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Impact of pitch angle fluctuations on airborne lidar forward sensing along the flight direction
摘要： Airborne lidar forward sensing along the ?ight direction can serve for noti?cation of clear air turbulence (CAT) and help to prevent injuries or fatal air accidents. The validation of this concept was presented in the framework of the DELICAT (DEmonstration of LIdar-based CAT detec- tion) project. However, the strong variations in signal level, which were observed during the DELICAT measurements but not explained, sometimes indicated the need of a bet- ter understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources perti- nent to this technique, related to ?uctuations of the ?ight pa- rameters, which may lead to strong signal variations caused by the random deviations of the sensing beam from the for- ward ?ight trajectory. We analyze the variations in backscat- tered lidar signal caused by ?uctuations of the most impor- tant forward-sensing ?ight parameter, the pitch angle. The ?uctuation values considered in the paper correspond to the error limits of the compensational gyro platform used in civil aviation. The part of the pitch angle ?uctuations not com- pensated for by the beam-steering device in the presence of aerosol concentration variations can lead to noticeable signal variations that can be mistakenly attributed to wind shear, turbulence, or fast evolution of the aerosol layer. We formu- late the criteria that allow the recognition of signal variations caused by pitch angle ?uctuations. In?uence of these ?uc- tuations is shown to be stronger for aerosol variations on smaller vertical scales. An example of DELICAT observa- tions indicating a noticeable pitch angle ?uctuation impact is presented.
关键词： clear air turbulence,signal variations,pitch angle fluctuations,aerosol clusters,airborne lidar
更新于2025-09-09 09:28:46
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3-D measurement of 2-D jet by 3-D 3-C SPIV
摘要： The stereo PIV, commonly known as the SPIV, is capable of measuring all the three velocity components in a two-dimensional measurement plane and thus classi?ed as 2-D 3-C PIV. Even though the SPIV utilizes a set of two cameras, the positions of the measurement points are assumed to be located in the central plane of the illuminated volume, which is usually the zone of laser light sheet. This assumption makes the SPIV versatile and its calibration procedure very handy. But, if the three-dimensional orientation of the cameras can be calibrated precisely, the three-dimensional position of the particles that may be distributed in the depth of the illuminated zone can be calculated. The authors of this study have proposed a new algorithm that enables the fully three-dimensional measurement by SPIV and it can be classi?ed as 3-D 3-C SPIV. Presently, the three-dimensional turbulent measurement of a plane jet has been carried out with the use of presently proposed 3-D 3-C SPIV and the transverse distributions of the turbulence statistics have been obtained in the depth direction of 3-D 3-C SPIV. The results are compared with those by standard PIV, i.e., 2-D 2-C PIV, and thus, the accuracy and the applicability of 3-D 3-C SPIV have been evaluated.
关键词： 2-D jet,Stereo PIV,Turbulence statistics,3-D measurement,Camera calibration,Scheimp?u¨g condition
更新于2025-09-09 09:28:46
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Impact of transmission techniques in asymmetric RF/FSO system over Nakagami- <i>m</i> and gamma-gamma fading channels with pointing errors
摘要： In this paper, the performance of transmission techniques on the fixed‐gain amplify‐and‐forward–based asymmetric radio frequency/free space optical (RF/FSO) communication system is studied. The RF link and FSO link are, respectively, modeled by the Nakagami‐m and Gamma‐Gamma fading channels under the effect of zero boresight and non‐zero boresight pointing errors subject to heterodyne detection. Maximum ratio transmission (MRT) and orthogonal space‐time block coding (OSTBC) techniques are the transmission scenarios considered at the system source while selection combining is used for reception at the relay and destination for the signal detection. Moreover, a unified cumulative density distribution (CDF) of end‐to‐end signal‐to‐noise ratio is derived for the system. By utilizing this channel statistical CDF, the closed‐form expressions for the outage probability and average bit error rate for the M‐ary phase shift keying modulation are then obtained for the system. The analysis presented illustrates that both the atmospheric turbulence and pointing error significantly degrade the system performance. Based on this, the MRT transmission technique offers a better performance compared with the OSTBC techniques under the same system conditions. The accuracy of the analytical results is verified by Monte‐Carlo simulations.
关键词： transmit diversity,free space optical,Nakagami‐m fading,pointing errors,atmospheric turbulence
更新于2025-09-09 09:28:46
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Polar-Coded MIMO FSO Communication System Over Gamma-Gamma Turbulence Channel With Spatially Correlated Fading
摘要： In this paper, for the first time to the best of our knowledge, a polar-coded multiple-input multiple-output (MIMO) free space optical (FSO) communication system is proposed to combat turbulence-induced fading. Here, the spatially correlated fading, which dominates the MIMO FSO system aggravation, is taken into consideration in this polar-coded MIMO FSO system. To evaluate our scheme, the ergodic capacity of the gamma–gamma modeled MIMO FSO turbulence channel with and without spatially correlated fading is studied. The results of ergodic capacity and Monte Carlo simulation show that the polar-coded multiple photodetector scheme can effectively mitigate the fading induced by turbulence, and the polar-coded multiple optical sources scheme is robust to the spatially correlated turbulence. Further, more than 20 dB net coding gain (NCG) is achieved for the polar-coded MIMO FSO scheme in a strong turbulence condition with spatially correlated fading. Compared with low density parity check codes, polar codes achieve a 0.4–1.6 dB NCG improvement under a strong turbulence condition with and without spatially correlated fading, which verifies that our scheme is a promising candidate for combating turbulence-induced fading.
关键词： MIMO FSO,Gamma-gamma turbulence,Spatially correlated,Polar codes
更新于2025-09-09 09:28:46
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The development of lower-atmosphere turbulence early in a solar flare
摘要： We present the first observational study of the onset and evolution of solar flare turbulence in the lower solar atmosphere on an unprecedented time scale of 1.7 s using the Interface Region Imaging Spectrograph observing plasma at a temperature of 80,000 K. At this time resolution, nonthermal spectral line broadening, indicating turbulent velocity fluctuations, precedes the flare onset at this temperature and is coincident with net blue-shifts. The broadening decreases as the flare brightens and then oscillates with a period of ~10 s. These observations are consistent with turbulence in the lower solar atmosphere at the flare onset, heating that region as it dissipates. This challenges the current view of energy release and transport in the standard solar flare model, suggesting that turbulence partly heats the lower atmosphere.
关键词： Interface Region Imaging Spectrograph,nonthermal spectral line broadening,turbulence,lower solar atmosphere,solar flare
更新于2025-09-09 09:28:46
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Propagation of Optical Coherence Vortex Lattices in Turbulent Atmosphere
摘要： Propagation properties in the turbulence atmosphere of the optical coherence vortex lattices (OCVLs) are explored by the recently developed convolution approach. The evolution of spectral density distribution, the normalized M2-factor, and the beam wander of the OCVLs propagating through the atmospheric turbulence with Tatarskii spectrum are illustrated numerically. Our results show that the OCVLs display interesting propagation properties, e.g., the initial Gaussian beam distribution will evolve into hollow array distribution on propagation and ?nally becomes a Gaussian beam spot again in the far ?eld in turbulent atmosphere. Furthermore, the OCVLs with large topological charge, large beam array order, large relative distance, and small coherence length are less affected by the negative effects of turbulence. Our results are expected to be used in the complex system optical communications.
关键词： optical coherence vortex lattices,evolution properties,atmospheric turbulence,free-space optical communications
更新于2025-09-04 15:30:14
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Real-time selection for free-space measurement device independent quantum key distribution
摘要： In consideration of the time-dependent transmittance caused by atmospheric turbulence, we combine real-time selection (RTS) method with measurement device independent quantum key distribution (MDI-QKD). The modi?ed scheme ?lters out the intervals with high channel transmittance and thus in turn increases the secure key rate. The optimal threshold of post-selection is determined by an iterative algorithm in advance, which balances the decrease in the total number of signals and the increase in average transmittance. Simulation results show that our modi?ed scheme has apparent advances in both maximum tolerant loss and secure key rate compared to the original MDI-QKD protocol. The MDI-QKD with RTS even performs better when the level of turbulence becomes larger. Furthermore, the modi?ed scheme is more stable against the statistical ?uctuation as well.
关键词： Quantum key distribution,Measurement device independent quantum key distribution,Atmospheric turbulence
更新于2025-09-04 15:30:14
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Performance Analysis of Hybrid RF/FSO System using BPSK-SIM and DPSK-SIM over Gamma-Gamma Turbulence Channel with Pointing errors for Smart City Applications
摘要： The in wireless communication -Free space optics (FSO) offers lots of merits over radio frequency (RF) links due to its license free bandwidth, ease of installation, high security features, and viable cost for short distance communication. It's high speed data rate and immunity against Electromagnetic Interference (EMI) makes FSO the emerging technology of today. But, FSO is not always reliable especially during Atmospheric conditions like fog, rain, mist and snow. Hence, in account a new technique of Hybrid FSO/RF, this includes advantages of both FSO and RF technologies. Through this paper intend to perform an extensive analysis of the error and misalignment effects encountered in Line Of Sight (LOS) communication. Pointing error and turbulence effects are the main drawback parameters for our analysis. For this purpose I have taken into consideration different modulation techniques-Binary Phase Shift keying-Subcarrier Intensity Modulation (BPSK-SIM), Differential Phase Shift Keying- Subcarrier Intensity Modulation (DPSK-SIM) communication system with reference to OOK Modulation. The novel expressions for outage probability and BER for both FSO and RF system are derived which uses Rician Channel and 16QAM Modulation scheme alongside Hybrid FSO/RF system for weak, moderate, strong turbulence regimes using Meijer-G Function.
关键词： Free space optics,Hybrid RF/FSO,Gamma-Gamma Channel,Atmospheric turbulence
更新于2025-09-04 15:30:14
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[IEEE 2018 11th International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP) - Budapest (2018.7.18-2018.7.20)] 2018 11th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP) - Modeling and Throughput Analysis of FSO Systems using GBN-ARQ and AR Transmission over Atmospheric Turbulence Channels
摘要： This paper addresses the error-control issue in free-space optical (FSO) communications. In particular, we model and analyze the throughput performance of FSO systems over atmospheric fading channel when the go-back-N (GBN) automatic repeat request (ARQ) and adaptive rate (AR) transmission schemes are employed. For this purpose, we first build the model to capture the time-varying behavior in atmospheric fading channels. We then use that channel model to develop the burst error model for GBN-ARQ with AR in order to analytically derive the system throughput. Numerical results quantitatively show the impacts of atmospheric turbulence on the throughput and confirm the effectiveness of the GBN/ARQ. Monte Carlo simulations are also performed to validate the analytical results.
关键词： throughput performance,go-back-N (GBN) automatic repeat request (ARQ),atmospheric turbulence,adaptive rate (AR) transmission,Free-space optical (FSO) communications
更新于2025-09-04 15:30:14
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Incoherent Shock and Collapse Singularities in Non-Instantaneous Nonlinear Media
摘要： We study the dynamics of a partially incoherent optical pulse that propagates in a slowly responding nonlinear Kerr medium. We show that irrespective of the sign of the dispersion (either normal or anomalous), the incoherent pulse as a whole exhibits a global collective behavior characterized by a dramatic narrowing and amplification in the strongly non-linear regime. The theoretical analysis based on the Vlasov formalism and the method of the characteristics applied to a reduced hydrodynamic model reveal that such a strong amplitude-incoherent pulse originates in the existence of a concurrent shock-collapse singularity (CSCS): The envelope of the intensity of the random wave exhibits a collapse singularity, while the momentum exhibits a shock singularity. The dynamic behavior of the system after the shock-collapse singularity is characterized through the analysis of the phase-space dynamics.
关键词： non-linear waves,optical turbulence,shockwaves,partially coherent beams and pulses,Vlasov formalism,coherence,rogue waves
更新于2025-09-04 15:30:14