Computerized ionospheric tomography with the IRI model

Abstract 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

GIM-TEC adaptive ionospheric weather assessment and forecast system

The Ionospheric Weather Assessment and Forecast (IWAF) system is a computer software package designed to assess and predict the world-wide representation of 3-D electron density profiles from the Global Ionospheric Maps of Total Electron Content (GIM-TEC). The unique system products include daily-hourly numerical global maps of the F2 layer critical frequency (foF2) and the peak height (hmF2) generated with the International Reference Ionosphere extended to the plasmasphere, IRI-Plas, upgraded by importing the daily-hourly GIM-TEC as a new model driving parameter. Since GIM-TEC maps are provided with 1- or 2-days latency, the global maps forecast for 1 day and 2 days ahead are derived using an harmonic analysis applied to the temporal changes of TEC, foF2 and hmF2 at 5112 grid points of a map encapsulated in IONEX format (-87.5°:2.5°:87.5°N in latitude, -180°:5°:180°E in longitude). The system provides online the ionospheric disturbance warnings in the global W-index map establishing categories of the ionospheric weather from the quiet state (W=±1) to intense storm (W=±4) according to the thresholds set for instant TEC perturbations regarding quiet reference median for the preceding 7 days. The accuracy of IWAF system predictions of TEC, foF2 and hmF2 maps is superior to the standard persistence model with prediction equal to the most recent ‘true’ map. The paper presents outcomes of the new service expressed by the global ionospheric foF2, hmF2 and W-index maps demonstrating the process of origin and propagation of positive and negative ionosphere disturbances in space and time and their forecast under different scenarios.Peer ReviewedPostprint (author's final draft

Investigation of Hourly and Daily Patterns for Lithosphere-Ionosphere Coupling Before Strong Earthquakes

The ionosphere can be characterized with its electron density distribution which is a complex function of spatial and temporal variations, geomagnetic, solar and seismic activity. An important measurable quantity about the electron density is the Total Electron Content (TEC) which is proportional to the total number of electrons on a line crossing the atmosphere. TEC measurements enable monitoring variations in the space weather. Global Positioning System (GPS) and the network of world-wide receivers provide a cost-effective solution in estimating TEC over a significant proportion of global land mass. In this study, five earthquakes between 2003-2008 that occurred in Japan with different seismic properties, and the China earthquake in play 2008 are investigated. The TEC data set is investigated by using the Kullback-Leibler Divergence (KLI), Kullback-Leibler Distance (KLD) and L2-Norm (L2N) which are used for the first time in the literature in this context and Cross Correlation Function (CCF) which is used in the literature before for quiet day period (QDP), disturbed day period (DDP), periods of IS days before a strong earthquake (BE) and after the earthquake (AE). In summary, it is observed that the CCF, KLD and L2N between the neighbouring GPS stations cannot be used as a definitive earthquake precursor due to the complicated nature of earthquakes and various uncontrolled parameters that effect the behavior of TEC such as distance to the earthquake epicenter, distance between the stations, depth of the earthquake, strength of the earthquake and tectonic structure of the earthquake. KLD, KLI and L2N are used for the first time in literature for the investigation of earthquake precursor for the first time in literature and the extensive study results indicate that for more reliable estimates further space-time TEC analysis is necessary over a denser GPS network in the earthquake zones.Wo

Nvis Hf Signal Propagation In Ionosphere Using Calculus Of Variations

Modeling Near Vertical Incidence Sounding (NVIS) High Frequency (HF) signal propagation in the ionosphere is important. Because, ionosondes which are special types of radars probing the ionosphere with certain HF frequencies (between 2 and 30 MHz), work mostly in NVIS mode (where elevation angle is between 89 and 90 degrees). In this work, we are going to propose a new method for NVIS wave propagation in the ionosphere by discretizing the NVIS wave propagation path into mediums in which the refractive index changes linearly, where we solve the ray propagation in each medium analytically using calculus of variations and use Snell's Law at medium changes. The main advantage of the proposed solution is the reduced computational complexity and time. This algorithm can be used to simulate and compare the behavior of vertical ionosondes together with other ray tracing algorithms. (C) 2018 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.WoSScopu

Lonolab-Ray: A Wave Propagation Algorithm for Anisotropic and Inhomogeneous Ionosphere

A high-frequency (HF) radio wave propagation model based on ray tracing for anisotropic, inhomogeneous, and time-varying ionosphere is developed to form the basis for a user-friendly modular algorithm. The algorithm IONOLAB-RAY applies the principles of ray tracing through the spherical 3D grid model of the ionosphere. The physical parameters of the ionosphere in each voxel are obtained from the ionosphere model IRI-Plas-G. The refractive index of each voxel is calculated with the Appleton-Hartree formula, which includes electron cyclotron frequency, collision frequency, and anisotropicity due to the geomagnetic field. The developed propagation algorithm allows the input of total electron content (TEC) values to update the ionospheric model to the current conditions. This capability has an important impact on the characterization of the ionosphere under geomagnetic storm conditions. The algorithm can be further developed to represent typical HF transmitter and receiver antenna properties and to calculate the electric field strength, time delay, and polarization rotation at the receiver location.WoSScopu

Regional Ionospheric Trend Statistics: Ionolab-Pdf

The ionosphere plays an important role for HF bands, space-based positioning systems, satellite communication, and propagation. The variability of the ionosphere has a complex spatiotemporal characteristic, which depends on statistical parameters. Total electron content (TEC) is one of the major observables for investigating and determining this variability. In this study, spatiotemporal within-the-hour statistical characteristics of TEC are determined for Turkey, which is located in midlatitude, using the TEC estimates from the Turkish National Permanent GPS Network Active between the years 2009 and 2012. The TEC values have strong hourly, seasonal, and positional dependence on east-west direction, and the trend shifts according to sunrise and sunset hours. It is observed that TEC is distributed predominantly as lognormal and Weibull probability density functions (pdf). Within-the-hour pdf estimates are grouped into ionospheric seasons such as March equinox, summer, winter, and September equinox. In winter and summer seasons lognormal and during equinox seasons Weibull distributions are observed more frequently. For spatial analysis, all TEC values within the same hour and in the same region are combined in order to improve the reliability and accuracy of pdf estimates. Statistical characterization of TEC over Turkey will contribute to developing a regional and seasonal random field model, which will be used in HF channel characterization and space weather risk analysis.WoSScopu

Regularized Estimation of Tec from Gps Data for Certain Midlatitude Stations and Comparison with the Iri Model

Regularized estimation of Total Electron Content (Reg-Est) is a novel technique which can combine signals from all the satellites for a given instant and given station and estimate the vertical TEC (VTEC) values for any desired period without missing any important features in the temporal or spatial domain. The preprocessed signals from all the satellites that are received for a certain time period are weighted according to their positions with respect to the local zenith. A two step regularization algorithm combines these signals and provides smooth VTEC estimates for the desired time period which can be as short as half an hour or as long as 24 h. The estimation algorithm is tried on VTEC values obtained from six midlatitude stations for the quiet and disturbed days of October, 2003. Within this period, the same estimation parameter set is used for all stations and time periods. When the regularized estimation results are compared with those from IRI-2001, JPL, CODE, UPC and ESA, best accordance is observed with JPL, UPC and CODE estimates. IRI computations usually provide a better fit for the night values. It is observed that the results from the regularized estimation algorithm are highly accurate in detecting disturbances and irregularities for various time scales and stations. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.Wo

Estimation Of Hmf2 And Fof2 Communication Parameters Of Ionosphere F2-Layer Using Gps Data And Iri-Plas Model

F2-layer is the most important and characteristic layer of the ionosphere in the propagation of high frequency (HF) waves due to the highest level of conductivity in the propagation path. In this study, the relation of Total Electron Content (TEC) with the maximum ionization height (hmF2) and the critical frequency (foF2) of F2-layer are investigated within their defined parametric range using the IRI model extended towards the plasmasphere (IRI-Plas). These two parameters are optimized using daily observed GPS-TEC (IONOLAB-TEC) in an iterational loop through Non-Linear Least Squares (NLSQ) optimization while keeping the physical correlation between hmF2 and foF2 parameters. Optimization performance is examined for daily (24-hour) and hourly TEC optimizations separately. It is observed that hourly TEC optimization produces results with much smaller estimation errors. As a result of the hourly optimization, we obtain the hourly hmF2 and foF2 estimates as they are the optimization parameters. Obtained hmF2 and foF2 estimates are compared with the ionosonde estimates for various low, middle and high latitude locations for both quite and disturbed days of ionosphere. The results show that hmF2 and foF2 estimates obtained from IRI-Plas optimization (IRI-Plas-Opt) and ionosonde are very much in agreement with each other. These results also signify that IRI-Plas provides a reliable background model for ionosphere. With the proposed method, it is possible to build a virtual ionosonde via optimization of IRI-Plas model using the observed TEC values.Wo

IONOLAB-Fusion: Fusion of Radio Occultation into Computerized Ionospheric Tomography

In this study, a 4-D, computerized ionospheric tomography algorithm, IONOLAB-Fusion, is developed to reconstruct electron density using both actual and virtual vertical and horizontal paths for all ionospheric states. The user-friendly algorithm only requires the coordinates of the region of interest and range with the desired spatio-temporal resolutions. The model ionosphere is formed using spherical voxels in a lexicographical order so that a 4-D ionosphere can be mapped to a 2-D matrix. The model matrix is formed automatically using a background ionospheric model with an optimized retrospective or near-real time manner. The singular value decomposition is applied to extract a subset of significant singular values and corresponding signal subspace basis vectors. The measurement vector is filled automatically with the optimized number of ground-based and space-based paths. The reconstruction is obtained in closed form in the least squares sense. When the performance of IONOLAB-Fusion across Europe was compared with ionosonde profiles, a 26.51% and 32.33% improvement was observed over the background ionospheric model for quiet and disturbed days, respectively. When compared with GIM-TEC, the agreement of IONOLAB-Fusion was 37.89% and 31.58% better than those achieved with the background model for quiet and disturbed days, respectively

Lonolab-Map: An Automatic Spatial Interpolation Algorithm for Total Electron Content

Investigation of the variability of total electron content (TEC) is one of the most important parameters of the observation and monitoring of space weather, which is the main cause of signal disturbance in space-based communication, positioning, and navigation systems. TEC is defined as the total number of electrons on a ray path. The Global Positioning System (GPS) provides a cost-effective solution for the estimation of TEC. Due to various physical and operational disturbances, TEC may have temporal and spatial domain gaps. Global ionospheric maps (GIMs) provide worldwide TEC with 1-to 2-h temporal resolution and 2:5 degrees x 5 degrees spatial resolution in latitude and longitude, respectively. The GIM-TEC with the highest possible accuracy can be obtained 10 days after the recording of the signals. Therefore, a high-resolution and accurate interpolation of TEC is necessary to image and monitor the regional distribution of TEC in near-real time. In this study, a novel spatiotemporal interpolation algorithm with automatic gridding is developed for 2-D TEC imaging by data fusion of GPS-TEC and GIM-TEC. The algorithm automatically implements optimum spatial resolution and desired temporal resolution with universal kriging with linear trend for midlatitude regions and ordinary kriging for other regions. The theoretical semivariogram function is estimated from GPS network data using a Matern family, whose parameters are determined with a particle swarm optimization algorithm. The developed algorithm is applied to the Turkish National Permanent GPS Network (TNPGN-Active), a dense midlatitude GPS network. For the first time in the literature, high spatial resolution TEC maps are obtained between May 2009 and May 2012 with a 2.5min temporal update period. These TEC maps will be used to investigate the spatiotemporal variability of the ionosphere over the diurnal and annual trend structure, including seasonal anomalies and geomagnetic and seismic disturbances over ionosphere.WoSScopu