Improved precision of virtual height measurements with coherent radio pulse sounding based on the maximum likelihood method

The paper describes a new technique that improves precision of the virtual height measurements by a coherent pulse sounding of the ionosphere. Proposed technique is based on the method of maximum likelihood that matches expected and observed spectral domain signatures of the signal intermixed with the noise. Computer simulations show that our technique allows measurements of the echo virtual height with ∼100 m precision even at a much coarser step of the height sampling in the sounder. In experiment, we expect an average 300 m precision of the virtual height measurements for single echoes received during periods of little spread due to ionospheric irregularities.     

Variations in the ionospheric scale height parameter at the F2 peak over Grahamstown,South Africa

The Grahamstown, South Africa (33.3°S, 26.5°E) ionospheric field station operates a Lowell digital pulse ionospheric sounder (Digisonde) whose output includes scaled parameters derived from the measured ionogram. One of these output parameters is the ionospheric scale height parameter (H), and this paper presents an analysis of the seasonal, diurnal, and solar activity variations of this parameter over the Grahamstown station. Ionosonde data from three years 2002, 2003, and 2004 were used in this study. The data was subjected to a general trend analysis to remove any outliers and then the monthly median data were used to explore the different variations. The results of this analysis were found to be similar to what has already been presented in the literature for low latitude stations, and are presented as well as the correlation at this mid-latitude station between the H parameter, the IRI shape parameter (B0), and the peak electron density (NmF2).     

The topside sounder database – Data screening and systematic biases

The ionospheric topside sounder measurement database developed at the US National Space Science Data Center (NSSDC) is a valuable source of information when investigating the composition and complex dynamics of the upper ionosphere. The database is increasingly used by many scientists around the world for both research and development of empirical models. However, there is always a danger of indiscriminately using the data without properly assessing the data quality and applicability for a given purpose. This paper is concerned with the issue of data screening and pre-processing of the Alouette/ISIS topside sounder database. An overview of the original database availability and formatting is given and the use of solar and geomagnetic indices is discussed. Data screening procedures, concerning detection and handling of erroneous profiles, are also presented. Special attention is drawn to the systematic biases observed in the database and the possibilities for their removal.     

CAS-DIS数字电离层测高仪系统研制

提出了将射频直接采样数字化的接收方式应用于数字电离层测高仪系统的设计方案, 并详细介绍了该方案在CAS-DIS (Chinese Academy of Sciences, Digital Ionosonde)电离层测高仪系统的应用, 给出了CAS-DIS电离层测高仪在中国武汉电离层探测标校试验场和武汉-北京斜向电离层探测系统中的实验结果. 观测结果表明, 采用本文提出的数字化方案设计的电离层测高仪系统性能优越, 且可以分别工作在垂直和斜向探测模式, 满足了电离层探测的多种应用需求.      

Effect of HF emission of the topside sounder transmitter aboard the COSMOS-1809 satellite on the ionospheric plasma

The experiment on investigation of effect of the HF emission (300 W) by the dipole antenna on the ionospheric plasma was carried out onboard the COSMOS-1809 satellite (1987). The sounder accelerated particles (SAP) at the electron cyclotron harmonics n · ω_cs and in the frequency region of antenna resonance were detected by the charged particle spectrometer.     

基于时频分析的电离层多模污染抑制研究

研究高分辨线性时频分析方法和联合时频分析方法，利用这两种方法对高频电离层返回散射探测的多个传播模式实测数据，对污染抑制效果进行分析.结果显示，这两种基于时频分析的方法针对多模和相位污染同时存在的复杂情况，与其他常规方法相比均使杂波谱得到了较好的锐化，目标信号在二维时频域较清楚地显出.因此可以证明，上述两种方法具有很高的应用价值，对天波超视距雷达海上低速目标检测具有重要意义.      

一种用于电离层TEC监测的GNSS信号载波跟踪算法

全球卫星导航系统（GNSS）是电离层TEC监测中应用最普遍的手段. 目前方法通常是在传统导航用途的GNSS接收机输出的原始观测量基础上,经过数据后处理得到电离层TEC信息,其GNSS信号的跟踪处理算法依然采用GNSS导航接收机的算法. 针对GNSS系统用于电离层TEC监测的特殊性,提出一种称为GNSS双频信号和差联合跟踪的新算法,与传统方法相比,该算法直接跟踪电离层TEC的变化,可以提高电离层TEC跟踪的灵敏度和TEC的观测精度,改善电离层TEC监测性能.      

匹配滤波方法在非相干散射雷达测量空间碎片中的应用

利用2010年3月25日欧洲非相干散射雷达的常规电离层探测数据, 分析了实测数据中接收功率突变的现象. 高度发生在500 km以上的功率突变一般不是由于地球物理影响产生的, 而要归因于卫星或空间碎片. 采用匹配滤波方法实现了对空间碎片的探测和提取, 利用相干积累的方法提高了信噪比, 共获得363个碎片, 得到了碎片随高度和时间的分布. 结果表明, 碎片高度分布主要集中在800 km左右, 与欧洲非相干散射组织现有的碎片数据具有一致性.      

基于声光技术的扩频信号循环谱的检测

扩频信号是被伪随机码调制的周期信号,因此它应被建模为循环平稳随机信号.基于循环平稳随机信号的谱相关理论,提出了一种采用基于声光信号处理技术的扩频信号循环谱检测方法.通过研究扩频信号的循环频率、循环功率谱和基于时间积分的声光循环平稳处理模型,利用声光循环平稳时间积分相关器,在循环频率处对扩频信号的声光相关函数(循环谱)进行检测.相比于传统的能量检测方法,循环谱检测方法具有更好的检测性能;对于噪声和干扰,无论是平稳的、还是非平稳的,循环谱检测方法都具有较强的抗干扰能力,仿真结果证明了其有效性.     

Thermal and TEC anomalies detection using an intelligent hybrid system around the time of the Saravan,Iran, (Mw = 7.7) earthquake of 16 April 2013

A powerful earthquake of M_w = 7.7 struck the Saravan region (28.107° N, 62.053° E) in Iran on 16 April 2013. Up to now nomination of an automated anomaly detection method in a non linear time series of earthquake precursor has been an attractive and challenging task. Artificial Neural Network (ANN) and Particle Swarm Optimization (PSO) have revealed strong potentials in accurate time series prediction. This paper presents the first study of an integration of ANN and PSO method in the research of earthquake precursors to detect the unusual variations of the thermal and total electron content (TEC) seismo-ionospheric anomalies induced by the strong earthquake of Saravan. In this study, to overcome the stagnation in local minimum during the ANN training, PSO as an optimization method is used instead of traditional algorithms for training the ANN method. The proposed hybrid method detected a considerable number of anomalies 4 and 8 days preceding the earthquake. Since, in this case study, ionospheric TEC anomalies induced by seismic activity is confused with background fluctuations due to solar activity, a multi-resolution time series processing technique based on wavelet transform has been applied on TEC signal variations. In view of the fact that the accordance in the final results deduced from some robust methods is a convincing indication for the efficiency of the method, therefore the detected thermal and TEC anomalies using the ANN + PSO method were compared to the results with regard to the observed anomalies by implementing the mean, median, Wavelet, Kalman filter, Auto-Regressive Integrated Moving Average (ARIMA), Support Vector Machine (SVM) and Genetic Algorithm (GA) methods. The results indicate that the ANN + PSO method is quite promising and deserves serious attention as a new tool for thermal and TEC seismo anomalies detection.     

电离层中小尺度扰动的多频多普勒快速探测与分析

电离层反射回波的高频多普勒频移测量是研究电离层扰动的重要方法之一.利用基于单频信号相位变化率测量的实时探测方法,获取连续高精度多频多普勒频高图,以实现中小尺度电离层扰动的快速探测研究.该方法被应用于敏捷式高性能电离层无线电多频探测系统样机平台.经验证在该平台上获取的多普勒频移分辨率可达0.039Hz,频高图探测周期最短小于1min.基于这种快速探测方法和平台在武汉观测站进行了较长时间的观测和数据采集,获得高精度多普勒频移并反演出电子浓度等值面法向运动速度,得到电离层反射寻常波的多普勒频高图和垂直扰动速度等信息,进而推演出电离层扰动随时间和空间的实时变化特征.对这些时域信号进行频谱分析,初步结果显示这些扰动主要是由极区活动激发的中国中部地区冬季出现率较高的中尺度TID.此外,对三种常用的电离层扰动反演分析方法进行了对比研究,结果显示电离层扰动的变化趋势基本一致,说明观测数据和研究方法可靠稳定,为多频多点电离层扰动的传播特性观测研究提供了基础.      

中国区域电离层异常数据野值检测

针对中国区域连续运行参考站接收机野值会干扰星基增强系统（SBAS）电离层异常事件提取的问题，提出了一种基于电离层垂直延迟时间梯度的野值检测方法。首先，介绍了电离层延迟数据的提取方法；然后，论述了依据野值和电离层异常的不同时空相关特性进行野值检测的方法，并用单双频定位误差结果验证了野值检测的正确性；最后，对检测结果进行了分析。结果表明:该方法可有效区分野值和电离层异常；在中国大陆构造环境监测网络200多个参考站中，野值检测所剔除的参考站数目在3~10个，对电离层穿透点空间分布的影响在可接受范围内。      

广义Digisonde漂移分析原理

本文给出一种广义Ｄｉｇｉｓｏｎｄｅ漂移分析，取消了ＤＤＡ的假设限制，使Ｄｉｇｉｓｏｎｄｅ漂称测量能应用于更为复杂的运动形式，如电离层研究中的剪切、辐散或辐辏等运动形式。文中阐述了ＧＤＤＡ的原理并给出一些模拟实验结果。     

Digisonde observations of TIDs with frequency and angular sounding technique

A technique for studying ionospheric wavelike phenomena, primarily AGW/TID events, is developed based on the solution of the problem of radio wave propagation in ionospheric plasma disturbed by wavelike processes. A perfectly reflecting surface model is used for representing TIDs propagating at ionospheric heights. This technique is a generalization of the Frequency-and-Angular Sounding (FAS) method developed earlier for oblique TID diagnostics using transmitters of opportunity. Trial measurements were made in November 2003 with two DPS-4 systems at Millstone Hill Observatory, providing experimental validation of the developed method by comparing the results of disturbance diagnostics to those simultaneously obtained with the original (oblique) FAS method. The TID parameters recovered during the November 2003 campaign suggest that the observed disturbances predominately propagated equatorward which likely indicates their sources to be in the auroral region. The equatorward propagating AGW/TIDs are typical for disturbed geomagnetic conditions which were observed during the campaign. Implementation of the generalized FAS technique in the DPS sounder allowed development of a dedicated data acquisition system for ionospheric disturbance diagnostics. Routine measurements with the developed technique using the existing world-wide network of Digisondes (GIRO) will make it possible to conduct large-scale studies of the AGW/TID phenomena.     

基于GPS的广州地区电离层不规则体纬向漂移速度计算分析

利用广州站组建的两台短间距GPS电离层闪烁监测仪的观测数据, 分别对GPS卫星信号强度用功率谱和短间距台链互相关性两种方法计算了3次闪烁事件电离层不规则体的漂移速度. 分析结果表明, 同一不规则体会引起两台站闪烁事件的同时发生, 两种方法测量不规则体漂移速度通常在50～160m/s之间, 平均大小均在120m/s左右, 且纬向漂移速度在闪烁初期起伏较明显, 速度随闪烁时间有下降的趋势, 夜间纬向漂移方向由西向东, 广州地区漂移速度特性符合低纬其他地区不规则体漂移速度特征, 两种计算方法合理有效.      

Ionospheric effects of the solar flares as deduced from global GPS network data

Results derived from analysing the ionospheric response to faint and bright solar flares are presented. The analysis used novel technology of a global detection of ionospheric effects from solar flares as developed by the authors (Afraimovich, 2000a; Afraimovich, 2000b), on the basis of phase measurements of the total electron content (TEC) in the ionosphere using an international GPS network. The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in the TEC which is determined from GPS data, simultaneously for the entire set of visible GPS satellites at all stations used in the analysis. This technique is useful for identifying the ionospheric response to faint solar flares (of X-ray class C) when the variation amplitude of the TEC response to separate line-on-sight (LOS) is comparable to the level of background fluctuations. The dependence of the TEC variation response amplitude on the flare location on the Sun is investigated.     

Doppler observations of infrasonic waves of meteorological origin at ionospheric heights

Ionospheric effects of meteorological origin observed by the continuous HF Doppler sounder over the Czech Republic are reported in this paper. We focused on detection of waves of periods 1–10 min. We discuss the influence of dynamics and intensity of active weather systems on the occurrence of short period waves and dependence of the observed ionospheric effects on the height of reflection of the sounding radio wave. We observed 3–5 min waves during a severe weather event in summer and 2.5–4 min waves during a severe weather event in winter. We excluded possible geomagnetic origin of these oscillations by the analysis of fluctuations of the local geomagnetic field. In eight cases of 10, wave activity in the analysed period range was not significantly increased comparing to quiet days. The intensity of weather systems as well as the location of potential sources of waves towards the points of HF Doppler shift observation influence significantly the occurrence of infrasonic waves in the ionosphere. The results in Central Europe differ considerably from those previously obtained in North America. As a possible reason, we discuss different intensity and dynamics of weather systems in both regions.     

Evaluation of COMPASS ionospheric model in GNSS positioning

As important products of GNSS navigation message, ionospheric delay model parameters are broadcasted for single-frequency users to improve their positioning accuracy. GPS provides daily Klobuchar ionospheric model parameters based on geomagnetic reference frame, while the regional satellite navigation system of China’s COMPASS broadcasts an eight-parameter ionospheric model, COMPASS Ionospheric Model(CIM), which was generated by processing data from continuous monitoring stations, with updating the parameters every 2 h. To evaluate its performance, CIM predictions are compared to ionospheric delay measurements, along with GPS positioning accuracy comparisons. Real observed data analysis indicates that CIM provides higher correction precision in middle-latitude regions, but relatively lower correction precision for low-latitude regions where the ionosphere has much higher variability. CIM errors for some users show a common bias for in-coming COMPASS signals from different satellites, and hence ionospheric model errors are somehow translated into the receivers’ clock error estimation. In addition, the CIM from the China regional monitoring network are further evaluated for global ionospheric corrections. Results show that in the Northern Hemisphere areas including Asia, Europe and North America, the three-dimensional positioning accuracy using the CIM for ionospheric delay corrections is improved by 7.8%–35.3% when compared to GPS single-frequency positioning ionospheric delay corrections using the Klobuchar model. However, the positioning accuracy in the Southern Hemisphere is degraded due apparently to the lack of monitoring stations there.     

Power variation analysis of echo signals from ionospheric reflectors

A series of Power Virtual Height measurements (PVH) of radio echoes reflected from the ionosphere were acquired at a given frequency during the period 3–22 January 2008 with the purpose of studying the slow fading variations through time of the ionospheric channel. To obtain PVH data, an ionospheric vertical sounding system was suitably adapted to work at a single fixed frequency. PVH measurements were recorded between two routine ionospheric vertical soundings, providing a data type that enables evaluation of fading fluctuation through time. The time stability of the ionospheric layers is determined by analyzing the level of the received signal power within a chosen threshold. In this paper the fading behaviour and its characteristics are described, considering only temporal periodicity above 0.5 s. In a further analysis a relation is demonstrated between the recorded fading and the time stability of the signal within a fixed interval of values.     

An improved approach to model ionospheric delays for single-frequency Precise Point Positioning

PPP with low-cost, single-frequency receivers has been receiving increasing interest in recent years because of its large amount of possible users. One crucial issue in single-frequency PPP is the mitigation of ionospheric delays which cannot be removed by combining observations on different frequencies. For this purpose, several approaches have been developed, such as, the approach using ionospheric model corrections with proper weight, the GRAPHIC (Group and Phase Ionosphere Calibration) approach, and the method to model ionospheric delays over a station with a low polynomial or stochastic process. From our investigation on the stochastic characteristics of the ionospheric delay over a station, it cannot be precisely represented by either a deterministic model in the form of a low-order polynomial or a stochastic process for each satellite, because of its strong irregular spatial and temporal variations. Therefore, a novel approach is developed accordingly in which the deterministic representation is further refined by a stochastic process for each satellite with an empirical model for its power density. Furthermore, ionospheric delay corrections from a constructed model using GNSS data are also included as pseudo-observations for a better solution. A large data set collected from about 200 IGS stations over one month in 2010 is processed with the new approach and several commonly adopted approaches for validation. The results show its significant improvements in terms of positioning accuracy and convergence time with a negligible extra processing time, which is also demonstrated by data collected with a low-cost, handheld, single-frequency receiver.