I. Sayin

Synthetic TEC Mapping with Ordinary and Universal Kriging

Spatiotemporal variations in the ionosphere affects the HF and satellite communications and navigation systems. Total Electron Content (TEC) is an important parameter since it can be used to analyze the spatial and temporal variability of the ionosphere. In this study, the performance of the two widely used Kriging algorithms, namely Ordinary Kriging (OrK) and Universal Kriging (UnK), is compared over the synthetic data set. In order to represent various ionospheric states, such as quiet and disturbed days, spatially correlated residual synthetic TEC data with different variances is generated and added to trend functions. Synthetic data sampled with various type of sampling patterns and for a wide range of sampling point numbers. It is observed that for small sampling numbers and with higher variability, OrK gives smaller errors. As the sample number increases, UnK errors decrease faster. For smaller variances in the synthetic surfaces, UnK gives better results. For increasing variance and decreasing range values, usually, the errors increase for both OrK and UnK.

Investigation of optimum temporal update periods for regional TEC monitoring

Monitoring of the ionospheric variability is necessary for improving the performance of communication, navigation and positioning systems. Total Electron Content (TEC) is an important parameter since it can be used to analyze the variability of the ionosphere. Due to sparse distribution of samples, TEC can not be directly obtained everywhere on the ionosphere and accurate interpolation methods are needed for generating TEC maps. The optimum temporal update period (TUP) for regional Total Electron Content (TEC) monitoring has to be determined for capturing significant variations. In this study, the wide sense stationary (WSS) period is investigated whether it is possible to use the estimated WSS periods as the TUPs of the regional TEC maps. The sliding window statistical analysis method is used to estimate the WSS period. The WSS periods are compared with the difference between the maps obtained by Ordinary Kriging (OK) method. It is observed that the maximum WSS periods are following the length of time delays between the TEC maps at which the first significant difference occurs. The minimum WSS periods have to be used if the small variations are needed to be monitored within the day of interest. The WSS periods can be used as the TUPs for the regional TEC monitoring and can be adaptively adjusted.

Synthetic TEC Mapping with Kriging and Random Field Priors

Total electron content (TEC) can be used for analyzing the variability of the ionosphere in space and time. In this study, spatial interpolation is implemented by Kriging and random field priors (RFP), which are widely used in geostatistics. Performance of Kriging and RFP methods are analyzed on synthetic TEC data for different trend functions, sampling patterns, sampling numbers, variance and range values of covariance function which is used to simulate the synthetic data, by comparing the normalized errors of interpolations. In regular sampling patterns, as opposed to random sampling, the normalized average error is very close to each other for all methods and trend assumptions. The error increases with variance and decreases with range. As the number of samples increase, the normalized error also decreases.

Web based IONOLAB TEC Estimation

Because of the effects on radio waves, monitoring the ionosphere gains significant importance. Monitoring of the ionosphere becomes possible by estimating Total Electron Content (TEC) which is the major variable expressing ionospheric effects. Although International Reference Ionosphere (IRI) and International GNSS Service (IGS) analysis centers can generate TEC estimates, these estimates either have low temporal resolution or are based on emprical data. IONOLAB (www.ionolab.org) TEC estimation method provides robust estimations with high temporal resolution compared to IGS analysis centers. In this work, robust TEC estimations with high temporal resolution generated by IONOLAB method are presented by an easily accessible, user-friendly application. The application has a layered modular design. By means of this modular design, possible changes regarding the estimation method can be easily adapted.

Automatic TEC mapping using a GPS network and GIM-TEC

Ionosphere is an important layer of atmosphere, extending from 60 km to 1000 km altitude. The primary source of ionization is solar radiation. Yet, various factors from geomagnetic field to atmospheric structure complicate the ionization and recombination processes. Ionosphere has a spatio-temporal variable structure which is also, inhomogeneous, anisotropic, and dispersive. In order to determine the effects of the ionosphere over the HF and satellite communication and space-based positioning signals, the structure of the ionosphere must be understood and the variability of the ionosphere must be continuously monitored. Total Electron Content (TEC) is one of the most important observables for monitoring Space Weather. Global Positioning System (GPS) Networks provide cost-effective solution for estimation of TEC. Due to various physical or operational disturbances, TEC data which is estimated from GPS may have gaps in space and time. In order to obtain regular and dense TEC values in a given region, TEC can be interpolated spatially which is called as TEC map. A widely used source of TEC maps can be obtained as Global Ionosphere Maps (GIM) from International GNSS (Global Navigation Satellite Systems) Service (IGS) website (ftp://igscb.jpl.nasa.gov). The spatial resolution of GIM maps are 2.5° in latitude and 5° in longitude, and typical temporal resolution of GIM is 2 h. In this study, an automatic spatial interpolation algorithm is developed for TEC maps using combination of a GPS Network and GIM-TEC. With the developed algorithm, high space and time resolution maps can be obtained automatically. The developed technique is applied to a midlatitude regional GPS Network, namely, Turkish National Permanent GPS Network (TNPGN-Active). At the first step, GPS-TEC values are estimated as IONOLAB-TEC (www.ionolab.org) and the missing TEC data are interpolated with spatial interpolation method from GIM-TEC. The algorithm utilizes two different Kriging algorithms, isotropic Universal Kriging with linear trend for midlatitude regions and Ordinary Kriging for other regions. The theoretical semivariogram function is chosen to be from the Matern Family which includes most of the other semivariogram functions and the Matern parameters are estimated using Particle Swarm Optimization (PSO). Using the proposed automatic TEC mapping method with ingestion of GIM-TEC is applied to TNPGN-Active and high spatio-temporal resolution TEC maps are obtained between May 2009 and May 2012 with 2.5 minute temporal update period, and 0.25° to 0.3° spatial resolution, in latitude and longitude, respectively.

Computation of Fresnel integral by Fractional Fourier Transform methods

Fractional Fourier Transform (FrFT) is the generalization of the ordinary Fourier Transform (FT) and a subclass of the Linear Canonical Transform (LCT). FrFT is used in many fields, where ordinary FT has already found applications, such as signal processing, optics, telecommunications, noise filtering, beamforming, and solution of differential equations.

Application of Fractional Fourier Transform to finite difference time domain method

With the improvement in the computer speed and memory, Numerical Methods are frequently used in the solution of electromagnetic problems. Numerical Methods can be classified as the frequency domain and the time domain based methods. While the time domain methods are suitable for modeling of the transient response and wideband problems, the frequency domain methods are suitable for modeling of the steady state response and narrow band problems. A numerical method that has the advantages of both time and frequency domain approaches can be developed. Applying Fractional Fourier Transform in space and/or time can reduce the computational complexity for some cases. The Fractional Fourier Transform is a generalization of the continuous Fourier Transform. In last decades, there are several studies and applications concerning this transform. Generally, it is used in signal processing and noise filtering. In this study, Fractional Fourier Transform is applied to the Maxwells Equations for the first time in literature. Finite difference equations are obtained by the application of finite difference approximation to the differential equations.

A study of Lithosphere-ionosphere coupling using TUSAGA active TEC estimates

In this study, the disturbances in the ionosphere due to the seismic activity are investigated by using Total Electron Content estimates obtained from TUSAGA Active GPS stations in Turkey. Two earthquakes with same geophysical properties occurred on Northern Anatolia Fault are chosen for the study. TEC estimates are compared with each other using correlation coefficient (IK), symmetric Kullback-Leibler Distance (KLD) and L2-Norm (L2N) for geomagnetically and seismically quiet days of ionosphere and the earthquakes days. It is observed that IK values of quiet days are highly correlated in quiet days. IK values of earthquake days decrease down to 0.2 in earthquake days. KLD values of earthquake days are 10 times greater than those of the quiet days. In order to form a proper earthquake precursor alarm signal, more earthquakes with different properties have to be investigated in the future.