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Orbital angular momentum (OAM) multiplexing/demultiplexing technology is crucial in increasing the data transmission rate for acoustic communication. However, the existing acoustic OAM multiplexing/demultiplexing is still limited to eight channels, and its combination with other communication techniques has not been verified experimentally. Here, we experimentally demonstrate the demultiplexing of up to 40 data channels using OAM multiplexing combined with wavelength-division multiplexing (WDM). The proposed demultiplexing method is improved from the virtual rotating receiver method used to detect the rotational Doppler effect of the OAM waves by a static array of microphones. The improved method has overcome the challenges of insufficient response, cross-talk, and signal aliasing, which often hinder the existing demultiplexing methods in the low-frequency region. The proposed demultiplexing method can be used to quickly decode the massive information concealed in a large number of acoustic OAM-WDM channels. Our work also shows practical prospects in underwater communication applications, especially in long-range communication using acoustic waves at low frequencies. [ABSTRACT FROM AUTHOR]

This research aimed to improve diagnosis of non-alcoholic fatty liver disease (NAFLD) by deep learning with ultrasound Images and reduce the impact of the professional competence and personal bias of the diagnostician. Three convolutional neural network models were used to classify and identify the ultrasound images to obtain the best network. Then, the features in the ultrasound images were extracted and a new convolutional neural network was created based on the best network. Finally, the accuracy of several networks was compared and the best network was evaluated using AUC. Models of VGG16, ResNet50, and Inception-v3 were individually applied to classify and identify 710 ultrasound images containing NAFLD, demonstrating accuracies of 66.2%, 58.5%, and 59.2%, respectively. To further improve the classification accuracy, two features are presented: the ultrasound echo attenuation coefficient (θ), derived from fitting brightness values within sliding region of interest (ROIs), and the ratio of Doppler effect (ROD), identified through analyzing spots exhibiting the Doppler effect. Then, a multi-input deep learning network framework based on the VGG16 model is established, where the VGG16 model processes ultrasound image, while the fully connected layers handle θ and ROD. Ultimately, these components are combined to jointly generate predictions, demonstrating robust diagnostic capabilities for moderate to severe fatty liver (AUC = 0.95). Moreover, the average accuracy is increased from 64.8% to 77.5%, attributed to the introduction of two advanced features with domain knowledge. This research holds significant potential in aiding doctors for more precise and efficient diagnosis of ultrasound images related to NAFLD. [ABSTRACT FROM AUTHOR]

Optical orthogonal time frequency space (OTFS) is better at handling Doppler shifts and multipath fading, making signals more reliable and valuable in places with much movement beyond the fifth generation (B5G). Using practical power amplifiers at the transmitter causes power inefficiency and signal distortion because of the OTFS system’s high peak-to-average power ratio (PAPR), severely reducing system efficiency. Combining partial transmit sequence (PTS) and selective mapping (SLM), a technique known as PTS+SLM, reduces peak power. While SLM generates numerous phase-modulated signal candidates and chooses the one with the lowest PAPR, PTS separates the signal into sub-blocks and optimizes their phases to decrease peak power. With few changes to the signal structure, this dual strategy effectively reduces PAPR while improving power spectral density (PSD) efficiency. As a result, we ensure the accuracy and dependability of the transferred data by maintaining the bit error rate (BER). Fractal optimization methods could be applied to these algorithms. For example, fractal-inspired optimization techniques might be used to explore the phase space more effectively or to discover new phase sequences that result in lower PAPR. According to the simulation findings, the suggested PTS+SLM method works better than the traditional PTS and SLM methods. [ABSTRACT FROM AUTHOR]

The Doppler effect reveals the law that light waves undergo frequency changes in interacting with motion, which is highly significant in velocity detection and has applications in fields such as astrophysics, aerospace, and advanced manufacturing. A typical Doppler velocimetry involves illuminating a moving object with interference fringes generated based on phase gradients while detecting the frequency shift of scattered light to determine the velocity. Beyond the spatial phase distributions, the spatial amplitude is a unique dimension of light fields that can be directly controlled, but its application prospects in motion detection are rarely revealed, particularly in both the magnitude and orientation of velocity measurements. In this work, a remote vector velocimeter based on spatially structured amplitude fields is proposed for monitoring angular velocities of objects in situ. Guided through a 40 km seven‐core fiber, the structured beams with spatially‐distributed amplitude are constructed at the remote fiber facet by adjustable mode excitation in outer cores, and the Doppler signals reflected by the target are collected and transmitted back by the inner core, enabling the remote measurement of rotational motion vectors with a probe‐signal‐integrated configuration. These results suggest the great potential of spatial amplitude fields in motion detection, the cost‐efficient and compact velocimetry may contribute to the communities of optical sensing and engineering. [ABSTRACT FROM AUTHOR]

This paper proposes a specific algebraic structure and demonstrates its nature as an extension field, enabling the construction of Golomb Costas (GC) arrays. It provides detailed instructions and examples for constructing GC arrays using this extension field, along with a corresponding flowchart. Additionally, the paper conducts a thorough analysis, incorporating calculations and comparisons, to evaluate the autocorrelation of a GC array derived from the extension field compared to that of a diagonal frequency hopping array. The analysis reveals the superior autocorrelation properties of GC arrays based on the extension field. Furthermore, the paper establishes a mathematical model for the signal coded by the frequency hopping array and subsequently simulates and compares the ambiguity function of the signal coded by a GC array with that of a signal coded by a diagonal frequency hopping array. This comparison underscores the thumbtack ambiguity function of frequency hopping signal coded by a GC array. Moreover, the paper thoroughly investigates the relationship between the correlation function of GC arrays and the roots of an algebraic equation in a finite field, and strictly proves the ideal autocorrelation properties of Golomb Costas arrays. [ABSTRACT FROM AUTHOR]

In today's modern and globalized world, connectivity is a key factor for businesses, production facilities, sensor networks, and ordinary people. However, there are still populated areas which are not covered by ground-based telecommunications infrastructure. This is where telecommunication satellite constellations come in, as they can provide coverage to remote and uninhabited regions and fill existing connectivity gaps to ensure data transfer. LoRa is one of the technologies designed for data transmissions over long distances with low power consumption. Alongside with other technologies of the Low-Power Wide Area Network family, it is widely used for Internet of Things applications. LoRa chirp spread spectrum modulation is robust against the Doppler frequency shifts encountered in low earth orbits, and it has already been used in IoT satellite communications. Due to the low transmitted signal power, the achieved data rate is not high, making it a suitable technology for telecommunications payloads on CubeSat platforms for messaging services. As compared to existing traditional communication satellite systems, CubeSat constellations are low-cost and may offer an affordable connectivity service to developing regions. This study is divided in two parts. In Part I the demand model is built based on the population distribution not covered by cell towers. The LoRa link performance is analyzed, considering the impact of LoRa channel parameters variation, such as spreading factor and channel bandwidth, while satellite orbital height, transmission antenna beamwidth, and transmitter peak power have a direct impact on the payload mass. Among thousands of possible configurations, 73 feasible payload designs have been downselected. In Part II of the study, the satellite mass and the total system cost are estimated based on the payload parameters obtained. Messages transmission simulation via a constellation is conducted in order to identify optimal constellation architectures for messaging service, as well as the main drivers of the system economic profitability. The presented analysis results provide a deeper understanding of LoRa connectivity advantages and limitations together with the performance drivers, which will support the optimization of future LoRa-based satellite communication systems and other IoT satellite constellations. • A general framework for communications satellite constellation is suggested. • LoRa technology feasibility for messaging service via satellites has been proved. • LoRa channel capacity is sufficient for 1 message per day per uncovered user. • SF drives achievable channel capacity, orbit height and antenna beamwidth. • 73 payloads were downselected, with 2x2-8x8 patch antennas and transceivers of 1-2W. [ABSTRACT FROM AUTHOR]

This study aimed to explore the Doppler shift at different wavelengths in the IRIS solar spectrum and analyze the evolution and statistical properties of longitudinal oscillations with damping in chromosphere and transition region bright points (BPs). Understanding the damping mechanism and statistical properties of oscillations in BPs is crucial for gaining insights into the dynamics and energy transport within the solar atmosphere.This study explores the Doppler shift at different wavelengths in the solar spectrum of the Interface Region Imaging Spectrograph (IRIS) and implements a comprehensive consideration of Doppler velocity oscillations in the IRIS channels. This comprehensive consideration reveals a propagating periodic perturbation in a large number of chromosphere and transition region (TR) bright points (BPs). To the best of our knowledge, this is the first investigation of the longitudinal oscillations with damping in BPs using comprehensive consideration of the Doppler velocity at various wavelengths. The phenomena of attenuation in the red and blue Doppler shifts of the solar wavelength range were seen several times during the experiments. We utilized deep learning techniques to examine the statistical properties of damping in network and internetwork BPs, as well as active, quiet areas, and coronal hole areas. Our results revealed varying damping rates across different regions, with 80% of network BPs exhibiting damping in quiet areas and 72% in coronal hole areas. In active areas, the figure approached 33%. For internetwork BPs, the values were 65%, 54%, and 63% for quiet areas, coronal hole areas, and active regions, respectively. The damping rate in active regions is twice as high at Internetwork's BPs. The damping components in this study were computed, and the findings show that the damping at all points is underdamped. The observed damping process suggests the propagation and leaking of energetic waves out of TR bright points, potentially contributing to the energy transport from the bright magnetic footpoints to the upper chromosphere, transition region, and corona. [ABSTRACT FROM AUTHOR]

Brown dwarfs and planetary-mass companions display rotationally modulated photometric variability, especially those near the L/T transition. This variability is commonly attributed to top-of-atmosphere (TOA) inhomogeneities, with proposed models including patchy thick and thin clouds, planetary-scale jets, or chemical disequilibrium. Surface mapping techniques are powerful tools to probe their atmospheric structures and distinguish between models. One of the most successful methods for stellar surface mapping is Doppler imaging, where the existence of TOA inhomogeneities can be inferred from their varying Doppler shifts across the face of a rotating star. We applied Doppler imaging to the nearest brown dwarf binary WISE 1049AB (also known as Luhman 16AB) using time-resolved, high-resolution spectroscopic observations from Gemini IGRINS, and obtained for the first time H - and K -band simultaneous global weather map for brown dwarfs. Compared to the only previous Doppler map for a brown dwarf in 2014 featuring a predominant mid-latitude cold spot on WISE 1049B and no feature on WISE 1049A, our observations detected persistent spot-like structures on WISE 1049B in the equatorial to mid-latitude regions on two nights, and revealed new polar spots on WISE 1049A. Our results suggest stability of atmospheric features over time-scale of days and possible long-term stable or recurring structures. H - and K -band maps displayed similar structures in and out of CO bands, indicating the cold spots not solely due to chemical hotspots but must involve clouds. Upcoming 30-m extremely large telescopes will enable more sensitive Doppler imaging of dozens of brown dwarfs and even a small number of directly imaged exoplanets. [ABSTRACT FROM AUTHOR]

We report the discovery of the new accreting millisecond X-ray pulsar SRGA J144459.2−604207 using data of the SRG/ART-XC. The source was observed twice in February 2024 during the declining phase of the outburst. The timing analysis revealed a coherent signal near 447.9 Hz modulated by the Doppler effect due to the orbital motion. The derived parameters for the binary system are consistent with a circular orbit with a period of ∼5.2 h. The pulse profiles of the persistent emission, showing a sine-like part during half a period with a plateau in between, can be well modeled by emission from two circular spots that are partially eclipsed by the accretion disk. Additionally, during our observations with an exposure of 133 ks, we detected 19 thermonuclear X-ray bursts. All bursts have similar shapes and energetics, and none show any signs of an expanding photospheric radius. The burst recurrence times decreases linearly from ∼1.6 h at the beginning of observations to ∼2.2 h at the end and anticorrelate with the persistent flux. The spectral evolution during the bursts is consistent with the models of the neutron star atmospheres that are heated by accretion and implies a neutron star radius of 11–12 km and a distance to the source of 8–9 kpc. We also detected coherent pulsations during the bursts and showed that the pulse profiles differ substantially from those observed in the persistent emission. However, we could not find a simple physical model explaining the pulse profiles detected during the bursts. [ABSTRACT FROM AUTHOR]

Compact laser interferometers with large dynamic range are one of the core emerging tools to improve low frequency performance in gravitational wave detectors by providing local displacement sensing with sub 1 pm Hz - 0.5 precision. Strong sinusoidal frequency modulations are used in such laser interferometers to create heterodyne-like photodetector signals from which the phase and other parameters, such as the absolute distance, can be extracted. The nested sinusoidal function in such signals is a challenge for the real-time parameter estimation in low-noise applications. In this article, we present an algorithm to calculate exact signal parameters in a non-iterative way from such interferometric signals. The algorithm makes use of a recurrence relation between Bessel functions to enable a direct extraction of modulation parameters from the signal. Additionally, the algorithm is capable of dealing with high phase dynamics where the Doppler-shift of the signal becomes relevant and can limit the range and precision of the parameter estimation, if not accounted for. Simulations show that the algorithm is computationally efficient, can be well parallelised and the phase estimation is close to optimal precision given by the Cramer–Rao lower bound of the signal parameters. [ABSTRACT FROM AUTHOR]

Solar-induced chlorophyll fluorescence (SIF) is highly correlated with photosynthesis and can be used for estimating gross primary productivity (GPP) and monitoring vegetation stress. The far-red band of the solar Fraunhofer lines (FLs) is close to the strongest SIF emission peak and is unaffected by chlorophyll absorption, making it suitable for SIF intensity retrieval. In this study, we propose a retrieval window for far-red SIF, significantly enhancing the sensitivity of data-driven methods to SIF signals near 757 nm. This window introduces a weak O2 absorption band based on the FLs window, allowing for better separation of SIF signals from satellite spectra by altering the shape of specific singular vectors. Additionally, a frequency shift correction algorithm based on standard non-shifted reference spectra is proposed to discuss and eliminate the influence of the Doppler effect. SIF intensity retrieval was achieved using data from the GOSAT satellite, and the retrieved SIF was validated using GPP, enhanced vegetation index (EVI) from the MODIS platform, and published GOSAT SIF products. The validation results indicate that the SIF products provided in this study exhibit higher fitting goodness with GPP and EVI at high spatiotemporal resolutions, with improvements ranging from 55% to 129%. At low spatiotemporal resolutions, the SIF product provided in this study shows higher consistency with EVI and GPP spatially. [ABSTRACT FROM AUTHOR]

In this article, a deep neural network (DNN)-based underwater acoustic (UA) communication receiver is proposed. Conventional orthogonal frequency-division multiplexing (OFDM) receivers perform channel estimation using linear interpolation. However, due to the significant delay spread in multipath UA channels, the frequency response often exhibits strong non-linearity between pilot subcarriers. Since the channel delay profile is generally unknown, this non-linearity cannot be modeled precisely. A neural network (NN)-based receiver effectively tackles this challenge by learning and compensating for the non-linearity through NN training. The performance of the DNN-based UA communication receiver was tested recently in river trials in Western Australia. The results obtained from the trials prove that the DNN-based receiver performs better than the conventional least-squares (LS) estimator-based receiver. This paper suggests that UA communication using DNN receivers holds great potential for revolutionizing underwater communication systems, enabling higher data rates, improved reliability, and enhanced adaptability to changing underwater conditions. [ABSTRACT FROM AUTHOR]

Enhancing rotational measurement resolution and broadening the detectable spectral range are two critical and unresolved matters within the realm of motion perception. The rotational Doppler effect (RDE) is combined with the harmonic generation process to create a rotational measurement scheme that offers flexible detection wavelength conversion, exponential improvement of measurement resolution, and real‐time display of detection results. In the experiments, a cascaded second harmonic generation process is employed to attain a fourfold enhancement in rotational resolution and demonstrate how low‐cost silicon‐based detectors can be used for real‐time detection of infrared objects. This scheme employs a Gaussian beam within the nonlinear process to achieve high conversion efficiency, thereby enabling potential for subsequent cascade amplification. Additionally, it is fully compatible with existing RDE schemes, allowing for co‐amplification of rotational resolution at both the front‐end and back‐end. This research could offer a more precise and cost‐effective method for remote sensing detection. [ABSTRACT FROM AUTHOR]

Estuaries and coasts runoff plays a crucial role in the dynamics of the ocean system. However, the current emphasis on synthetic aperture radar (SAR) retrieved current velocities has been on studying strong boundary currents and large-scale ocean circulation, resulting in relatively limited observational research on estuarine current velocities. This study aims to observe the current velocities in the Yangtze River estuary area using Sentinel-1A SAR data. The findings indicate a high agreement between the current velocities retrieved by Sentinel-1A and those observed at hydrographic stations, showcasing the characteristics of current velocity distribution associated with seafloor topography. This highlights the significant potential of SAR in observing estuarine current velocities, contributing to the analysis of benthic topography and the statistics of river runoff. [ABSTRACT FROM AUTHOR]

Rotational Doppler shift of a circularly polarized wave impinging normally on a rotating anisotropic surface, causes scattered waves with frequency shift equals twice the surface rotation frequency. We show that virtual rotational Doppler shift can be realized in transmission line platforms through a time-varying junction. In a system consisting of a pair of decoupled but identical transmission lines, voltage waves with a 90-degree phase difference between the two lines mimic a circularly polarized wave. A junction, comprising three time-varying capacitors and a static two-port network, connects the two lines and acts as a synthetically rotating anisotropic surface. As a result, the reflected and transmitted voltage (or current) waves undergo a frequency shift equal to twice the synthetic rotation frequency. Utilizing this effect, a full frequency converter is then proposed by augmenting the synthetically rotating capacitive junction with a dispersive phase shifter, followed by a short circuit. The system efficiently converts the incident tone into a single down- or up-converted tone, with amplification observed in the case of up-conversion. The frequency converter is subsequently employed to design a magnetic-free isolator. Circuit simulations with both ideal and switch-based time-varying capacitors match theoretical predictions. [ABSTRACT FROM AUTHOR]

Aiming at the problem of difficulty in detecting the faults of sound source during driving, the received signal was affected by Doppler effect, which caused the problem of frequency shift, frequency band spread and amplitude modulation, and in addition, signal was in a low signal-noise ratio (SNR) which caused by strong environmental noise, which seriously affected the extraction of the target sound signal, a fault detection method for train bearings based on Doppler signal correction and short-step long-wave beamforming was proposed. Firstly, based on the relative motion relationship between sound source and microphone and Morse acoustic theory, the interpolation time series and amplitude reduction formulas were obtained to recover the distorted signal modulated by Doppler effect. Then, a short-step beamforming method based on microphone array was proposed, which intercepted the signal in segments and enhanced each segment in its expected direction. Finally, the effectiveness of this method under different signal-to-noise ratio was studied by simulation. The rail edge fault detection experiment was designed, and the rolling bearing with inner ring crack defect was analyzed experimentally. Combining with simulation and experiment, the effectiveness of this method was verified. The research result shows that comparing with the original low SNR signal collected by 8 microphones, the signal is corrected in both time domain and frequency domain, and the SNR is increased by 11. 5 dB. The proposed method can recover the Doppler distortion signal and effectively extract the fault characteristics of mobile sound sources under strong interference environment. [ABSTRACT FROM AUTHOR]

In this article, the spin transfer between two ferromagnetic insulators is studied. There is a narrow gap between the ferromagnetic insulators so that they are weakly interacting with each other. One of the ferromagnetic insulators is moving at a constant speed while the other is at rest; hence, the system is out of equilibrium. In the presence of the shearing motion, the interaction amplitude is periodically modulated at the Doppler frequency. A unitary transformation allows us to regard the periodic modulation of the interaction amplitude as an effective potential, which drives the spin transfer. The amount of the spin current is controlled by the spectral overlap and the carrier population difference between the two ferromagnetic media. If the spectra of the two ferromagnets are moderately broadened, the overlap in the spectral domain increases, enlarging the spin current. However, too much broadening spoils the spectral overlap and, hence, the spin current. This implies that there is an optimal condition for maximizing the spin transfer. [ABSTRACT FROM AUTHOR]

X-ray sources for a range of wavelengths are being considered for in situ calibration of X-ray Imaging Crystal Spectrometers (XICSs) and for monitoring line shifts due to changes in the crystal temperature, which can vary during experimental operation over a day [A. Ince-Cushman et al., Rev. Sci. Instrum. 79, 10E302 (2008), L. Delgado-Aparicio et al., Plasma Phys. Control. Fusion 55, 125011 (2013)]. Such crystal temperature dependent shifts, if not accounted for, could be erroneously interpreted as Doppler shifts leading to errors in plasma flow-velocity measurements. The x-ray sources encompass characteristic x-ray lines falling within the wavelength range of 0.9–4.0 Å, relevant for the XICSs on present and future fusion devices. Several technological challenges associated with the development of x-ray sources for in situ calibration are identified and are being addressed in the design of multiple x-ray tubes, which will be installed inside the spectrometer housing of the XICS for the JT-60SA tokamak. These x-ray sources will be especially useful for in situ calibration between plasma discharges. In this paper, laboratory experiments are described that were conducted with a Cu x-ray source, a heated quartz (102) crystal, and a pixelated Pilatus detector to measure the temperature dependent shifts of the Cu Kα1 and Kα2 lines at 1.5405 and 1.5443 Å, respectively, and to evaluate the 2d-lattice constant for the Bragg reflecting crystal planes as a function of temperature, which, in the case of in situ wavelength calibration, would have to be used for numerical analysis of the x-ray spectra from the plasma. [ABSTRACT FROM AUTHOR]

A 16 channel spectroscopy system has been installed on DIII-D to provide information about the energy distribution of the atomic neutrals using the Doppler shift and broadening of passive Balmer-α emission. The lines of sight are dominantly in the toroidal direction, with tangencies near the lowest point on closed magnetic flux surfaces moving from the lower divertor upward into the confined plasma. This allows the system to provide details of the neutrals as they undergo various atomic physics processes while traveling upward from the lower divertor. The spectrally resolved measurements provide several advantages that complement typical optical filter based measurements of hydrogenic spectral lines. These include more direct measurements of the neutrals that fuel deeper in the plasma and capturing neutral velocity distribution information via the Doppler broadened and shifted line. In addition, the spectral separation of higher energy "thermal" neutrals and bright cold emission from the scrape-off layer allows for a more straightforward calculation of the underlying neutral densities based on the emission because of the dependence of the thermal emission on confined plasma properties that are approximately flux functions and well measured. [ABSTRACT FROM AUTHOR]

Leveraging the ultra‐wideband advantages of the terahertz band, Integrated sensing and communication (ISAC) facilitates high‐precision sensing demands in human smart home applications. ISAC channel characteristics are the basis for ISAC system design. Currently, the ISAC channel is divided into target and background channels. Existing researches primarily focus on the attributes of human target itself, e.g. radar cross‐section and micro‐Doppler effect. However, the impact of human target on neither the pathloss characteristic of background channel nor the multipath propagation characteristic of target channel is considered. To address the gap, we conduct indoor channel measurements at 105 GHz to investigate the ISAC channel characteristics with the impact of human target. Firstly, by analysing the power angular delay profiles with and without human target, the changes in quantity and power of multipath components (MPCs) are observed. Then, a parameter called power control factor is proposed to evaluate the human target impact on pathloss, thereby modifying the existing pathloss model of background channel. Eventually, the MPCs belonging to target channel are extracted within target‐oriented power delay profile to count the power proportion of each bounce MPCs of the target‐Rx link, which supports the necessity of multi‐bounce (indirect) paths modelling in target channel. [ABSTRACT FROM AUTHOR]

• HF & VHF radars, making measurements of resident space objects, detect Doppler oscillations. • The Doppler oscillations are interpreted as waves propagating in the ionosphere. • Statistical analysis shows the waves have a frequency distribution in the 0.2–1 Hz range. • The waves are most likely observed during day with higher frequencies in weaker ionospheres. • The wave frequency and occurrence distributions suggest the most likely waves are EMIC waves. Radars performing observations of resident space objects (RSO) measure Doppler variations in wave events of several minutes duration, with frequencies in the 0.2–1 Hz range peaking near 0.5–0.7 Hz, consistent with variations in electron density induced by waves in the intervening ionosphere. The two mid-latitude radars used were a Very High Frequency (VHF) radar operating at 55 MHz, and a High Frequency (HF) radar operating at 30 MHz, both in southern Australia. The VHF radar wave observations exibited a peak in wave occurrence in the post-dawn sector (0600–1200 local solar time). The seasonal occurrence of the waves had a strong minimum during winter compared with the other seasons. Comparison between observations in 2018/19 (near solar minimum) and 2021/22 (mid-rise to cycle 25 peak) suggests wave occurrence is anti-correlated with sunspot number and hence with EUV and ionospheric strength. Generally the waves had higher frequencies during night than day, and low sunspot number than mid solar cycle, when there was a weaker ionosphere. Given the oscillation frequency of the wave events, the most likely geophysical phenomena that is consistent with the observations are electro-magnetic ion cyclotron (EMIC) plasma waves in the Pc1 (0.2–5 Hz) frequency range. No other candidate geophysical disturbance appears to fit the wave characteristics. The majority of geomagnetic field-guided transverse Pc1 EMICWs project from the outer magnetosphere down onto the polar ionosphere, where they can convert to compressional fast-mode waves propagating parallel to the Earths surface in the ionospheric F2 layer waveguide, both equatorwards to mid-latitudes and polewards. It is likely the majority of the Doppler oscillations in the Pc1 frequency range observed by the radars at mid-latitudes are these ducted compressional waves. However, sources of transverse EMICWs from the magnetosphere onto the mid-latitude ionosphere do exist, and these may cause some of the observed oscillations. The micro-physics of these compressional waves causing Doppler oscillations in radio observations is not inconsistent with the history of ionospheric Doppler measurements and theory, although the radar trans-ionospheric radio propagation and the observed waves being higher frequency than previous studies is different. If the observed Doppler oscillations are compressional EMICW in the ionospheric waveguide then several of the statistical results can be explained. The anti-correlation of the wave occurrence with sunspot number (and resultant ionospheric strength) can be attributed to lower ionospheric attenuation at the higher latitudes, between where geomagnetically field-guided transverse EMIC waves initially enter the high-latitude ionospheric waveguide from the magnetosphere above, and their observation at mid-latitudes. The observation of higher frequency waves during low sunspot number may also be explained by a source effect in the magnetosphere that preferentially selects the higher frequency field-guided transverse EMIC waves to propagate down to the ionosphere. Comparisons will be shown with results from ground and space based magnetometers of compressional waves in the waveguide, highlighting consistent results and areas where the measurement techniques differ. Theory suggests the radar Doppler measurements are far more sensitive to variations in in-situ ionospheric electron density than magnetic field variations. This agrees with existing literature which highlights the very strong contribution from the compressional component. Theory also suggests that the Doppler sensitivity of a radar to ionospheric electron density variations is frequency dependent, with lower frequencies being more sensitive. This is borne out by the measurements, with the HF radar being more sensitive than the VHF radar. [ABSTRACT FROM AUTHOR]

It has been demonstrated that sound waves carrying orbital angular momentum undergo frequency modulation after being reflected by a rotating object. In this paper, we, observed the rotational Doppler shift phenomenon of the acoustic vortex in an underwater environment by using a Fresnel-spiral zone plate for the construction of the focused acoustic vortex with controllable topological charge. The positive and negative rotation speeds of the spinning object are precisely determined with the rotational frequency shift and the acoustic vortex's topological charge. The rotational Doppler effect of acoustic vortex provides a possibility to improve the detection and recognition accuracy of Autonomous Underwater Vehicles with propellers. [ABSTRACT FROM AUTHOR]

Low-Earth-orbit (LEO) satellites are widely acknowledged as a promising infrastructure solution for global Internet of Things (IoT) services. However, the Doppler effect presents a significant challenge in the context of long-range (LoRa) modulation uplink connectivity. This study comprehensively examines the operational efficiency of LEO satellites concerning the Doppler weather effect, with state-of-the-art artificial intelligence techniques. Two LEO satellite constellations—Globalstar and the International Space Station (ISS)—were detected and tracked using ground radars in Perth and Brisbane, Australia, for 24 h starting 1 January 2024. The study involves modelling the constellation, calculating latency, and frequency offset and designing a hybrid Iterative Input Selection–Long Short-Term Memory Network (IIS-LSTM) integrated model to predict the Doppler weather profile for LEO satellites. The IIS algorithm selects relevant input variables for the model, while the LSTM algorithm learns and predicts patterns. This model is compared with Convolutional Neural Network and Extreme Gradient Boosting (XGBoost) models. The results show that the packet delivery rate is above 91% for the sensitive spread factor 12 with a bandwidth of 11.5 MHz for Globalstar and 145.8 MHz for ISS NAUKA. The carrier frequency for ISS orbiting at 402.3 km is 631 MHz and 500 MHz for Globalstar at 1414 km altitude, aiding in combating packet losses. The ISS-LSTM model achieved an accuracy of 97.51% and a loss of 1.17% with signal-to-noise ratios (SNRs) ranging from 0–30 dB. The XGB model has the fastest testing time, attaining ≈0.0997 s for higher SNRs and an accuracy of 87%. However, in lower SNR, it proves to be computationally expensive. IIS-LSTM attains a better computation time for lower SNRs at ≈0.4651 s, followed by XGB at ≈0.5990 and CNN at ≈0.6120 s. The study calls for further research on LoRa Doppler analysis, considering atmospheric attenuation, and relevant space parameters for future work. [ABSTRACT FROM AUTHOR]

We analyze the feasibility of so-called radial surface currents as derived from Sentinel-1 Wave mode Doppler shift observations to support the geodetic estimation of the mean dynamic topography (MDT). Broadly defined as subtracting the geoid from the mean sea surface, this separation problem suffers from data inhomogeneities and usually requires strong prior assumptions about the MDT's smoothness. Recent advancements in calibration and current retrieval methodology for Sentinel-1 data give reason to assess potential gains from this observation type, which can be linked to the gradient of the MDT under geostrophic approximation. Due to current lack of long time-series, we synthesize 10 years of observations from daily surface currents grids and include these in a parametric least-squares adjustment of the MDT. Our results utilizing the synthetic data in place of smoothness constraints are promising, showing 2 cm RMS agreement with a state-of-the-art MDT model. [ABSTRACT FROM AUTHOR]

A digital phase-locked loop (DPLL) based on sliding window infinite Fourier transform (SWIFT) and αSWIFT is proposed to extract the vibration and velocity parameters from the phase-modulated carrier signal. The phase of a carrier signal gets modulated when it is reflected from a vibration object due to the Doppler effect. Therefore, the phase-modulated carrier signal contains the Doppler signal that has the vibration information. The SWIFT or αSWIFT structure is designed from sliding discrete-time Fourier transform (DTFT) where the exponential window provides higher weightage to recent samples. It acts as a phase detector in the proposed DPLL, and its bandwidth can be varied based on the values of the time-constant. A controller unit, along with a bi-quad oscillator, is introduced in the proposed DPLL structure to remove any leakages in the system and to synchronize the system units in the parameter estimation. The proposed scheme is implemented in MATLAB/SIMULINK environment and validation for real-time applications is done using dSPACE 1202 MicrolabBox. [ABSTRACT FROM AUTHOR]

Compared with the traditional two‐step localization methods, the direct position determination (DPD) method is more robust at low signal‐to‐noise ratio. However, to guarantee the optimal location result, the computational complexity of DPD with grid search is too high, especially for moving targets. Therefore, a new DPD method is proposed for moving targets with low computational complexity. First, using a proposed cost function, the position information is obtained from the received array signals directly, which enhances the accuracy of localization. Second, utilizing the method of successional difference the velocity is extracted, which avoid the multi‐dimensional grid search and reduces the computational complexity. Simulation results demonstrate that the proposed method reduces the computational complexity greatly and outperforms the conventional methods. [ABSTRACT FROM AUTHOR]

In detecting hypersonic vehicles, the radar echo signal is coupled with an intra-pulse Doppler frequency (I-D frequency) component caused by relative motion of a plasma sheath (PSh) and the vehicle, which can induce the phenomenon of a ghost target in a one-dimensional range profile. In order to investigate the I-D frequency generated by the relative motion of a PSh, this study transforms a linear frequency modulated (LFM) signal into a single carrier frequency signal based on echo signal equivalent time delay-dechirp processing and realizes high resolution and fast extraction of the I-D frequency coupled with the frequency-domain echo signal. Furthermore, by relying on the computation of the surface flow field of the RAMC-II Blunt Cone Reentry Vehicle, the coupled I-D frequency in the radar echo signal of a PSh-enveloped target under circumstances of typical altitudes and carrier frequencies is extracted and further investigated, revealing the variation law of I-D frequency. The key findings of this study provide a novel approach for suppressing anomalies in radar detection of PSh-enveloped targets as well as effective detecting and as robust target tracking. [ABSTRACT FROM AUTHOR]

The objective of this paper is to analyze the performance of two possible architectures of tethered satellite systems, used as a platform for a distributed radar sounder. The first architecture consists in a cross-track oriented tethered satellite system, controlled and stabilized by exploiting the aerodynamic forces generated by the interaction with the rarefied atmosphere in Low Earth Orbit. The second architecture involves a tethered satellite system controlled through gyroscopic stabilization, obtained by spinning the system about an axis contained in the orbital plane. After a brief survey of radar sounding techniques, the methodology is introduced for describing the geometry of the systems and their characteristics, the performance of the two architectures are then compared with each other and with the current state of the art. By analyzing the modeled nominal behavior, it is shown that the two proposed architectures can achieve continuous or multiple observations, respectively, at maximum cross-track resolution, during one orbit, minimizing clutter noise. This is a considerable improvement of performance versus a formation flight architecture which can typically achieve only up to four observations per orbit. Finally, the advantages and disadvantages of each architecture are studied, and their possible mission scenarios are discussed. • A Tethered Satellite System is proposed, flying with tether in cross-track direction. • The tether tension is generated by aerodynamic lift forces on plates at its extrema. • Antenna elements distributed along the tether enable radar sounding applications. • Top radar sounding performances are achieved everywhere along the orbit. • An alternative gyroscopic tether system is studied for planets with no atmosphere. [ABSTRACT FROM AUTHOR]

Solar loops are hot and magnetic structures that often have transverse displacements. In this research, we study the characteristics of three solar loops using the images observed in the radiation region with the Hinode/XRT telescope on 11 September 2017. Fluctuations of solar loops can be traced using two methods: image and spectral processing. In the first method, displacements and disturbances are measured in images, while in the second method, periodic patterns are searched in Doppler shifts by spectrometers. Our analysis method is based on image processing and solar loop seismology. In this method, the time slices can be prepared from the images and then analyzed using the wavelet method. The measurement results of time series for three loops with lengths of 113040, 79128, and 62800 km were obtained in the range of 3–18, 3–6, and 3–16 min, respectively. The phase velocity of these oscillations was calculated in the range of 209–1256, 438–879.2, and 130–697.7 km s−1 for three loops, respectively. Also, Alfven speeds were determined as 150–1012, 316–708.8, and 94–562.5 km s−1. The magnetic field related to these fluctuations was calculated by the seismological method with the approximation of the thin cylindrical model for the loops measuring 2.47–16.52, 3.6–11.56, and 1.1–9.18 Gauss. Our findings suggest that oscillating movements in the loops can be interpreted as Kink transverse oscillations. [ABSTRACT FROM AUTHOR]

The common opinion that the Lorentz transformation distorts the Minkowski spacetime arises from a mathematically incorrect neglect of its non-diagonal terms. When the non-diagonal Lorentz transformation is properly treated, the time dilation and the space distortion disappear. Consequently, the Lorentz metric produced by the Lorentz transformation is identical to the Minkowski metric. The spacetime distortion in moving inertial frames is properly described by the so-called Doppler metric produced by the Doppler transformation. This transformation assumes the existence of a preferred frame and depicts the Doppler shift of light for a moving light source and/or observer. The Doppler metric is Lorentz invariant and leaves the Maxwell's equations unchanged from their form in the Minkowski spacetime. Moreover, the Doppler metric is experimentally confirmed because it yields the null result in the Michelson–Morley experiment and other interferometric experiments. The suitability of the Doppler metric for describing light properties in Special and General Relativity is supported by recent cosmological observations of the cosmic microwave background. [ABSTRACT FROM AUTHOR]

Chromospheric condensations (CCs) are a prominent feature of flare footpoint heating in the solar flare standard model, yet their timescales and velocities are not well understood. Fisher derived several important analytical relationships, which have rarely been examined with modern spectral observations. The Interface Region Imaging Spectrograph (IRIS) provides a wealth of flare data with a high enough cadence to sufficiently capture CC evolution. We analyzed Doppler shifts in Mg ii 2791 and Fe ii 2814 from a sample of flare footpoint pixels observed by IRIS to compare with Fisher's analytics and recent flare models. We found a detection lifetime of 1 minute occurs in 50% of the sample, with Mg ii showing several pixels with longer values and Fe ii almost categorically shorter, and both growing with the maximum velocity, v max. The shifts' half-life is commonly <40 s and is inversely related to v max, indicating that the first half of the CC evolution has more efficient kinetic energy loss. The lifetime's wide range and growth with v max indicate that the footpoint atmospherics and heating scenarios can vary more widely than first postulated in Fisher. Around 90% of the sample had observable acceleration periods, lasting an average of 38 and 32 s for Mg ii and Fe ii, respectively. These acceleration periods, as well as serving as flare model diagnostics themselves, could potentially be used to calculate other model diagnostics such as the initially accelerated mass. [ABSTRACT FROM AUTHOR]

Context. Periodic quasi-stellar objects (QSOs) are considered as candidates of supermassive binary black hole (BBH) systems in galactic centers. The periodicity of their light curves can be interpreted as being due to the Doppler boosting caused by the rotation of the two black holes (BHs). Further confirmation of these candidates may require different lines of observational evidence. Aims. Assuming the Doopler boosting scenario, in this paper we investigate the (coherent) variations of broad emission lines (BELs) and continuum light curves for active BBH systems surrounded by a circumbinary broad-line region (cBLR) and focus on their dependence on the eccentric orbital configuration. Methods. We calculated the variation of continuum light according to the motion of BBHs on elliptical orbits, with simplified orbital orientation for demonstration, the Doppler enhanced or weakened photoionization of each BLR cloud by the central BBH sources and its variation by assuming a shifted Γ-distribution of BLR clouds for a simple BLR geometry, and finally obtain the coherent variation of the continuum and the BELs. Results. We find that the amplitude and variation pattern of the continuum light curves and the evolution of the BEL profiles both sensitively depend on the eccentric orbital configuration of BBH systems, especially when the eccentricity is high. If only the secondary BH is active, the variation amplitudes of continuum light curves and BELs both increase with increasing BBH inclination angles and orbital eccentricities, but decrease with increasing BBH mass ratio. If both BHs are active, the asymmetry in the ionization of BLR clouds at different areas caused by the Doppler boosting effect of the secondary BH is weakened due to that of the primary BH at the opposite direction, which leads to systematically smaller variation amplitudes of both continuum light curves and BELs compared with those in the cases where only secondary the BH is activated. Conclusions. The coherent variations of the BEL profiles with the continuum light for those periodic QSOs provide an important way to confirm the existence of BBHs in their center. Future joint analysis of the light curves and multi-epoch observed BEL profiles for periodic QSOs may lead to the identification of a number of BBH systems. [ABSTRACT FROM AUTHOR]

Radar sensors, leveraging the Doppler effect, enable the nonintrusive capture of kinetic and physiological motions while preserving privacy. Deep learning (DL) facilitates radar sensing for healthcare applications such as gait recognition and vital-sign measurement. However, band-dependent patterns, indicating variations in patterns and power scales associated with frequencies in time–frequency representation (TFR), challenge radar sensing applications using DL. Frequency-dependent characteristics and features with lower power scales may be overlooked during representation learning. This paper proposes an Enhanced Band-Dependent Learning framework (E-BDL) comprising an adaptive sub-band filtering module, a representation learning module, and a sub-view contrastive module to fully detect band-dependent features in sub-frequency bands and leverage them for classification. Experimental validation is conducted on two radar datasets, including gait abnormality recognition for Alzheimer's disease (AD) and AD-related dementia (ADRD) risk evaluation and vital-sign monitoring for hemodynamics scenario classification. For hemodynamics scenario classification, E-BDL-ResNet achieves competitive performance in overall accuracy and class-wise evaluations compared to recent methods. For ADRD risk evaluation, the results demonstrate E-BDL-ResNet's superior performance across all candidate models, highlighting its potential as a clinical tool. E-BDL effectively detects salient sub-bands in TFRs, enhancing representation learning and improving the performance and interpretability of DL-based models. [ABSTRACT FROM AUTHOR]

Gravity wave (GW) momentum flux spectra help to uncover the mechanisms through which GWs influence momentum transfer in the atmosphere and provide crucial insights into accurately characterizing atmospheric wave processes. This study examines the momentum flux spectra of GWs in the troposphere (2–14 km) and stratosphere (18–28 km) over Koror Island (7.2°N, 134.3°W) using radiosonde data from 2013–2018. Utilizing hodograph analysis and spectral methods, the characteristics of momentum flux spectra are discussed. Given that the zonal momentum flux spectra of low-level atmospheric GWs generally follow a Gaussian distribution, Gaussian fitting was applied to the spectral structures. This fitting further explores the seasonal variations of the zonal momentum flux spectra and the average spectral parameters for each month. Additionally, the GW energy is analyzed using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) satellite data and compared with the results of the momentum flux spectra from radiosonde data, revealing the close negative correlation between wave energy and wave momentum for stratospheric GW changing with time. The findings indicate that the Gaussian peak shifts more eastward in both the troposphere and stratosphere, primarily due to the absorption of eastward-propagating GWs by the winter tropospheric westerly jet and critical layer filtering. The full width at half maximum (FWHM) in the stratosphere is larger than in the troposphere, especially in June and July, as the spectrum broadens due to propagation effects, filtering, and interactions among waves. The central phase speed in the stratosphere exceeds that in the troposphere, reflecting the influences of Doppler effects and background wind absorption. The momentum flux in the stratosphere is lower than in the troposphere, which is attributed to jet absorption, partial reflection, or the dissipation of GWs. [ABSTRACT FROM AUTHOR]

Velocimetry of a motion target within 3‐D space is highly desirable in numerous applicable realms, ranging from explosion and shock wave physics, aerospace engineering to astronomical surveys. However, it is challenging to achieve synchronous, real‐time, and photon‐counting 3‐D velocimetry in modern frameworks as they either require separate multi‐directional detections, and cumbersome calculation processes or are confined to achieve in situ measurements. Here, a new conceptual paradigm is proposed to circumvent these constraints using orbital‐angular‐momentum (OAM)‐driven multi‐point Doppler effect at the photon‐counting level. This scheme, emanating from a single‐direction launch of an on‐demand engineered sequence OAM light mode onto a motion surface, enables simultaneous and independent detections of time‐varying Doppler photon‐count events from three orthogonal echo light paths. Concretely, at the range of motion velocity of 0.25–0.5 ms−1, the relative measurement errors of this proof‐of‐principle prototype are below 1.5%, thus achieving high‐accuracy 3‐D velocimetry at the photon‐counting level for the first time. The exploration of the OAM‐photon‐counting 3‐D velocimetry techniques provides unprecedented advantages in potential applications of synchronous, real‐time, high‐efficiency, and long‐range quantum lidar. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence (LIF) Doppler spectroscopy using an optical vortex beam with an asymmetric intensity distribution, referred to as aOVLIF, is proposed as a new method to measure plasma flow velocity. LIF spectra were calculated numerically using typical laboratory low-temperature plasma parameters, and it was revealed that an ion flow across the beam produces a frequency shift of the spectra. This method also has the capability of temperature measurements. The propagation effects of asymmetric optical vortex beams are discussed assuming an actual experiment, and it is found that the sensitivity to the transverse flow velocity is approximately unchanged. The aOVLIF method, which exploits the inhomogeneous phase structure of optical vortices, can be applied to the determination of three-dimensional velocity vectors and promises to enhance the usefulness of conventional LIF spectroscopy using plane waves. [ABSTRACT FROM AUTHOR]

In experiments with neutral beam injection on the Globus-M2 spherical tokamak, sequences of long-lasting harmonics of toroidal Alfvén modes were discovered, equidistant from each other in frequency and shifted from zero by a constant value. Using microwave Doppler backscattering diagnostics, the central localization of toroidal modes was determined. In this work, the possibility of "splitting" of toroidal harmonics due to the Doppler shift caused by the toroidal rotation of the plasma is being discussed. It is found that the unshifted frequency of the toroidal Alfvén mode obtained from the spectrum of the magnetic probe signal is in good agreement with the frequency of the mode calculated at the mode location radius, and the toroidal rotation frequency, also determined from the spectrum of the magnetic probe signal, correlates well with the rotation frequency measured using charge exchange spectroscopy diagnostics, but differs by a constant amount. Possible reasons for the discrepancies are being discussed. [ABSTRACT FROM AUTHOR]

This paper presents a novel, cost-effective technique for estimating the Doppler effect in the time domain using a single pulse and subsequently leveraging the precise Doppler value to recover the radar waveform. The proposed system offers several key advantages over existing techniques, including the ability to calculate the target speed without any frequency ambiguity and the ability to detect a wide range of target speeds. These two features are not available in any existing techniques, including the conventional moving target detection (MTD) processor. To ensure improved accuracy and robust estimation, the system employs ensemble and iterative techniques by recursively and efficiently reducing the Doppler residues from the signal. Furthermore, the proposed system demonstrates effective signal recovery of a well-known bi-phase code shape at low signal-to-noise ratios in just a few iterations. The performance evaluation of the new algorithm demonstrates its practicability and its superiority over traditional radar systems. Implementation on software-defined radio (SDR) reveals that the proposed system excels in Doppler estimation and signal recovery at low SNRs, demonstrating promising results. [ABSTRACT FROM AUTHOR]

Spread spectrum technology has been widely employed for positioning and communicating with autonomous underwater vehicles (AUVs), but conventional spread spectrum sequences lack confidentiality and reliability in UWA channel. Considering the limitations of conventional sequences and the characteristics of underwater acoustic (UWA) channel, a composite chaotic orthogonal sequence (CCOS) based on the UWA channel is proposed. The confidentiality of the CCOS is superior to that of the m‐sequence, while the autocorrelation performance of the CCOS is superior to that of the orthogonal sequence. Moreover, the CCOS can compensate for the imbalance of logistic chaotic sequence when assigned certain initial values. Acquisition is a crucial component of accessing the spread spectrum signal; therefore, the acquisition performance indicates the applicability of the CCOS. The source–target model is established to simulate communication with an underwater moving target. To simulate the acquisition process, a parallel algorithm based on fast Fourier transform is adopted, and the entire simulation process is completed based on the BELLHOP ray acoustic model. Through data processing, the Doppler shift error is less than half of the frequency‐search element. Furthermore, the acquisition probabilities of the CCOS with different numbers of bits are over 90%, which demonstrates the reliability of the CCOS. [ABSTRACT FROM AUTHOR]

Upper-atmosphere winds from a meteor radar and a field-widened Michelson interferometer, co-located at the Polar Environment Atmospheric Research Laboratory in Eureka, Nunavut, Canada (80° N, 86° W) are compared. The two instruments implement different wind-measuring techniques at similar heights and have very different temporal and spatial observational footprints. The meteor radar provides winds averaged over a ∼ 300 km horizontal area in 3 km vertical bins between 82 and 97 km on a 1 h cadence. The E-Region Wind Interferometer II (ERWIN) provides airglow-weighted winds (averaged over volumes of ∼ 8 km in height by ∼ 5 km radius) from three nightglow emissions (O(1S) , oxygen green line, 557.7 nm , 97 km ; an O2 line, 866 nm , 94 km ; and an OH line, 843 nm , 87 km) on a ∼ 5 min cadence. ERWIN's higher precision (1–2 ms-1 for the O(1S) and OH emissions and ∼ 4 ms-1 for the O2 emissions) and higher cadence allows more substantive comparisons between winds measured by meteor radar and Doppler shifts in airglow emissions than previously possible for similar meteor radar/airglow Doppler shift comparisons using Fabry–Perot interferometers. The best correlation is achieved using Gaussian weighting of meteor radar winds with peak height and vertical width being optimally determined. Peak heights agree well with co-located SABER airglow observations. Offsets between the two instruments are ∼ 1–2 ms-1 for the O2 and O(1S) emissions and less than 0.3 ms-1 for the OH emission. Wind directions are highly correlated with a ∼ 1:1 correspondence. On average, meteor radar wind magnitudes are ∼ 40 % larger than those from ERWIN. Gravity wave airglow brightness weighting of observations is discussed. Non-quadrature phase offsets between the airglow weighting and gravity wave associated wind and temperature perturbations will result in enhanced or reduced layer-weighted wind amplitudes. [ABSTRACT FROM AUTHOR]

The performance of Doppler velocity logs (DVLs) in terms of velocity estimate error is directly linked to the geometry of the beam and the pulse transmitted. Beyond a specific transmitted bandwidth, the phase‐shift beamformer can introduce significant errors in velocity estimation. To delineate the operating mechanism of phase‐shift errors within a phased array of acoustic DVLs, the correlation between bottom echo and velocity distribution, in conjunction with the power‐weighted function, was initially examined predicated on spectral estimation theory. Subsequently, numerical and analytical models of the Gaussian‐shaped Doppler spectrum were formulated. The models are employed to evaluate the velocity estimation inaccuracies attributed to phase shifts in extant DVLs, and the comparative results with field experiments corroborate the model's efficacy in forecasting errors. The theoretical findings evaluate the performance limitations of the current phased array transducer design and provide insights for developing new designs. Pool experimental results show that this design effectively reduces the velocity estimation error caused by phase shift under static conditions and in the presence of Doppler frequencies to a level of almost complete elimination of the error compared to conventional configurations. [ABSTRACT FROM AUTHOR]

The adoption of baffles is relatively common in the construction of culverts, to assist with the upstream passage of migrating fish species. However, there still is a lack of systematic studies of the complicated hydraulic conditions induced by the baffles to optimize the designs. Herein, near-full-scale physical modeling was performed, focusing on the oscillation and instability of open-channel flow in a fish-friendly culvert equipped with full-height sidewall baffles. High-resolution measurements of the instantaneous flow velocity were obtained using an acoustic Doppler velocimeter. The physical results were marked by the existence of some low-frequency oscillations. A triple decomposition technique was applied to the free-surface and velocity time series. The low-pass components confirmed a unique flow structure, consisting of a high-velocity zone in the main channel and a low-velocity flow reversal within the lateral cavities. The band-pass components corresponded to the low-frequency flow oscillations, highlighting the complicated transverse interactions between the lateral cavity and the main channel. The high-pass velocity components were related to the true turbulence characteristics. This study provides a quantitative data set in support of the sustainable design of culverts to assist with upstream fish migration in artificial and natural fast waterways. [ABSTRACT FROM AUTHOR]

A general formulation is employed to study and quantitatively ascertain the effect of plasma broadening of intrinsic autoionizing (AI) resonances in photoionization cross sections. In particular, R-matrix data for iron ions described in the previous paper in the RMOP series (RMOP-II, hereafter RMOP2) are used to demonstrate underlying physical mechanisms due to electron collisions, ion microfields (Stark), thermal Doppler effects, core excitations, and free–free transitions. Breit–Pauli R-matrix cross sections for a large number of bound levels of Fe ions are considered, 454 levels of Fe XVII, 1184 levels of Fe XVIII and 508 levels of Fe XIX. Following a description of theoretical and computational methods, a sample of results is presented to show significant broadening and shifting of AI resonances due to extrinsic plasma broadening as a function of temperature and density. The redistribution of AI resonance strengths broadly preserves their integrated strengths as well as the naturally intrinsic asymmetric shapes of resonance complexes which are broadened, smeared and flattened, eventually dissolving into the bound-free continua. [ABSTRACT FROM AUTHOR]

Optical quantum memories are key elements in modern quantum technologies to reliably store and retrieve quantum information. At present, they are conceptually limited to the optical wavelength regime. Recent advancements in x-ray quantum optics render an extension of optical quantum memory protocols to ultrashort wavelengths possible, thereby establishing quantum photonics at x-ray energies. Here, we introduce an x-ray quantum memory protocol that utilizes mechanically driven nuclear resonant 57Fe absorbers to form a comb structure in the nuclear absorption spectrum by using the Doppler effect. This room-temperature nuclear frequency comb enables us to control the waveform of x-ray photon wave packets to a high level of accuracy and fidelity using solely mechanical motions. This tunable, robust, and highly flexible system offers a versatile platform for a compact solid-state quantum memory at room temperature for hard x-rays. [ABSTRACT FROM AUTHOR]

How to enhance the desired signal with low signal-to-noise ratio (SNR) is a difficult problem in the estimation process of the direction-of-arrival (DOA) of the target scattering wave signal. In this paper, the feasibility of spatial spectrum estimation in the Range-Doppler (RD) domain is analyzed in principle, and the SNR gain expression of weak scattering wave signal is derived when constructing multi-snapshots virtual array data. On this basis, the mutual eigenvector singular value decomposition (MESVD) method based on RD domain mode excitation is proposed, which can robustly and effectively estimate the direction of the coherent weak signals. Simulation experiments verify that the RD domain spectral estimation method has the ability to simultaneously obtain the direction of multiple weak target scattering waves, and the direction-finding accuracy can reach the Cramer–Rao bound (CRB) of conventional spectral estimation method. The results of Monte Carlo experiments show that the root-mean-square-error (RMSE) of azimuth estimation of RD domain spatial spectrum estimation method is 5.76° lower than that of a conventional multiple signal classification (MUSIC) method. In addition, the practicability of the proposed method is demonstrated by comparing the DOA estimation results of a set of real data with Automatic Dependent Surveillance-Broadcast (ADS-B) data. [ABSTRACT FROM AUTHOR]

High-frequency (HF) radar plays a crucial role in the detection of far-range, stealth, and high-speed targets. Nevertheless, the echo signal of such targets typically exhibits a low signal-to-noise ratio (SNR) and significant amplitude fluctuations because their radar cross-section (RCS) accounting for the HF band is in the resonance region. While enhancing detection performance often requires long-time integration, existing algorithms inadequately consider the impact of amplitude fluctuation. In response to this challenge, this article introduces an improved approach based on coherent and non-coherent integration. Initially, coherent integration, employing the generalized Radon Fourier transform (GRFT), is utilized to derive a candidate detection set of targets' range–time trajectories. This involves a joint solution for range migration (RM) and Doppler frequency migration (DFM) through a multi-parameter motion model search. Subsequently, the removal of low SNR pulses, followed by non-coherent integration, is implemented to mitigate amplitude fluctuation, referred to as Amplitude Fluctuation Suppression (AFS), and refine the detection outcomes. Both simulation and experiment results are provided to prove the effectiveness of the proposed AFS-GRFT algorithm. [ABSTRACT FROM AUTHOR]

Sea surface wind, waves, and currents are the three basic parameters that describe the dynamic process of sea surface, and they are coupled with each other. To more accurately describe large-scale ocean motion and extract the ocean dynamic parameters, we adopt the spaceborne Doppler measurement to estimate the radial Doppler velocity generated by the sea surface motion. Due to the presence of wind and waves, the Doppler spectrum will be formed, shifted and broadened. Pulse-pair phase interference is used to obtain the Doppler spectrum from the sea surface echo. We simulate the Doppler spectrum with different look geometry and ocean states in a spaceborne condition. In this paper, the Doppler centroid variations are estimated after reducing the platform Doppler velocity under different observation conditions. With the increase in wind speed, the measured Doppler shift increases, and the simulated Doppler centroid accuracy is estimated. In addition, the measurement error along the trace direction is at the maximum, and the error in the cross-track is the smallest. At moderate wind-wave conditions, the Doppler velocity offset can be less than 0.1 m/s. [ABSTRACT FROM AUTHOR]

This paper presents the results of bats detected with marine radar and their validation with acoustic detectors in the vicinity of a wind turbine with a hub height of 120 m. Bat detectors are widely used by researchers, even though the common acoustic detectors can cover only a relatively small volume. In contrast, radar technology can overcome this shortcoming by offering a large detection volume, fully covering the rotor-swept areas of modern wind turbines. Our study focused on the common noctule bats (Nyctalus noctula). The measurement setup consisted of a portable X-band pulse radar with a modified radar antenna, a clutter shielding fence, and an acoustic bat detector installed in the wind turbine's nacelle. The radar's detection range was evaluated using an analytical simulation model. We developed a methodology based on a strict set of criteria for selecting suitable radar data, acoustic data and identified bat tracks. By applying this methodology, the study data was limited to time intervals with an average duration of 48 s, which is equal to approximately 20 radar images. For these time intervals, 323 bat tracks were identified. The most common bat speed was extracted to be between 9 and 10 m/s, matching the values found in the literature. Of the 323 identified bat tracks passed within 80 m of the acoustic detector, 32% had the potential to be associated with bat calls due to their timing, directionality, and distance to the acoustic bat detector. The remaining 68% passed within the studied radar detection volume but out of the detection volume of the acoustic bat detector. A comparison of recorded radar echoes with the expected simulated values indicated that the in-flight radar cross-section of recorded common noctule bats was mostly between 1.0 and 5.0 cm2, which is consistent with the values found in the literature for similar sized wildlife. [ABSTRACT FROM AUTHOR]