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The largest tsunamis are generated by seafloor uplift resulting from rupture of offshore subduction-zone megathrusts. The rupture of the shallowest part of a megathrust often produces unexpected outsize tsunami relative to their seismic magnitude. These are so called 'tsunami earthquakes', which are difficult to identify rapidly using the current tsunami warning systems, even though, they produce some of the deadliest tsunami. We here introduce a new method to evaluate the tsunami risk by measuring ionospheric total electron content (TEC). We examine two Mw 7.8 earthquakes (one is a tsunami earthquake and the other is not) generated in 2010 by the Sunda megathrust, offshore Sumatra, to demonstrate for the first time that observations of ionospheric sounding from Global Navigation Satellite System (GNSS) can be used to evaluate the tsunamigenic potential of earthquakes as early as 8 min after the mainshock. [ABSTRACT FROM AUTHOR]

This paper summarizes the definition and validation of two complementary new strategies, to invert incomplete Global Navigation Satellite System Radio‐Occultation (RO) ionospheric measurements, such as the ones to be provided by the future EUMETSAT Polar System Second Generation. It will provide RO measurements with impact parameter much below the Low Earth Orbiters' height (817 km): from 500 km down approximately. The first presented method to invert truncated RO data is denoted as Abel‐VaryChap Hybrid modeling from topside Incomplete Global Navigation Satellite System RO data, based on simple First Principles, very precise, and well suited for postprocessing. And the second method is denoted as Simple Estimation of Electron density profiles from topside Incomplete RO data, is less precise, but yields very fast estimations, suitable for Near Real‐Time determination. Both techniques will be described and assessed with a set of 546 representative COSMIC/FORMOSAT‐3 ROs, with relative errors of 7% and 11% for Abel‐VaryChap Hybrid modeling from topside Incomplete Global Navigation Satellite System RO data and Simple Estimation of Electron density profiles from topside Incomplete RO data, respectively, with 20 min and 15 s, respectively, of computational time per occultation in our Intel I7 PC. Key Points: New techniques are proposed for obtaining electron density profiles from incomplete radio‐occultation dataAVHIRO technique generates accuracies at 7% relative error, with computing time of 20 min in an Intel i7 standard PCSEEIRO generates accuracies at 11% relative error, about 60 times faster than AVHIRO, suitable for real‐time applications [ABSTRACT FROM AUTHOR]

High-frequency surface wave radar (HFSWR) has been widely applied in sea-state monitoring, and its performance is known to suffer from various unwanted interferences and clutters. Radio frequency interference (RFI) from other radiating sources and ionospheric clutter dominate the various types of unwanted signals because the HF band is congested with many users and the ionosphere propagates interference from distant sources. In this paper, various orthogonal projection schemes are summarized, and three new schemes are proposed for interference cancellation. Simulations and field data recorded by experimental multi-frequency HFSWR fromWuhan University are used to evaluate the cancellation performances of these schemes with respect to both RFI and ionospheric clutter. The processing results may provide a guideline for identifying the appropriate orthogonal projection cancellation schemes in various HFSWR applications. [ABSTRACT FROM AUTHOR]

We consider the behavior of the parameters of the ionospheric E layers according to the vertical sounding at the Sodankylä observatory and oblique sounding at the Lovozero (Murmansk region)-Gor'kovskaya station (Leningrad region) path during a superstorm of March 17, 2015. Temporal and spatial behavior of these parameters is compared. It was found that the storm significantly distorted the normal course of variations of the sporadic E layer characteristics. Specific behavior of the layers during a storm at points separated by about 300 km was detected. With the help of ray tracing calculations using the IRI model, oblique sounding ionograms were constructed for the radio path analyzed. Primary attention is given to the maximum usable frequency of the F layer-MUF- F . Additionally, for the disturbed conditions where there is only a high-power E layer on the experimental ionograms, the values of MUF- E and the ratio K =MUF- E / f E for various cutoff frequencies f E of the E layer and its altitudes $$ {h}_{E_s} $$ are calculated within the framework of the well-known approximations. Calculations for the case of weak disturbance and semitransparent E layers are carried out with the IRI model adapted to the current diagnostics parameters. It was found that the calculated and experimental values of MUF- F are close to each other or coincide, while this cannot be said about MUF- E . The calculated and experimental values of MUF- E can be matched in the model of mirror reflection from a flat layer for intense layers and the model of the E layer for thick E layers of low intensity . [ABSTRACT FROM AUTHOR]

The SIRM model proposed by Zolesi et al. (1993, 1996) is an ionospheric regional model for predicting the vertical-sounding characteristics that has been frequently used in developing ionospheric web prediction services (Zolesi and Cander, 2014). Recently the model and its outputs were implemented in the framework of two European projects: DIAS ( DIgital upper Atmosphere Server ; http://www.iono.noa.gr/DIAS/ ) (Belehaki et al., 2005, 2015) and ESPAS ( Near-Earth Space Data Infrastructure for e-Science ; http://www.espas-fp7.eu/ ) (Belehaki et al., 2016). In this paper an updated version of the SIRM model, called SIRMPol, is described and corresponding outputs in terms of the F2-layer critical frequency ( fo F2) are compared with values recorded at the mid-latitude station of Rome (41.8°N, 12.5°E), for extremely high (year 1958) and low (years 2008 and 2009) solar activity. The main novelties introduced in the SIRMPol model are: (1) an extension of the Rome ionosonde input dataset that, besides data from 1957 to 1987, includes also data from 1988 to 2007; (2) the use of second order polynomial regressions, instead of linear ones, to fit the relation fo F2 vs. solar activity index R 12 ; (3) the use of polynomial relations, instead of linear ones, to fit the relations A 0 vs. R 12 , A n vs. R 12 and Y n vs. R 12 , where A 0 , A n and Y n are the coefficients of the Fourier analysis performed by the SIRM model to reproduce the values calculated by using relations in (2). The obtained results show that SIRMPol outputs are better than those of the SIRM model. As the SIRMPol model represents only a partial updating of the SIRM model based on inputs from only Rome ionosonde data, it can be considered a particular case of a single-station model. Nevertheless, the development of the SIRMPol model allowed getting some useful guidelines for a future complete and more accurate updating of the SIRM model, of which both DIAS and ESPAS could benefit. [ABSTRACT FROM AUTHOR]

For decades, high-frequency (HF) radar has played an important role in sensing the Earth's environment. Advances in radar technology are providing opportunities to significantly improve the performance of HF radar, and to introduce more applications. This paper presents a low-power, small-size, and multifunctional HF radar developed by the Ionospheric Laboratory of Wuhan University, referred to as theWuhan Ionospheric Oblique Backscattering Sounding System (WIOBSS). Progress in the development of this radar is described in detail, including the basic principles of operation, the system configuration, the sounding waveforms, and the signal and data processing methods. Furthermore, its various remote sensing applications are briefly reviewed to show the good performance of this radar. Finally, some suggested solutions are given for further improvement of its performance. [ABSTRACT FROM AUTHOR]

The retrieval of profiles of meteorological variables from radio occultation observations requires knowledge of bending angle profiles up to heights of no less than 60-70 km. Because of the residual error of the ionospheric correction, retrieved profiles become too noisy by a height of about 40 km. In order to invert the bending angle profiles, the statistical optimization is used. This makes it possible to construct an optimal linear combination of the a priori estimate of the average bending angle profile and a posteriori noisy estimate based on observations. The estimate of the average bending angle profile for the given coordinates and the time of year is usually based on the climatological atmospheric model. MSIS and CIRA models that have been used for this purpose are now obsolete and do not describe the global changes in the atmospheric state. The model of average bending angles BA-IAP (Bending Angle-Institute of Atmospheric Physics) is built based on the processing of the array of COSMIC radio occultation observations during 2006-2013. The proposed model is statistically validated based on the COSMIC database. It is shown that our model describes the average bending angle profiles more accurately than the MSIS model. [ABSTRACT FROM AUTHOR]

Most ionospheric models can calculate vertical total electron content (VTEC) predictions, but only a few are suitable for calculating slant total electron content (STEC). This ionospheric magnitude is generally measured for electron content determinations, with VTEC particularly corresponding to an elevation of 90°. This is generally obtained by applying a mapping function to STEC measurements, which leads to important calculation errors. Moreover, the equatorial region has unique characteristics, such as the fountain effect and the equatorial electrojet, which lead to significant errors in the model’s calculations. In this paper, the Tucumán ionospheric model (TIM) is presented as a novel alternative for calculating the STEC in low-latitude regions (−24 to 24 dip latitude). The model is based on spatial geometry where the considered trajectory is segmented, and the corresponding electron density calculations for the resulting segment end points are determined using the semi-empirical low-latitude ionospheric model (SLIM) with reference to their corresponding magnetic coordinates and height. Finally, the electron density values are integrated along the path to obtain the STEC. This work describes the TIM and tests their STEC predictions for five ray paths around the world (totaling 16 cases under study), which are compared with experimental data from satellites and with those calculated by the NeQuick model. Moreover, the TIM performance for VTEC predictions is also checked and compared with VTEC data obtained from Global Positioning System (GPS) signals, IRI model, and NeQuick model predictions, for six GPS receiver stations during the equinox and solstice (totaling 12 cases studied). Comparisons of the TIM predictions with experimental data show that 53% of the calculation has, in general, deviations <30%. For the considered cases, TIM reproduces the experimental data better than the other models. [ABSTRACT FROM AUTHOR]

The ionospheric phenomena which significantly influenced radio propagation during March 17–19, 2015 are considered in the study. The data of oblique ionospheric sounding (OIS) were analyzed at six radio paths. These paths are located in the zone of North Siberia in Russia and have different lengths: from 1000 to 5000 km. The results are the following. The magnetic storm drastically changed the character of radio propagation at all the considered paths: in most cases the reflections from the ionospheric F2-layer were changed by the reflections only from the sporadic Es-layer. The parameters of movement of the disturbance front were estimated on the basis of OIS data of the paths. The average velocity of the front movements from east to west was about V = 440 m/s. Even the moderate growth of riometer absorption within the region of radio paths’ locations, resulted in loss of multihop modes in the signal reflections from sporadic layers. It also resulted in a sharp decrease of signal strength at the paths. Real distance–frequency characteristics (DFC) of the paths were compared to DFC calculated on the basis of International Reference Ionosphere (IRI) model. It was revealed that on a quiet day of March, 15th, the real and the calculated DFC are similar or coincide in the majority of cases. During the disturbed days of March, 17–19, most commonly observed are the significant differences between the calculated and the experimental data. The most pronounced difference is revealed while estimating the character of OIS signals’ reflections from Es-layers. [ABSTRACT FROM AUTHOR]

This paper presents comparisons of variability and accordance of ionospheric parameters foF2, hmF2, M3000F2, B0, B1 over the middle latitude station in Warsaw (52.21°N, 21.06°E). Examination included observational data from Space Research Centre ionosonde in Poland and International Reference Ionosphere (IRI) 2012 model, for the time period of increasing solar activity from 2009 to 2012. The analysis concerned: trend, monthly median differences in twenty-four hours variability, local minima and maxima. Results are presented as tables of semi-annual data, and plots of difference in four-year period. The study indicated good agreement of foF2 and hmF2 parameters. Underestimations of B1 and M3000F2, variability of B0 parameter in Bil-2000, Gul-1987, and ABT-2009 option, were taken into consideration. [ABSTRACT FROM AUTHOR]

The interpretation of planar retarding potential analyzers (RPA) during ionospheric sounding rocket missions requires modeling the thick 3D plasma sheath. This paper overviews the theory of RPAs with an emphasis placed on the impact of the sheath on current-voltage (I-V) curves. It then describes the Petite Ion Probe (PIP) which has been designed to function in this difficult regime. The data analysis procedure for this instrument is discussed in detail. Data analysis begins by modeling the sheath with the Spacecraft Plasma Interaction System (SPIS), a particle-in-cell code. Test particles are traced through the sheath and detector to determine the detector's response. A training set is constructed from these simulated curves for a support vector regression analysis which relates the properties of the I-V curve to the properties of the plasma. The first in situ use of the PIPs occurred during the MICA sounding rocket mission which launched from Poker Flat, Alaska in February of 2012. These data are presented as a case study, providing valuable cross-instrument comparisons. A heritage top-hat thermal ion electrostatic analyzer, called the HT, and a multi-needle Langmuir probe have been used to validate both the PIPs and the data analysis method. Compared to the HT, the PIP ion temperature measurements agree with a root-mean-square error of 0.023 eV. These two instruments agree on the parallel-to-B plasma flow velocity with a root-mean-square error of 130 m/s. The PIP with its field of view aligned perpendicular-to-B provided a density measurement with an 11% error compared to the multi-needle Langmuir Probe. Higher error in the other PIP's density measurement is likely due to simplifications in the SPIS model geometry. [ABSTRACT FROM AUTHOR]

Vertical sounding is a widely used technique to obtain ionosphere measurements, such as an estimation of virtual height versus frequency scanning. It is performed by high frequency radar for geophysical applications called “ionospheric sounder” (or “ionosonde”). Radar detection depends mainly on targets characteristics. While several targets behavior and correspondent echo detection algorithms have been studied, a survey to address a suitable algorithm for ionospheric sounder has to be carried out. This paper is focused on automatic echo detection algorithms implemented in particular for an ionospheric sounder, target specific characteristics were studied as well. Adaptive threshold detection algorithms are proposed, compared to the current implemented algorithm, and tested using actual data obtained from the Advanced Ionospheric Sounder (AIS-INGV) at Rome Ionospheric Observatory. Different cases of study have been selected according typical ionospheric and detection conditions. [ABSTRACT FROM AUTHOR]

The main purpose of this paper is to investigate the response of the ionospheric F layer in the American and African sectors during the intense geomagnetic storm which occurred on 30 September–01 October 2012. In this work, we used observations from a chain of 20 GPS stations in the equatorial, low- and mid-latitude regions in the American and African sectors. Also, in this study ionospheric sounding data obtained during 29th September to 2nd October, 2012 at Jicamarca (JIC), Peru, São Luis (SL), Fortaleza (FZ), Brazil, and Port Stanley (PST), are presented. On the night of 30 September–01 October, in the main and recovery phase, the h´F variations showed an unusual uplifting of the F region at equatorial (JIC, SL and FZ) and mid- (PST) latitude stations related with the propagations of traveling ionospheric disturbances (TIDs) generated by Joule heating at auroral regions. On 30 September, the VTEC variations and foF2 observations at mid-latitude stations (American sector) showed a long-duration positive ionospheric storm (over 6 h of enhancement) associated with large-scale wind circulations and equatorward neutral winds. Also, on 01 October, a long-duration positive ionospheric storm was observed at equatorial, low- and mid- latitude stations in the African sector, related with the large-scale wind circulations and equatorward neutral winds. On 01 and 02 October, positive ionospheric storms were observed at equatorial, low- and mid-latitude stations in the American sector, possibly associated with the TIDs and an equatorward neutral wind. Also, on 01 October negative ionospheric storms were observed at equatorial, low- and mid-latitude regions in the American sector, probably associated with the changes in the O/N 2 ratio. On the night of 30 September–01 October, ionospheric plasma bubbles were observed at equatorial, low- and mid- latitude stations in the South American sector, possibly associated with the occurrence of geomagnetic storm. [ABSTRACT FROM AUTHOR]

An algorithm for the solution of the inverse problem of vertical ionosphere sounding and a mathematical model of noise filtering are presented. An automated system for processing and analysis of spectrograms of vertical ionosphere sounding based on our algorithm is described. It is shown that the algorithm we suggest has a rather high efficiency. This is supported by the data obtained at the ionospheric stations of the so-called 'AIS-M' type. [ABSTRACT FROM AUTHOR]

Two high-latitude high-frequency (HF) paths of oblique-incidence ionospheric sounding were selected: Gor'kovskaya (GRK)-Dixon Island (DIK) and GRK-Lovozero (LOZ).The following propagation parameters were studied: F2MOF and F2MUF for the main signal, F2MOFl for lateral signals, EsMOF for a signal reflected from the sporadic ionospheric layer, and MOFm for the propagation modes of the M (or N) type. Here MOF and MUF are the maximum observed and usable frequencies, respectively. It was found that the reciprocal theorem is satisfied on two radio paths depending on the studied propagation parameter: the path length and location, time of day, and degree of magnetospheric disturbance. It was found that the reciprocal theorem is satisfied in 78, 50, and 30% of cases on the first path for the F2MUF values in the daytime under moderately disturbed conditions, in the morning, and at night, respectively. For the MOFm and moderately disturbed conditions on this path, the reciprocal theorem is satisfied in 40, 35, and 20% of cases in the daytime, morning, and at night, respectively. For the F2MUF values on the second path, reciprocity is satisfied in 73% of cases in the daytime and under moderately disturbed conditions and in 30% of cases at night during a quiet period. On this path, reciprocity is satisfied in 33% of all cases at night under strongly disturbed conditions. An unusual fact was also detected when ionograms of oblique-incidence ionospheric sounding were analyzed: the F2MUF values in the LOZ-GRK direction are almost always slightly (by 1-4%) larger than in the GRK-LOZ direction. A similar difference in the F2MUF values in the forward and backward directions can apparently be explained by the steep horizontal gradient in the electron density distribution from north to south in the reflection region of this path. Such a situation is not observed on the DIK-GRK line. [ABSTRACT FROM AUTHOR]

The paper presents a system designed for automatic measurement of signal level in ionospheric channels based on a spectrum analyzer controlled by a software application developed for fast and flexible sounding. It enables choosing of desired frequencies, bandwidths, times and duration of monitoring and customized recordings. The measured values of two ionospheric channels were compared against the ones provided by the prediction of the commercial application Voice of America Coverage Analysis Program (VOACAP). Sounding recordings proved that automatic measurement is able not only to assess realistic received power versus time continuously but also to distinguish fading periods. [ABSTRACT FROM AUTHOR]

Ambiguity resolution of phase measurements during Precise Point Positioning (PPP) in global navigation satellite systems (GNSSs) is implemented. The underlying feature of the design matrix of singular systems of GNSS linearized equations allowing one to estimate the user coordinates with ambiguity resolution of phase measurements is revealed. Details of the algebraic methods of overcoming the rank deficiency associated with the procedure are considered. Observation models at GPS and GLONASS initial frequencies are proposed and investigated. A filtering method allowing one to exclude the influence of ionospheric signal delays on estimates of user coordinates in processing of the measurement at initial GPS and GLONASS frequencies is described. Significant reduction of the PPP convergence period due to ambiguity resolution of phase measurements is shown. Comparison of the quality of user positioning with the use of decoupled satellite clocks calculated from global and local network solutions is presented. [ABSTRACT FROM AUTHOR]

Mars Express, the first European interplanetary mission, carries the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) to search for ice and water in the Martian subsurface. Developed by an Italian–US team, MARSIS transmits low-frequency, wide-band radio pulses penetrating below the surface and reflected by dielectric discontinuities linked to structural or compositional changes. MARSIS is also a topside ionosphere sounder, transmitting a burst of short, narrow-band pulses at different frequencies that are reflected by plasma with varying densities at different altitudes. The radar operates since July 2005, after the successful deployment of its 40 m antenna, acquiring data at altitudes lower than 1200 km. Subsurface sounding (SS) data are processed on board by stacking together a batch of echoes acquired at the same frequency. On ground, SS data are further processed by correlating the received echo with the transmitted waveform and compensating de-focusing caused by the dispersive ionosphere. Ground processing of active ionospheric sounding (AIS) data consists in the reconstruction of the electron density profile as a function of altitude. MARSIS observed the internal structure of Planum Boreum outlining the Basal Unit, an icy deposit lying beneath the North Polar Layered Deposits thought to have formed in an epoch in which climate was markedly different from the current one. The total volume of ice in polar layered deposits could be estimated, and parts of the Southern residual ice cap were revealed to consist of ≈ 10 m of CO 2 ice. Radar properties of the Vastitas Borealis Formation point to the presence of large quantities of ice buried beneath the surface. Observations of the ionosphere revealed the complex interplay between plasma, crustal magnetic field and solar wind, contributing to space weather studies at Mars. The presence of three-dimensional plasma structures in the ionosphere was revealed for the first time. MARSIS could successfully operate at Phobos, becoming the first instrument of its kind to observe an asteroid-like body. The main goal pursued by MARSIS, the search for liquid water beneath the surface, remains elusive. However, because of the many factors affecting detection and of the difficulties in identifying water in radar echoes, a definitive conclusion on its presence cannot yet be drawn. [ABSTRACT FROM AUTHOR]

Studies were performed on three oblique ionospheric sounding (OIS) paths: Gor'kovskaya-Lovozero (St. Petersburg) with a length of 890 km, Sodankyla-Gor'kovskaya (800 km), and Sodankyla-Lovozero (360 km). The data for March 17 and April 14, 2012, the days during the recovery phase of the corresponding magnetic storms, have been analyzed. According to the observations performed at Sodankyla, riometer absorption in the morning-daytime hours was high against a background of very weak magnetic disturbances registered with a magnetometer; a high absorption level was also typical of the second day but during a substantial magnetic disturbance. The signal propagation mode structure and intensity on different paths were compared for the indicated days. The main results achieved are as follows. The OIS signal mode structure at weak (for April 14, 2012) and strong (for March 17, 2012) absorption substantially differed when magnetic disturbances were weak at the same instant. Diffuse reflections from the F2 layer were observed on the first two paths during a magnetic disturbance at night of April 14, 2012. At the same time, diffusivity was absent on the same paths under quiet magnetic conditions in the morning and daytime hours on March 17, 2012. A short-term abrupt increase in the maximum observed frequency of the Es layer (MOF Es) by 30-80% was registered half an hour before a substantial absorption burst on the first path. Signal reflections from the sporadic Es layer were observed only on the first path on March 17, 2012, and April 14, 2012, during the absorption maximum ( A = 6 dB) according to the Sodankyla data, and the signal characteristics differed on those days. [ABSTRACT FROM AUTHOR]

We present the results of processing and analysis of more than 4500 events of radio occultation sounding of the Earth's atmosphere observed in the course of the COSMIC experiment on the limb path 'satellite-ionosphere-satellite'. Events observed in December 2011 (when a number of solar flares occurred) and in January 2012 (when a strong solar proton event took place) were analyzed. It is shown that small-scale variations of electron density increase in polar latitudes, equatorial region, and midlatitudes of the southern hemisphere in January 2012. In the same period, an increase of large-scale variations of electron density is observed during daylight hours in the equatorial region and in the southern hemisphere. No noticeable distinctions in comparison with days of quiet Sun were observed in December 2011. [ABSTRACT FROM AUTHOR]

Based on data from the Japanese Kokubunji and Wakkanai stations of vertical sounding of the ionosphere, the variations in the foF2 critical frequency prior to the strong earthquakes of March 9 and 11, 2011 ( M 7.2 and 9.0, respectively), are analyzed. It is found that significant positive disturbances of foF2 had been recorded approximately one day before the first earthquake. Notably, at the Irkutsk reference station, which is located about 3300 km from the earthquake epicenters, there were no significant disturbances of foF2. This suggests that the effects of increased foF2, observed at the Kokubunji and Wakkanai stations, were probably caused by the earthquake preparation processes. The seismo-ionospheric manifestations of the stronger earthquake on March 11, 2011, even if they took place, were hidden by the geomagnetic storm's effects. [ABSTRACT FROM AUTHOR]

In this study, Global Positioning System (GPS) simulation is used to evaluate the effect of varying GPS signal power levels on the accuracy of two handheld GPS receivers; Garmin GPSmap 60CSx (evaluated GPS receiver) and Garmin GPSmap 60CS (reference GPS receiver). Both receivers employ the GPS L1 coarse acquisition (C/A) signal. With decreasing GPS signal power level, probable error values increase due to decreasing carrier-to-noise density (C/N0 levels for GPS satellites tracked by the receiver. Varying probable error patterns are observed for readings taken at different locations and times. This is due to the GPS satellite constellation being dynamic, causing varying GPS satellite geometry over location and time, resulting in GPS accuracy being location / time dependent. For all the readings taken, the evaluated GPS receiver recorded lower probable error values as compared to the reference GPS receiver. This occurred as the evaluated GPS receiver has higher receiver sensitivity, and hence, is able to obtain lower position dilution of precision (PDOP) values. In addition, it has lower receiver noise, reducing the value of its user equivalent ranging error (UERE). [ABSTRACT FROM AUTHOR]

An ionosonde is a kind of high-frequency skywave radar developed for ionospheric research and HF channel management. A novel low-power multifunctional ionospheric sounding system, i.e., the Wuhan Ionospheric Comprehensive Sounding System (WICSS), is presented in this paper. Software-defined-radio-based power and low transmitting power are the two most significant features of the system. The system can achieve vertical-incidence, oblique backscatter, and oblique-incidence sounding within low transmitting power. The application of pulse compression and coherent integration allows WICSS to measure ionospheric parameters with high resolution and very low power. The background of ionospheric sounding is introduced. Conventional and modern ionospheric sounding theories are described. The radar waveforms, system structure, and major system parameters are depicted in detail. Typical experimental results are shown at the end of this paper. [ABSTRACT FROM AUTHOR]

By comparing the data of ionospheric radio sounding at Petropavlovsk and Magadan stations, we eliminated the identical disturbances induced by geomagnetic storms and identified the ionospheric responses to seismic activity. It was found that strong earthquakes in the area of Petropavlovsk station are preceded by positive ionospheric variations which are more pronounced the higher the magnitude of the earth-quake. [ABSTRACT FROM AUTHOR]

This paper considers the results of an experiment conducted in May 2002 on the radio path St. Petersburg-Spitsbergen with a length of D = 2150 km and equipped with instrumentation for oblique ionospheric sounding (OIS). The features of OIS ionograms at various degrees of magnetic activity are revealed, and a comparison of these ionograms with the vertical sounding (VS) data at the Sodankyla observatory (Finland) is performed. This observatory is located near the reflection point of the path in question. In order to estimate the auroral absorption, riometer data from the Sodankyla observatory were included in the analysis of the OIS and VS ionograms. The merits of the OIS method as a diagnostic tool of ionospheroic plasma in comparison with VS at high latitudes are demonstrated. These merits are mainly formulated for magnetically disturbed periods: in the presence of B events (black-outs), anomalous sporadic Es formations, and a series of other events. [ABSTRACT FROM AUTHOR]

We present the results of experimental studies of propagation of short radio waves on a long transequatorial path of Laverton (Australia) — Rostov-on-Don, which were obtained with the help of an ionosonde/direction finder with chirp modulation of the signal. It is shown that conditions for propagation of anomalous signals by means of sideband reflection of radio waves from the Himalayan Hills and the Plateau of Iran and also due to scattering of radio waves from the high-latitude ionosphere of the northern hemisphere are realized on the given path. The propagation of radio waves is modeled with allowance for their scattering by anisotropic magnetic field-aligned irregularities of a high-latitude ionosphere, which are located on the northern wall of the main ionospheric trough of the F layer. It is shown that the results of the experiment agree well with the calculated data. [ABSTRACT FROM AUTHOR]

Abstract: The Mars advanced radar for subsurface and ionospheric sounding (MARSIS) on Mars Express is the first high-frequency sounding radar operating from orbital altitudes since the Apollo 17 Lunar Sounder flown in 1972. The radar operates from a highly elliptical orbit but acquires data only from altitudes lower than 1200km. The periapsis altitude is 250km. This radar has been succesfully operating since August 2005. The radar is a dual channel low-frequency sounder, operates between 1.3 and 5.5MHz (MegaHertz) with wavelengths between 230 and 55m in free space for subsurface sounding and between 0.1 and 5.5MHz (wavelengths between 3000 and 55m) for ionospheric sounding. The subsurface sounder can operate at one or two-frequency bands out of four available bands at either like or cross polarization. The subsurface sounding radar transmits radio frequency (RF) pulses of 250μs duration through a 40m dipole antenna. The return echoes are then converted to digital form and temporarily stored on board for some digital processing. A second antenna, a monopole, provides reception for the cross-polarized return and its data are processed by a second channel. This processing reduces the data rate produced by the instrument to rates allowed by the spacecraft communications channel. These processed returns are then sent to Earth by the telecommunications system on the spacecraft. The advances in digital data acquisition and processing, since 1972, have enabled this technique to be used in a compact spacecraft science instrument.. This sounder has obtained returns from several kilometers below the surface of the Mars. The ionospheric sounder operates at altitudes greater than 800km in a mode that sweeps the entire 0.1–5.5MHz range. During ionospheric sounding, the transmitter sends a 91μs tone at 127 pulses per second rate. The frequency sweep takes 7.3s to complete the 0.1–5.5MHz range. Operational aspects of the instrument are described, including the selection of frequency bands and receive antenna selection, which are based on the expected solar zenith angle. The process of data take planning as well as data archiving are described. Results of both subsurface and ionospheric sounding are presented. [Copyright &y& Elsevier]

Abstract: Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation. [Copyright &y& Elsevier]

The article offers information on a research conducted in order to determine the effectiveness of the combined use of L- and C-band measurements in suppressing the ionospheric error. For the research, code- and carrier-phase measurements were used in the linear combinations for obtaining ionospheric elimination, large wavelengths, and a low noise level. The research has revealed that the combination of L- and C-band measurements reduces the noise level of ionosphere-free code-only combinations. The combination reduces the noise level by a factor 4.5 as compared to pure L-band combinations.

We model theoretically the received spectrum in the case of sounding of the ionospheric HF radio channel by a chirp signal. It is shown that the result of processing of an individual time sample of the received signal is equivalent to the sounding of the radio channel by a complex narrow-band pulsed signal such that the group delays of its propagation modes determine the maxima in the received spectrum. We analyze the quadrature components of realizations of the received signal at the intermediate frequency at the bandpass-filter output in the receiving channel of the chirp ionosonde. The results of our analysis show the possibility of reconstructing the transfer function of a HF radio channel in the sounding-frequency band for the delay range determined by the characteristics of the intermediate-frequency bandpass filter. We propose a method for reconstructing the transfer function of the ionospheric radio channel, which involves supplementing the circuit of primary processing of the signal by a corrective digital filter with specified amplitude-frequency and phase-frequency characteristics. The proposed method can be used for all operating regimes of the chirp ionosonde in the case of digital recording and processing of signals. [ABSTRACT FROM AUTHOR]

The ice-rich south polar layered deposits of Mars were probed with the Mars Advanced Radar for Subsurface and Ionospheric Sounding on the Mars Express orbiter. The radar signals penetrate deep into the deposits (more than 3.7 kilometers). For most of the area, a reflection is detected at a time delay that is consistent with an interface between the deposits and the substrate. The reflected power from this interlace indicates minimal attenuation of the signal, suggesting a composition of nearby pure water ice. Maps were generated of the topography of the basal interlace and the thickness of the layered deposits. A set of buried depressions is seen within 300 kilometers of the pole. The thickness map shows an asymmetric distribution of the deposits and regions of anomalous thickness. The total volume is estimated to be 1.6 × 106 cubic kilometers, which is equivalent to a global water layer approximately 11 meters thick. [ABSTRACT FROM AUTHOR]

We describe a mathematical model for a signal-and-noise mixture at the output of a chirp-ionosonde receiver. A method for lumped-noise attenuation based on using the criterion of detection of rough errors in experimental measurements is proposed. The results of numerical and full-scale experiments on efficiency estimation of the proposed method are presented. [ABSTRACT FROM AUTHOR]

Abstract: The goal of this study is to reveal some ionospheric forerunners which occur prior to the expansion phase of any storm or substorm. Another task is to compare variations of two main ionospheric parameters, f 0 F2 and h max F2, on two chains of ionosondes located in Europe and North America during the interval 26–28 October 2003. This interval precedes the Halloween storm and includes its onset. Analysis is based on ionospheric data of the SPIDR system. The key findings are as follows. A positive peak of delta f 0 F2 occurs some hours before onset of a magnetic storm or substorm, where delta is a deviation from the quiet value. This peak can serve as a forerunner of disturbance development. Delta f 0 F2 peak amplitude reaches the sizes 20–100% of f 0 F2 values. Similarities of delta f 0 F2 variations on two chains are: (i) the delta f 0 F2 peak mentioned above occurs simultaneously as a rule before disturbances on both chains; (ii) similar delta f 0 F2 variations are observed on all of the mid-latitude (or high-latitude) ionosondes of each chain. Delta h max F2 variations are approximately the opposite of the f 0 F2 variations. Their amplitudes fall in the range 15–25% from the average h max F2 value. This investigation is useful and important for the short-term prediction of Space Weather states. [Copyright &y& Elsevier]

In this paper, a cascaded correction and suppression method of reducing ionospheric phase path contamination and sea clutter to enable detection of targets travelling at speeds near the Bragg Doppler is addressed. The Hankel rank reduction (HRR) technique based on singular value decomposition (SVD) has been used to estimate the ionospheric phase distortion and suppress the sea clutter. Simulation results show that such a technique is helpful for the worse conditions when the target masking effect happens even after ionospheric phase decontamination. Finally, an attempt to combine another phase decontamination algorithm based on the piecewise polynomial phase modeling with the clutter cancellation stage for faster phase fluctuation is discussed briefly and some results are given. [ABSTRACT FROM AUTHOR]

During the past decade, South African scientists have pioneered research in the field of ionospheric modelling using the technique of neural networks (NNs). Global ionospheric models have always been insufficient for the South African region owing to an historical paucity of available data. Within the past 10 years, however, three new ionospheric sounders have been installed locally and are operating continuously. These sounders are located at Grahamstown (33.3°S, 26.5°E), Louisvale (28.5°S, 21.2°E) and Madimbo (22.4°S, 30.9°E). The addition of a modern sounder at Grahamstown enlarged the ionospheric database for this station to 30 years, making this archive a considerable asset for ionospheric research. Quality control and online availability of the data has also added to its attraction. An important requirement for empirical modelling, but especially for employing NNs, is a large database describing the history of the relationship between the ionosphere and the geophysical parameters that define its behaviour. This review describes the path of South African ionospheric modelling over the past 10 years, the role of NNs in this development, the international collaborations that have arisen from this, and the future of ionospheric modelling in South Africa. [ABSTRACT FROM AUTHOR]

We consider issues relating to the nonstationarity of an oblique ionospheric sounding signal. Experimental data obtained with the Doppler goniometric facility of the Kazan State University are used. The equipment, methods of measurement, and the algorithms used for analysis of the experimental data are described. Typical coherence ranges of an ionospheric signal measured in the daytime at different frequencies on the Moscow – Kazan path are presented. Diurnal variations in the coherence range and the diurnal mean distributions of the Doppler shift and its drift velocity are analyzed. The relative contribution of short- and long-period variations to the signal distortions is examined using wavelet transform. [ABSTRACT FROM AUTHOR]

We present the results of experimental studies of variations in the maximum usable frequency (MUF) on the latitudinal (England-Moscow and Khabarovsk-Nizhny Novgorod) and meridional (Cyprus-Moscow) paths. It is found that the quasi-period of MUF variations ranges from 20 min to several hours in spring 2002. The absolute value of the MUF varied in the range from 0.2 to 2 MHz. We show that variations in distance-frequency characteristics of HF signals propagated over the long-distance latitudinal path are strongly affected by quasi-periodic disturbances being the ionospheric response to acoustic-gravity waves excited by the terminator. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Extracting and parameterizing ionospheric waves globally and statistically is a longstanding problem. Based on the multichannel maximum entropy method (MMEM) used for studying ionospheric waves by previous work, we calculate the parameters of ionospheric waves by applying the MMEM to numerously temporally approximate and spatially close global-positioning-system radio occultation total electron content profile triples provided by the unique clustered satellites flight between years 2006 and 2007 right after the constellation observing system for meteorology, ionosphere, and climate (COSMIC) mission launch. The results show that the amplitude of ionospheric waves increases at the low and high latitudes (∼0.15 TECU) and decreases in the mid-latitudes (∼0.05 TECU). The vertical wavelength of the ionospheric waves increases in the mid-latitudes (e.g., ∼50 km at altitudes of 200–250 km) and decreases at the low and high latitudes (e.g., ∼35 km at altitudes of 200–250 km). The horizontal wavelength shows a similar result (e.g., ∼1400 km in the mid-latitudes and ∼800 km at the low and high latitudes). [ABSTRACT FROM AUTHOR]

Modulated high frequency (HF) heating of the ionosphere provides a feasible means of artificially generating extremely low frequency (ELF)/very low frequency (VLF) whistler waves, which can leak into the inner magnetosphere and contribute to resonant interactions with high energy electrons. Combining the ray tracing method and test particle simulations, we evaluate the effects of energetic electron resonant scattering driven by the discrete, multi-frequency artificially generated ELF/VLF waves. The simulation results indicate a stochastic behavior of electrons and a linear profile of pitch angle and kinetic energy variations averaged over all test electrons. These features are similar to those associated with single-frequency waves. The computed local diffusion coefficients show that, although the momentum diffusion of relativistic electrons due to artificial ELF/VLF whistlers with a nominal amplitude of ∼ 1 pT is minor, the pitch angle scattering can be notably efficient at low pitch angles near the loss cone, which supports the feasibility of artificial triggering of multi-frequency ELF/VLF whistler waves for the removal of high energy electrons from the magnetosphere. We also investigate the dependences of diffusion coefficients on the frequency interval (Δf) of the discrete, multi-frequency waves. We find that there is a threshold value of Δf for which the net diffusion coefficient of multi-frequency whistlers is inversely proportional to Δf (proportional to the frequency components Nw) when Δf is below the threshold value but it remains unchanged with increasing Δf when Δf is larger than the threshold value. This is explained as being due to the fact that the resonant scattering effect of broadband waves is the sum of the effects of each frequency in the ‘effective frequency band’. Our results suggest that the modulation frequency of HF heating of the ionosphere can be appropriately selected with reasonable frequency intervals so that better performance of controlled precipitation of high energy electrons in the plasmasphere by artificial ELF/VLF whistler waves can be achieved. [ABSTRACT FROM AUTHOR]