and height correction values. In this paper, we focus on the case study of inclined sporadic E (E_s) layers in the high-latitude ionosphere based on available CHAMP RO data. Assuming that the internal gravity waves (IGWs) with the phase-fronts parallel to the ionization layer surfaces are responsible for the tilt angles of sporadic plasma layers, we have developed a new technique for determining the parameters of IGWs linked with the inclined E_s structures. A small-scale internal wave may be modulating initially horizontal E_s layer in height and causing a direction of the plasma density gradient to be rotated and aligned with that of the wave propagation vector k. The results of determination of the intrinsic wave frequency and period, vertical and horizontal wavelengths, intrinsic vertical and horizontal phase speeds, and other characteristics of IGWs under study are presented and discussed.     

射频接收机系统级建模中的噪声谱分析

提出了一种在有限的系统参数下进行射频接收机系统建模时,各子电路噪声参数的分析和计算方法.从噪声功率谱的等效计算出发,给出了加性高斯白噪声通过有噪线性电路的分析方法.以一通信电台为例进行了分析计算,在给定有限的系统参数(信噪比,灵敏度,增益等)下分析出合理的各子电路的噪声参数,并进行建模仿真验证,理论结果与仿真结果较好吻合,表明了分析方法的有效性.该噪声功率谱的等效分析计算方法对射频系统建模有很好的指导作用.      

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.      

D-region ionospheric disturbances associated with the Extremely Severe Cyclone Fani over North Indian Ocean as observed from two tropical VLF stations

The D-region ionospheric disturbances due to the tropical cyclone Fani over the Indian Ocean have been analysed using Very Low Frequency (VLF) radio communication signals from three transmitters (VTX, NWC and JJI) received at two low latitude stations (Kolkata-CUB and Cooch Behar-CHB). The cyclone Fani formed from a depression on 26th April, 2019 over the Bay of Bengal (Northeastern part of the Indian Ocean) and turned into an extremely severe cyclone with maximum 1-min sustained winds of 250 km/h on 2 May, 2019 which made landfall on 3 May, 2019. Out of six propagation paths, five propagation paths, except the JJI-CHB which was far away from the cyclone track, showed strong perturbations beyond $3\sigma$ level compared to unperturbed signals. Consistent good correlations of VLF signal perturbations with the wind speed and cyclone pressure have been seen for both the receiving stations. Computations of radio signal perturbations at CUB and CHB using the Long Wave Propagation Capability (LWPC) code revealed a Gaussian perturbation in the D-region ionosphere. Analysis of atmospheric temperature at different layers from the NASA’s TIMED satellite revealed a cooling effect near the tropopause and warming effects near the stratopause and upper mesosphere regions on 3 May, 2019. This study shows that the cyclone Fani perturbed the whole atmosphere, from troposphere to ionosphere and the VLF waves responded to the disturbances in the conductivity profiles of the lower ionosphere.     

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.      

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.     

电离层Es对HF和VHF传播信号的影响

Es层是存在于电离层中的电子密度非常高的偶发E层（Sporadic E）,其电子密度可达常规E层的100倍.电离层Es能够反射原本穿透F层的VHF低频段（30~150MHz）无线电波,而且对HF高频段（10~30MHz）无线电波传播具有显著影响.运用垂测和斜测观测数据,研究HF频段Es层电波传播特征,得到了不同类型及不同高度Es层的衰减系数.根据f₀E_s的日变化规律,可得HF频段衰减系数的日变化规律,进而分析并得到Es层对短波传播的影响.不存在电离层Es时,通常无法通过电离层实现VHF超远距离通信.为了对VHF链路通过电离层Es的传播衰减进行定量分析,根据EBU多条链路的观测结果,拟合并建立了电离层Es衰减模型.将该模型、ITU模型和观测数据进行对比,发现本文建立的模型准确度更高.利用建立的模型,对电离层Es不同临频f₀E_s条件下接收信号场强和电压随传播距离的变化进行了计算,结果可为VHF链路设计及建立提供参考.