Ionospheric Tomography Technique Research Articles

An improved method using adaptive smoothing for GNSS tomographic imaging of ionosphere.

Global navigation satellite system (GNSS) is a well-established sensors in the recent ionosphere research. By comparing with classical meteorological equipments, the GNSS application can obtain more reliable and precious ionospheric total electron content (TEC) result. However, the most used GNSS ionospheric tomography technique is sensitive to a priori information due to the sparse and non-uniform distribution of GNSS stations. In this paper, we propose an improved method based on adaptive Laplacian smoothing and algebraic reconstruction technique (ALS-ART). Compared with traditional constant constraints, this method is less dependent on a priori information and adaptive smoothing constraints is closer to the actual situation. Tomography experiments using simulated data show that reconstruction accuracy of ionospheric electron density using ALS-ART method is significantly improved. We also use the method to do the analysis of real observation data and compare the tomography results with ionosonde observation data. The results demonstrate the superiority and reliability of the proposed method compared to traditional constant constraints method which will further improve the capability of obtaining precious ionosphere TEC by using GNSS.

A new parameterized approach for ionospheric tomography

GNSS ionospheric tomography technique is capable to reconstruct the high-quality 3D ionospheric electron density (IED) images with a relatively low cost. We present a new parameterized approach for refining the voxel-based ionospheric tomography modeling. This approach is different to most other voxel-based techniques as they assume a homogeneous IED distribution in each voxel that is unreasonable for the tomography modeling. In this method, IED of any point within a voxel is determined via vertically exponential interpolation and horizontally inverse distance weighted interpolation from the IED values at the eight corners of that voxel. The parameterized tomography is tested with real data collected over the period of June 1–30, 2015 from 45 GPS stations in south China. The superiority of the new parameterized method is verified by comparison with the traditional nonparametric method. The new parameterized method outperforms the traditional method by 12%, 10%, 5% and 2% for vertical resolutions of 25 km, 50 km, 75 km and 100 km, respectively, in the self-consistency validation by GPS data. Such improvements are 20%, 24%, 22% and 16%, respectively, when assessed by the Swarm in situ IEDs. In terms of the vertical layer discretization, configurations using the resolution of 25 km generally performs better than the other three vertical resolutions. Overall, the parameterized method using a vertical resolution of 25 km achieves the best performance from the comprehensive comparisons with ionospheric data derived by GPS, ionosonde and Swarm satellites.

Ionospheric tomography for the study of relative X‐ and O‐mode power over low‐ and mid‐latitude ionosphere

Realising the importance of the ionospheric effects on low Earth orbit (LEO) satellite signals, tomographic reconstruction method has been applied to study the relative X- and O-mode power distribution at an altitude of LEO satellites over the low- and mid-latitude regions of the Earth. Eight regions of the ionosphere in Northern and Southern Hemisphere have been reconstructed tomographically based on the Ionex TEC data and direct measurement TEC data over West Bengal, India and subsequently ray tracing of very high frequency waves has been performed. Then, based on the magnetoionic theory, the relative X-mode power distribution has been investigated. Evaluation of the results and observations related to the existence of X-mode only band and narrow O-mode only band ensures the possibility of application of ionospheric tomography technique for study of relative absorption in the ionosphere. The outcome of this research will greatly assists the researchers to interpret the ionospheric effects on radio waves.

Real-Time Ionosphere Monitoring by Three-Dimensional Tomography over Japan

This paper reports on the development of a real-time three-dimensional ionospheric tomography technique using GNSS measurements. GPS measurement data from 200 GPS receivers distributed throughout Japan are analyzed in real time to calculate ionospheric Total Electron Contents (TECs). The three-dimensional ionospheric electron density distribution was reconstructed using the tomography technique from the slant TEC data. The tomography results were found to be in good agreement with both the electron density profiles obtained by radio occultation measurements by the COSMIC satellite and the ionospheric peak densities obtained by vertical radio sounding measurements (ionosondes). The real-time ionospheric tomography system commenced operation in March 2016 and it continuously produces three-dimensional electron density distributions over Japan every 15 min with latency of about 6 min. This system is a very powerful tool for monitoring the ionosphere in real time, which could both improve the performance of GNSS-based systems and further the understanding of ionospheric variations.

Variable pixel size ionospheric tomography

A novel ionospheric tomography technique based on variable pixel size was developed for the tomographic reconstruction of the ionospheric electron density (IED) distribution. In variable pixel size computerized ionospheric tomography (VPSCIT) model, the IED distribution is parameterized by a decomposition of the lower and upper ionosphere with different pixel sizes. Thus, the lower and upper IED distribution may be very differently determined by the available data. The variable pixel size ionospheric tomography and constant pixel size tomography are similar in most other aspects. There are some differences between two kinds of models with constant and variable pixel size respectively, one is that the segments of GPS signal pay should be assigned to the different kinds of pixel in inversion; the other is smoothness constraint factor need to make the appropriate modified where the pixel change in size. For a real dataset, the variable pixel size method distinguishes different electron density distribution zones better than the constant pixel size method. Furthermore, it can be non-chided that when the effort is spent to identify the regions in a model with best data coverage. The variable pixel size method can not only greatly improve the efficiency of inversion, but also produce IED images with high fidelity which are the same as a used uniform pixel size method. In addition, variable pixel size tomography can reduce the underdetermined problem in an ill-posed inverse problem when the data coverage is irregular or less by adjusting quantitative proportion of pixels with different sizes. In comparison with constant pixel size tomography models, the variable pixel size ionospheric tomography technique achieved relatively good results in a numerical simulation. A careful validation of the reliability and superiority of variable pixel size ionospheric tomography was performed. Finally, according to the results of the statistical analysis and quantitative comparison, the proposed method offers an improvement of 8% compared with conventional constant pixel size tomography models in the forward modeling.

Research on ionospheric tomography based on variable pixel height

A novel ionospheric tomography technique based on variable pixel height was developed for the tomographic reconstruction of the ionospheric electron density distribution. The method considers the height of each pixel as an unknown variable, which is retrieved during the inversion process together with the electron density values. In contrast to conventional computerized ionospheric tomography (CIT), which parameterizes the model with a fixed pixel height, the variable-pixel-height computerized ionospheric tomography (VHCIT) model applies a disturbance to the height of each pixel. In comparison with conventional CIT models, the VHCIT technique achieved superior results in a numerical simulation. A careful validation of the reliability and superiority of VHCIT was performed. According to the results of the statistical analysis of the average root mean square errors, the proposed model offers an improvement by 15% compared with conventional CIT models.

Application of ionospheric tomography for high frequency oblique incidence ray tracing

<p>Recognizing the applicability of tomographic imaging in the ionosphere, the reconstruction method has been applied to generate more realistic low latitude ionospheric background based on the Ionex TEC data and also direct measurement TEC data over West Bengal, India. Using these backgrounds, ray tracing of high frequency waves has been performed. Influence of Earth’s magnetic field and also the existence of horizontal gradient of electron concentration in the ionosphere are considered separately for analyzing the ray tracing algorithm. Evaluation of ray tracing parameters and maximum usable frequency for different ground range ensures the possibility of application of ionospheric tomography technique for accurate ray tracing. The analysis shows accurate ray tracing for HF waves, thus defeating the restriction of choice of ionospheric model for analytical ray tracing methods. Performance analysis of the ray tracing technique presented in this paper ensures the consistency of speed with the other methods.</p>

Three‐dimensional GPS ionospheric tomography over Japan using constrained least squares

AbstractA new three‐dimensional GPS ionospheric tomography technique is developed that uses total electron content (TEC) data from the dense Global Position System (GPS) receiver network, GPS Earth Observation Network (GEONET) in Japan, and it will not require an ionospheric model as the initial guess that will bias the reconstruction of electron density. The GEONET is operated by Geospatial Information Authority of Japan and consists of more than 1200 receivers; this high density and wide coverage helps to reconstruct the electron density distribution in the ionosphere with high spatial resolution. This tomography technique uses a constrained least squares fit to reconstruct the three‐dimensional electron density distributions. This method is different to most other techniques as they require a background ionospheric model as an initial guess that could bias the reconstructed electron density. It rather uses a prior condition that the electron density should not exceed a certain value that is determined by the restrain parameter, which is derived from the NeQuick model. Its independency of the initial guess from a model will make it useful even in disturbed conditions. This paper presents results that are obtained by using this new tomographic technique. The reconstruction of three‐dimensional ionospheric tomograms is demonstrated using the GPS data, and the reliability and robustness are checked with simulated tomograms obtained using the synthetic GPS‐TEC data produced using NeQuick model.

Grid-based representation and dynamic visualization of ionospheric tomography

Abstract. The ionosphere is a dynamic system with complex structures. With the development of abundant global navigation satellite systems, the ionospheric electron density in different altitudes and its time variations can be obtained by ionospheric tomography technique using GNSS observations collected by the continuously operating GNSS tracking stations distributed over globe. However, it is difficult to represent and analyze global and local ionospheric electron density variations in three-dimensional (3D) space due to its complex structures. In this paper, we introduce a grid-based system to overcome this constraint. First, we give the principles, algorithms and procedures of GNSS-based ionospheric tomography technique. Then, the earth system spatial grid (ESSG) based on the spheroid degenerated octree grid (SDOG) is introduced in detail. Finally, more than 400 continuously operating GNSS receivers from the International GNSS Service are utilized to realize global ionospheric tomography, and then the ESSG is used to organize and express the tomography results in 4D, including 3 spatial dimensions and time.

Some features of the equatorial anomaly revealed by ionospheric tomography

The equatorial anomaly in ionization density has been imaged using the computerized ionospheric tomography technique applied to data from a low‐latitude ionospheric tomography network. Examples of images representative of typical conditions during equinox and low solar flux are presented and shown to exhibit some characteristic features which have not been observed directly previously. The EA core, comprising the highest density region of the EA, is shown to exhibit a characteristic structure and asymmetry. These characteristics are quantified using measures of the ionospheric slab thickness and altitude, and they are discussed in light of the fountain mechanism which is responsible for formation of the EA.

Estimating dynamic ionospheric changes without a priori models

We present a novel artificial‐neural‐network‐based computerized ionospheric tomography technique, capable of imaging through time in three‐dimensional space. Total electron content (TEC) data collected from a satellite passing over the region of interest are used to train a neural network. The trained network creates estimates of the difference in ionospheric electron density between time steps based on the TEC data. Application of the difference estimate at each time step to the previous electron density image results in a time‐varying ionospheric electron density estimate. Experimental results on synthetic data are presented that demonstrate that the algorithm is capable of detecting short‐term localized disturbances in the ionosphere.