Abstract With the ongoing transition for geodetic Very Long Baseline Interferometry (VLBI) from the use of legacy S/X telescopes to the VLBI Global Observing System (VGOS), it is necessary to connect the new VGOS network to the existing legacy S/X telescopes. One way to do this is through specially designed short‐baseline interferometry sessions with the aim to obtain local‐tie vectors between these telescope generations at observatories that host both legacy S/X and new generation VGOS telescopes. The dismantling of the legacy S/X telescope NYALES20 at the Ny‐Ålesund Geodetic Earth Observatory in August 2023 was preceded by around one and a half years of short‐baseline interferometric measurements, referred to as NYTIE, to the new VGOS telescopes NYALE13S and NYALE13N. The results of these measurements are used to connect the old and new networks at this observatory. We present both group and phase delay results of this dedicated measurement campaign. We also analyze the same baselines with data that can be obtained from regular IVS‐R1 and IVS‐R4 sessions. We find a significant discrepancy between group and phase delay positions in the vertical component. Sensitivity tests attribute this discrepancy to session‐dependent differential troposphere effects rather than to telescope deformation modeling. Both group and phase delay results are consistent with a classical geodetic survey using total station and GNSS once the formal uncertainties are rescaled to reflect the observed session‐to‐session scatter and a residual modeling systematic of 7 mm is taken into account.
Abstract Oblique high‐frequency (HF) ionospheric measurements contain valuable information about electron density structure, but remain difficult to assimilate operationally due to the computational cost of ray tracing. PyRayHF is an open‐source, fully Python‐based framework that addresses such difficulties through the implementation of efficient forward operators for HF propagation and parameter retrieval tailored to data‐assimilation (DA) applications. PyRayHF implements magneto‐ionic vertical tracing and a hierarchy of two‐dimensional oblique ray tracing methods, which are verified against one another for internal consistency and validated through comparative analysis with a full three‐dimensional Jones‐Stephenson ray tracer. Validation results show that all solvers reproduce key observables, including group path and apex location, with typical errors below 10% when compared to the reference solution. For group path, the primary observable in oblique ionogram interpretation, simple stratified Snell's‐law formulations perform comparably to gradient‐based solvers while being several orders of magnitude faster, making them suitable for DA workflows. To enable efficient assimilation, PyRayHF combines a fast vertical forward operator with a midpoint‐based oblique‐to‐vertical transformation. For transmitter–receiver separations of 1,000–2,000 km, midpoint‐derived errors remain below 10%. PyRayHF further provides a parameter‐minimization framework that retrieves F2‐layer parameters directly from midpoint‐converted observations, producing anchor‐point measurements suitable for direct assimilation into parametrized systems such as ANCHOR.
Abstract During deep space missions, maintaining reliable communication between a spacecraft and Earth becomes particularly challenging during superior solar conjunctions, when the spacecraft passes behind the Sun relative to Earth. The highly ionized solar corona and solar wind create plasma inhomogeneities that severely affect radio signals, leading to phase scintillation, amplitude fading, and signal loss. These effects intensify as the Sun‐Earth‐Probe (SEP) angle decreases, resulting in frequent disruption of phase‐locking techniques crucial for successful signal demodulation. This paper comprehensively analyzes the communication subsystem to ensure reliable tracking, telemetry, and command (TT&C) links during superior solar conjunctions. By analyzing the effects of solar plasma on radio links, the paper proposes new operational methods and subsystem architectures that enhance the robustness of RF communication even in very harsh conditions. The solutions include modulation, coding, non‐coherent demodulation methods, and alternative channel models to mitigate scintillation effects and maintain stable communication links, particularly at low SEP angles. These strategies offer promising ways to improve data transmission reliability and maximize mission success during challenging solar conjunction phases.
Abstract Terahertz (THz) technology, operating in the 0.1–10 THz band, has gained increasing relevance in wireless communications, biomedical diagnostics, and wearable systems. Its ability to provide high‐resolution imaging and non‐invasive sensing has made it a promising tool in healthcare. However, strong absorption of THz waves by water‐rich biological tissues raises critical concerns regarding biological safety. A key metric for evaluating electromagnetic exposure is the Specific Absorption Rate (SAR), which quantifies the rate of energy deposition in tissues. This review examines the role of THz antennas, sensors, and absorbers in shaping electromagnetic field distributions and their implications on SAR. While flexible and compact THz antennas and sensors demonstrate potential for applications such as COVID‐19 detection, fall monitoring, and biosensing, most studies rely on simplified phantoms or simulations, limiting biological realism. Similarly, metamaterial absorbers enhance device sensitivity and control field localization but may inadvertently introduce resonant hotspots that increase localized SAR. Current research highlights a trade‐off between device performance and biological safety, further constrained by incomplete dielectric data sets and lack of standardized measurement techniques. This review underscores the urgent need for experimental validation, localized SAR metrics, and updated safety frameworks to ensure the safe deployment of THz‐enabled biomedical technologies.
Abstract This paper presents the creation of an ultra‐miniaturized implanted meander lined antenna for Wireless Body Area Network (WBAN) applications. The employed antenna is especially intended for incorporation with implantable medical network, and it functions in the 2.45 GHz covering the Industrial, Scientific, and Medical (ISM) frequency range. In order to extend the electrical length of the proposed design, the meander line‐like radiating components function as radiators and are connected to the ground section via a shorting pin. For improved impedance matching and compactness, the suggested antenna makes use of via (shorting pins), meander line configurations, and coaxial feedline. Extreme miniaturization, impedance matching within lossy biological tissues, radiation efficiency, and compliance with Specific Absorption Rate (SAR) safety guidelines are some of the significant challenges faced by implantable antennas that are specifically addressed by the proposed design. With the meander‐line topology and shorting pins, the antenna can be made smaller while maintaining dependable performance for biomedical implant applications. The antenna's minimal overall tiny size of 4 × 4 × 0.5 mm 3 allow it to be incorporated into modern medical and compact appliances. The antenna has been evaluated in tissues using an actual human body phantom simulator and at various levels. The suggested antenna was constructed, optimized, and modeled with the layers of skin, fat, liver, muscle, and brain phantom in account. Additionally, the antenna's SAR is determined and examined at both 1 and 10 g conditions. With the maximum permitted power input of 6.17 mW (1 g) and 36.3 mW (10 g) at 2.42 GHz, the SAR values are within the safety standards. The suggested antenna facilitates an energy‐efficient solution and runs at low power levels. Biomedical applications can benefit from this small, minimized implanted antenna.
Abstract This paper explores the impact of natural phenomena on signal data transmission across various ionospheric communication channels. The study focuses on three primary phenomena: meteor trails prevalent in the middle‐ and lower‐latitude ionosphere, bubble and plume structures observed around the equatorial ionosphere, and magnetic‐storm‐induced plasma structures excited at higher latitudes. These phenomena contribute substantially to two types of fast fading: flat and frequency‐selective. The relationships between signal intensity, signal intensity scintillation index, and the ‐factor of fast frequency‐selective fading are analytically derived. The mutual spectral characteristics of these parameters are also analyzed, considering their dependence on the carrier frequency of signals passing through channels affected by fading caused by natural clutter phenomena. Furthermore, the paper investigates the close relationship between the ‐factor of fast fading, which characterizes multiplicative noise, and data stream parameters such as capacity, spectral efficiency, and bit error rate. The effects of natural clutter phenomena on signal data transmission are analyzed across various ionospheric communication channels with fading at different latitudes. The study considers two practical frequency bands: HF , commonly used for “ionospheric weather” monitoring and ionospheric radiolocation using ionosondes, and UHF/SHF typically employed for land‐satellite communication links.
Abstract Early detection of tumors improve treatment efficiency and the prognosis of tumor recurrence. Microwave imaging (MWI) is an effective modality in detection of tumor. Yet, this is challenged by strong signal attenuation and scattering caused by the dielectric heterogeneity of the human head. In this paper, we report a left‐hand circularly polarized stacked antenna operating in the L‐band (1.1–1.6 GHz) designed to improve signal penetration and reduce multipath interference. The reported antenna comprises of an electromagnetic coupling (EMC) stacked configuration with dual radiating patches separated by a foam spacer, a modified ground plane, and low‐loss Rogers RT/duroid 6010 substrates. Strategic slotting in both the radiating patches and ground plane improves current distribution and impedance matching. This results in an average VSWR of 1.2 and a minimum return loss of dB at resonance. Full‐wave simulations and experimental validation demonstrate stable circular polarization, improved radiation sensitivity, and enhanced detection capability for deep‐seated tumors as small as 2 mm. These findings indicate that the proposed antenna design is well‐suited for integration into non‐invasive MWI‐based diagnostic systems.
Abstract This letter provides the design of a planer ultra‐wideband horizontally polarized (HP) omnidirectional antenna. To obtain the omni‐directional radiation performance, a circular array composed of 12 Vivaldi units is proposed. And a 1‐to‐12 ultrawideband power divider is designed to achieve ultra‐wide impedance matching bandwidth. The antenna is designed and measured with bandwidth of 134.3% (1.96–9.98 GHz) for |S 11 | < −10 dB. And a qualified omnidirectional radiation performance with gain variation ≤3 dB is achieved over the frequency of 1.96–8.1 GHz. Meanwhile, the peak gain of 3.42 dBi and a compact size of π × (0.43 λ 0 ) 2 × 0.0066 λ 0 (λ 0 is the lowest working frequency of the antenna) are achieved for this antenna.
Abstract Accurate specification of the ionosphere is essential for mitigating the impacts of space weather on communication, navigation, and surveillance systems. ANCHOR is a newly developed ionospheric data assimilation (DA) model from the U.S. Naval Research Laboratory that fuses measurements from ionosondes, radio occultation, and Global Navigation Satellite System (GNSS) receivers into a PyIRI‐based background via a Kalman filter. To support model validation in both reproduction and standardization, PyVALION (Python VALidation for IONosphere) is introduced—a community‐oriented tool built on a forward‐operator framework that projects model output into observation space for statistically robust comparison. Currently, PyVALION directly supports validation of models producing F2, F2, and thickness parameters and , with extension to other parameters for future releases. ANCHOR's performance is assessed using electron density parameters (F2 and F2) from the Global Ionospheric Radio Observatory (GIRO) ionosonde network. Four multi‐day geophysical events are analyzed to evaluate model behavior under varying solar and geomagnetic conditions. Results show that ANCHOR significantly reduces both bias and root mean square error (RMSE) relative to the PyIRI background. Separating day and night conditions reveals that background bias is larger during daytime, and data assimilation proves particularly effective at correcting this bias when it is most significant. A controlled synthetic experiment confirms that forward‐operator errors in PyVALION are negligible compared to observed residuals, validating its use as a precision‐grade evaluation tool. These findings establish PyVALION as a reproducible and scalable framework for ionospheric model assessment and confirm ANCHOR's capability to enhance real‐time ionospheric specification.
Abstract Accurately estimating the propagation direction of plasma waves is essential for preserving space infrastructure, as it provides insight into the global spatial distribution of plasma in geospace. This study proposes an improved wave distribution function (WDF) estimation algorithm based on the conventional Markov random field (MRF) model. In the MRF‐based WDF method, the optimal WDF is obtained by minimizing an objective function that balances the discrepancy between theoretical and observed spectral matrices with the smoothness of the WDF. We introduce a data‐driven approach to automatically determine the weighting between these terms by minimizing the generalization error. Validation using computer‐generated data demonstrates that the proposed method yields high‐quality WDF estimates, as confirmed through visual evaluation.
Abstract Some known factors influencing the accuracy of D‐ and E‐region ionospheric radio absorption estimates at auroral latitudes, determined from Riometer (Relative Ionospheric Opacity Meter) measurements of incident cosmic noise, are briefly reviewed. Then, measured and modeled height distributions of electron concentration at auroral latitudes, corresponding to various levels of auroral radio‐wave absorption determined by riometers, are analyzed. It is first confirmed that riometer absorption can be equated to vertical incidence absorption on the upward or downward legs. Then the frequency dependence of absorption is derived for the Medium Frequency (MF) and High Frequency (HF) ranges at vertical incidence. In both cases, absorption values at different frequencies are calculated using the phase‐integral method, incorporating realistic height distributions of effective electron‐neutral particle collision frequencies. The results indicate that the frequency dependence deviates from that observed at low and middle latitudes, particularly when absorption levels are high. It is proposed that the found changes to the frequency dependence for auroral latitudes may be used for practical estimations of absorption on MF and HF propagation predictions at vertical incidence, based on the widely available riometer observations. In a companion paper, formulae are proposed to determine oblique‐incidence absorption in MF‐HF propagation predictions using vertical‐incidence estimates.
Abstract With the rapid expansion of LTE (Long Term Evolution) cellular and IoT (Internet of Things) enabled systems, there is a growing demand for compact, low‐cost and flexible antennas. Present antennas are commonly built using rigid substrates which are not suitable for flexible applications and offers more signal reflection rather than far field radiation. Proposed work addresses this demand by a lightweight foam substrate based flexible antenna that operates at 1.74 GHz LTE‐band 4 and additional 1.4 GHz LTE—band 11 to support (Narrow Band) NB‐IoT applications. A trident shaped antenna built using foam with 1.2 mm thickness and permittivity of 1.15. Stagewise analysis in terms of electrical and geometrical is focused to improvize current distribution, radiation characteristics with lesser reflections. Gain and radiation efficiency of about 3.9 dB and 90.6% is achieved with the final stage antenna, with an impedance bandwidth of 570 MHz. Simulated results are validated with the measured results using vector network analyzer and anechoic chamber. Simulated and measured return loss is about −42.3 and—31 dB respectively. To verify the flexibility and wearability of the antenna, conformal and SAR simulations are performed and analyzed for the real time deployment in flexible IoT and LTE cellular applications.
Abstract Artificial intelligence (AI) system design for brain tumor diagnosis by utilizing Magnetic Resonance Imaging (MRI) demands more than accuracy; it also entails the development of robust, understandable, and medically trustworthy systems capable of handling heterogeneous imaging situations. The existing models are usually task‐oriented, that is, either segmenting or classifying brain tumors in MRI images, and cannot easily generalize when faced with images from other machines and procedures. With the goal of tackling these limitations, this research seeks to develop the TRUST Brain Intelligence Framework. This proposed model is a unified architecture integrating Adaptive Contrast Harmonizer, attention‐enhanced DeepLabV3+ segmentation, EfficientNetV2‐based Scale Fusion Grader, Monte Carlo Dropout uncertainty estimation, and Grad‐CAM++ explainability. Entropy‐driven pre‐processing enhances intensity consistency and tumor boundary preservation, whereas attention‐guided segmentation and adaptive multi‐scale grading facilitate discriminative feature learning. Empirical results using the BRISC and Brain Tumor MRI data sets reveal that the proposed approach significantly outperforms traditional approaches based on convolutional neural networks and segmentation. The proposed model was found to be highly effective in terms of Dice, Intersection Over Union, accuracy, and F1‐score, and it exhibited consistent performance across heterogeneous MRI modalities. Uncertainty‐aware predictions and visual explanations further improved clinical interpretation of the results, making the framework more suitable for trustworthy AI‐assisted neuro‐oncology decision support.
This research proposes a simulation-based framework for electromagnetic (EM) wave propagation in near- and far-field regimes under varying atmospheric conditions. Within a compact-range model, temperature, pressure, and humidity profiles from radiosondes are employed to create height-dependent refractive index distributions. The propagation in a stratified dielectric medium is investigated using a frequency-domain full-wave formulation of Maxwell's equations. The transmission coefficient (S21), beamwidth, and phase delay are all analyzed numerically. The results demonstrate attenuation fluctuations of up to 12.4%, beam broadening of 3–5°, and an RMSE of 0.5–0.83 dB relative to standard conditions. The suggested approach increases prediction accuracy by ∼35% over static models and incorporates refractivity-induced phase variations. The study is based on simulation, with future experimental validation expected.
This study proposes a wave distribution function (WDF) estimation method based on minimizing the Bregman divergence in the function space. The proposed method is entirely data-driven and requires no hyperparameter tuning during the estimation process. It can be regarded as an extension of the maximum entropy method. Whitening the spectral matrix was found to significantly enhance the estimation accuracy. By replacing the likelihood or loss function of the spectral matrix with its whitened counterpart, the estimated solutions were significantly improved. This method enables the precise estimation of WDFs from electromagnetic field observations, establishing a technical foundation for diagnosing space plasma environments and forecasting space weather. To address the uncertainty arising from the limited snapshots, we derive the posterior distribution of the spectral matrix. The Bregman divergence-based framework ensures stable and interpretable estimation under both physical and statistical constraints. Numerical experiments confirmed that whitening markedly improved both accuracy and robustness in realistic random wave scenarios.
Abstract This work presents a technique to monitor ionospheric absorption in the HF radio band using ionosondes. For this purpose, the ionosondes were run in a radio‐silent mode, recording not reflections of their own emitted signals but oblique signals from transmitters of opportunity. We present first the quiet‐time data, showing what the typical diurnal variations in the spectrograms are. We then demonstrate that enhancements of ionospheric absorption resulting from either solar X‐rays or particle precipitation events can be detected in these data. We show several examples of such signatures in the spectrograms, in time series integrated over a wide band of frequencies, and in small frequency band time series. The aim of this paper is to introduce this type of observations and demonstrate the principle of its use for absorption monitoring.
Field-aligned irregularities (FAIs) are the signatures of plasma turbulence and convection in the mid- and high-latitude F-region ionosphere, and also provide coherent backscatter targets for HF radars. To serve irregularity generation and characterization studies, we conducted experiments at the High-frequency Active Auroral Research Program (HAARP) in August 2023 with the goal of identifying the optimal HAARP beam pattern for reliable generation of intense FAIs over a large geographic region. The HAARP beam patterns we tested were the commonly-used narrow beam, also known as L0, as well as the wider L1 and L2 “twisted” beam patterns. The size and intensity of the FAI region generated by each HAARP beam pattern was quantified using the Kodiak Island Super Dual Auroral Radar Network (SuperDARN) radar. Stimulated electromagnetic emissions (SEE) from heater wave-FAI scattering were also recorded using a receiver located near HAARP. The L1 beam pattern was found to produce the strongest SuperDARN backscatter over the largest region. Although the heater frequency was intended to be tuned a few hundred kHz below the F-region critical frequency (foF2) during each experiment, difficulty in estimating foF2 during the campaign likely resulted in HAARP heating at significantly different frequency ranges around foF2 during each experiment. Although this additional free parameter complicated data analysis for this study, the SuperDARN and SEE measurements have led to further inquiry into the role heater frequency plays in the artificial generation of FAIs.
Radio occultation (RO) is a very powerful technique as it offers great opportunities to study planetary atmospheres, providing information about their ionosphere and neutral atmosphere. Standard methods use a radio link at S and/or X bands between a spacecraft orbiting a planet and a ground station on the Earth. At Mars, such measurements have been conducted since the 60s. Three most recent data sets are from Mars Global Surveyor, Mars Express and the Mars Atmosphere and Volatile Evolution satellites. Furthermore, the possibility to obtain information about the Martian atmosphere with mutual RO events, using data from NASA Mars Odyssey and Mars Reconnaissance Orbiters, has been demonstrated by NASA scientists in 2015. Taking advantage of two European spacecraft in orbit around Mars, the ESA is currently performing an experiment that consists of mutual radio occultations between Mars Express (MEX) and ExoMars Trace Gas Orbiter (TGO). In preparation for MEX and TGO data inversion and analysis, a simulation-based strategy has been adopted and an algorithm, including the associated software, able to retrieve vertical electron density profiles from Doppler shift measurements has been implemented and validated. Subsequently, to test the mentioned algorithm with experimental data, the same three mutual RO events considered by NASA scientists have been re-processed. In this work, the research activities carried out through the simulation studies and the results obtained by the application of the mentioned inversion algorithm to experimental data are presented.
It is important to specify the state of the ionosphere for Over-the-Horizon communications and surveillance. Both vertical and oblique ionograms provide valuable data to this end. Oblique ionograms can probe the ionosphere in locations where it would be impractical to build a vertical ionosonde, but their measurements are not straightforward to assimilate. There are essentially two assimilation methods: the first is to transform the oblique ionogram to the corresponding vertical ionogram and assimilate it at the link midpoint. There are a few different ways to perform this transform, all of which introduce errors. The second method is to ingest the oblique ionogram directly. This method does not make these assumptions, but is more computationally expensive and is currently only implemented in the GPS Ionospheric Inversion data assimilation model. This paper quantifies the errors in the first method. This is done by simulating oblique ionograms as well as vertical ionograms at the oblique link midpoint using numerical raytracing. The oblique ionograms are transformed and compared to the verticals. Differences in the critical frequency and the height at which it occurs are studied as a function of latitude, local time, oblique link direction, and mode (O or X). This is done for ground ranges of 2,500, 1,250, 600, 300, 150, and 75 km. For 2,500 km links, we find Root Mean Square Errors for critical frequency of 0.4 MHz (9.9%), and 22 km (9.3%) for hmF2. Shorter links are less affected, but still have significant errors for ground distances over 300 km.
Recent advancements in wireless communication technologies and Internet-enabled devices have accelerated the demand for compact, efficient antennas suitable for body-centric applications. This study presents a compact, high-performance dual-frequency antenna featuring an artificial magnetic conductor (AMC) surface for next-generation wireless systems. A modified gear-wheel radiating patch on top and a partial ground plane on the bottom comprise the antenna, making it a three-layer antenna. A circular patch overlays 12 radiating teeth, enabling a dual wideband response. The AMC surface comprises a 4 & times; 4 array of unit cells, each incorporating slotted square and rectangular patches. The unit cells exhibit double-negative (DNG) properties and operate in three frequency regions. The zero-degree reflection phase occurs at 2.5 GHz, 4.1 GHz, and 6.5 GHz. The integrated antenna covers impedance bandwidths of 71.79% (2.0-4.24 GHz) and 41.67% (5.13-7.83 GHz), encompassing key wireless frequency bands, while maintaining compact dimensions of 0.554 lambda 0 & times; 0.554 lambda 0 & times; 0.135 lambda 0 at the lowest resonant frequency. A maximal gain of 8.26 dBi, a low Specific Absorption Rate (SAR) of 0.0528 W/kg, and a high FBR of 33.85 dB at 4.1 GHz are key performance characteristics. VSWR values below 2 in both bands indicate exceptional impedance matching. Experimental validation using a Vector Network Analyzer (VNA) and an anechoic chamber confirms the modeling results. The proposed antenna, owing to its low SAR and wideband performance, is suitable for body-centric wireless systems operating in standard frequency bands such as ISM, Wi-Fi 6E, and C-band.
