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• A phase error correction method (IFOA-JMEC) for InIRA images is proposed. • Ionospheric scintillation and atmospheric turbulence are main errors source for InIRA. • Correcting spatial-variant Error in range by dividing subblocks and interpolating. • Background atmosphere has little influence on InIRA height measurement accuracy. • Phase screen models for ionospheric scintillation and atmospheric turbulence. The interferometric imaging radar altimeter (InIRA) is a newly developed microwave remote sensing system in recent years. It is of great significance to analyze the main sources of its height measurement error through simulation, and to develop the corresponding image phase error correction methods. This can provide basis and reference for the subsequent correction of the real images phase error. In this study, according to the interaction principle between atmospheric interference and InIRA radar electromagnetic waves, the influence of background ionosphere and background troposphere on height measurement error is analyzed by introducing ionosphere and troposphere delays into echo signals respectively. The impact of ionospheric scintillation and atmospheric turbulence on the height measurement error is analyzed by building multi-layer phase screen models. Then according to the analysis results, a phase error correction method for InIRA images combined by the improved fruit fly optimization algorithm (IFOA) and the joint minimum entropy criterion (JMEC) is proposed. The image is divided into several sub-images with negligible spatial variation of phase error according to the principle of first range and then azimuth. IFOA is used to search the optimal phase compensation values iteratively to compensate the phase of all sub-images. When the joint entropy of the sub-images reaches the minimum value, the phase error correction of the whole InIRA image is completed. Experimental results show that the proposed method can correct the phase of InIRA images with inconspicuous features and polluted by weak ionospheric scintillation and atmospheric turbulence relatively quickly. After correction, the height measurement error can be reduced from decimeter level to centimeter level. The correction accuracy and efficiency of the proposed method are superior to those of the method combined by particle swarm optimization algorithm and joint minimum entropy criterion (PSO-JMEC), the method combined by genetic algorithm and joint minimum entropy criterion (GA-JMEC), the method combined by phase gradient autofocus and Map Drift (PGA-MD), and improved PGA-MD algorithm. [ABSTRACT FROM AUTHOR]

• The optimized structure of a high-altitude balloon venting butterfly valve is proposed. • High-altitude drifting trajectory of floated balloon in Hunan from night to day is predicted. • Flow characteristics under different valve openings and low pressures are presented. With the demand of near-space exploration, it is required to continually improve the flat drifting capability of high-altitude latex balloons. To extend the flat drift time of latex balloons at a high altitude, it is necessary to design a gas release device to control the volume of gas discharged from the balloon. However, there are few such studies, especially on the type of high-altitude latex balloon exhaust valve and the control of release gas flow rate by balloon height. In this study, based on the high-altitude atmospheric data in Hunan Province, China, the release process of gas in high-altitude balloons in the fluid turbulence mode was determined using Reynolds method. The dynamic characteristics of two types of release valves, ball valve and butterfly valve, during the opening and closing and the steady-state flow characteristics of butterfly valve under a certain valve opening were simulated using gas motion equations and slip grid technology. By comparing the flow rate of these two types of valves at the same time, the adjustable time range under the same flow rate, as well as the ascending and drifting trajectory of the latex balloon, it was determined that the performance of butterfly valve is more suitable for the high-altitude latex balloon gas release system. Finally, the butterfly valve was applied to the high-altitude balloon gas release system, and a solution was designed for day and night flat drift balloons, which provide a reference for the design of high-altitude long-duration latex balloons. [ABSTRACT FROM AUTHOR]

Microwave signals, for example, those from Global Navigation Satellite System (GNSS) and Very Long Baseline Interferometry (VLBI) , are affected by tropospheric turbulence in such a way that the random fluctuations of the atmospheric index of refractivity correlate the phase measurements. A proper modeling of correlations is mandatory to avoid biased analysis, particularly when statistical tests are used. In this contribution, we analyze single differences (SD) computed from Global Positioning System (GPS) phase observations for which the between receiver clock error could be strongly mitigated by a specific common clock setting. We estimate specific parameters from the power spectral density (psd), which is directly related to the correlation function, with the debiased Whittle maximum likelihood and investigate their dependencies with the satellite geometry (elevation, azimuth angles) and the time of the day. We show that (i) the estimated slopes of the psd follow the one predicted by the Kolmogorov turbulence theory and (ii) the cut-off at high frequencies shows daily variations that may be linked with the strength of the turbulence. Based on these findings, we derive an improved spectral density model for GPS phase SD. The results of this study contribute to improving the stochastic description of random effects impacting VLBI and GNSS phase observations by studying variations of parameters from the von Karman spectrum. [ABSTRACT FROM AUTHOR]

The Moon is depleted in volatile elements and compounds, and lunar samples exhibit a wide range of Cl isotopic compositions, which is believed to result from the volatilization of metal chlorides (e.g., NaCl, KCl, and FeCl 2). However, the Cl isotopic fractionation behavior during volatilization is not well constrained, particularly for metal chlorides. Furthermore, the effect of metal chloride evaporation on metal isotopes is poorly known. In the present study, we performed NaCl and KCl sublimation experiments to study Cl and K isotopic fractionations at temperatures ranging from 923 K to 1061 K and at pressures of 7×10–5 bar to 1 bar in an N 2 atmosphere. The isotope fractionation factors of 37/35Cl(α gas–solid) from NaCl sublimation experiments are 0.9985±0.0002, 0.9958±0.0004, and 0.99807±0.00004 at 1, 10–2, and 7×10–5 bar, respectively. Those of 41/39K(α gas–solid) and 37/35Cl(α gas–solid) from KCl sublimation experiments are 0.99884±0.00004 and 0.9988±0.0003 at 1 bar, 0.9977±0.0002 and 0.9972±0.0003 at 10–2 bar, and 0.9989±0.0002 and 0.9989±0.0001 at 7×10–5 bar, respectively. Chlorine and K isotopes fractionate more at 10–2 bar than at 7×10–5 bar and 1 bar. The saturation index in all the sublimation experiments was >95%, which resulted in near-equilibrium isotopic fractionation at the sublimation interface. Therefore, the isotopic fractionation was controlled by mass transfer processes in the gas and solid phases. The isotopic fractionation at 10–2 bar was controlled by the chemical diffusion of sublimated gas in an N 2 atmosphere with almost no convection effect, (i.e., Pe number close to zero), whereas the isotopic fractionation at 1 bar was suppressed by atmospheric convection with a turbulence factor of 0.4±0.1 (i.e., Pe number >1). The extremely high sublimation rate and the very slow diffusion in the sublimating solid at 7×10–5 bar suppressed isotopic fractionations. Based on our experimental results, calculations using Cl/K and Na/K in lunar materials reveal that degassing of KCl contributed very little (<0.2‰) to the K isotopic fractionation (>0.58‰) during lunar magma ocean degassing. The Cl isotopic fractionation factor from lunar samples is similar to our results at 10–2 bar. This similarity of Cl isotope fractionation indicates that there may have been a transient atmosphere above the lunar magma ocean. [ABSTRACT FROM AUTHOR]

• This paper presents a novel learning-based framework offering short temporal spanning to support dynamic scenes. • The framework has two concatenated modules to remove geometric distortions and to refine micro details, respectively. • We exploit complex-valued convolutions extracting phase information, altered by atmospheric turbulence. • The proposed method efficiently mitigates the atmospheric turbulence distortion and outperforms existing methods. Atmospheric turbulence distorts visual imagery and is always problematic for information interpretation by both human and machine. Most well-developed approaches to remove atmospheric turbulence distortion are model-based. However, these methods require high computation and large memory making real-time operation infeasible. Deep learning-based approaches have hence gained more attention but currently work efficiently only on static scenes. This paper presents a novel learning-based framework offering short temporal spanning to support dynamic scenes. We exploit complex-valued convolutions as phase information, altered by atmospheric turbulence, is captured better than using ordinary real-valued convolutions. Two concatenated modules are proposed. The first module aims to remove geometric distortions and, if enough memory, the second module is applied to refine micro details of the videos. Experimental results show that our proposed framework efficiently mitigates the atmospheric turbulence distortion and significantly outperforms existing methods. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence has been confirmed as the primary source affecting the quality of ground-based telescope image. To reduce the effect of atmosphere, a good site should be selected, and adaptive optics (AO) should be installed for the telescope. In general, the daytime atmospheric turbulence is more intense than that at night under the effect of solar radiation. Numerous solar telescopes have built AO systems worldwide. Conventional AO is only capable of improving the image quality in a small field of view, whereas it cannot satisfy the needs of a large field of view. The novel wide field adaptive optical system is capable of achieving a large field of view and high-resolution images, whereas the atmospheric turbulence profile should be accurately detected, which is the prerequisite and key parameter of the novel AO system. Moreover, the astronomical high-resolution technology in accordance with the turbulence imaging theory requires more detailed detection of turbulence. Accordingly, a brief review about the latest detection technology of the daytime optical turbulence profile is valuable for astronomical observations. Besides, the parameters of atmospheric turbulence are briefly introduced. Subsequently, SNODAR, SHABAR, MOSP, DIMM+, A-MASP, and other detection technologies of the stratified atmospheric turbulence for daytime are primarily presented, and the advantages and disadvantages of the different technologies are summarized. [ABSTRACT FROM AUTHOR]

To assess the impact of Doppler frequency shift and channel crosstalk on the performance of a four-channel free-space optically switched communication network, particularly in scenarios involving low Earth orbit satellites, we derived a bit error rate model under the influence of satellite-ground Doppler frequency shift and channel crosstalk in atmospheric turbulence. This model focuses on studying a four-channel distributed feedback (DFB) laser array through numerical simulation. We initially analyzed the effect of crosstalk on the system's switching time in the absence of Doppler frequency shift. Our research revealed that as the channel spacing increased, the impact of crosstalk on the system's optical switching time decreased. We utilized a primary power compensation solution, adjusting the output optical power for different channel spacings, to mitigate the influence of crosstalk on the system's switching time. After eliminating the effects of crosstalk, we further examined the impact of Doppler frequency shift on the system's switching time for different channel spacings. We proposed a secondary power compensation solution to mitigate the effects of both crosstalk and Doppler frequency shift on the system's switching time. Finally, through simulation experiments, we set the output optical power of the laser to 10.42mw when switching from channel 1 to channel 2, 9.05mw when switching from channel 1 to channel 3, and 8.68mw when switching from channel 1 to channel 4, ensuring a uniform switching time of 10ns. Our research results provide a solid theoretical foundation for enhancing the performance of free-space optically switched communication networks and for mitigating the impact of Doppler frequency shift on system switching time. [ABSTRACT FROM AUTHOR]

The design of a free-space optical (FSO) link between an unmanned aerial vehicle (UAV) and a high-altitude platform (HAP) in the practical scenario of generalized pointing errors (PE) and beam wandering (BWAN) is addressed. We propose to select appropriately the system parameters such as optical beamwidth at the UAV and its flight altitude so as to minimize the outage probability. First , we characterize the channel statistics by taking into account the joint influence of attenuation loss, weak atmospheric turbulence and generalized PE particularly impaired by BWAN. Second , we formulate the closed-form expressions for outage probability, where we also derive an asymptotic expression at high signal-to-noise ratios (SNR)s. Third , we derive the optimum beamwidth at the UAV leading to the instantaneous minimum outage probability under generalized PE and BWAN. Finally , we address the optimal flight altitude of the UAV to avoid strong turbulence in order to ensure efficient data transmission. Our simulation results validate both the effectiveness and the accuracy of the derived asymptotic outage probability and the adequate choice of beamwidth and flight altitude to minimize the average outage performance. For instance, our results show that at an SNR of 20 dBm, the link availability is increased from 20% (with nominal beamwidth) to more than 70% by adopting the proposed beamwidth optimization. [ABSTRACT FROM AUTHOR]

New experimental technique have been proposed to discuss the turbulence impact reduction using beam shaping technique. In first phase of experiments, turbulence Impacted Vortex beam shaping technique has been introduced for a propagating beam which is effected by Kolmogorov type lab based turbulence simulator using Pseudo Random Phase Plate (PRPP). The new technique is executed with a spatially filtered collimated Gaussian beam that is propagated through computer controlled rotating PRPP to introduce the lab based turbulence on the beam profile. The turbulence impacted beam is then propagated through Vortex Phase Plate (VPP) to shape that beam into different topological charged Laguerre Gaussian modes and further collimated using two lens based configuration. We have measured the scintillation index of the spatial intensity profile collected from CCD. Comparative studies have been done by placing the image plane in five different positions on the propagation path of the beam so that we can detect Collimated, diverging and converging outputs. Four topological charges of LG beams and Gaussian beam have been used to envisage results. [ABSTRACT FROM AUTHOR]

This paper explores the potential of relay-assisted transmission to mitigate fading in free-space optical (FSO) systems. Our investigation focuses on assessing the performance of a dual-hop relay system for Terahertz/FSO (THz/FSO) communication. We develop an aggregated channel model incorporating various factors, including pointing errors, channel fading, and deterministic path loss. Specifically, the THz link encounters α − μ fading with pointing errors, while the FSO link experiences Gamma–Gamma fading and pointing errors. We utilize amplify-and-forward relaying and derive mathematical expressions for the probability density function and cumulative distribution function of the end-to-end signal-to-noise ratio, considering the combined effects of channel fading and pointing errors. Employing the derived statistical expressions, we evaluate crucial performance metrics for the considered relaying schemes, including the outage probability and the average bit error rate. To validate our analytical findings, we conducted Monte-Carlo simulations. The results demonstrate a strong correlation between the performance of the system and the particular pointing errors and fading parameters linked to the THz/FSO connections. • A relay-assisted transmission is considered to mitigate fading in free-space optical (FSO) systems. • The investigation focuses on the performance of a dual-hop relay system for Terahertz/FSO (THz/FSO) communication. • An aggregated channel model incorporating various factors, including pointing errors, channel fading, and deterministic path loss is presented. • Focusing on amplify-and-forward relaying, expressions for the end-to-end signal-to-noise ratio are proposed. • Based on the proposed models, performance metrics for the considered relaying schemes are evaluated. [ABSTRACT FROM AUTHOR]

In this paper, we propose a beam pointing algorithm to find optimal beam coordinates maximizing link performance in multiple-input multiple-output (MIMO) free space optical communications (FSOC) with spatial diversity. We first derive the received signal-to-noise ratio (SNR) with spatial diversity and analyze the convexity condition of SNR function with respect to transmit beam coordinates. Then, we obtain the beam coordinates maximizing the received SNR through an alternating optimization (AO) algorithm and Newton's method. However, the proposed algorithm requires instantaneous channel state information (CSI), which requires a high signaling overhead in MIMO FSOC. Thus, instead of exploiting instantaneous CSI, we utilize statistical CSI and propose a robust beam pointing algorithm with respect to atmospheric turbulence and pointing error. Numerical results show that our proposed algorithm outperforms conventional schemes in a dynamic FSOC network. Furthermore, we confirm that the optimal coordinates which are robust to the channel conditions approach to the centroid of receiver lenses as the strength of turbulence and pointing error increases. [ABSTRACT FROM AUTHOR]

Despite extensive research on the effect of atmospheric stability, the influence of convective rolls and thermal plumes in convective boundary layer (CBL) on urban winds and turbulence remains merely understood. Using amplitude modulation (AM), we examine the multi-scale interaction among turbulence intensities, fluxes, and large-scale CBL structures in unstable urban boundary layers (UBLs). Nine sets of large-eddy simulation (LES) are conducted, contrasting the CBL behavior over idealized urban morphology from neutral to free convection. This study, comparing the stability-dependent AM coefficient, reveals a tight modulation of small-scale turbulence and fluxes at urban canopy layer (UCL) by different types of characteristic CBL structures. Wavelet analysis corroborates their interactions in the time-frequency domain. Moreover, the correlation between the AM and the coherent CBL structures in the mixed layers is unveiled. Additionally, building-induced secondary flow structures are observed to dominate the modulation of small-scale turbulence residing within. UCL building-scale turbulence exhibits a higher susceptibility to modulation compared with intermediate scales. Notably, strong convection can invert the phase relationship between large scales and UCL turbulence. These results signify the crucialness of considering CBL coherent structures in the development of land-surface-parameterization schemes as well as the cautious implementation of predictive models in urban CBLs. • Amplitude modulation in full-scale urban CBLs transitioning from neutral to strong convection is examined. • Small-scale turbulence and fluxes in UCL are significantly modulated by the large-scale CBL structures. • Building wakes attenuate the modulation by CBL structures in the building vincinity. • Building-size turbulent processes are more susceptible to the large-scale streamwise modulation. • Stability profoundly modifies the phase relationship between large- and small-scale signals in UCL. [ABSTRACT FROM AUTHOR]

Optical transmission through atmospheric turbulence is a critical issue for establishing optical communication based on photonic spatial degrees of freedom. Refractive index fluctuations by a turbulent atmosphere cause structural distortion on the transmitted laser beams, resulting in crosstalk between adjacent spatial modes. We study a deep-learning technique to compensate for the turbulence effect. In particular, we mainly focus on improving the correction performance according to the selection of a probe beam. Different modes of Laguerre-Gaussian beams, including the lowest order (Gaussian), are employed as optical probes, and their abilities to compensate for turbulence effects are compared. Our results show that the Gaussian probe beam achieves a higher average correlation coefficient and a lower error rate, i.e., better correction performance, despite the spot size smaller than higher-order modes. Finally, to demonstrate the origin of the performance difference, we examine residual phases by spatial position and discuss the correction coverages. • A generative model for correcting atmospheric turbulence is presented. • Different modes of Laguerre-Gaussian beams are employed as optical probes. • Correction performance for each probe beam is comparatively analyzed. • The importance of spatial analysis on correction performance is discussed. [ABSTRACT FROM AUTHOR]

Simulated turbulence phase screens have been widely used in a series of optical experiments, but their speed and accuracy still need to be improved. Because of that, We use sparse spectral models for atmospheric turbulence simulation, and we use the Computational Unified Device Architecture (CUDA) to accelerate the calculation process. Finally, we load the calculated turbulence model onto the Liquid Crystal Atmospheric Turbulence Simulator (LC-ATS). The atmospheric turbulence model was used as a wavefront phase screen, and that model was deployed to the Liquid Crystal Spatial Light Modulator (LC-SLM). In 512 × 512 resolution, it takes 1.36 ms per frame, and it reaches 423 Hz on the liquid crystal on silicon (LC-OS). • The sparse spectral method can be used to simulate the atmosphere turbulence data in real-time. • The Liquid Crystal Atmospheric Turbulence Simulator (LC-ATS) can simulate the atmosphere in real-time. • The Computational Unified Device Architecture (CUDA) are used on GPU to improving the speed of atmospheric data calculation. [ABSTRACT FROM AUTHOR]

Swell waves, characterized by the long wavelength components generated by distant weather systems or storms, exert a significant influence on various air–sea interaction processes, thereby impacting weather and climate systems. Over recent decades, substantial progress has been achieved in comprehending the dynamics of swell waves and their implications for air–sea interactions. This paper presents a comprehensive review of advancements and key findings concerning surface swell waves and their interactions with the atmosphere. It encompasses a range of topics, including wave growth theory, the effects of swell waves on air–sea momentum, heat, and mass fluxes, as well as their influence on atmospheric turbulence and mixed layer processes. The most important characteristics of the swell impact (where it differs from wind sea conditions) are the wave-induced upward component of the surface stress leading to alteration of total surface stress, generation of a low-level wind maxima or changed wind profile and change of scale and behaviour of turbulence properties (turbulence kinetic energy and integral length scale). Furthermore, the paper explores the modelling of swell dissipation, the integration of swell influences in weather and climate models, and the broader climatic implications of surface swell waves. Despite notable advances in understanding swell processes, persistent knowledge gaps remain, underscoring the need for further research efforts, which are outlined in the paper. • A comprehensive review of studies on swells and their role in air–sea interactions. • The implementation of swell waves into climate models is summarized. • Some knowledge gaps about swell impacts on air–sea interaction are identified. [ABSTRACT FROM AUTHOR]

Lidar-assisted wind turbine control has been proven to be beneficial for turbine load reduction. It relies on the preview of incoming turbulence provided by a nacelle lidar, which allows the turbine controller to react to the flow disturbances before their impact on the turbine. When assessing its benefits, previous studies mainly use the standard turbulence parameters suggested by the IEC 61400-1 standard and assume Taylor's frozen hypothesis. Based on atmospheric turbulence observations, the parameters of the Mann spectral turbulence model differ significantly from the values recommended in the standard, and they vary, e.g., with atmospheric stability. Also, turbulence evolution from upstream to the rotor position conflicts with the frozen turbulence hypothesis, and its effect on the preview quality by a nacelle lidar still needs quantification. This work presents a simple method to extend the Mann model to account for the temporal evolution of turbulence using a space-time spectral velocity tensor. Under various atmospheric stability conditions, we derive the space-time tensor parameters and evaluate the space-time tensor using data from a five-beam lidar and a meteorological mast, we found good agreement between the space-time tensor and the measurement data. In addition, a numerical method for simulating a four-dimensional (4D) space-time velocity field based on the space-time tensor is presented. In the end, we analyze the importance of including the temporal evolution of turbulence for assessing lidar wind preview quality. Lidar-assisted wind turbine control has been proven to be beneficial for turbine load reduction. It relies on the preview of incoming turbulence provided by a nacelle lidar, which allows the turbine controller to react to the disturbances prior their impact on the turbine. This work presents a simple method to extend the Mann model to account for the temporal evolution of turbulence using a space-time spectral velocity tensor. Under various atmospheric stability conditions, we derive the space-time tensor parameters and evaluate the space-time tensor using data from a five-beam lidar and a meteorological mast, we found good agreement between the space-time tensor and the measurement data. We analyze the importance of including the temporal evolution of turbulence for assessing lidar wind preview quality. Figure 1: Typical nacelle lidar measurements over a wind field. The evolved turbulence (more transparent) illustrates the actual rotor disturbance while the upstream turbulence is observed by the lidar. A Gaussian range weighting shape/function is shown for LOS measurements (the LOS speeds along the beam are weight averaged by this function to get lidar LOS measurement). [Display omitted] [ABSTRACT FROM AUTHOR]

• First-time imager observation of concentric mesospheric gravity waves over Srinagar, Kashmir, India. • Concentric gravity waves are linked with lower atmospheric deep convection. • Above 9 0 km height, the waves got dissipated and created turbulence in the atmosphere. Using the NARL airglow imager in the Indian subtropical station of Srinagar, Kashmir, India, we report the characteristics of the concentric deformed circular pattern of gravity waves detected at ∼85 km height. OH molecular emission intensities from the height of ∼85 km as observed by the imager in the wide band optical filter of centre wavelength of 840 nm show concentric deformed circular gravity waves drifting northwestward. Using Atmospheric Infrared Sounder (AIRS) image at the centre wavelength of 8.1 µm, deep convective activity around the imager site in the lower atmosphere is recognized as the source mechanism of the observed concentric gravity waves in the mesosphere. The determined phase speed, direction and period of these concentric gravity waves (CGWs) is ∼47 m/s, 150° north of east and ∼10 min (intrinsic period ∼8 min) respectively. At heights of ∼97 km [OI (1S)] and ∼250 km [OI (1D)], these concentric gravity waves are sporadically visible depending on the possible vertical movement of airglow layer height. The dissipated portions of these concentric gravity waves get converted into turbulence at these heights. Using anelastic gravity wave dispersion relation, decreasing horizontal wavelength from the centre of concentric gravity waves is explained. This result is novel in the sense that it disagrees with most of the earlier interpretations of such observations. [ABSTRACT FROM AUTHOR]

The transmission of vortex beam in wireless optical communication is susceptible to atmospheric turbulence (AT) and oceanic turbulence (OT), leading to crosstalk among different modes and posing significant challenges in the recognition of orbital angular momentum (OAM) modes. This study introduces a novel integration of transfer learning with the VGG16 neural network to form a transfer learning method, which is adeptly applied to the recognition of OAM modes between AT and OT speckle images. The model is trained the model with a substantial amount of data from AT speckle dataset, and then test the performance of recognizing OAM modes using OT speckle dataset. The recognition accuracy, precision, recall and F1-score for both 5 OAM modes and 8 OAM modes using the method are studied, with additional exploration into the impact of training with hybrid datasets on the model's efficacy. The results show that the transfer learning method is effective to recognize OAM modes from speckle images affected by OT. It is observed that training with a mixed dataset fosters a greater degree of similarity between the source domain data and the target domain data, thereby enabling the model to capture intricate features and enhance recognition accuracy. The approach delineated in this paper highlights the versatility of employing transfer learning method for OAM modes recognition across analogous domains, offering a robust solution for effective recognition of OAM modes. • Transfer learning was integrated with the VGG16 to recognize OAM modes between atmospheric turbulence and oceanic turbulence. • The model effectively recognizes 5 and 8 OAM modes in oceanic turbulence with a transmission distance of 100 m. • Mixed dataset training enhances the model's ability to capture complex features and improves accuracy. [ABSTRACT FROM AUTHOR]

In this paper, we show that speckle averaging helps to reduce the measurement error associated with a Shack–Hartmann wavefront sensor (SHWFS); however, this reduction is rendered ineffective with increasing beacon anisoplanatism. We do so operating in a weak-scintillation regime, where the SHWFS offers robust performance, and using in-plane translation of the illuminated rough surface to accomplish frame-to-frame speckle diversity. Understanding these trade-space limitations is critical when performing wavefront sensing with noncooperative, extended-source beacons. • We derive analytic estimates for RMS wavefront error/aberration in vacuum/turbulence. • We present a new modeling framework to simulate partially correlated speckle frames. • We report simulation results in wavefront error/aberration and irradiance skewness. • We demonstrate the benefits of speckle averaging under isoplanatic conditions. • We identify potential drawbacks of speckle averaging in anisoplanatic conditions. [ABSTRACT FROM AUTHOR]

• Loess Plateau turbulence intermittency features burst and quiescent periods. • Submeso has a significantly greater effect on quiescent than burst periods. • Energy closure during burst (quiescent) periods is 68 % (17 %) during nighttime. Turbulence intermittency is a challenge facing in the field of atmospheric boundary layer (ABL) and micrometeorology. We employed an automated algorithm for the separation and reconstruction of Sub-Mesoscale and Turbulent motions (SMT) to examine the basic characteristics of turbulence intermittency driven by sub-mesoscale motion over the complex underlying surface of the Loess Plateau. The findings revealed that sub-mesoscale motion has a significant influence on turbulence statistical parameters. We analyzed five cases and found that the turbulent intermittency events were characterized by quiescent and burst periods. During the quiescent (burst) period, the turbulent transport weakened (strengthened), turbulence fluctuations weakened (strengthened), atmospheric stability increased (decreased), and turbulent energy decreased (increased). These bursts can be triggered by energy conversion from sub-mesoscale to turbulent motion. Actual observations revealed atmospheric conditions where turbulent intermittency events are more likely to occur: wind speed U < 3.5 ms−1, wind speed gradient ΔU/ΔZ < 0.2 s −1, temperature gradient ΔT/ΔZ > -5.1 K/100 m, or bulk Richardson number Ri b > -0.1. The influence of turbulence intermittency on the classical energy non-closure issue over the Loess Plateau was explored further. The results show that the presence of sub-mesoscale motions contributed to energy closure, with an energy closure of 78 % during daytime and 36 % during nighttime. And different periods of turbulent intermittency events affected energy non-closure differently, the energy closure during the burst (quiescent) period was approximately 98 % (70 %) during daytime and 68 % (17 %) during nighttime, approaching closure and far exceeding the overall closure rate during the burst period, whereas significantly low during the quiescent period. This suggests that turbulence intermittency is a very important factor causing energy non-closure over complex underlying surfaces, especially in stable boundary layer (SBL) during nighttime. The results are highly significant for a better comprehension of turbulence intermittency and surface-atmosphere interactions over complex underlying surface. [Display omitted] [ABSTRACT FROM AUTHOR]

Active yaw control (AYC) is a promising technique to improve the power performance of wind farms. In this work, a novel analytical model for yawed wind turbines, considering the effects of yaw angle, turbulence intensity and thrust coefficient, is developed to predict the velocity deficit and the wake deflection. The model is derived based on the mass and momentum conservation, and the assumption of the cosine distribution for the velocity deficit in turbine wakes. The comparisons with wind-tunnel measurements of different yaw angles and large-eddy simulation (LES) of different atmospheric turbulence levels show that the model predictions are in good agreement with the experimental and numerical results. A wind-farm controller is developed by incorporating the newly developed wake model, the Linear Sum (LS) cumulative wake model and the Serial-Refine power optimization method. The developed controller is applied to the power optimization in the Horns Rev offshore wind farm. It is found that yaw control is not required for the last-row turbines whose wakes do not influence the downstream ones. All turbines are found to yaw in the same direction due to the secondary wake steering, which is favorable to the wind-farm control. Moreover, the AYC technique is more beneficial to the power improvement around the full-wake wind directions associated with shorter turbine spacing. The local maximum of power improvement occurs at the wind angles that slightly deviate from the full-wake ones (± 5 ∘ in the Horns Rev wind farm). • A novel analytical yawed wake model is developed. • The model is validated by wind-tunnel experiments and large-eddy simulations. • An active yaw control (AYC) framework is developed. • The AYC technique is applied to wind-farm power optimization. [ABSTRACT FROM AUTHOR]

This paper introduces an assessment framework for the effectiveness of free-space optical (FSO) communication systems that combine multilevel modulation with the use of orbital angular momentum (OAM) for data stream multiplexing to enhance system bandwidth efficiency. The proposed system integrates pulse-position modulation (PPM) and dense wavelength-division multiplexing (DWDM) while addressing challenges from optical nonlinearity and atmospheric disturbances. This work aims to refine FSO deployment by merging N-encoded adaptive spatial modulation (SM) with L-ary spatial PPM (SPPM) and by adopting a diversity approach that utilizes both multiple-input multiple-output (MIMO) and multiple-input single-output (MISO) configurations in an M-to-1 ratio. The inclusion of SM in FSO communications alleviates signal degradation caused by atmospheric turbulence, yielding a significant reduction in the bit error rate (BER) compared to other systems. The proposed configuration of 2-SM/4-SPPM combined with a 2 × 1 MISO setup achieves a BER of 10 ‐ 9 for a high-capacity communication range of 2 km, and a 4 × 4 MIMO OAM in FSO at a BER of 10 ‐ 9 for a 200-m high-capacity free-space optical link, demonstrating promising results. Additionally, the system utilizes sequential signal relay and an advanced form of PPM to compensate for atmospheric and alignment-related signal losses, thereby extending the potential reach of FSO communication. Numerical analysis confirms that the system setup, which features a greater mode spacing of 2 units for OAM +1, +3, +5, and +7 modes, is less affected by power penalties due to interchannel interference and maintains stable performance even with lateral displacements of up to 0.75 mm. When comparing systems with different antenna array sizes, specifically 8 × 8 and 4 × 4, the capacity of all three systems is greater with the larger 8 × 8 antenna array. This study introduces an innovative approach that integrates advanced diversity multiplexing techniques with hybrid DWDM, incorporating N-encoded SM/SPPM. To enhance the system's robustness, the optical modulation index (OMI) is meticulously optimized within the advanced diversity multiplexing framework, employing hybrid MIMO N-SM/SPPM modulation schemes for FSO OAM communication systems. The approach is tailored to specifically counteract the impact of atmospheric turbulence. Simulation results underscore a marked enhancement in the signal's stability and integrity, charting the course for high-capacity, resilient optical wireless communications that can effectively navigate the challenges posed by atmospheric turbulence. These findings lay the groundwork for a theoretical capacity framework for OMI/OAM-based hybrid MIMO/MISO communication networks. The numerical results also indicate that the throughput of OAM-enhanced MIMO systems surpasses that of conventional MIMO configurations beyond a certain distance threshold. This study is significant in improving last-mile Internet services and backhaul connections. • Proposed hybrid modulation enhances system in all conditions, boosting efficiency and reliability. • Study introduces approach merging diversity techniques with hybrid DWDM-FSO, incorporating N-encoded SM/SPPM. • Findings lay groundwork for theoretical capacity framework for OMI/OAM-based hybrid MIMO/MISO FSO communication network. • Combining OAM, MIMO, RF-DWDM, and advanced modulation schemes aims to enhance FSO system performance. • The study suggests using Hybrid DWDM, SCM, and SPPM modulation to boost system performance in varied conditions. [ABSTRACT FROM AUTHOR]

Imaging quality is highly important for astronomical observations. The imaging quality of a telescope depends not only on the capabilities of the telescope itself but also on its location. When distant stars pass the atmosphere, due to the presence of atmospheric turbulence, the imaging quality becomes degraded. The atmospheric coherence length is the most commonly used parameter to evaluate the influence of the local atmospheric turbulence on telescopes. It is used to describe the degree of atmospheric attenuation of image quality. For the preliminary estimation of atmospheric daytime seeing from a lot of candidate sites, we have developed a portable S-DIMM (PS-DIMM),which is mainly composed of a light-duty tracking mount, a 9 cm aperture lens barrel and a Canon 60D SLR camera. With the PS-DIMM the daytime seeing of a site can be well estimated by measuring the relative displacement variance of the edge positions at both ends of the Sun's diameter. The principle of PS-DIMM is introduced in this paper. The simultaneous seeing measurements by it and the traditional S-DIMM were carried out at the Fuxian Lake Solar Station, showing a very well correlated between them but with an offset of 2.6 cm. In contrast to the traditional S-DIMM, PS-DIMM offers the advantages of reduced weight and enhanced portability. The utilization of PS-DIMM has played an important role in the early stages of the site survey for the next-generation solar telescopes. • A portable S-DIMM (PS-DIMM) was developed to measure daytime seeing for site survey. • The PS-DIMM has the advantages of light weight and portability compared with the traditional S-DIMM instrument. • The simultaneous measurements by PS-DIMM and S-DIMM showed a very well systematic deviation between them. [ABSTRACT FROM AUTHOR]

• Optical fields of two beams propagating in atmospheric turbulence are derived. • Effect of C n 2 on phase structure and light intensity distribution is analyzed. • Effects of C n 2 and m on the target OAM power proportion are analyzed. • A RAPVB is more suitable for short range AOC. • An APVBA is more suitable for long distance AOC. Based on the model of multiple random phase screens of atmospheric turbulence, the optical fields of a ring Airyprime vortex beam (RAPVB) and an Airyprime vortex beam array (APVBA) propagating in atmospheric turbulence are expressed in the form of operators. The effect of the refractive index structure constant on normalized light intensity and vortex phase for a RAPVB and an APVBA propagating in atmospheric turbulence is examined. The influences of the atmospheric propagation distance, the refractive index structure constant, and the topological charge on the target orbital angular momentum (OAM) power proportion of a RAPVB and an APVBA are analyzed. Anyway, the topological charge initially carried by a RAPVB and an APVBA can be successfully detected on the observation plane. This study indicates that a RAPVB and an APVBA with the saturated beam number are equivalent when used for analysis of target OAM power proportion. Further, this study is beneficial to the optimal design of atmospheric optical communication (AOC) systems. A RAPVB is more suitable for short distance AOC, while an APVBA is more suitable for long distance AOC. [ABSTRACT FROM AUTHOR]

In this paper, we study the beam evolution, size, scintillation index, and signal-to-noise ratio (SNR) of Finite Energy Airy-Hermite-Hollow Gaussian Beam (FAHHGB) in turbulence. We utilize numerical turbulence modeling for the transmission medium. We observe that FAHHGB evolves rapidly in turbulence. In addition, we see that beam size can be controlled by adjusting the beam order, hollowness, and scaling parameters. We obtain more concentrated beams when the scaling parameter increases. In terms of scintillation, the beam provides significantly low scintillation. Especially when the hollowness parameter is high, we found that the scintillation index of FAHHGB remains below the Gauss beam. This means that FAHHGB has higher SNR than Gauss beam, particularly at long distances. We anticipate that this complete analysis will be helpful for free space optics link designers. [ABSTRACT FROM AUTHOR]

• A convenient approach for solving the influence of atmospheric turbulent for the beacon. • An experimental method is used to verify the effectiveness of this approach under the condition of atmospheric turbulent. • Our work justifies the laser communication systems design, which is to provide better guidance for system design. Atmospheric turbulence in the laser propagation path induces phase distortion and adversely affects the acquisition of the beacon spot center for satellite–ground laser links. We introduce an optical flow method to track multiple feature points and obtain the overall motion state of the light spot to predict the inter-frame offset and offset direction of the center position of the light spot. We combined rough and specific selections to improve the feature point selection and proposed a feature point exit and supplement mechanism suitable for an atmospheric turbulence environment. The algorithm outperformed traditional algorithms in terms of stability and accuracy in experimental verification, providing a simple and high-precision solution for predicting beacon spot centers under turbulent atmospheric conditions. [ABSTRACT FROM AUTHOR]

• A new compact and intelligent instrument structure for the monitoring of meteorological environmental elements in complex transport networks. • Develop a novel optical pathway framework for measuring both visibility and atmospheric turbulence intensity concurrently. • The feasibility of the scheme is proved by comparison experiments with existing commercial instruments and experimental data. This design provides a reliable, integrated and intelligent solution for monitoring the road surface meteorological environment. The number of transport network systems in the world is increasing and the transport network passes through different geographical environments, there is an urgent need to develop an environmental meteorological monitoring equipment for the transport network security. So, in this study, a new intelligent instrument for monitoring the visibility and turbulence meteorological environmental parameters (temperature, pressure, humidity, etc.) of traffic networks (IVMPT) is developed. The feasibility of simultaneous measurement of visibility, turbulence and various meteorological elements is verified. The atmospheric visibility is measured based on the extinction principle and the atmospheric turbulence in a certain range based on the light intensity scattering principle. The commercial Visibility instrument (DNQ2-1) and Micro-temperature Sensor are used to compare with our new instrument. The results show that the difference between the visibility and turbulence measurements and the comparison instrument is 3% and 8.4%, respectively. After comparative examination, the new instrument meets the design requirements. In addition to environmental meteorological monitoring applications in the transport network, the instrument provides a new scientific instrument for the study of aerosol-turbulence interactions in basic science issues. [ABSTRACT FROM AUTHOR]

The Costas loop is an important technology for synchronous demodulation in space coherent communication. However, due to the limitation of the loop device and processing technology, it is difficult to achieve high-precision locking, which results in the Costas loop phase deviation and further affects the quality of the demodulation signal. In this paper, a closed-form bit error rate (BER) model is derived to evaluate the Costas loop phase deviation effect on BER performance in the space differential phase shift keying (DPSK) modulation and heterodyne detection communication system, considering laser phase noise, atmospheric turbulence, and detector noise. Based on the BER model, analytical solution simulation is conducted. The results show that the BER performance deteriorates varying with the increase of the Costas loop phase deviation. In addition, we research how to better balance the laser linewidth and Costas loop accuracy in the system design to ensure good communication status. We find that the wider the laser linewidth, the more stringent the Costas loop accuracy. Furthermore, the BER performance versus the Costas loop phase deviation under different satellite-to-ground communication system parameter settings including divergence angle, receiving aperture, signal laser power, and data rate are analyzed. This work provides valuable references for space optical coherent communication systems design. [ABSTRACT FROM AUTHOR]

We present an intercomparison of a full year of turbulence measurements and simulations at Østerild, a site in northern Denmark with relatively flat terrain and high surface roughness, where a high-quality tall meteorological mast is deployed. Both sonic and cup anemometers are mounted on booms on the mast from 7 up to 244 m, thus covering the range of heights in which modern wind turbines operate. The simulations were performed using the Weather Research and Forecasting model in a multiscale setup, with large-eddy simulations (LESs) nested one-way within mesoscale simulations. The mesoscale domains thus simulated the evolving weather, while the two innermost domains used an LES closure, intending the largest scales of turbulence to be explicitly resolved. For a selected day, we show that the simulated turbulence is accurately resolved within the innermost LES domain, and agrees well with the observations at all vertical levels. For the full year, we show that the innermost domain accurately reproduces mean wind speed, direction, and turbulence levels, whereas the mesoscale simulations have difficulties matching the frequency of occurrence of both low and high turbulence ranges when compared to the observations. The largest differences between the simulated turbulence from the innermost domain and the observations are found under low wind speed conditions close to the surface, particularly during nighttime where the simulated mean wind and turbulence levels are higher and lower, respectively, than the observations, potentially due to the inability of the LES mesh to resolve the very small scales of turbulence and associated momentum transport that those features support. [Display omitted] • We show the ability of an atmospheric model to resolve turbulence for a full year. • The model's simulated turbulence can be used for assessment of site conditions. • The energy content of the model's resolved turbulence is captured by the simulations. • Power density is accurately predicted within large-eddy simulation domains. [ABSTRACT FROM AUTHOR]

The influence of atmospheric turbulence inflow on coupled dynamic behaviors and wakes of floating offshore wind turbine (FOWT) is pronounced, especially as blade size increases. In this study, we explore the effects of atmospheric inflow on aero-hydrodynamics and wake characteristics of a spar-type FOWT. A well-validated in-house computational fluid dynamic (CFD) solver, FOWT-UALM-SJTU, is utilized to conduct the numerical simulations. To generate the realistic atmospheric inflow condition, the large eddy simulation (LES) with long-duration is employed. Two other cases involving uniform inflow and shear inflow are conducted to provide some comparable results. Compared to uniform inflow and shear inflow, the power and thrust of FOWT under atmospheric inflow display greater instability, with their power spectrums exhibiting higher intensity in high frequency region. The variation of yaw moment in atmospheric scenario is significantly drastic, leading to a remarkable response in platform yaw motion. Notably, the dominant frequency in this scenario is the blade passage frequency, as opposed to the incident wave frequency observed in the uniform and shear scenarios. The atmospheric inflow promotes the manifestation of wake breakdown and wake meandering, resulting in the faster wake recovery of FOWT. Besides, the meandering in the far wake is more significant compared to uniform and shear scenarios. The absence of vortex rings in atmospheric scenario is observed, along with the presence of more complex and smaller vortices in far wake. Our findings highlight the remarkable impacts of atmospheric inflow on dynamic responses and wake evolution of FOWT, and it is recommended that the atmospheric inflow be taken into account when assessing wake interactions between multiple FOWTs. • The LES is used to generate the physically realistic atmospheric turbulence inflow. • The fully coupled aero-hydrodynamics of FOWT are considered. • Wake characteristics of FOWT are investigated, extending as far as 9D downstream. [ABSTRACT FROM AUTHOR]

• Temporal models of Gamma-Gamma fading FSO links with pointing errors are built. • Level crossing rate and state transition probability are derived theoretically. • Misalignment strength has special effects on the Max point of level crossing rate. • Actual rather than generic covariance functions are considered. • Experiments verify the theory with averaged relative errors below 6.7 %. The knowledge of atmospheric channel is quite imperative for free-space optical (FSO) communications. To capture the time-varying essence of intensity fluctuations in an FSO connection, we propose a finite-state Markov (FSM) model for FSO links affected by Gamma-Gamma distributed atmospheric turbulence and pointing errors. First, the level crossing rate (LCR) of the unified intensity fluctuation is derived considering the non-generic channel autocorrelation characteristics, based on which the state transition probability (STP) matrix of the FSM model is calculated. Then, numerical simulations are presented for different strength parameters and frequency parameters of the channel. The results show that the misalignment strength does not affect the ratio of the level where the LCR reaches its maximum to the mean level. Finally, a series of well-designed experiments are carried out to verify the theory. As a result, the theoretical predictions conform to the experimental data, with averaged STP relative errors below 6.7 %. [ABSTRACT FROM AUTHOR]

• The study offers insights into atmospheric turbulence during intense grass fires. • These findings are significant as they provide the first-ever in-situ turbulence data collected during Red Flag Warnings. • The data establishes an upper limit for turbulence in prescribed grass fires, offering valuable reference points. • These results have the potential to enhance turbulence schemes in fire behavior models, contributing to better fire predictions. • This study underscores the complexities of studying fire type effects in a real-world context. A wildland fire experiment was conducted on a 40-ha tall grass prairie in Texas to study fire-atmosphere interactions. The experiment coincided with a Red Flag Warning, signifying specific weather conditions associated with an elevated risk of wildfire ignition and rapid spread. This provided a rare opportunity to observe wildland fire behavior and its impact on atmospheric flows under conditions deliberately avoided in similar experiments. This study analyzed the high frequency meteorological data collected during the experiment using three-dimensional sonic anemometers mounted on a 43-m tower (at 5.8, 10 and 20 m) and on three 10-m towers (at 5.3 m) within the burn block. The tower placements were carefully chosen to capture the impact of both head and flank fires on the mean and turbulent flows. The hypothesis is that fire-induced perturbations in the lower atmosphere, quantified through parameters like turbulent kinetic energy (TKE) and vertical fluxes, would be substantially stronger than those observed in similar wildland fires under less fire-prone atmospheric conditions. While this hypothesis was validated for heat flux and TKE, it did not hold for momentum fluxes or friction velocity. Additionally, the observations failed to confirm the hypothesis that head fires would produce more intense perturbations than flank fires. Given the homogeneity of fuel, terrain and meteorological elements across the burn unit, this underscores the challenge of disentangling the effects of fire type from other natural factors in a real-world setting. Nevertheless, the finding holds promise for improving turbulence parameterizations in fire-behavior and smoke dispersion models. [ABSTRACT FROM AUTHOR]

As a study on the flight of an interplanetary spacecraft back to Earth, the approximate analytical solutions for the atmospheric trajectory from reentry interface to pull-up point are derived. In future research, the solutions will be used as the theoretical tools for designing an analytical predictor-corrector guidance algorithm to control the pull-up flight. To derive the solutions, the highly nonlinear reentry dynamics is simplified first by planning the profile of drag-coefficient as an inversely proportional function of energy and using atmospheric density as a new state variable to replace altitude. Then, in order to decouple the dynamical equations, reference atmospheric density and reference flight-path angle are introduced to replace some real state variables while the differences between the reference and real variables are treated as small perturbations. Finally, a perturbation method is proposed to decompose the dynamics into two analytically solvable sub-systems. Thus the approximate analytical solutions are derived successfully. The simulation results demonstrate that the new solutions are more accurate than the existing solutions, because the new solutions give full consideration to the influence of Earth's curvature and rotation. • Analytical trajectory solutions for atmospheric pull-up phase of interplanetary return flight are derived. • A novel flight dynamics is proposed by taking atmospheric density as a state variable. • Drag coefficient is planned skillfully as an inversely proportional function of energy. • The flight dynamics is decoupled by introducing some reference state variables. • The decoupled dynamics is decomposed into two analytically solvable subsystems by a perturbation method. [ABSTRACT FROM AUTHOR]

The helical phasefront and orbital angular momentum (OAM) of vortex electromagnetic waves (VEMW) have attracted extensive attention in expanding communication capacity and enhancing radar imaging resolution. However, the VEMW is sensitive to atmospheric turbulence (AT), which will cause helical phasefront distortion and OAM mode diffusion. Hence, a convolutional neural network (CNN) based method for AT compensation is proposed. Specifically, the developed CNN automatically learns the AT characteristic parameters from the distorted VEMW phasefront and reconstructs the equivalent AT phase screen, and then, compensation is achieved by loading the inverse of the predicted phase on the distorted VEMW. The simulations show that the trained CNN has strong generalization capability and can perform high-quality AT prediction for different turbulence strengths and OAM modes, after compensation, the distorted phase is repaired and the mode purity is significantly improved. Moreover, comparing with the conventional AT compensation method, the proposed method shows smaller computational burdens and higher compensation accuracy. [ABSTRACT FROM AUTHOR]

Knowledge of the dynamics of wind turbine wakes and its dependence on the incoming boundary layer is fundamental to optimize and control the power production of wind farms. This work aims at investigating the effect of inflow turbulence on the wake of the NREL-5MW wind turbine. Sparsity-Promoting Dynamic Mode Decomposition (SP-DMD) is performed on snapshots extracted from large-eddy simulations of the turbine wake, for detecting the most dynamically-relevant flow structures in the presence or absence of inflow turbulence. We demonstrate that inflow turbulence generated by a precursor simulation radically changes the most dynamically-relevant flow structures. For the laminar-inflow case the DMD modes selected by the SP algorithm have high wavenumbers and are spatially localized. When turbulence is added at the inflow, these high-frequency modes are superseded by low-frequency modes lying in the frequency range of the wake meandering and filling the whole domain, mostly corresponding to those dynamically relevant for the precursor simulation. These results show that, in the presence of inflow turbulence, coherent structures linked to endogenous mechanisms such as tip and root vortices loose their dynamical relevance in favour of those exogenously excited by turbulence, indicating that low-dimensional models of turbine wakes should take into account atmospheric turbulence. • Sparsity promoting DMD is performed for the first time on an NREL-5MW wind turbine wake. • Incoming turbulence radically changes the most dynamically-relevant flow structures in the turbine's wake. • In the absence of incoming turbulence, high-to moderate-frequency modes are selected. • In the presence of turbulence, all relevant modes have frequency in the range of the wake meandering. [ABSTRACT FROM AUTHOR]

This paper investigates a special atmospheric glide problem where the gliding speed is close to the first cosmic velocity (FCV). Here, the centrifugal force and gravity almost balance each other, and thus in the local vertical direction the lift force required to maintain the glide is almost zero. However, all the existing analytical solutions for 3-D gliding trajectories were derived under the condition that the vertical component of lift is large enough, and thus become invalid for the special glide case. To overcome the shortage of the existing studies, we abandon the vertical component of lift, but instead plan the profile of drag acceleration (a D) to derive the analytical solutions for the 3-D special gliding trajectory. By simplifying the nonlinear equations of motion properly, a reduced-order system is obtained and governed by a D and L 2 /D, where L 2 /D represents the ratio of the local horizontal component of lift to drag. Subsequently, by proposing a perturbation method to expand the system in Taylor series with finite terms, two analytically solvable subsystems are developed. As a result, the analytical trajectory solutions are obtained successfully. Additionally, as verified by the simulation results, the new solutions are also applicable to the conventional glide problem where the speed is much less than the FCV, and found to be much more accurate than the existing solutions because the new solutions give full consideration to the coupling of the longitudinal and lateral equations. • This glide problem is special as the speed is close to the first cosmic velocity. • 3-D analytical trajectory solutions are derived based on the drag profile. • The coupling of longitudinal and lateral differential equations is fully considered. • Some difference expressions are created to weaken the effect of coupling terms. • Two analytically solvable sub-systems are developed by a perturbation method. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence significantly impacts the performance of optical systems, causing wavefront distortions that degrade image quality and limit the capabilities of remote sensing, astronomical observations, and free-space communication. The phenomenon of angular anisoplanatism, where the angular extent of the isoplanatic patch becomes limited due to varying turbulence conditions across the field of view, poses a substantial challenge in achieving high-resolution imaging of extended objects through turbulent atmospheres. This paper presents a comprehensive investigation into angular anisoplanatism using a laboratory-based environment that employs a pseudo-random phase plate to generate Kolmogorov turbulence. Angular anisoplanatism has been studied by generating two beams using a robust cyclic interferometer type arrangement, such that one beam is considered as a reference and the other beam as a target. Here, we have investigated experimentally the correlation of Zernike coefficients, mean square error (MSE), and beam centroid displacement, as a function of the angular separation of two beams. Furthermore, we have also reported the correlation of Zernike coefficients of the Laguerre Gaussian beam with different charges and eventually compared its MSE with that of the Gaussian beam. • Anisoplanatism studied in the Kolmogorov dynamic turbulence using a pseudo-random phase plate. • Angular anisoplanatism has been studied with a robust cyclic interferometer, generating reference and target beams. • Correlation of Zernike coefficients and mean square wavefront error used to characterize anisoplanatism. • Comparative study of anisoplanatism of Gaussian beams and Laguerre Gaussian beams of different charges. [ABSTRACT FROM AUTHOR]

In this paper, a composite channel model for Free-Space Optical (FSO) communication in marine environments is studied and the effects of different weather conditions such as fog and rain on communication performance metrics such as Signal-to-Noise Ratio (SNR), Bit Error Rate (BER) and Packet Loss Rate (PLR) are analyzed. The closed-form expressions for the Average Bit Error Rate were derived, considering different models of fading channels. The corresponding turbulence probability distribution function (such as LN, GG, K distributions) is selected based on the different intensities of turbulence according to the scintillation index. Compared with other weather conditions, FSO is easily affected by foggy weather. Additionally, considering the features of marine platforms, the effects of ships and wind speed on the FSO communication performance are studied and analyzed. The simulation results show that FSO is susceptible to the effects of wind speed. The presented composite channel model has been verified by experiments carried out in relevant sea areas to illustrate the effectiveness of the proposed channel model. • To address the unique conditions of sea-based mobile platforms, a composite channel model for wireless optical transmission over sea is proposed. • Based on the environmental conditions of marine mobile communication, an analysis of the impact of weather conditions as well as ship speed and wind speed on the performance of FSO communication is conducted. • To validate the proposed channel model, experimental tests were conducted in typical maritime environments. FSO communications between surface ships were performed under different weather conditions. [ABSTRACT FROM AUTHOR]

For atmospheric flows, the limitations of the eddy viscosity assumption are even more critical in cases involving complex terrain and features, where secondary strains are influential. Reynolds Stress Models (RSMs) can account for these effects, but their numerical stiffness and crucially the lack of established guidelines for modelers regarding the appropriate boundary conditions and model settings, as exist for eddy-viscosity models (EVMs), have led to limited adoption. In order to circumvent the shortcomings of EVMs, the focus has been directed at large eddy simulations (LES), which have greater computational requirements that can be prohibitive. Thus, this work aims to extend the traditional methodology that has been applied to adapt EVMs to atmospheric flows to an RSM that incorporates near-ground effects. This results in suggestions for new model constants and novel boundary conditions, which can easily be adapted according to local atmospheric parameters and employed by modelers. The performance of the proposed RSM is compared with four distinct EVMs through real-life cases representing two distinct complex terrain configurations. The results showed that the RSM indeed outperformed the EVMs, and it was found that only its anisotropic formulation could capture more complex flow features that were observed experimentally. • A common method for CFD wind simulations is extended to a Reynolds stress model. • Novel, site-specific constants and boundary conditions are suggested for the model. • The Reynolds stress model outperforms typical models in complex terrain. • The Reynolds stress model can reproduce complex flows at a low computational cost. [ABSTRACT FROM AUTHOR]

This study investigates the probability density function (PDF) of angle of arrival (AoA) fluctuations of a laser beam propagating through convective air turbulence in two settings: indoor turbulence and atmospheric surface layer. The indoor turbulence is generated by an electric heater with a 50 cm × 100 cm surface area placed horizontally on the bottom. The atmospheric turbulence experiments are conducted over a level at 60 cm height from an asphalted area using a slightly divergent laser beam that propagates along 350 m. Image motion monitoring of four sub-apertures installed in front of the receiving telescope aperture is used to record the AoA fluctuations and their PDFs over the entrance pupil at the sub-aperture locations for both types of turbulence. The observations are made in two perpendicular directions to the propagation of the laser beam. The experimental PDFs of the AoA fluctuations are fitted with Gaussian, Lognormal, Weibull, and Gamma distribution functions. The results show that the Weibull and Gamma models are more consistent with the experimental data for the indoor convective air and atmospheric turbulence respectively. For the indoor convective turbulence, the full width at half maximum and the peak location of the measured PDFs increase with the heater temperature, as well as the goodness of fit, which saturates at high temperatures. • The probability density function of fluctuations in the angle of arrival of a laser beam is explored. • The study involves convective indoor air and atmospheric turbulence. • The Weibull model show greater consistency for indoor convective air turbulence. • The Gamma model show greater consistency for atmospheric air turbulence. [ABSTRACT FROM AUTHOR]

• The autocorrelation operation is used to achieve the equiphase superposition of the signals output by the elements of the array detector. • An experiment based on the distortion phase generated by an atmospheric turbulence simulator to examine the performance of the new compensation method. • The method proposed in this manuscript may become a universal laser heterodyne detection mean to overcome the influence of spatial phase distortion. Spatial distortion phase seriously deteriorates the performance of heterodyne detection system. Effective phase compensation is necessary to reduce the impact of the distortion phase. Compared with the compensation method based on the combination of an optimization algorithm and sequence shifting, that by use of autocorrelation does not require repetitive iterations and avoids false alarms, resulting in better stability and real-time properties. In this paper, the distortion phase is generated by an atmospheric turbulence simulator to examine the performance of the new compensation method. The experiment results show that compared with the single point detector method, the new method can even increase the SNR by 50.3 dB, which confirms the effectiveness of the compensation method. Due to easy implementation and excellent performance, it is possible that the new method will become a universal means to compensate for the spatial distortion phase in the heterodyne detection. [ABSTRACT FROM AUTHOR]

Free space optical communication has become increasingly popular in the past decade due to its terahertz bandwidth, unlicensed spectrum, and enhanced security features. This technology has been attracting significant interest from the network community due to its broad applications in future military and civilian protocols. Recently, the use of orthogonal spatial states of the light field, including those with nonzero orbital angular momentum, has increased communication system capacities and bit-transfer rates per photon. However, atmospheric turbulence disrupts the orthogonal spatial modes over propagation. We present a novel deep learning architecture, cGULnet, to mitigate the effects of atmospheric turbulence on spatial multiplexing of Laguerre–Gaussian structured light modes. The model predicts the phase correction patterns required for an adaptive optical system. It is a U-shaped architecture trained adversarially to a patch discriminator similar to conditional generative adversarial neural-networks. The network is trained for exact phase recovery systems without approximating Zernike polynomials, enabling its use in highly phase-sensitive but high-speed spatial multiplexed communication systems. The proposed architecture can predict the successive three frames in a single shot using the previous ten frames. The method was evaluated for a wide range of beam waist sizes in a simulated Laguerre–Gaussian spatial mode multiplexed 1 km free space optical communication link. We have considered 20 dB optical signal-to-noise ratio, with turbulence conditions parameterized by the refractive index structure parameter of C n 2 ≤ 5 × 1 0 − 15 m − 2 3 . It was found to reduce the bit error rate to the International Telecommunication Union recommended forward error correction limit of 3. 8 × 1 0 − 3 under high turbulence conditions. [Display omitted] • Conditional adversarial neural network compensates time delay in adaptive optics for atmospheric correction. • Study predicts complete Zernike order for phase-sensitive spatially multiplexed free space optical communication, outperforming existing literature. • Results showcase bit error rate in simulated 1km 4-Laguerre Gaussian mode 4-QAM multiplexed 15 Gbps link under ITU recommendations. [ABSTRACT FROM AUTHOR]

Optical parameters of atmospheric turbulence have a significant impact on high angular resolution, adaptive optics, and site testing. The single star SCIDAR technique provides vertical profiles of these parameters, including the refractive index structure constant C n 2 (h). Its is based on the analysis of single stars scintillation. This study introduces a new approach for real-time measurement of atmospheric parameters using the Single Star SCIDAR. The key element of this approach is the utilization of a modified power spectrum of atmospheric speckles, that have more significant variation with altitude. By using that modified power spectrum, an objective function is computed, and the minimization process is performed using the Active-Set algorithm. With this approach, we successfully obtained real-time vertical profiles of C n 2 (h) with good accuracy. The processing takes approximately 3 s per profile, and the recovery rate (i.e. sum of C n 2 (h)) is about 95%. The reliability of this approach is validated through simulation results and a comparison with data obtained from sounding balloons. These validations confirm the accuracy and credibility of this method, which can be useful in many practical applications. • Novel approach to extract C n 2 (h) from the power spectrum of atmospheric speckles. • Real-time atmospheric parameter profiles. • Maintaining high accuracy in produced C n 2 (h) profiles despite reduced processing time. [ABSTRACT FROM AUTHOR]

The coherent laser signals of optical wireless communication suffer dramatic intensity fluctuations due to the atmospheric turbulence effect. A larger receiver aperture could only increase the average laser intensity without adaptive optics. Adaptive optics and wavefront reconstruction are popular in laser wireless transmission systems. Still, the alternative spatial diversity receivers are more convenient and economical to mitigate the irradiance scintillation, in which the antenna array combines multiple signals in digital signal processing or analog laser beams coherent combination. We provide channels correlated spatial diversity models and compare the performances of digital and optical combinations. In addition, reception experiments with a 19-aperture optical combination for horizontal 2 km transmission showed a ten times increase in average optical intensity and decreased scintillation index to 16 %. The large-scale spatial diversity receivers of 64-aperture that adopt hybrid optical and digital combinations are proposed to reduce the complexity of digital processing or cascaded optical combination structure. • Provide spatial diversity antenna design theory and experimental validation. • Compared the turbulence suppression effects of digital combination and optical combination. • Give the high-speed modulated signals' synchronization solution in an optical combination. • Extend the scale of spatial diversity and give a hybrid signals processing method. • Compared the applications of traditional adaptive optics and spatial diversity antennas. [ABSTRACT FROM AUTHOR]

In the field of free-space optical communication (FSOC), the sensitivity of the receiver can be affected by atmospheric turbulence, leading to light-intensity scintillation or beam drift. This paper offers a solution to this challenge by proposing a few-mode heterodyne receiver with a few-mode optical fiber amplifier (FM-OFA). The FM-OFA is used in the few-mode heterodyne receiver to enhance the output power of different modes of signal light, and the amplified signal light is detected by the few-mode heterodyne detection based on Kramers–Kronig (KK) relations, which effectively suppresses the atmospheric turbulence effect and improves the sensitivity of the receiver. In addition, we build an FSOC system and verify the sensitivity performance of this receiver under different turbulence conditions. The experimental results show that the sensitivities of the few-mode heterodyne receiver with an FM-OFA are −42.6dBm, −41.3dBm, and −40.7dBm, respectively, which are 8.8 dB, 8.5 dB, and 9.3 dB higher than those of the few-mode heterodyne receiver without an FM-OFA at average BER = 10−3 under low, moderate, and high turbulence intensities. • Proposing a high-sensitivity and turbulence-resistant few-mode heterodyne receiver. • Introducing the single-mode KK coherent receiver into the few-mode field. • Improved receiver sensitivity with a dual-clad few-mode fiber amplifier. • The sensitivity of the receiver is less than −40 dBm under different turbulence. [ABSTRACT FROM AUTHOR]

Turbulence effects can seriously affect the propagation of structured light, especially for orbital angular momentum (OAM) modes, causing spread and crosstalk. We derive an analytical solution to the mode spectrum of a Laguerre-Gaussian beam (LGB) propagating in weak compressible turbulence. The influence of the radial number and turbulence strength in the mode spectrum on the probability of LGB propagation is mainly considered. The results show that the weak compressible turbulence interferes strongly with the LGB, and this effect is mainly reflected in the sharp drop of the signal mode energy at short transmission distance. We find that the pure radial mode has a higher signal mode energy than the pure OAM mode, which means that the pure radial mode is more resilient to compressible turbulence than the pure OAM mode. In addition, the increase of radial number can improve the performance of the OAM dimension, but on the other hand, it reduces the probability of the radial dimension. Finally, we discussed the propagation properties of higher-order modes. Our research clarifies the interaction of OAM mode and radial mode in turbulence, and provides a theoretical basis for applications based on radial modes in the field of free-space communication. • The analytical solution of the mode spectrum of Laguerre-Gauss beam (LGB) carrying radial indices in weak compressible turbulence is obtained. • The impact of compressible turbulence on the propagation characteristics of LGB mode was clarified, and the correlation between radial number and OAM number was established. [ABSTRACT FROM AUTHOR]

Despite extensive past research efforts, the influence of turbulence on suspension bridge flutter stability has not been fully understood yet. Moreover, the role of large-scale turbulence has been overlooked in predicting the critical wind velocity, despite experimental and numerical studies indicating that atmospheric turbulence can have either a stabilising or a destabilising effect. This study investigates the parametric effects of large-scale turbulence on flutter stability based on the 2D Rational Function Approximation model, considering the Hardanger Bridge, in Norway, as a case study. First, a Monte Carlo method is used to analyse the bridge stability under various turbulent wind conditions, considering turbulence intensity and integral length scale as key parameters. The results highlight the sensitivity of flutter stability to these turbulence parameters, as well as essentially its independence of the spanwise correlation of the parametric excitation. The role of the variation in the wind angle of attack is found to be largely dominant compared to that in the wind velocity magnitude. Subsequently, Floquet multipliers are employed to study the influence of periodic parametric excitations on bridge stability, assuming sinusoidal fluctuations of wind velocity magnitude and angle of attack. This simplified scenario helps us understand the various parametric excitation mechanisms and explain the results of the more realistic Monte Carlo approach. The study emphasises the key role played in flutter stability and buffeting response by the so-called "average parametric effect", associated with the mean of the aerodynamic derivatives as nonlinear functions of the slowly-varying angle of attack. The preponderance of this effect in most realistic turbulent wind scenarios also explains the negligible impact of correlation of the parametric variation in the angle of attack. Finally, an equivalent linear time-invariant model is proposed to account for the parametric effects of turbulence in a simple way, yielding good results and offering a new perspective on the use of classical self-excited force coefficients. • Large-scale turbulence can parametrically affect bridge flutter stability. • Parametric effects of wind angle of attack are dominant compared to those of wind velocity magnitude. • Spanwise correlation of the parametric excitation does not play a significant role. • Aerodynamic derivatives averaged along the angle of attack are crucial for bridge stability. • Simple equivalent time-invariant model proposed for flutter and buffeting calculations. [ABSTRACT FROM AUTHOR]

• In this paper, we proposed a CNN-OAM mode classifier to detect OAM modes of distorted high-order (radial index p > 0) and zero-order (radial index p = 0) vortex beams. • The effect of propagation distance, atmospheric turbulence intensity and radial index on the detection accuracy of the CNN-OAM mode classifier was analyzed. • For 5AT (atmospheric turbulence)-mixed dataset, the CNN-OAM mode classifier has an average detection accuracy of 97.3%. • The CNN-OAM mode classifier used in this paper can offer a novel approach to the detection of high-order vortex beams. Vortex beams provide an innovative multiplexing mechanism for the field of optical communication due to its specific orthogonality. The precise and efficient detection of orbital angular momentum (OAM) modes carried by vortex beams is an essential technology for OAM multiplexing communication systems. An OAM mode classifier based on convolutional neural network (CNN) is proposed for the mode detection of distorted high-order vortex beams (radial index p > 0). The effect of different atmospheric turbulence (AT) intensities, radial indexes, and propagation distances on the detection accuracy of the OAM modes are investigated. The ability of the classifier models for generalization after training on both specific and mixed AT dataset is analyzed. The results show that the higher-order vortex beams have a greater detection accuracy in AT environments. For the high-order vortex beam carrying OAM modes, the CNN-OAM mode classifier has an average detection accuracy of 97.3% and can detect up to 30 OAM modes under any AT intensities ranging from 1 × 10−16 m−2/3 to 1 × 10−14 m−2/3 at a propagation distance of 1000 m. The great accuracy and extensive detection range of the CNN-OAM mode classifier proposed in this paper can offer a novel approach to the recognition and detection of high-order vortex beams. [ABSTRACT FROM AUTHOR]

In this paper, taking the atmospheric turbulence into account, two low-complexity algorithms which can jointly compensate the frequency offset (FO) and phase noise (PN) caused by laser line-width and atmospheric turbulence were proposed in coherent free-space optical (FSO) communications with polarization division multiplexing QPSK (PDM-QPSK) scheme. By introducing phase-conjugated counterparts for training symbols or information symbols on one of two orthogonal polarization states, the FO and PN were sensed simultaneously for a transmitted block or for each symbol. Simulations and experiments were performed to evaluate the BER performance. The results both manifested significant bit-error-rate (BER) performance improvement. • Two novel algorithms mitigate the FO and PN jointly in coherent FSO communications. • The phase-conjugated symbols on one polarization states sense the FO and PN jointly. • Results of simulations and experiments showed significant performance improvement. [ABSTRACT FROM AUTHOR]

• We explore the propagation characteristics of multimode laser beams in a turbulent atmosphere. • We employ the Huygens–Fresnel integral to develop analytical formulations. • Average intensity distribution, kurtosis parameter, beam spread, and the average transmittance of multimode beams are examined. The investigation focuses on studying the propagation characteristics of multimode lasers in the turbulent amosphere. By employing the Huygens-Fresnel integral, we develop analytical formulations for various propagation parameters. These include the average intensity distribution, kurtosis parameter, beam spread, and the average transmittance of multimode beams in turbulent atmosphere. Our findings reveal that as the propagation distance or the structure constant of the atmosphere increases, i.e., turbulence becomes stronger, the kurtosis parameter and the beam spread increase. The multimode beam exhibits a Gaussian like intensity profile when the propagation distance is significantly increased or when the structure constant becomes sufficiently large. For the case of the Gaussian beam, the kurtosis parameter is found to be 3. The multimode beam's kurtosis parameter rises as the turbulence becomes stronger and eventually approaches 3. Raising the mode content leads to a rise in the average transmittance; however, it leads to a decline in the Kurtosis parameter and the beam spread. [ABSTRACT FROM AUTHOR]