Receiver Pair Research Articles

Combined application of FTAN and cross-spectra analysis to ambient noise recorded by a microseismic monitoring network

A case study of seismic interferometry applied to a small microseismic monitoring network is here presented. The main objectives of this study are (i) to quantify the lateral variability of shear-wave velocities in the studied area, and (ii) to investigate the bias produced by noise directionality and non-stationarity in the velocity estimate. Despite the limited number of stations and the short-period character of the seismic sensors, the empirical Green’s functions were retrieved for all station pairs using two years of passive data. Both group and phase velocities were derived, the former using the widespread frequency-time analysis, the latter through the analysis of the real part of the cross-spectra. The main advantage of combining these two methods is a more accurate identification of higher modes, resulting in a reduction of ambiguity during picking and data interpretation. Surface wave tomography was run to obtain the spatial distribution of group and phase velocities for the same wavelengths. The low standard deviation of the results suggests that the sparse character of the network does not limit the applicability of the method, for this specific case. The obtained maps highlight the presence of a lower velocity area that extends from the centre of the network towards southeast. Group and phase velocity dispersion curves have been jointly inverted to retrieve as many shear-wave velocity profiles as selected station pairs. While the average model can be used for a more accurate location of the local natural seismicity, the associated standard deviations give us an indication of the lateral heterogeneity of seismic velocities as a function of depth. Finally, the same velocity analysis was repeated for different time windows in order to quantify the error associated to variations in the noise field. Errors as large as 4% have been found, related to the unfavorable orientation of the receiver pairs with respect to strongly directional noise sources, and to the very short time widows. It was shown that using a one-year time window these errors are reduced to 0.3%.

All-optical platform for ultrasound transmission matrix measurements

Piezoelectric ultrasound transducers are constrained by size, bandwidth, and angular response, limiting their ability to fully characterize the acoustic properties of objects. In this study, we introduce a novel modular all-optical platform for ultrasound generation and detection to overcome these limitations, demonstrating wideband operation (>50 MHz), omnidirectional response, and high signal fidelity. Ultrasound generation is performed via the optoacoustic effect by illuminating an optically absorbing coating with spatially modulated pulsed light, and ultrasound detection is carried out using a silicon-photonic acoustic detector. By illuminating patterns that span a basis and scanning the detector, the full transmission matrix is measured, consisting of the acoustic waveforms for all the transmitter–receiver pairs in the measurement geometry. Our method is experimentally demonstrated in transmission mode for beam steering, beam focusing, and imaging, achieving excellent agreement with the theory.

Combinatorial-restless-bandit-based transmitter–receiver online selection of distributed MIMO radar with non-stationary channels

In a distributed multiple-input multiple-output (MIMO) radar for tracking moving targets, optimizing sensible selections of the transmitter–receiver pairs is crucial for maximizing the sum of signal-to-interference-plus-noise ratios (SINRs), as it directly affects the tracking accuracy. In solving the trade-off between exploitation and exploration in non-stationary channels, the optimization problem is modeled by a restless multi-armed bandits model. This paper regards the estimated SINR mean reward as the state of an arm (transceiver channel). The SINR reward of each arm is estimated based on whether it is probed. A closed loop is established between SINR rewards and the dynamic states of targets, which are estimated via the interacting multiple model-unscented Kalman filter. The combinatorial optimized selection of transmitter–receiver pairs at each time is accomplished by using the binary particle swarm optimization with the SINR index fitness function, where the index represents the upper bound on the confidence of the SINR reward. Above all, a multi-group combinatorial-restless-bandit closed-loop (MG-CRB-CL) algorithm is proposed. Simulation results for different scenarios are provided to verify the effectiveness and superior performance of MG-CRB-CL.

Performance Evaluation of Carrier-Frequency Offset as a Radiometric Fingerprint in Time-Varying Channels.

The authentication of wireless devices through physical layer attributes has attracted a fair amount of attention recently. Recent work in this area has examined various features extracted from the wireless signal to either identify a uniqueness in the channel between the transmitter-receiver pair or more robustly identify certain transmitter behaviors unique to certain devices originating from imperfect hardware manufacturing processes. In particular, the carrier frequency offset (CFO), induced due to the local oscillator mismatch between the transmitter and receiver pair, has exhibited good detection capabilities in stationary and low-mobility transmission scenarios. It is still unclear, however, how the CFO detection capability would hold up in more dynamic time-varying channels where there is a higher mobility. This paper experimentally demonstrates the identification accuracy of CFO for wireless devices in time-varying channels. To this end, a software-defined radio (SDR) testbed is deployed to collect CFO values in real environments, where real transmission and reception are conducted in a vehicular setup. The collected CFO values are used to train machine-learning (ML) classifiers to be used for device identification. While CFO exhibits good detection performance (97% accuracy) for low-mobility scenarios, it is found that higher mobility (35 miles/h) degrades (72% accuracy) the effectiveness of CFO in distinguishing between legitimate and non-legitimate transmitters. This is due to the impact of the time-varying channel on the quality of the exchanged pilot signals used for CFO detection at the receivers.

Solar activity dependence for the relationship between nighttime medium-scale traveling ionospheric disturbance and sporadic E (Es) layer activities in summer during 1998–2019 over Japan

To investigate solar activity dependence of the coupling between medium-scale traveling ionosphere disturbance (MSTID) and sporadic E (Es) layer, we analyzed the total electron content (TEC) obtained from a Japanese global positioning system (GPS) receivers and ionosonde at Kokubunji (35.7° N, 139.5° E) in Japan during the summer period of May–August from 1998 to 2019. To obtain perturbation TEC caused by MSTIDs, the detrended TEC is calculated by subtracting 1-h moving averages from the measured TEC for each pair of GPS satellite and receiver. The detrended TEC data are mapped on to the geographical coordinates to make detrended 2-D maps with spatial resolution of 0.15° × 0.15° in longitude and latitude. The MSTID activity is defined as a ratio of the standard deviation to the background TEC over Kokubunji in Japan. Day-to-day variations of the MSTID activity during summer nights was compared to Es layer parameters [critical frequency (foEs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{o}Es$$\\end{document}) and Δfo-b≡foEs-fbE\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\Delta f}_{o-b}\\equiv {f}_{o}Es-{f}_{b}E$$\\end{document}, where fbEs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{b}}}Es$$\\end{document} is blanketing frequency] derived from ionosonde station at Kokubunji. We have found that the correlation coefficient between the MSTID activity and foEs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{o}Es$$\\end{document}(Δfo-b\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\Delta f}_{o-b}$$\\end{document}) between 1998 and 2019 is 0.5 3 (0.46) on average, suggesting that there is an electrodynamical coupling between the Es layer and F region could generate nighttime MSTIDs. We also have found that the correlation coefficient positively correlates with solar activity. This finding indicates that in the high solar activity conditions, when the growth rate of Perkins instability is relatively low, generation of the polarization electric fields in the Es\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Es$$\\end{document} layer could play a more important role to grow MSTIDs than in the low solar activity conditions.Graphical

GNSS-Based Narrow-Angle UV Camera Targeting: Case Study of a Low-Cost MAD Robot.

One of the key challenges in Multi-Spectral Automatic Diagnostic (MAD) robot design is the precise targeting of narrow-angle cameras on a specific part of the equipment. The paper shows that a low-cost MAD robot, whose navigation system is based on open-source ArduRover firmware and a pair of low-cost Ublox F9P GNSS receivers, can inspect the 8 × 4 degree ultraviolet camera bounding the targeting error within 0.5 degrees. To achieve this result, we propose a new targeting procedure that can be implemented without any modifications in ArduRover firmware and outperforms more expensive solutions based on LiDAR SLAM and UWB. This paper will be interesting to the developers of robotic systems for power equipment inspection because it proposes a simple and effective solution for MAD robots' camera targeting and provides the first quantitative analysis of the GNSS reception conditions during power equipment inspection. This analysis is based on the experimental results collected during the inspection of the overhead power transmission lines and equipment inspections on the open switchgear of different power plants. Moreover, it includes not only satellite, dilution of precision, and positioning/heading estimation accuracy but also the direct measurements of angular errors that could be achieved on operating power plants using GNSS-only camera targeting.

Shear Wave Velocity Determination of a Complex Field Site Using Improved Nondestructive SASW Testing.

The nondestructive spectral analysis of surface waves (SASW) technique determines the shear wave velocities along the wide wavelength range using Rayleigh-type surface waves that propagate along pairs of receivers on the surface. The typical configuration of source-receivers consists of a vertical source and three vertical receivers arranged in a linear array. While this approach allows for effective site characterization, laterally variable sites are often challenging to characterize. In addition, in a traditional SASW test configuration system, where sources are placed in one direction, the data are collected more on one side, which can cause an imbalance in the interpretation of the data. Data interpretation issues can be resolved by moving the source to opposite ends of the original array and relocating receivers to perform a second complete set of tests. Consequently, two different Vs profiles can be provided with only a small amount of additional time at sites where lateral variability exists. Furthermore, the testing procedure can be modified to enhance the site characterization during data collection. The advantages of performing SASW testing in both directions are discussed using a real case study.

GNSS signal ray-tracing algorithm for the simulation of satellite-to-satellite excess phase in the neutral atmosphere

Traditionally, GNSS space-based and ground-based estimates of tropospheric conditions are performed separately. It leads to limitations in the horizontal (e.g., a single space-based radio occultation profile covers a 300 km slice of the troposphere) and vertical resolution (e.g., ground-based estimates of troposphere conditions have spacing equal to stations’ distribution) of the tropospheric products. The first stage to achieve an integrated model is to create an effective 3D ray-tracing algorithm for the satellite-to-satellite (radio occultation) path reconstruction. We verify the consistency of the simulated data with the RO observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-1) Data Analysis and Archive Center (CDAAC) in terms of excess phase and bending angle. The results show that our solution provides an effective RO excess phase, with a relative error varying from 35% at the height of 25–30 km (1.0–1.5 m) to 0.5% at heights 5–10 km (0.1–1 m) and 14 to 2% at heights below 5 km (2–14 m). The bending angle retrieval on simulated data attained for high-resolution ray-tracing, bias lower than 2% with respect to the observed bending angle. The optimal solution takes about 1 s for one transmitter–receiver pair with a tangent point below 5 km altitude. The high-resolution processing solution takes 3 times longer.

Sensitivity-Adjustable force sensors based on one-dimensional superlattice structures

A proposed force sensor contains a one-dimensional periodic superlattice and piezoceramic wave generator–receiver pair. For force sensing, the strain induced by an applied force in the phononic structure causes stop band frequency variations because the sound velocity varies with the induced strain (stress). When the input wave frequency coincides with the margins of the stop band, it results in noticeable variations in the wave transmission rate with the applied strain. As a result, sensitivity, represented by the transmission rate variation, can be adjusted by introducing distinct acoustic wave frequencies within a phononic structure. A high sensitivity level is suitable for the detection of light forces. Conversely, when dealing with large force measurements, the sensitivity should be reduced to prevent saturation. This study presents theoretical models for analyzing the variation of the transmission rate with frequency and the variation of the stop band with different acoustic parameters. To enhance the signal reaching the receiver, a piezoceramic disk whose axial resonance frequency corresponds to the frequency of the stop band margin is designed. The results indicated that the sensitivity of a prototype of the designed force sensor varied from 15 to 45 mV/N under a compressive force of 50 N.

Investigating Diurnal and Seasonal Cycles of Vegetation Optical Depth Retrieved From GNSS Signals in a Broadleaf Forest

AbstractVegetation Optical Depth (VOD) has emerged as a valuable metric to quantify water stress on vegetation's carbon uptake from a remote sensing perspective. However, existing spaceborne microwave remote sensing platforms face limitations in capturing the diurnal VOD variations and global products lack site‐level validation against plant physiology. To address these challenges, we leveraged the Global Navigation Satellite System (GNSS) L‐band microwave signal, measuring its attenuation by the canopy of a temperate broadleaf forest using a pair of GNSS receivers. This approach allowed us to collect continuous VOD observations at a sub‐hourly scale. We found a significant seasonal‐scale correlation between VOD and leaf water potential. The VOD diurnal amplitude is affected by soil moisture, plant transpiration and leaf surface water. Additionally, VOD can help independently estimate plant transpiration. Our findings pave the way for a deeper understanding of response of the vegetation to water stress at finer temporal scales.

The Marmara Sea basin as a regional depression constrained from ambient noise correlation tomography

SUMMARY We computed a 3-D shear wave velocity model of the Marmara Sea region from ambient noise tomography. The correlations of up to 8 yr of vertical-component seismic recordings from 80 broad-band stations provided Rayleigh wave group velocity measurements in the period band 6–21 s at more than 1400 selected virtual source–receiver pairs. Rayleigh wave group velocity maps were used to derive a shear wave velocity model through simulated annealing inversion. The resulting crustal model provides coverage of the Marmara Sea along with its surrounding regional tectonic features. This allows for an investigation of the spatial extents of the Marmara Sea on a scale larger than that of basins. The low-velocity structures of the Marmara Sea and the Thrace Basins are coeval to a depth of approximately 9 km. The crustal velocities beneath the Marmara Sea basins exhibit a low vertical gradient and smooth horizontal variations. The regional tectonic structures, such as Istranca Massif, Istanbul and Sakarya Zones, display sharp velocity contrasts with the lower velocity crust beneath the Marmara Sea. The observed low crustal velocities, along with depth variations of the velocity isosurfaces (i.e. 3.4 km s−1) indicate that the Marmara region is a structural depression much deeper and larger than the three basins of the North Marmara Trough. The North Anatolian Fault Zone is unlikely to be the primary factor contributing to the origin of this significant depression, as the basin's development appears to have occurred before the fault propagated into the region.

Fastener hole inspection using 2D phased array

Fastener hole cracks, resulting from stress concentration around the hole, are potential sources of failure in mechanical structures. Traditionally, inspection involves manually moving a single ultrasonic probe around the hole or using an eddy current probe to move through the hole. However, single ultrasonic probe inspection is time-consuming and needs skilful operators. Eddy current inspection requires the removal of the fastener in advance and does not allow easy crack sizing and visualization. This paper proposes a novel approach that combines a 2D phased array with a coupling block to enable complete fastener hole inspection from a single array position. A single full matrix capture (FMC) data set is collected, from which 3D volumetric images are formed. To obtain clear images of the geometry and potential defects, half-skip and multi-skip total focusing method imaging algorithms are investigated together with transmitter–receiver pairs selection methods to detect different target features. Experimental results demonstrate that the proposed inspection approach can eliminate dead zone problems and capture all target information from a single inspection position. These methods show great potential in enhancing the efficiency and reliability of fastener hole inspections.

Adversarial AI applied to cross-user inter-domain and intra-domain adaptation in human activity recognition using wireless signals.

In recent years, researchers have successfully recognised human activities using commercially available WiFi (Wireless Fidelity) devices. The channel state information (CSI) can be gathered at the access point with the help of a network interface controller (NIC card). These CSI streams are sensitive to human body motions and produce abrupt changes (fluctuations) in their magnitude and phase values when a moving object interacts with a transmitter and receiver pair. This sensing methodology is gaining popularity compared to traditional approaches involving wearable technology, as it is a contactless sensing strategy with no cumbersome sensing equipments fitted on the target with preserved privacy since no personal information of the subject is collected. In previous investigations, internal validation statistics have been promising. However, external validation results have been poor, due to model application to varying subjects with remarkably different environments. To address this problem, we propose an adversarial Artificial Intelligence AI model that learns and utilises domain-invariant features. We analyse model results in terms of suitability for inter-domain and intra-domain alignment techniques, to identify which is better at robustly matching the source to target domain, and hence improve recognition accuracy in cross-user conditions for HAR using wireless signals. We evaluate our model performance on different target training data percentages to assess model reliability on data scarcity. After extensive evaluation, our architecture shows improved predictive performance across target training data proportions when compared to a non-adversarial model for nine cross-user conditions with comparatively less simulation time. We conclude that inter-domain alignment is preferable for HAR applications using wireless signals, and confirm that the dataset used is suitable for investigations of this type. Our architecture can form the basis of future studies using other datasets and/or investigating combined cross-environmental and cross-user features.

Stoneley wave detection by acoustic Interferometry: Estimation of shear velocity of a geological formation

Both active and passive full waveform acoustic loggings (FWAL), complemented by a flow log, were conducted in a borehole of an experimental site located in the Cher region (France).. The acoustic tool used for the FWAL experiments is a flexible monopole tool holding a pair of piezoelectric receivers and a magnetostrictive transducer. The tool was modified to perform both active and passive FWAL. For passive acoustic logging, several runs were recorded to obtain a set of acoustic noise sections. As the noise is simultaneously recorded by two receivers of the tool, an interference noise section was elaborated by correlating or deconvolving the pair of signals and then summing these pairs of acoustic traces at each depth. This procedure, which can be interpreted as an interferometry analysis, points out the presence of low-frequency waves identified as Stoneley waves. The velocity and RMS amplitude of the Stoneley wave were computed at each depth. It is shown that: 1- the Stoneley wave velocity obtained in passive mode can be used to estimate the shear velocity of the formation, 2- the RMS amplitude and velocity variations of the Stoneley waves are strongly correlated with the variations of the flowmeter.

Noise correlation analysis: the role of the inflection points in the formation of zero-time waves

SUMMARY The cross-correlation function (CCF) of seismic noise recorded at two stations often exhibits a prominent burst of energy near zero time. However, the excitation mechanism of such unexpected signals remains unclear. In this study, we demonstrate that this phenomenon arises from constructive interference of body waves radiating from sources near the mid-vertical plane between two receivers. This interference is driven by the occurrence of an inflection point effect in the traveltime difference between waves reaching the two receivers. When noise source distribution is asymmetric, the cumulative effect of such sources results in the generation of zero-time waves. It is noteworthy that a noise source distribution that is either uniform or follows a cosϕ pattern, with ϕ denoting the azimuth from the centre of the receiver pair, can counteract the formation of these waves. Additionally, our investigation reveals that this constructive interference does not occur in the correlation of 2-D surface waves. We used numerical simulation to support our findings. In conclusion, this study offers a theoretical explanation for the emergence of zero-time waves on the CCF and provides efficient means of investigating the extraction of inter-receiver waves with uneven noise source distribution.

The Effect of Multiple Cycles of Surfactant-Alternating-Gas Process on Foam Transient Flow and Propagation in a Homogeneous Sandstone

Summary Years of laboratory studies and field tests show that there is still uncertainty about the ability of foam to propagate deep into a reservoir. Many factors have been identified as potential causes of nonpropagation, the most concerning being the lack of sufficient pressure gradient required to propagate foam at locations far from the point of injection. Most researchers that investigated foam propagation did so by coinjecting surfactant and gas. Coinjection offers limited information about transient foam processes due to limitations in the experimental methods needed to measure foam dynamics during transient flow. Foam injection by surfactant-alternating-gas (SAG) has proven to be more effective and common in field application. Repeated drainage and imbibition cycle offer a more favorable condition for the quick generation of foam. Foam can also be propagated at a lower pressure gradient in SAG mode. The objective of this study is to experimentally investigate how transient foam dynamics (trapping, mobilization, and bubble texture) change with multiple cycles of SAG and also with distance from the point of injection. A pair of X-ray source and receiver, differential pressure transducers, and electrical resistance sensors were placed along a 27-cm long, homogeneous, and high-permeability (KL = 70 md) Berea sandstone core. Foam was then generated in situ by SAG injection and allowed to propagate through the core sample under a capillary displacement by brine (brine injection rate = 0.5 cm3/min, Nca = 3×10-7). By use of a novel analytical method on coreflood data obtained from axial pressure and saturation sensors, we obtained trapped foam saturation, in-situ foam flow rates, apparent viscosities, and inferred qualitative foam texture at different core sections. We then observed the following: (i) Maximum trapped foam is uniform across the core sections, with saturation ranging from 47% to 52%. At the vicinity of foam injection, foam apparent viscosity is dominantly caused by gas trapping. At locations farther away, foam apparent viscosity is dominated by both gas trapping and refinement of foam texture. (ii) Cyclic injection of foam further enhances the refinement of foam texture. (iii) Textural refinement increases foam apparent viscosity as it propagates away from the point of injection. (iv) As the foam strength increases, the average gas flow rate in the core sample decreases from 0.5 cm3/min to 0.06 cm3/min. (v) There is no stagnation of foam as remobilization of trapped gas occurs during each cycle at an average flow rate of 0.002 cm3/min.

Damage imaging using multipath-scattered Lamb waves under a sparse reconstruction framework

This paper presents a damage sparse imaging method using multipath-scattered Lamb waves. It leverages a large number of echoes and reverberations in the recorded signal that may be usually ignored in conventional methods. First, reflections of Lamb waves at free edges are viewed as waves transmitted from a virtual transducer which is located at the mirror point of the actual one. On this basis, an optimized transducers-layout strategy is proposed based on the multipath propagation model of the Lamb wave. Benefiting from that, the direct damage-scattered wave and several waves scattered by both the damage and edges could be separately identified in the time domain, and further, each wave could be matched with a sensing path (either actual or virtual) in the expanded sensor network. Subsequently, a dictionary is constructed from the Lamb wave propagation and scattering model. By solving the sparse reconstruction problem, the pixel value of each point in the region of interest is obtained, and the whole area can be finally visualized. The proposed method is validated using experiments conducted on an aluminum plate with simulated damages. Results show that the damages can be correctly detected and accurately localized with only a single transmitter–receiver pair.

Soundscapes from deep-water moored receivers in the vicinity of the New England Seamounts

Acoustic noise interferometry in the ocean relies on synchronized measurements of ambient sound at spatially separated points to passively measure the acoustic travel times between receiver locations. A network of four autonomous, moored, near-bottom acoustic receivers were deployed in the vicinity of the New England Seamounts for 52 days as part of the larger New England Seamount Acoustics (NESMA) Pilot experiment. Receiver depths ranged from 2500–4475 m. The moored receivers provided stable observation platforms in a region with highly variable and occasionally strong currents. The noise interferometry network aimed to investigate the feasibility of utilizing passive acoustic remote sensing methods to study a highly dynamic ocean region with complex bathymetry. Strong ocean variability at the experimental site was induced by its proximity to the Gulf Stream. This paper presents the initial analysis of water-depth dependence and intermittency of ambient sound spectra and spectrograms on near-bottom receivers. The relation between the spectral features of ambient sound and ocean dynamics is explored. Additionally, the feasibility of using the onboard Chip Scale Atomic Clocks to synchronize the experimental data and calculate broadband noise cross-correlation functions for each of the receiver pairs is discussed.

Studying the Day‐To‐Day Effect of F Region Meridional Neutral Wind on the Formation of Post‐Sunset Equatorial Ionospheric Irregularities

AbstractThe unique properties of the equatorial ionosphere continue to be of great concern to users of space‐based technologies that rely on the ionosphere. Due to the lack of observational data, the influence of the F region meridional wind on the formation of the post‐sunset equatorial ionospheric irregularities has not yet been fully investigated. Therefore, in this work, the effect of dusk‐time F region meridional wind on the formation of post‐sunset equatorial ionospheric irregularities is studied. A pair of GNSS receivers from each of the three longitudinal sectors, such as Eastern Africa, Eastern South America, and Eastern Asia is considered to evaluate the rate of change of the TEC index (ROTI) in the years from 2010 to 2013. Observational meridional wind speed from the Gravity Field and Steady‐State Ocean Circulation Explorer satellite and electron density from the Communication/Navigation Outage Forecasting System are also used for the same longitudinal sectors. In the three longitudinal sectors, correlational analysis is carried out between the ROTI and the latitudinal gradient of the meridional wind speed. Based on observational data, we find that ionospheric irregularities are suppressed when the latitudinal gradient of the meridional wind is positive, while irregularities often occur when the latitudinal gradient is negative.

Estimation of the Azimuth Angle of the Arrival Direction for an Ultrasonic Signal by Using Indirect Determination of the Phase Shift

The paper presents and discusses a method of azimuth determination of ultrasonic echo arrival in air. The basis of the presented approach is the assumption that the received signal is a narrowband one. In this way, the direction of the signal arrival can be determined based on its phase shift using two receivers. When the distance between the receivers exceeds half of the wavelength of the received signal, a problem of ambiguity in determining the angle of arrival arises. To solve this, a method using multiple pairs of receivers was used. Its robustness and temperature dependence is analysed. The most important advantages of the presented approach are simplified computations and low hardware requirements. Experimental data made it possible to show that for strong echoes, the accuracy is higher than 0.5X. In the case of weak echos, it is reduced to about 2X. Because the method is based on phase shift measurement, the ultrasonic sonar that uses this method can be compact in size. Moreover, owing to the theoretical analysis, certain properties of the mutual location of the receivers were found and formally proved. They are crucial for determining proper receivers’ inter-distances.