Oblique Ionograms Research Articles

NOIRE-Net–a convolutional neural network for automatic classification and scaling of high-latitude ionograms

Millions of ionograms are acquired annually to monitor the ionosphere. The accumulated data contain untapped information from a range of locations, multiple solar cycles, and various geomagnetic conditions. In this study, we propose the application of deep convolutional neural networks to automatically classify and scale high-latitude ionograms. A supervised approach is implemented and the networks are trained and tested using manually analyzed oblique ionograms acquired at a receiver station located in Skibotn, Norway. The classification routine categorizes the observations based on the presence or absence of E− and F-region traces, while the scaling procedure automatically defines the E− and F-region virtual distances and maximum plasma frequencies. Overall, we conclude that deep convolutional neural networks are suitable for automatic processing of ionograms, even under auroral conditions. The networks achieve an average classification accuracy of 93% ± 4% for the E-region and 86% ± 7% for the F-region. In addition, the networks obtain scientifically useful scaling parameters with median absolute deviation values of 118 kHz ±27 kHz for the E-region maximum frequency and 105 kHz ±37 kHz for the F-region maximum O-mode frequency. Predictions of the virtual distance for the E− and F-region yield median distance deviation values of 6.1 km ± 1.7 km and 8.3 km ± 2.3 km, respectively. The developed networks may facilitate EISCAT 3D and other instruments in Fennoscandia by automatic cataloging and scaling of salient ionospheric features. This data can be used to study both long-term ionospheric trends and more transient ionospheric features, such as traveling ionospheric disturbances.

An Updating of the IONORT Tool to Perform a High-Frequency Ionospheric Ray Tracing

This paper describes the main updates characterizing the new version of IONORT (IONOsperic Ray Tracing), a software tool developed at Istituto Nazionale di Geofisica e Vulcanologia to determine both the path of a high frequency (HF) radio wave propagating in the ionospheric medium, and the group time delay of the wave itself along the path. One of the main changes concerns the replacement of a regional three-dimensional electron density matrix, which was previously taken as input to represent the ionosphere, with a global one. Therefore, it is now possible to carry out different ray tracings from whatever point of the Earth’s surface, simply by selecting suitable loop cycles thanks to the new ray tracing graphical user interface (GUI). At the same time, thanks to a homing GUI, it is also possible to generate synthetic oblique ionograms for whatever radio link chosen by the user. Both ray tracing and homing GUIs will be described in detail providing at the same time some practical examples of their use for different regions. IONORT software finds practical application in the planning of HF radio links, exploiting the sky wave, through an accurate and thorough knowledge of the ionospheric medium. HF radio waves users, including broadcasting and civil aviation, would benefit from the use of the IONORT software (version 2023.10).

Statistical and simulation study on the separation in junction frequencies between ordinary (O) and extraordinary (X) wave in oblique ionograms

The most important aim in interpreting an oblique ionogram is to obtain the accurate Junction Frequencies (JFs) of the ordinary (O) and extraordinary (X) mode. This requires the correct identification of O- and X-mode traces, so it is very helpful and worthy to grasp the relative position between the two modes. This paper presents a statistical and simulation study of the separation in JFs between O- and X-waves based on observed oblique ionograms over three mid-latitude paths within China and a 3D ray-tracing program. The dependence on local time, season, geomagnetic activity, O-wave JF and group path, solar activity, direction, and length of propagation is investigated. The main conclusions are as follows: (a) the separation on east–west path is susceptible to ionospheric variability, while the separation on north–south path does not show a significant correlation with local time and season; (b) a general diurnal tendency and a summer anomaly on east–west propagation are first proposed and discussed, which may be related to the diurnal variation of hmF2 above the reflection point and the strong lower layers below the reflection point; (c) the separation varies approximately as a cosine function with the propagation direction owning two maxima in the north–south direction and two minima in the east–west direction; (d) the variation patterns of the separation with the propagation length are obviously not the same in different directions. In the case of east–west propagation, the separation decreases to a minimum near ground range of 2000 km and then increases very slowly with increasing ground range, while it monotonically increases for the north–south propagation path.Graphical

Variable-step 3D numerical ray-tracing method for simulating oblique ionogram via inversion analysis

Oblique ionogram inversion is an important tool for understanding the ionosphere. Vertical plasma density profiles are the most common inversion results for the majority of the inversion algorithms. However, the inversion of the dynasonde ionogram provides not only the vertical plasma density profile, but also two tilts profiles directly related to the horizontal gradients of the plasma density. The main reason is that the dynasonde ionogram inversion uses a 3D numerical ray-tracing technique that considers the geomagnetic field and the horizontal inhomogeneity of the ionosphere. The theory and the ray-tracing performance of the 3D numerical ray-tracing method are presented. The ray-tracing method uses a variable-step strategy to speed up the simulation. And three plasma density models with traveling ionospheric disturbances and small-scale irregularities and without them are considered when using the presented ray-tracing method to simulate oblique ionograms. The simulation results show that the ionosphere disturbances can eventually be embodied in the simulated oblique ionograms.

A parallel optimization 3D numerical ray-tracing method for the fast and accurate simulation of disturbed oblique ionogram

A novel parallel optimization 3D numerical ray-tracing method is proposed in this paper. The method takes the geomagnetic field and the horizontal in-homogeneity of the ionosphere into account. It works in both O-mode and X-mode. High-frequency radiowaves and oblique ionograms are simulated by the method under three different plasma density models with travelling ionospheric disturbances and small-scale irregularities and without them. Both the parallel optimization algorithm (MultiStart) and the parallel traversal algorithm are used during the simulation, which allows multiple rays to be traced simultaneously to shorten the simulation time. The simulation results show the proposed 3D numerical ray-tracing method can sensitively and accurately capture the changes that geomagnetic field, travelling ionospheric disturbances, and small-scale irregularities bring to the high-frequency radiowaves and the oblique ionograms. It is a useful tool for restoring different oblique ionogram characteristics under various parameters on the plasma density models.

Ionogram Echo Extraction Based on the Convolutional Neural Networks

AbstractThe extraction of ionospheric echo plays a key role in the research of radio wave propagation theory and is the basis of the design, application, and test of the radio engineering system. However, it is too complicated to achieve because of several negative factors, such as environmental noise, thermal noise, ionospheric time‐varying dispersion characteristics, and so on. In this paper, a novel ionospheric echoes extraction model, referred to as IECAENet, is proposed based on the convolutional neural networks. First, VGGNet is introduced as the classification module to extract echoes from different kinds of ionograms, including vertical ionograms, oblique ionograms, and backscatter ionograms. Then, the echoes extraction module is designed. It considers not only the ionogram features but also an explicit hint, named echo mask, that expresses the location of the echoes. Finally, the residual learning technology and skip connection structure are introduced to improve the performance of the module. Experimental results on a real data set indicate that IECAENet outperforms the baselines for the ionospheric echoes extraction. Using the false alarm probability that IECAENet has an optimal performance as the benchmark, the detection probabilities of the vertical ionogram, oblique ionogram, and backscatter ionogram are improved by 22.18%, 22.56%, and 6.67%, respectively, compared with the traditional methods.

A Method for Automatic Inversion of Oblique Ionograms

In this study, a method is proposed to carry out automatic inversion of oblique ionograms to extract the parameters and electron density profile of the ionosphere. The proposed method adopts the quasi-parabolic segments (QPS) model to represent the ionosphere. Firstly, numerous candidate electron density profiles and corresponding vertical traces were, respectively, calculated and synthesized by adjusting the parameters of the QPS model. Then, the candidate vertical traces were transformed to oblique traces by the secant theorem and Martyn’s equivalent path theorem. On the other hand, image processing technology and characteristics of oblique echoes were adopted to automatically scale the key parameters (the maximum observable frequency and minimum group path, etc.) from oblique ionograms. The synthesized oblique traces, whose parameters were close to autoscaled parameters, were selected as the candidate traces to produce a correlation with measured oblique ionograms. Lastly, the proposed algorithm searched the best-fit synthesized oblique trace by comparing the synthesized traces with oblique ionograms. To test its feasibility, oblique ionograms were automatically scaled by the proposed method and these autoscaled parameters were compared with manual scaling results. The preliminary results show that the accuracy of autoscaled maximum observable frequency and minimum group path of the ordinary trace of the F2 layer is, respectively, about 91.98% and 86.41%, which might be accurate enough for space weather specifications. It inspires us to improve the proposed method in future studies.

Characteristics of Anomalous Radio Propagation before and after the 2011 Tohoku-Oki Earthquake as Seen by Oblique Ionograms

The characteristics of anomalous radio propagation in the frequency 1 - 30 MHz before and after the 2011 Tohoku-Oki earthquake are firstly revealed by using oblique ionograms for the pass from Wakkanai to Kokubunji which is closest to the epicenter. An oblique ionogram with a wavy-shape-trace was observed at 06:30 UTC on 11 March 2011 after the 2011 Tohoku-Oki earthquake. The velocity of northward-propagating disturbance caused this wavy-shape-trace is estimated to be 130 m/s. This wave-shape-trace shows very clear signature appearing in the oblique ionograms as the characteristic of strong ionospheric disturbances triggered by the earthquake. An oblique ionogram with a steep slopy-shape-trace was observed at 04:45 UTC on 11 March 2011 one hour before the 2011 Tohoku-Oki earthquake. This slopy-shape-trace is investigated as the signatures of preseismic ionospheric anomaly. This anomalous oblique ionogram with a slopy-shape-trace is examined with the slope ratio of virtual height to sweep-frequency, and the difference between monthly median foF2 and hourly value foF2 at Wakkanai and Kokubunji. These features appearing in oblique ionograms suggest that it is useful for studying the signatures of preseismic ionospheric anomaly.

Generalized Force Approach to Point-to-Point Ionospheric Ray Tracing and Systematic Identification of High and Low Rays

A variant of the direct optimization method for point-to-point ionospheric ray tracing is presented. The method is well suited for applications where the launch direction of the radio wave ray is unknown, but the position of the receiver is specified instead. Iterative transformation of a candidate path to the sought-for ray is guided by a generalized force, where the definition of the force depends on the ray type. For high rays, the negative gradient of the optical path functional is used. For low rays, the transformation of the gradient is applied, converting the neighborhood of a saddle point to that of a local minimum. Knowledge about the character of the rays is used to establish a scheme for systematic identification of all relevant rays between the given points, without the need to provide an accurate initial estimate for each solution. Various applications of the method to isotropic ionosphere demonstrate its ability to resolve complex ray configurations including 3-D propagation and multi-path propagation where rays are close in the launch direction. Results of the application of the method to ray tracing between Khabarovsk and Tory show good quantitative agreement with the measured oblique ionograms.

Modeling the HF Ray Trajectories and Vertical and Oblique Ionograms in the Artificially Disturbed Ionosphere Based on Radiotomographic Data

This paper presents the results of numerical simulation of the propagation of HF radio waves in the artificially disturbed ionosphere during launch of the Tsyklon-3 carrier rocket from the Plesetsk cosmodrome on December 18, 1991, and an industrial high-power explosion with a magnitude of M = 2.4 on the Kola Peninsula on April 8, 1990. A specially developed software package in Python programming language allows three-dimensional modeling of ray trajectories in the geometric optics approximation by solving a bicharacteristic system for the eikonal equation. The geomagnetic field is specified according to the IGRF model, and the ionospheric electron density distribution is specified according to radio tomography data. The ionogram modeling is performed using a three-dimensional shooting method based on the Nelder—Mead simplex minimization. It is shown that the artificial ionospheric irregularities that arise during the periods of the considered events are quasi-wave perturbations that have a significant effect on the ray trajectories of HF radio waves and ionograms of vertical and oblique sounding. This effect manifests itself in the form of characteristic peculiarities of ray trajectories with multiple reflections from the local maxima of the electron density and additional traces on ionograms caused by the passage of traveling ionospheric disturbances above the sounding system.

Passive ionospheric radar builds with USRP N210

Abstract This paper presents first results and test of passive detection of Digisonde signal obtained at the Průhonice station, Czech Republic. Described technique can be used to analyze signals transmitted by the Digisonde and further received by simple loop antenna. The receiving system is based on the software defined radio (SDR USRP N210). The device works as the two channels receiver. This method will be employed to record oblique ionograms at several locations in the Czech Republic forming a simple array for detection of fine structures within ionospheric plasma using common DPS 4D transmitter. Using relatively inexpensive passive detection helps to increase a number of reflection points which leads to a better spatial coverage of the ionosphere sounding. The device represents a complementary measurement for detection of signal transmitted by Digisondes in European sector.

A Novel Ionospheric Sounding Network Based on Complete Complementary Code and Its Application.

In this paper, complete complementary code (CCC) sequences are applied to a High Frequency (HF) ionospheric sounding network. Ionosondes distributed at multiple locations use the mutually orthogonal CCC sequences to conduct vertical soundings synchronously. At the same time, thanks to the omnidirectional antennas, every station can receive the oblique echoes transmitted from the others. Due to the orthogonality between the code sequences, both vertical and oblique ionograms can be simultaneously obtained and completely separated. Through this method, the sounding efficiency can be enhanced, and the inversion difficulty can be reduced. Further, by using the data assimilation method, vertical and oblique sounding results can be combined to obtain a wide range of regional ionospheric characteristics. To verify the performance of this kind of sounding network, validation experiments are implemented to demonstrate that vertical and oblique ionograms can be obtained independently at the same time and inverted separately and that the maps of foF2 parameters obtained by using the data assimilation method provide more details than single vertical or oblique sounding.

Use of Hazard Function for Signal Detection on Ionograms

This letter considers a new method for sample detection of ionospheric propagation modes at the receiver output of a chirp ionosonde. The method is based on the use of the hazard function. Within the framework of the two-component mixture model, it is shown that the hazard function can be used to detect samples of the second component in the mixture when the corresponding fraction of samples in the total number is small. The implementation of the method is carried out using training sets. An average normalized hazard function is constructed for the training set not containing the signal. Normalization is carried out in such a way that the hazard function can be considered as a probability density function of a certain random variable. We used Pearson’s statistic to test the agreement between the average normalized hazard function of samples that do not contain the signal and an arbitrary normalized hazard function. The critical value of the statistic was determined using the Neyman–Pearson criterion. The effectiveness of the proposed method has been tested for the determination of the lowest and maximum observed frequencies of radio paths from oblique ionospheric sounding ionograms obtained in the Russian network of circumpolar radio paths. The method suggested in this letter can be used not only when processing ionograms but also for the detection of samples of signals of arbitrary nature against a background noise, provided that statistical signal-to-noise ratios are similar to those considered here.

Oblique Ionograms Automatic Scaling Algorithm OIASA application to the ionograms recorded by Ebro observatory ionosonde

The oblique ionograms automatic scaling algorithm (OIASA), developed at the INGV for the identification of trace of oblique ionograms, allows the determination of the maximum usable frequency (MUF) for communication between the transmitter and receiver, automatically rejecting poor quality ionograms. A test of the algorithm using data from a campaign of oblique soundings between Dourbes (50.1°N, 4.6°E) and Roquetes (40.8°N, 0.5°E) has been performed. Oblique soundings from three different campaigns have been studied, covering different geomagnetic conditions, in order to study the behavior of the autoscaling algorithm under quiet and perturbed geomagnetic periods. The reported results demonstrate that OIASA performance is not influenced by geomagnetic or ionospheric activity conditions. This demonstrates a satisfactory performance of the automatic scaling algorithm, even under different geomagnetic conditions, the potential application of OIASA as a near-real-time tool for ionospheric monitoring purposes, and its performance for tracking ionospheric effects caused by space weather events.

Separation of O/X Polarization Modes on Oblique Ionospheric Soundings

AbstractThe oblique‐incidence sounder (OIS) is a well‐established instrument for determining the state of the ionosphere, with several advantages over vertical‐incidence sounders (VIS). However, the processing and interpretation of OIS ionograms is more complicated than that of VIS ionograms. Due to the Earth's magnetic field, the ionosphere is birefringent at radio frequencies and a VIS or OIS will typically see two distinct ionospheric returns, known as the O and X modes. The separation of these two modes on a VIS, using a polarimetric receive antenna, is a well‐established technique. However, this process is more complicated on an OIS due to a variable separation in the phase difference between the two modes, as measured between the two arms of a polarimetric antenna. Using a polarimetric antenna that can be rotated and tilted, we show that this variation in phase separation within an ionogram is caused by the variation in incidence angle, with some configurations leading to greater variation in phase separation. We then develop an algorithm for separating O and X modes in oblique ionograms that can account for the variation in phase separation, and we demonstrate successful separation even in relatively difficult cases. The variation in phase separation can also be exploited to estimate the incident elevation, a technique which may be useful for other applications of HF radio.

Application of the IRI model to the HF propagation model with optimization of the ionosphere parameters to day-to-day variation

The HF propagation model, North Ionospheric Model and Ray Tracing (NIM-RT) was developed and tested for a number of years by comparing measured vertical and oblique ionograms over a number of radio links (especially in high latitude area) with the simulated ionograms. The present paper extends the model in order to include:(a)Implementation of the data retrieved from the International Reference Ionosphere (IRI-2012) model into the software for radio channel modeling.(b)The algorithm for IRI data optimization to the real time condition.(c)Results of comparison between simulated and measured ionograms.Based on these updates, a new software tool called North Ionospheric Model with IRI and Ray Tracing (NIMIRI-RT) was developed, and a number of vertical ionograms corresponding to multiple ionospheric reflections was simulated. The vertical ionograms observed at various ionosondes were compared with the synthesized ionograms, generated by applying NIM-RT in conjunction with initial and optimized IRI data. The ionogram structure simulated by NIMIRI-RT based on the data retrieved from optimized IRI is more reminiscent to the observations than ionograms synthesized with the initial NIMIRI-RT without parameters optimization.

Vertical and oblique HF sounding with a network of synchronised ionosondes

A network of ionosondes in Europe has been established to monitor travelling ionospheric disturbances (TIDs) by simultaneously making vertical and oblique incidence HF sounding measurements. This network is the outcome of the Net-TIDE project, a collaboration between European Digisonde operators that have synchronised the sounding schedules of the Digisondes in order to record vertical and oblique ionogram traces simultaneously, and have added Digisonde-to-Digisonde (D2D) fixed frequency oblique-incidence measurements to the measurement schedule. The distances between the observatories involved in the project range from 500km to over 2000km. The technical feasibility of this network approach is explored. The challenge for the fixed-frequency D2D skymap measurements is the automatic selection of the sounding frequencies depending on the geometry of the sounding paths, the diurnal and seasonal ionospheric changes, and space weather induced events.

An automatic scaling method for obtaining the trace and parameters from oblique ionogram based on hybrid genetic algorithm

AbstractScaling oblique ionogram plays an important role in obtaining ionospheric structure at the midpoint of oblique sounding path. The paper proposed an automatic scaling method to extract the trace and parameters of oblique ionogram based on hybrid genetic algorithm (HGA). The extracted 10 parameters come from F2 layer and Es layer, such as maximum observation frequency, critical frequency, and virtual height. The method adopts quasi‐parabolic (QP) model to describe F2 layer's electron density profile that is used to synthesize trace. And it utilizes secant theorem, Martyn's equivalent path theorem, image processing technology, and echoes' characteristics to determine seven parameters' best fit values, and three parameter's initial values in QP model to set up their searching spaces which are the needed input data of HGA. Then HGA searches the three parameters' best fit values from their searching spaces based on the fitness between the synthesized trace and the real trace. In order to verify the performance of the method, 240 oblique ionograms are scaled and their results are compared with manual scaling results and the inversion results of the corresponding vertical ionograms. The comparison results show that the scaling results are accurate or at least adequate 60–90% of the time.

Multi-quasi-parabolic ionosphere model with EF-valley

<p>We improve the conventional multi-quasi-parabolic (MQP) model and develop two MQP models with EF-valley by applying the predicted EF-valley parameter of the newest version of the International Reference Ionosphere. On the basis of the conventional MQP model and these two improved MQP models with EF-valley, we generate one-hop oblique sounding ionograms between Wuhan and Beijing. Comparisons between the generated ionograms and the sounding ionogram in the experiments illustrate that the improved MQP models are more suitable for describing the true ionosphere than the conventional MQP model is. In contrast to the conventional MQP model, the improved MQP models can increase the accuracy of ray tracing and reduce the errors in shortwave single location and over-the-horizon-radar registration.</p>

A procedure for the reliability improvement of the oblique ionograms automatic scaling algorithm

AbstractA procedure made by the combined use of the Oblique Ionogram Automatic Scaling Algorithm (OIASA) and Autoscala program is presented. Using Martyn's equivalent path theorem, 384 oblique soundings from a high‐quality data set have been converted into vertical ionograms and analyzed by Autoscala program. The ionograms pertain to the radio link between Curtin W.A. (CUR) and Alice Springs N.T. (MTE), Australia, geographical coordinates (17.60°S; 123.82°E) and (23.52°S; 133.68°E), respectively. The critical frequency foF2 values extracted from the converted vertical ionograms by Autoscala were then compared with the foF2 values derived from the maximum usable frequencies (MUFs) provided by OIASA. A quality factor Q for the MUF values autoscaled by OIASA has been identified. Q represents the difference between the foF2 value scaled by Autoscala from the converted vertical ionogram and the foF2 value obtained applying the secant law to the MUF provided by OIASA. Using the receiver operating characteristic curve, an appropriate threshold level Qt was chosen for Q to improve the performance of OIASA.