Radio Science

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Plain Language Summary Estimations of effective path length (LE) for an Earth‐space path from Ku‐band rain attenuation and rain rate measurements at a tropical location Kolkata reveal that the variation of LE is controlled by the type of rain namely, stratiform and convective precipitation. Low rain rate events mostly associated with stratiform rain are identified by the presence of a melting layer as observed by micro‐rain Doppler radar. There is, however, no melting layer signature in convective precipitation which is characterized by high rain rates. For low rain rates measured at the receiver site, it is expected that higher rain rates will prevail at other points along the Earth‐space path. However, for high rain rates at the receiver end, comparatively lower rain rates are likely to occur along the satellite link. The present study shows that the ITU‐R model formulations for effective path length are inadequate to account for the larger variability of LE at low rain rates, underestimating rain attenuation for stratiform rain. Whereas for convective rain, the ITU‐R model overestimates LE and rain attenuation at higher rain rates. Proposed separate power‐law models for LE for the two types of rain provide rain attenuation estimations that agree well with actual experimental measurements.
 
Operating principle of the LoRaWAN Class A.
Variation of network energy consumption as a function of the number of end‐devices for scenario 0 and scenario 1.
Variation of network energy consumption as a function of the number of end‐devices for scenario 0 and scenario 2.
Variation of network energy consumption as a function of the number of end‐devices for scenario 0 and scenario 3.
Variation of network energy consumption as a function of the number of end‐devices for scenario 0, scenario 1, scenario 2 and scenario 3.
LoRa to say Long Range is one of the Low Power Wide Area Network technologies suitable for Internet of Things due to its low power consumption. In this paper we evaluate the energy consumption of a LoRa network using the LoRaWAN protocol which is based on the pure Aloha access method. We define a reference scenario (scenario 0), which consists in neglecting the energy consumption related to the different states of the end‐devices after transmission of a packet that does not require downlink acknowledgment and then we compare this scenario 0 to scenario 1, scenario 2 and scenario 3. Indeed the scenario 1 consists in considering the states of the end‐devices after transmission in uplink of a packet without acknowledgment, in the evaluation of the energy consumption. However, scenario 2 takes into account the different states of the end‐devices in scenario 1 with an acknowledgment on the first reception window. While in scenario 3 the acknowledgment is done on the second reception window always taking into account the end‐devices states. The simulation results with the LoRaSim tool have shown that the network energy consumption (NEC) increases significantly when considering the different states of the end‐device after uplink transmission of a packet without acknowledgment, but also that the increase of the NEC related to end‐device states is even more important if packets are acknowledged. Thus for a rigorous evaluation of the NEC, it is important to take into account the states of the end‐device after transmission of a packet but also the eventual acknowledgments in order to better predict the battery lifetime.
 
This paper proposes an efficient method to determine the material of spherical objects and the location of the receiving antenna relative to the object in bi‐static measurements using supervised learning techniques. From a single observation, we compare classification performances resulting from the application of several classifiers on different data types: the Ultra‐Wide Band scattered field in time and frequency domains and pre‐processed data from the singularity expansion method (SEM) which has seldom been used in classification because it is considered to be noise sensitive. We selected a robust SEM technique which is vector fitting to decompose the frequency response into complex natural resonances (CNRs) and residues. Indeed, CNRs are aspect independent and therefore, can be used to discriminate the objects. However, the residues associated to each pole depend upon the aspect angle, and hence, they were never exploited. In this paper, we propose a novel use of those residues. Additionally, we construct an original data set using SEM data in order to further improve the robustness to noise and the generalization capacity of the learning algorithms. The advantages of using SEM data for object classification are highlighted by comparing it with raw scattered field data in time and frequency domains where the classification algorithms are optimized in each case. The results are very promising, especially in terms of generalization, robustness to noise, and computation time, which are all reasons to take an interest in SEM for these purposes.
 
Today, microstrip antennas have received a lot of attention due to their unique features such as low weight and volume, easy and cheap construction, and the ability to adapt to planar structures. Of course, the important problem with these antennas is their small gain and bandwidth. In this paper, the gain and bandwidth of the microstrip antenna with a patch in the form of Antipodal Vivaldi are improved by using log‐periodic structures. Three 8‐element log‐periodic structures designed in a straight‐line, U‐shaped and V‐shaped are proposed, simulated and fabricated. The dimensions of these antennas are 287.2×48mm2 $287.2\times 48{\,\mathrm{m}\mathrm{m}}^{2}$, 182.5×160mm2 $182.5\times 160{\,\mathrm{m}\mathrm{m}}^{2}$ and 195×200mm2 $195\times 200{\,\mathrm{m}\mathrm{m}}^{2}$ respectively. The results of the measurements show that the bandwidth of the 8‐element straight‐line, U‐shaped and V‐shaped antennas are 3–18, 2.6–18, and 2.4–18 GHz respectively, and their maximum gain are 6.26, 8.3, and 7.61 dBi, respectively, which has increased the bandwidth by approximately 125% compared to the single element mode and increased the gain from 2.26 to 4.3 dBi. These quantities show that among the three antennas proposed in this article, the V‐shaped antenna has the highest bandwidth (15.6 GHz) and the U‐shaped antenna has the highest maximum gain (8.3 dBi). Of course, it is possible to achieve more gain by changing the angle between the branches of the V‐shaped antenna, but it may ruin the bandwidth.
 
This paper presents the design of an Ultra‐wide band‐stop Frequency Selective Surface (FSS), the building block of which makes use of a modified Jerusalem cross loop structure. The proposed FSS unit cell has a coupled configuration with a modified Jerusalem cross loop on the top and a square loop on the rear side to operate over an ultra‐wide bandwidth. The proposed FSS design offers band‐stop characteristics over the entire UWB operating bandwidth extending from 2.95 to 12 GHz with a reference transmission loss of 10 dB. The proposed FSS unit cell has a miniaturized profile of 0.05λ˳ × 0.05λ˳ where the λ˳ corresponds to the free space wavelength at the lowest operating frequency. The rotational symmetry of the proposed FSS makes it polarization‐independent under both TE and TM modes of operation. Also, the proposed FSS exhibits a good angular stability response up to 60°. The prototype FSS is fabricated and the simulated results are validated using experimental measurements. Thus, the proposed FSS is a promising candidate for a multitude of applications such as spatial filtering (Bayatpur & Sarabandi, 2010, https://doi.org/10.1109/LMWC.2009.2038517), antenna gain enhancement (Chatterjee & Parui, 2016, https://doi.org/10.1109/TAP.2016.2552543), and electromagnetic shields (Chiu et al., 2008, https://doi.org/10.1109/TEMC.2008.2004560) operating over the UWB spectrum.
 
The ionosphere contains many small‐scale electron density variations that are under represented in smooth physics‐based or climatological models. This can negatively impact the results of Observation System Simulation Experiments, which use a truth model to simulate data. This paper addresses this problem by using ionosonde data to study ionospheric variability and build a new truth model with empirically driven variations. The variations are studied for their amplitude, horizontal and vertical size, and temporal extent. Results are presented for different local times, seasons, and at solar minimum and solar maximum. We find that these departures from a smooth background are often as large as 25% and are most prevalent near 250 km in altitude. They have horizontal spatial extents that vary from a few hundred to a few thousand kilometers, and typically have the largest horizontal extent at high altitudes. Their vertical extents follow the same pattern of being larger at high altitudes, but they only vary from 10s of km up to 200 km in vertical size. Temporally, these variations can last for a few hours. A procedure for using these spatial and temporal distributions to add empirically driven variance to a smooth truth model is outlined. This process is used to make a truth model with representative variations, which is compared to ionosonde data as well as Global Positioning System Total Electron Content data that was not used to inform the model. The new model resembles the data much better than the smooth models traditionally used.
 
This paper presents an antenna architecture from Huygens sources placed in a middle cavity formed by a metal reflector and a partially reflective surface (PRS) of dielectric superstrates. The proposed antenna is excited by a pair of cross‐feedings to produce two frequency resonances in reflection coefficients and radiate unidirectional patterns. In contrast to conventional designs, the proposed PRS‐supported Huygens source antenna may effectively broaden the bandwidth and increase radiation gains with a low profile, including low cost. Simulation results show broadband radiation characteristics at 2.45 and 5 GHz bands, where the impedance bandwidths are in the ranges of 2.4–2.5 GHz and 4.8–5.8 GHz, respectively. In addition, the measurement results of the antenna prototype validate the simulation results in terms of reflection coefficients and radiation patterns. It is found that the gains are 5.0 dBi (2.45 GHz) and 3.0 dBi (5 GHz), where simulation and measurement results are agreed well for practical applications of wireless communications.
 
A procedure of designing ultra‐short conical horns corrected by lenses, is boarded at this work. The method takes advantages of the axisymmetric character of the conical horns to project the problem to a bidimensional mesh, reducing the computational time and making easy the optimization of the device. This approach is not used for most popular commercial codes which employ a 3D analysis of the whole antenna. Horn antennas exhibit some remarkable radiation characteristics as they are high directivity, low crosspolar level and symmetric patterns with low return losses on a reasonable bandwidth. The price to pay is handle devices very long and weighed, so it is demanding to find lines of design to shorten and make compact horns to be incorporated to the current communication systems. In this work, we show the advantages to use a double convex lens together with a dielectric core, added to the horn, to achieve a compact and shortened horn maintaining a good performance. Conical horns, of a size as small as one wavelength, can be achieved. To reach such shortening, a double convex lens and a dielectric core must be added to the antenna. The parameters of the final prototype, at 60 GHz, are the following: Length: ≈λ, Directivity = 20 dBi, Cross polar Peak = −19.1 dB, Beam width on copular plane = 18.3°, Side Lobe Levels lower than −20 dB (both H and E plane), Return losses = −30.7 dB, and presenting an smooth frequency behavior on a wide bandwidth.
 
Ka‐band (32 GHz) communications links utilized by the National Aeronautics and Space Administration (NASA) flight missions for science downlink are susceptible to degradation due to weather. In this study, a customized real‐time forecast system has been developed to predict zenith atmospheric noise temperature (Tatm) at the Deep Space Network (DSN) tracking sites using machine learning (ML). A random forest model is trained with the Global Forecast System (GFS) forecast and analysis datasets in addition to the Tatm measurements derived from on‐site advanced water vapor radiometers (AWVR). The real‐time forecast uncertainty is quantified for different error regimes using the Self‐Organizing Map method. The results show that the root mean square error (RMSE) of the 24‐hr Tatm prediction at Goldstone, CA increases with the increase of Tatm. At 90% of the time, the forecasts have RMSE (bias) of less than 3.50 K (0.16 K) for fair‐weather conditions with Tatm < 16 K. In comparison to the current approach in designing Ka‐band communications links, application of weather forecasts can increase data return to the downlink for 80% of the time. A downlink gain of up to 2.43 dB (75% more data) can be realized at 10° elevation angle when Tatm = 8.32 K.
 
The Aperture Array Verification System 2.0 is the latest technological demonstrator designed for the Square Kilometer Array radio telescope operating at low frequency (50–350 MHz). This is an aperture phased array, known as a “station” of the radio telescope, with a pseudo‐random distribution of 256 dual polarized log‐periodic antennas. Mutual coupling effects among the antennas are known to produce detrimental effects especially at the lower end of the operating frequency band. This paper presents a relationship between the antennas electromagnetic performance and their physical position inside the station, followed by an identification of the most critical distances and alignment‐angles between antennas. The calibration accuracy of the station and its directivity within the field of view of the radio telescope are recalculated after repositioning ∼20% of the 256 antennas showing clear improvements in dense regime with respect to the actual AAVS2.0 layout.
 
Ground scatter (GS) echoes in Super Dual Auroral Radar Network (SuperDARN) observations have been always expected to occur under high‐enough electron density in the ionosphere providing sufficient bending of high frequency radio wave paths toward the ground. In this study, we provide direct evidence statistically supporting this notion by comparing the GS occurrence rate for the Rankin Inlet SuperDARN radar and the F region peak electron density NmF2 ${N}_{m}{F}_{2}$ measured at Resolute Bay by the Canadian Advanced Digital Ionosonde and incoherent scatter radars RISR‐N/C. We show that the occurrence rate increases with NmF2 ${N}_{m}{F}_{2}$ roughly linearly up to about ∼4⋅1011m−3 $\sim 4\cdot {10}^{11}{\mathrm{m}}^{-3}$ and the trend saturates at larger NmF2 ${N}_{m}{F}_{2}$. One expected consequence of this relationship is correlation in seasonal and solar cycle variations of the GS echo occurrence rate and NmF2 ${N}_{m}{F}_{2}$. GS occurrence rates for a number of SuperDARN radars at middle latitudes, in the auroral zone and in the polar cap are considered separately for daytime and nighttime. The data indicate that the daytime occurrence rates are maximized in winter and nighttime occurrence rates are maximized in summer for middle latitude and auroral zone radars in the Northern Hemisphere, consistent with the winter anomaly (WA) phenomenon. The effect is most evident in the North American and Japanese sectors, and the quality of WA signatures deteriorates in the European and, especially, in the Australian sectors. The effect does not exist in the South American sector and in the polar caps of both hemispheres.
 
This paper employs the complete data of six ionospheric observation stations in China in the 24th solar activity cycle and researches the variation of frequency of the F2 layer (foF2) and the relationship with the solar activity indices. We discover that the dependence of foF2 on the sunspot number (R) and the solar radio flux of 10.7 cm wavelength (F10.7) is strongest in mid‐solar years, and weakest in high solar years. At the same time, the long‐term variation trends of foF2, R and F10.7 in the complete 24th solar activity cycle are analyzed using the mutual information method in information theory. We utilize the linear and nonlinear methods to compare the effects of long term change relationship between foF2 and R during the rising and falling phases of solar activity. The findings reveal that the sunspot number is more suitable to depict the long‐term trend of foF2 in China, and the relationship between them is better described by nonlinear quasi‐polynomial. This result has important implications for improving foF2 forecasts and the prediction accuracy of some ionospheric models.
 
A 2×2 multiple-input multiple-output (MIMO) antenna with super-ultra wideband, dual band-notched characteristics and high gain is proposed. The antenna element consists of a circular slot shape radiator fed by a tapered feedline on the top side of the substrate, whereas defected ground structure (DGS) is etched on the bottom ground plane. The DGS and tapered feedline has been designed to produce the super-UWB performance. Dual-band notch is achieved by etching a J-shape slot in the radiator and an I-shape slot at the edges of the ground plane. The dual-band notch prevents the electromagnetic interference allocated for other wireless applications. High isolation is realized by incorporating a plus-shape decoupling structure with four small arm shape stubs at the center of the ground plane. A prototype was fabricated to verify the simulated and measurement results. The proposed design achieves a super-ultra wide bandwidth of 165% (3.4 ∼ 35 GHz), isolation of 21 dB, and a peak gain of 13 dBi. The notched bands are at 6.6 GHz and 10 GHz. The envelope correlation coefficient (ECC) is less than 0.01. The overall size of the MIMO design is 0.58λ0L × 0.34λ0L (λ0L is the free-space wavelength at the lowest operating frequency).
 
We have recently introduced a modification of the multiple signal classification method for synthetic aperture radar. This method incorporates a user‐defined parameter, ϵ, that allows for tunable quantitative high‐resolution imaging. However, this method requires relatively large single‐to‐noise ratios (SNR) to work effectively. Here, we first identify the fundamental mechanism in that method that produces high‐resolution images. Then we introduce a modification to Kirchhoff Migration (KM) that uses the same mechanism to produce tunable, high‐resolution images. This modified KM method can be applied to low SNR measurements. We show simulation results that demonstrate the features of this method.
 
A new 36.17 MHz all‐sky meteor radar was installed at McMurdo Station Antarctica (77.8°S, 166.7°E) in February 2018 to provide wind measurements in the mesosphere and lower thermosphere (MLT) region (70–120 km). This instrument is the highest latitude meteor radar currently in operation in the southern hemisphere; it joins two other meteor radars within the Antarctic Circle. The radar will provide long‐term continuous wind measurements of the polar region, and contribute to a greater understanding of MLT dynamics. This work describes the radar hardware and its context with other instruments in the region. The paper provides an overview of the spatial and temporal variation in meteor echoes over the observation period of March 2018 through October 2021. It also provides an analysis of the mean winds and solar tides over the first three years of operation; including a description of an observed 12 hr summertime wind oscillation consistent with previously documented observations of a westward propagating 12 hr non‐migrating tide of zonal wavenumber 1.
 
The Harbin Engineering University, the People’s Republic of China, multifrequency multiple-path coherent radio system operates continuously and provides data for post analysis. The data collected during the solar eclipse of June 21, 2020 have been chosen for this study with the objectives to interpret the variations in the Doppler spectra, Doppler shift, and in the reflected radio wave amplitudes that are associated with the solar eclipse, establish the magnitude and find physical significance of these variations, determine the reduction in the electron density caused by the solar eclipse, and to estimate an increase in wave activity in the ionosphere. The eclipse was accompanied by Doppler spectrum diffuseness resulting from an increase in the number of rays, the temporal variations in the Doppler shift were observed to be bi-polar, asymmetrical, and anomalously small, with extreme Doppler shift magnitudes varying from –11 to –40 mHz and from 22 to 56 mHz. The duration of processes with negative Doppler shifts varied from 50 to 80 min, and the duration of processes with positive Doppler shifts changed from 30 to 80 min. The multi-hop propagation (from two to five hops) took place along all propagation paths, with a 360 to 560-km one-hop length, due to the anomalous radio wave propagation via the sporadic-E layer present about 80% of the time on June 21, 2020. The Doppler shift exhibited 4–18-min period quasi-sinusoidal variations with 20–10-mHz amplitudes.
 
Strong solar flare events can occur even during the decay phase of the solar cycle. During these events concurrent increases in the X‐ray and Enhanced UV (EUV) fluxes and solar radio bursts (SRBs) can be observed. The SRBs cover a large range of frequencies including the L band, giving rise to signal fades in the GNSS carrier‐to‐noise ratio and fluctuations in its amplitude and phase. The increases in the X‐ray, UV, and EUV fluxes cause increase in the ionospheric D, E, and F region electron densities. The aim of this work is to analyze the effects in the GNSS signal, in the ionosphere and in the magnetic field H component of the X9.3 and X1.3 solar flares that occurred on 06 and 07 September 2017, respectively. Data from a network of six GNSS receivers, two magnetometers, and four Digisondes are used in the analysis. Fades of about 5 and 10 dB were observed in the signals of GNSS L1 and L2/L5 frequencies, respectively. Significant positioning errors, were observed for the strongest X9.3 flare. A sudden increase in Total Electron Content with the rates of 2.5–5.0 TECU/min was observed. An increase in the E layer density gave origin to an increase in the Equatorial Electrojet intensity, whose signatures were observed in the H component of two magnetometers. Another observed effect was the ionospheric D region density increase that caused disruption in the Digisonde signal. As a consequence of the described effects, GNSS receivers may fail to produce accurate navigation solution.
 
The atmosphere plays a significant role in degrading Ka‐band (32 GHz) radio and optical communications for single antenna communications as well as degrading interferometric measurements such as used for radio astronomy and multi‐antenna arrays. Higher frequencies such as deep space Ka‐band (32 GHz) and near‐Earth K‐band (26 GHz) are more susceptible to this atmospheric degradation than at the lower frequency bands of S‐band (2.3 GHz) and X‐band (8.4 GHz). The various sources of atmospheric degradation include rain attenuation, cloud attenuation, gaseous attenuation, atmospheric noise temperature increase, and scintillation. For this study, we have focused on the strength of the turbulence caused principally by the wet atmosphere represented by the refractive index structure parameter Cn², which is a measure of the variance of the refractive index over small incremental distances along the signal path, and thus is a strong function of water vapor variations at microwave frequencies. Here we provide a comprehensive study of this parameter derived from years of Site Test Interferometer measurements acquired from a variety of climates. We found that the strongest diurnal variation and correlation of Cn² measurements with its model occurred for Goldstone, California representing a high‐desert climate, while weaker correlations were found for Kennedy Space Center representing a sub‐tropical climate, and Canberra, Australia and Madrid, Spain representing temperate climates. We quantified periods where the strongest correlation was observed such as Goldstone summers (as high as 0.7 for ∼3‐day periods) where the days are typically hot and humid while nights tend to be colder and drier.
 
A first stage of a sub‐array synthesis method is presented. The approach refers to a continuous aperture radiating multiple beams thanks to a phase‐only control and its aim is to determine the optimal partitioning of an aperture into sub‐apertures fed with a constant phase. The technique is based on a multi‐stage approach. Several synthesis tools are used in cascade to perform the multiple beam synthesis of a prefixed aperture. To improve the effectiveness of the method, an interleaved use of global and local optimizers is adopted and the number of the unknowns is progressively increased from stage to stage. A numerical analysis is reported to show the effectiveness of the method.
 
A novel computer vision‐based meteor head echo detection algorithm is developed to study meteor fluxes and their physical properties, including initial range, range coverage, and radial velocity. The proposed Algorithm for Head Echo Automatic Detection (AHEAD) comprises a feature extraction function and a Convolutional Neural Network (CNN). The former is tailored to identify meteor head echoes, and then a CNN is employed to remove false alarms. In the testing of meteor data collected with the Jicamarca 50 MHz incoherent scatter radar, the new algorithm detects over 180 meteors per minute at dawn, which is 2 to 10 times more sensitive than prior manual or algorithmic approaches, with a false alarm rate less than 1 percent. The present work lays the foundation of developing a fully automatic AI‐meteor package that detects, analyzes, and distinguishes among many types of meteor echoes. Furthermore, although initially evaluated for meteor data collected with the Jicamarca VHF incoherent radar, the new algorithm is generic enough that can be applied to other facilities with minor modifications. The CNN removes up to 98 percent of false alarms according to the testing set. We also present and discuss the physical characteristics of meteors detected with AHEAD, including flux rate, initial range, line of sight velocity, Signal‐to‐Noise Ratio, and noise characteristics. Our results indicate that stronger meteor echoes are detected at a slightly lower altitude and lower radial velocity than other meteors.
 
The Sanya incoherent scatter radar (SYISR) is a newly built active digital phased array, all solid‐state transmitting and digital receiving incoherent scatter radar in Sanya (18.3°N, 109.6°E). The radar frequency band is from 430 to 450 MHz. The Sanya site is a low latitude station in China dedicated for ionospheric investigation. Meanwhile, SYISR is also suitable for detecting a wide range of space debris because of its high power and flexible beam steering ability. In this paper, we first calculate the detectable lower limit size of space debris with respect to integration time and range based on SYISR parameters through theoretical simulation. Then, we selected several typical space debris, with a size near the theoretical detectable lower limit, to perform the experiment. We found that the estimated radar cross section versus range accorded well with the theoretical curve, which confirmed our simulation. Specifically, the detectable lower limit size of space debris is ∼8 cm in a range of 1,000 km given a threshold signal‐to‐noise ratio of 14 dB. This value decreases to ∼3 cm if a 200‐ms coherent integration is implemented. We further performed several experiments on objects with different inclinations using signals with 0.3 and 4 MHz under both static staring and tracking modes. In comparison with the two‐line element file predicted raw orbit, the tracking error of 0.3 and 4 MHz are 7 km and 400 m, respectively, without coherent integration in the processing. The study expands the function of the SYISR and should be beneficial to our ionospheric data inversion given the frequent occurrence of space debris around the SYISR.
 
Radio waves provide a useful diagnostic tool to investigate the properties of the ionosphere because the ionosphere affects the transmission and properties of high frequency (HF) electromagnetic waves. We have conducted a transionospheric HF‐propagation research campaign with a nanosatellite on a low‐Earth polar orbit and the EISCAT HF transmitter facility in Tromsø, Norway, in December 2020. In the active measurement, the EISCAT HF facility transmitted sinusoidal 7.953 MHz signal which was received with the High frEquency rAdio spectRomEteR (HEARER) onboard 1 Unit (size: 10 × 10 × 10 cm) Suomi 100 space weather nanosatellite. Data analysis showed that the EISCAT HF signal was detected with the satellite's radio spectrometer when the satellite was the closest to the heater along its orbit. Part of the observed variations seen in the signal was identified to be related to the heater's antenna pattern and to the transmitted pulse shapes. Other observed variations can be related to the spatial and temporal variations of the ionosphere and its different responses to the used transmission frequencies and to the transmitted O‐ and X‐wave modes. Some trends in the observed signal may also be associated to changes in the properties of ionospheric plasma resulting from the heater's electromagnetic wave energy. This paper is, to authors' best knowledge, the first observation of this kind of “self‐absorption” measured from the transionospheric signal path from a powerful radio source on the ground to the satellite‐borne receiver.
 
This article focuses on the new generation of geodetic very long baseline interferometry (VLBI), the VLBI global observing system (VGOS), and measurements carried out during the CONT17 campaign. It uses broadband technology that increases both the number and precision of observations. These characteristics make VGOS a suitable tool for studying the atmosphere. This study focuses on the effects of the ionosphere on VGOS signals using a model that incorporates and extends ideas originally published in Hobiger et al. (2006, https://doi.org/10.1029/2005RS003297). Our investigation revealed that the differential total electron content (dTEC) data product calculated with the VGOS post‐processing software had a sign error that fortunately, does not change the final values of the phase and group delay. Therefore, this study was a way to identify this problem within the dTEC product. After diagnosing and solving this problem, the underlying model was modified such that instead of considering a single unknown for the latitude gradient of the ionosphere, a time series of latitude gradients were considered that enhanced the resulting vertical total electron content (VTEC) estimates. For evaluation purposes, time series of VTEC at each station during the CONT17 campaign were compared with VTEC obtained from the global navigation satellite system (GNSS). The final agreement between VGOS and GNSS was between 1.1 and 5.9 TEC units (TECU).
 
We propose an accurate calibration method for short‐time waveform signals passed through a linear time‐invariant (LTI) system that has a non‐negligible group delay. Typically, the calibration process of waveform data is expressed by the deconvolution in the time domain, and there is an equivalent operation in the frequency domain with the Fourier transform. For the short‐time data, if the short‐time Fourier transform is applied to the waveform data in the calibration process, multiplying the data by a window function is highly recommended to reduce side‐lobe effects. However, the multiplied window function is also modified in the calibration process. We analyzed the modification mathematically and derived a method to eliminate the modification of the multiplied window function. In the method, calibrated data in the frequency domain are inverse‐transformed into waveform data at each frequency, divided by a modified window function at each frequency, and accumulated over the frequencies. The principle of this method derived quantitatively indicates that the calibration accuracy depends on the transfer function of the system, frequency resolution of the Fourier transform, type of the window function, and typical frequency of the waveform data. Compared with conventional calibration methods, the proposed method provides more accurate results in various cases. This method should be useful for the calibration of general radio wave signals passed through LTI systems as well as for the calibration of plasma waves observed in space.
 
The set of perfect Golomb‐Honeycomb arrays are plotted in the left panels, while the corresponding difference‐vector distributions are shown in the right panels. The amber‐blue colors show the conjugate pairs. The circles indicate the radius of maximum baseline. Note that the Manx array has a chiral opposite.
The “best” symmetric 7‐element array (from figure 6 in Grydeland and La Hoz (2006)), presented in a similar manner to Figure 1 in the top row and the “best” in the bottom row. Unlike the perfect Golomb‐Honeycomb arrays, these layouts have gaps inside the circle of maximum baseline‐length. Similar to the Manx array, these 7‐element arrays have chiral opposites.
A 2‐D discrete Fourier transform of the 6‐element Manx array. The intensity scale is linear and is normalized to the central peak.
The Manx array layout (top left) and its uv‐coverage (top right) are fractally expanded. The middle row is the configuration after the first expansion. This is repeated a second time for the layout/configuration in the third row. The colors in the uv‐coverage plots shows the increase in the occurrence of redundant baselines, for no‐redundancy (black), 6‐fold redundancy (red) and 36‐fold (yellow).
The nodes of the Manx array lie on the Reuleaux triangle of width 7s $\sqrt{7}s$, where s is the separation between close nodes.
Interferometry applications (e.g., radio astronomy) often wish to optimize the placement of the interferometric elements. One such optimal criterion is a uniform distribution of non‐redundant element spacings (in both distance and position angle). While large systems, with many elements, can rely on saturating the sample space, and disregard “wasted” sampling, small arrays with only a few elements are more critical, where a single element can represent a significant fraction of the overall cost. This paper defines a “perfect array” as a mathematical construct having uniform and complete element spacings within a circle of radius equal to the maximum element spacing. Additionally, the largest perfect non‐redundant array, comprising six elements, is presented. The geometry is described, along with the properties of the layout and situations where it would be of significant benefit to array application and non‐redundant masking designs.
 
Based on ray tracing in a smooth ionosphere described by the IRI‐2012 model we have inferred the seasonal‐diurnal dynamics of radio noise observed by four mid‐latitude high‐frequency (HF) radars. In the calculations, noise is assumed to be homogeneous and stationary, but the main contribution comes from the radar skip zone boundary due to focusing radiowaves effect. Noise absorption along the ray path is simulated from the IRI‐2012 electron density, and from the molecular nitrogen density and electron temperatures obtained from the NRLMSISE‐00 model. Earth magnetic field is not taken into account both in the absorption and ray‐tracing calculations due to insufficient accuracy of the ionospheric model. The model results are compared with experimental radar data, and good agreement between the two is demonstrated. It is shown that experimentally observed seasonal and diurnal dynamics of the noise correlates well with model predictions. We demonstrated saturation effect at low noise levels. The model makes it possible to estimate the amount of absorption in D‐ and E‐layers using noise observations at SuperDARN and SuperDARN‐like poleward‐oriented radars, especially at mid‐latitudes. This is important for the retrieval of long term variations in the electron density in the lower ionosphere, by using wide coverage provided by these radars' network. The model also makes it feasible to interpret vertical absorption by experimental noise observations, thereby significantly expanding the capability of HF radars to monitor the lower ionosphere, and to provide data for joint analysis with other data, obtained by these radars at E‐ and F‐layer heights.
 
To provide new insights into the relationship between geomagnetic conditions and plasma irregularity scale‐sizes, high‐latitude irregularity spectra are computed using a novel Incoherent Scatter Radar (ISR) technique. This new technique leverages: (a) the ability of phased array Advanced Modular ISR (AMISR) technology to collect volumetric measurements of plasma density, (b) the slow F‐region cross‐field plasma diffusion at scales greater than 10 km, and (c) the high dip angle of geomagnetic field lines at high‐latitudes. The resulting irregularity spectra are of a higher spatiotemporal resolution than what has been previously possible with ISRs. Spatial structures as small as 20 km are resolved in less than 2 minutes (depending on the radar mode). In this work, we focus on Resolute Bay ISR‐North (RISR‐N) observations operating in the “imaginglptight” mode. In addition to having an unprecedented view of the size and occurrence of irregularities as they traverse the polar cap, we find that as the F‐region plasma density decreases below approximately 2.5 × 10¹⁰ m⁻³ at 350 km altitude the spectral power shifts to scale‐sizes lower than 50 km. Additionally, near magnetic local noon, the spectral power shifts to scales greater than 50 km, and from 15 to 6 magnetic local time, the spectral power shifts to scales lower than 50 km. This reflects the role of photoionization dominating high‐latitude ionospheric structuring in the polar cap.
 
Mars Orbiter Subsurface Investigation Radar (MOSIR) is carried by China's first Mars probe, Tianwen‐1 orbiter, investigating the Martian subsurface stratification. Surface clutter from topography off‐nadir will overlap with the subsurface echoes, which affects the recognition of Martian subsurface reflections. Surface clutter simulation can effectively distinguish the nadir and off‐nadir radar echoes. In this paper, we choose the facet method to model the Mars surface topography and combine the roughness parameter with the radar backscatter function. We also provide an analytic expression of the echo phase considering the distance variation in the whole facet. The Chinese first Mars landing site is on Utopia Planitia, which is also one of the key investigating regions of MOSIR. Therefore, we also carried on surface clutter simulation of this region and generated simulation radargram with the Chirp Scaling algorithm. Furthermore, we use the contrast method to compensate for ionospheric error introduced by the NeMars Mars ionosphere model. Our surface clutter simulation program will significantly support MOSIR subsurface investigation, and provide a chance to verify the related data processing.
 
Six hundred hours of data from a receiver located at the Cape Verde Atmospheric Observatory at 15°N (dip latitude), has been used to explore the fading correlation of 300–360 MHz trans‐ionospheric signals from the MUOS satellite. Using these data, we have highlighted that the inter‐frequency correlation varies with the fading frequency; components at frequencies close to the Fresnel frequency tend to be well correlated over bandwidths between 15 MHz and greater than 20 MHz, but those at higher fading frequencies are only well correlated over bandwidths between 0.1 and 5 MHz at a correlation threshold of 0.7. When considered over all fading frequencies, flat fading is far more common than frequency selective fading, such that when the frequency separation is 5 MHz and when S4 lies between 0.7 and 0.8, the ratio is ∼16:1, when the separation is 10 MHz the ratio is ∼9:1 and when the separation is 15 MHz it is ∼7:1. Together, the results in this paper suggest that flat fading is the dominant fading mechanism for satellite communication systems, with bandwidths up to 15 MHz, operating in the high VHF and low UHF bands in the equatorial region. At still higher operating bandwidths we expect frequency selective fading to become dominant as the differentially delayed multipath components, occurring via Fresnel scale irregularities, cause destructive and constructive interference.
 
A numerical integration of the Sommerfeld integral is performed using the Schelkunoff formulation for cylindrical media. The Schelkunoff kernel for cylindrical media involves higher order modified Bessel functions with azimuthal summation over higher order modes. As such, the convergence characteristics of the cylindrical integral kernel are strongly dependent on complex linear combinations of higher order Bessel/Hankel/modified Bessel functions, compared to the case of the planar media where only a single Bessel/modified Bessel function of zeroth order is present. Two cylindrical configurations are analyzed using the new formulation, viz. a conducting cylinder and a dielectric‐coated conducting cylinder. The branch‐point singularity in the first configuration is removed using the angular transformation for the Sommerfeld/Schelkunoff formulations. A path deformation technique is used for the second configuration to address the problem of poles and branch‐point singularities on the real axis of integration. The in‐depth analysis of the cylindrical kernels and the integrals with variation in the location of the observation point clearly bring out the relative merits of both formulations for the cylindrical configurations, with the TE/TM coupling for the coated cylinder considered.
 
The Sun is a readily accessible source of wideband noise that can be used for the calibration of a wide variety of radio systems. Since 1979 the United States Air Force Radio Solar Telescope Network (RSTN) has performed solar flux observations around noon on eight frequencies between 245 and 15,400 MHz. These fluxes are disseminated by the National Ocean and Atmospheric Administration (NOAA) and are used by many radio and radar system operators to perform system calibration. Little analysis has been performed on the RSTN noon flux values. Here we review the calibration procedures of RSTN and perform statistical tests between the four sites comprising RSTN. For system operators who require precise system calibration the noted variations may be large enough to cast doubt on RSTN flux values for such calibration. However, for many communication systems a calibration within one or two dB is adequate. We discuss the reported variation both in linear and logarithmic terms. When it is possible to used smoothed data the inter‐site variation becomes less of a problem, although the correlation between sites at Ku band (15,400 MHz) still appears excessively large. We update some simple empirical models for wideband solar flux based purely on the 10 cm flux at any time, as these have also been used in solar radio calibration procedures. We give practical advice on the use of solar flux for radio system calibration and make some recommendations that we believe could improve the accuracy and utility of solar radio flux reports.
 
Convergence of different methods: (a) TE mode, (b) TM mode.
Time gain of different methods: (a) TE mode, (b) TM mode.
It is shown that there is a better choice for solution of equations appeared in the above‐mentioned paper from the aspects of convergence and CPU time.
 
In this paper equatorward propagating large scale traveling ionospheric disturbance (LSTID) on 17 March 2015 were investigated using the Statistical Angle‐of‐Arrival and Doppler Method for GPS (SADM‐GPS) radio interferometry technique in data‐scarce East Africa. To apply the SADM‐GPS method, 5 GPS arrays each with 3 GPS receivers arranged in a triangular geometry were used. Our results show that during 15:00–18:00 UT on 17 March, TIDs with mean horizontal velocities between 161.9 and 464.4 m/s were observed. Using the wavelet analysis, the periods of TIDs in a range of 51–69 min that qualify to LSTIDs were revealed. The peak‐to‐peak phase shift of detrended total electron content (TEC) over latitudes in this study confirms the equatorward TID propagation, which was obtained by the SADM‐GPS technique. A pair of magnetometers were used to infer E × B drift and an adequate agreement was found with Swarm satellites derived plasma density that enabled us to explain the behaviour of ionospheric irregularity during the main phase of the geomagnetic storm. Moreover, significant TEC enhancement (reaching ∼20%–105%) were captured by GPS arrays during the period of TID propagation. Nevertheless, the rate of change of TEC (ROT) and ROT index (ROTI) show wavy structures that reflect TID effects over ionospheric modulations during the post noon to evening hours of 17 March 2015.
 
The paper reports occurrences of postmidnight to early morning ionospheric depletions in total electron content (TEC) observed using the Low Earth Orbiting satellite beacon from Calcutta (22.57°N, 88.36°E geographic, and 32°N magnetic dip) situated around the northern crest of the equatorial ionization anomaly (EIA) in the Indian longitude sector for the period 2015–2016. These results are novel and contrary to the usual scenario of equatorial ionospheric irregularities being dominant during the early evening to pre‐midnight hours of equinoctial months. The TEC depletions, commonly called “bite‐outs,” exhibit amplitudes of 1–2.5 TECU, with maximum depletion found near the northern crest of the EIA around subionospheric coordinates 25.8°N and 82.2°E. These bite‐outs exhibited seasonal variabilities, with maximum occurrences found during local summer months. The observed TEC depletions during postmidnight hours point toward irregularities, which are not generated over the magnetic equator and drifted along magnetic field lines. A case study of the observation of these ionospheric disturbances has also been presented across a spectrum of frequencies, namely 150 and 400 MHz (Low Earth Orbit satellites), 250 MHz (Geostationary satellites), and 1.5 GHz (GPS—Medium Earth Orbit satellites) to establish the presence of irregularities of different scale sizes even during postmidnight hours.
 
k‐vector direction determined by propagation characteristics is crucial for understanding the global features of space plasma. Identifying the arriving wave model is a major factor in obtaining fast and accurate results in the direction finding of various plasma waves. If we can determine whether the observation data contain a significant natural wave or not, we can reduce the computational time for direction finding analysis by excluding noise‐only data. The conventional approach for identifying the arriving wave model assumes that all electromagnetic field sensors have same noise levels. However, the noise levels of electromagnetic field sensors on board scientific satellites can change owing to sensor degradation during long‐term instrument operation. Thus, the arriving wave model should be identified even when the noise levels of all electromagnetic field sensors are not equal. We proposed robustly identifying the arriving wave model by introducing a noise integration kernel that includes information about noise level ratios. Our proposed approach classifies a spectral matrix into three cases: noise model, single plane wave model, and multiple waves model. The proposed approach comprises the likelihood ratio test, and the identification result based on a statistical viewpoint. We conducted Monte Carlo simulations, and it was verified that the proposed approach can correctly derive the arriving wave model with high accuracy even with different sensor noise levels.
 
Plain Language Summary A large amount of images are retrieved by a specialized instrument designed to make observations in the ionosphere. These images are contaminated by instrumental noises. In order to recover useful signals from these noises, we train a deep learning model. A data set containing the labeled signals is used for both training and validating the model performance. The desired signals are manually labeled using a labeling software. By comparing the model predictions with the labels, the results show that the model can well identify the elongated, overlapping, or compact signals. The model is also capable of correcting some missing and incorrect labels. The performance of the model is sensitive to the data number of the corresponding labels fed to the model during training. The recovered useful signals are then used to estimate physical quantities that are important for the study of ionospheric physics.
 
The Chang'E‐5 (CE‐5) spacecraft was launched on 24 November 2020 with the purpose to implement unmanned lunar surface sampling and return. Very long baseline interferometry (VLBI) technique played an important role in real‐time precise trajectory determination and attitude determination of the CE‐5 spacecraft in the Earth to Moon transfer, circumlunar, landing, and ascending phases. However, tropospheric delay is one of the main error sources for VLBI observations which has to be corrected accurately for precise trajectory determination. To deal with this error properly, a prediction model (TRO_P) and a method with Global Navigation Satellite System observations (TRO_G) are proposed for tropospheric zenith delay correction for real‐time and 30 min latency trajectory determination of the CE‐5 spacecraft, respectively. The results demonstrate that the mean root mean square (RMS) of residual error of VLBI delay and delay rate for TRO_P are 0.62 ns and 0.75 ps/s at low elevation angle, respectively. Moreover, the improvement for the mean RMS of VLBI delay using TRO_G is 39.5% meaning from 0.43 to 0.26 ns compared with that using the method with meteorological data (TRO_S) at low elevation angle. This study can provide a reference for tropospheric delay calibration for trajectory determination of the spacecraft using VLBI or other techniques.
 
In this paper, the human milk dielectric constant by a microwave sensor has been evaluated. The proposed sensor is result of coupling between a simple microstrip line and a split ring resonator and its resonance frequency is 6 GHz. When up to 30 µl of milk are placed on the sensing area on the sensor, it causes the resonance frequency to shift. Six samples of human milk were measured by a network analyzer, then the dielectric constant of the samples were determined, and three similar samples were measured by the built-in sensor. The proposed microwave sensor has Q-factor of 38 and its sensitivity is 0.17%. The implemented sensor has acceptable performance compared to previous sensors and it can be used to determine the normal behavior of human milk with a simple method and for this work, it requires to small value of sample for measurement.
 
Since the publication of the special collection on Radio Channel Modeling for 5G Millimeter Wave Communications in the Built Environments new frequency bands, new antennas and new transmission techniques are being proposed to cope with the demanding requirements of beyond‐5G systems. In this commentary, we provide an overview of the new technology and on how it will impact on radiowave propagation characteristics, and the way radio channel models will be developed and used in the future.
 
A multi‐manifold based sparse representation (MMSR) algorithm which can preserve the local structure of the datasets in the reconstruction space with multiple descriptions is proposed for synthetic aperture radar (SAR) target configuration recognition. Different from the traditional manifold learning‐based algorithms that preserve the local structure of the datasets in the low‐dimensional space, the proposed MMSR guarantees local structure preserving of the datasets after feature extraction and sparse representation. The proposed MMSR can avoid corrupted relationships of the samples to ease the obstacle of target aspect angle sensitivity, which can realize more accurate SAR target configuration recognition. Satisfying results are obtained on the moving and stationary target acquisition and recognition database.
 
Oleg A. Tretyakov and his wife, Tetyana Tretyakova, lived in Istanbul from 1998 to 2022, until he passed away.
Oleg Alexandrovich Tretyakov, the Chair of Commission B of the URSI National Committee of Ukraine since 1993, a member of the IEEE, and Professor Emeritus of the Department of Theoretical Radiophysics, V. N. Karazin Kharkiv National University (KhNU), Ukraine, passed away on January 25, 2022, in Istanbul, at the age of 83. He died shortly after he was hospitalized and diagnosed with lung cancer on December 29, 2021.
 
The 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.
 
The Super Dual Auroral Radar Network (SuperDARN) currently consists of more than thirty high‐frequency (HF, 3–30 MHz) radars covering mid‐latitude to polar regions in both hemispheres. Their major task is to map ionospheric plasma circulation which provides information about the interactions between the solar wind and the near‐Earth's space plasma environment. One of the major factors defining radar data quality is the signal‐to‐noise ratio (SNR), which requires an accurate characterization of the HF noise. The standard SuperDARN data analysis software uses the SNR as part of a set of empirical procedures designed to remove low‐quality data from further analysis. In this study we found that the currently used empirical algorithm systematically underestimates the noise level by up to 40%. Based on comparison of theoretical and observational noise statistics, we resolve this issue by designing and validating a procedure for accurate background noise level estimation. We then propose a simple SNR threshold to replace the existing criteria for excluding low‐quality data. In addition, we show that several aspects of the radar operational regime design, as well as short‐lived anthropogenic radio interference, can adversely affect the quality of the noise estimates at selected radar sites, and we propose ways to mitigate these problems.
 
Terahertz (THz) communications are considered as a critical technology for the future wireless communication. The THz band (0.1–10 THz) offers ultra‐wide bandwidths and promises to satisfy the need for ultra‐high‐speed wireless communication. Although propagation characterization is necessary for the THz system design and validation, the response mechanism between the surface roughness and the THz wave is rarely studied. According to the Rayleigh Criterion, most surfaces in a physical environment may be considered to be rough. Scattering on rough surfaces will play a significant role in the THz propagation. Due to the difficulty of practical measurements at the THz band, we use a full‐wave simulation method to study the propagation characteristics of scattering on rough surfaces. First, the Monte Carlo method is used to model rough surfaces with different root‐mean‐squared heights δ and correlation lengths l. Then, with full‐wave simulations, frequency dependency, incident angle dependency, material dependency, and surface size dependency are discussed. By changing the roughness of surfaces, scattering on different rough surfaces at 300 GHz are simulated. Based on a large number of simulation results, we investigate the effect of δ and l on scattering on rough surfaces. Afterward, the cross‐polarization discrimination of each case is analyzed and counted to evaluate the depolarization effect. This study shows that scattering can become a prominent propagation mechanism with the growth of roughness. Besides, the depolarization effect becomes much more severe for a rougher surface. Studying THz propagation characteristics on rough surfaces is critical to accurate THz channel modeling, which further supports the design and deployment of THz communication systems.
 
Different types of incoherent scatter radar (ISR) echoes are observed associated with aurora, including some which have been interpreted as signatures of cavitating Langmuir turbulence (CLT). Akbari et al. (2013) https://doi.org/10.1002/jgra.50314 discussed two instances of correlation between CLT and naturally occurring radio emissions called medium frequency burst (MFB) which occur at substorm onsets. Based on that observation, radio detections of MFB from Toolik Lake Observatory have been applied to investigate occurrence of CLT in ISR data from Poker Flat Incoherent Scatter Radar and their possible correlation with MFB. Of 131 MFB events, 25 occurred within 15 min of an ISR echo detection, compared to 6 of 116 intervals of a control set with similar local time and seasonal distribution. The difference is significant at the 10⁻⁴ level, suggesting that ISR echoes are more probable during substorm onset times identified using MFB as a proxy. However, only four observed ISR echoes coincident with one MFB event showed both specific characteristics consistent with CLT. Furthermore, investigation of the angle of arrival of MFB suggests that the electromagnetic emissions do not originate from the plasma volume where the ISR detects the echoes. The small number of coincident ISR echoes and MFB is expected due to the different volumes in which the emissions and the echoes are detected. 50% of the MFB events occurred within 20 min of a substorm onset independently identified versus 8% of the control set intervals, confirming the correlation of MFB with substorm onsets.
 
In this paper, the performance of the Coupled Ocean‐Atmosphere Mesoscale Prediction System (COAMPS®) electromagnetic (EM) propagation forecast is assessed using numerous metrics such as forecast lengths, frequencies, ranges, and spatial resolutions. The modeled PL (Propagation Loss) for 2–40 GHz based on the COAMPS ®predicted refractivity profiles is evaluated by 1 month of range‐dependent measurements, which were collected during Coupled Air‐Sea Processes and Electromagnetic ducting Research (CASPER) East Campaign. PL data were collected using a shore‐to‐ship radio link, near the coast of Duck, NC. Mesoscale atmospheric refractivity predictions for the CASPER East operational area are provided by COAMPS®. Propagation loss predictions are computed from these refractivity profiles via a parabolic wave equation propagation code. The results show a good agreement between the COAMPS®‐predicted and the measured propagation loss for the northwest Atlantic region. The analyses show that the prediction error varies from 3 to 8 dB as the frequency increases from 2 to 40 GHz. In addition, COAMPS® successfully tracked the changes in the refractivity field even when the forecast length increased from 0 to 48 hr, with only a marginal degradation in EM prediction accuracy within the first 12 hr. Higher horizontal range resolution does not result in significant improvement in EM predictions as long as there are at least 2 M‐profiles that allows the EM propagation code to capture the first order horizontal variation. Low‐resolution, high order nested COAMPS® fields still produce good EM forecasts for this particular geometry.
 
A 1 × 4 array of printed dipole antenna (PDA) with a broadband rhomboid artificial magnetic conductor (AMC) is introduced. A suggested PDA using a pair of microstrip dipoles with unequal arms excited by a T‐shaped microstrip feedline expands the bandwidth in the measured range of 5.27–6.74 GHz. Then, the suggested 3 × 3 AMC reflector is inserted into the PDA to exhibit −10 dB measured impedance bandwidth from 4.20 to 7.11 GHz for WLAN and WiMAX applications. The suggested PDA with AMC compared to the PDA without AMC exhibits a size reduction of 46.72%, enhanced gain up to 8.4 dBi, and excellent impedance matching with unidirectional radiation patterns. The novel AMC unit cell operates at 6.20 GHz with an AMC bandwidth of 5.09–7.21 GHz. Also, by loading a 3 × 8 AMC periodic array on a FR4 substrate with 3 mm thickness underneath the four‐element array of PDA with a gap distance of 3 mm, a low‐profile wideband structure with enhanced radiation properties is achieved. The measured S‐parameters show the broad bandwidth from 4.26 to 7.28 GHz with enhanced gains and efficiencies of all elements. Besides, the suitable isolation between the array elements of more than 30 dB for multiple‐input multiple‐output (MIMO) systems is achieved by applying various polarized directions of PDAs and loading the 3 × 8 periodic AMC reflector. Finally, the proposed MIMO design introduces remarkable advantages of good size reduction and bandwidth enhancement, low‐profile antenna with high stable gain, high efficiency and large isolation, and unidirectional radiation patterns with reduced backside direction.
 
Propagation of high‐frequency (HF) radio signals is strongly dependent on the ionospheric electron density structure along a communications link. The ground‐based, HF space weather radars of the Super Dual Auroral Radar Network (SuperDARN) utilize the ionospheric refraction of transmitted signals to monitor the global circulation of E‐ and F‐region plasma irregularities. Previous studies have assessed the propagation characteristics of backscatter echoes from ionospheric irregularities in the auroral and polar regions of the Earth's ionosphere. By default, the geographic location of these echoes are found using empirical models which estimate the virtual backscattering height from the measured range along the radar signal path. However, the performance of these virtual height models has not yet been evaluated for mid‐latitude SuperDARN radar observations or for ground scatter (GS) propagation modes. In this study, we derive a virtual height model suitable for mid‐latitude SuperDARN observations using 5 years of data from the Christmas Valley East and West radars. This empirical model can be applied to both ionospheric and GS observations and provides an improved estimate of the ground range to the backscatter location compared to existing high‐latitude virtual height models. We also identify a region of overlapping half‐hop F‐region ionospheric scatter and one‐hop E‐region GS where the measured radar parameters (e.g., velocity, spectral width, elevation angle) are insufficient to discriminate between the two scatter types. Further studies are required to determine whether these backscatter echoes of ambiguous origin are observed by other mid‐latitude SuperDARN radars and their potential impact on scatter classification schemes.
 
Top-cited authors
Charles Werner
  • GAMMA Remote Sensing Research and Consulting AG
Toshitaka Tsuda
  • Kyoto University
Toru Sato
  • Kyoto University
Titus Roth
  • Lockheed Martin Corporation
Susumu Kato
  • Kyoto University