This PDF file contains the front matter associated with SPIE Proceedings Volume 11153, including the Title Page, Copyright Information, Table of Contents, Author and Conference Committee lists.

In the framework of a NATO research group Fraunhofer IOSB and partners conducted a field trial in an arid shrub land environment in southern New Mexico (USA). The group investigates environmental limitations of fielded EO-TDAs (Electro-Optical Tactical Decision Aids). Main objective of the trial was to study the impact of the atmosphere on imaging sensor performance with a focus on the effects of atmospheric extinction and near surface turbulence. An overview of the trial will be given, as well as an overview on EO-TDA development. Results of efforts to forecast the refractive index structure parameter using numerical weather prediction (NWP) models will be described, as well as the results of a perception study on the influence of turbulence on target acquisition ranges using MWIR imagery.

Due to the exponential increase in global data transfer, high-speed free space optical links between the ground and satellites are increasingly being envisaged. However, the availability of the optical link is affected by atmospheric attenuation and turbulence. To reliably estimate the overall optical system performance, accurate prediction of the link availability is required, which implies reliable characterization of the local atmospheric parameters. Such characterization will help for optimal selection of the optical ground station network and the prediction of their turbulence conditions. In this contribution, we propose several composite parametric models (profiles of Cn² and wind) based on analytical and bulk models fed by weather data. This flexible method can be applied to any location on the globe for an automatic characterization of the turbulent channel. In this paper we present the characterization methodology and the comparison of parametric and numerical models with in situ experimental measurements. Results show outstanding adequacy of the ground Cn² values obtained with a Bulk-model fed by the local weather station and those of micro-thermometer measurements and the Astro-Meso-NH model.

Results are reported from a study of propagation near-ground within the solar blind ultraviolet wavelength range, including measurements of ultraviolet transmission along a horizontal path. In addition a discussion is presented about ozone data measured at 41 stations spread across Sweden, collected and stored by the Swedish Meteorological and Hydrological Institute (SMHI) and the Swedish Environmental Protection Agency (SEPA). In connection with this a simple statistical analysis of occurrence of the ground ozone is done.

The transmission measurements were made along slightly inclined path of 1830 m during the period from mid- November 2018 to mid-January 2019. The transmission values were collected together with weather data including measurement of ozone content in the atmosphere.

A simple model for ultraviolet transmission that takes into account both visibility and measured ozone content has been developed. This model shows a difference to the model found in the Modtran program, often used to predict atmospheric transmissions in several wavelength bands including ultraviolet wavelengths. Overall, the new model shows a weaker connection between ultraviolet extinction, ozone content and visibility compared to Modtran.

Data from the countrywide ozone measurements as well as data collected in connection with transmission measurement showed variations in the atmospheric ground-level ozone content that are not easily explained from weather parameters alone. Reports, see for instance [2], state that it is volatile hydrocarbons and other molecules, both natural and anthropogenic, along with solar radiation that influence the amount of ozone. The variation of both volatile hydrocarbons and solar radiation together with a variation in wind direction and speed have an impact of varying transmission.

In practice all measured ozone values during conditions with low relative humidity were relatively high. There was also a high correlation between high ozone values and high wind velocities. The relation between ultraviolet extinction deduced from measurements and the corresponding values for the visual wavelength range and corresponding values calculated using Modtran show different relationships. The transmission measurements in the ultraviolet wavelengths range showed that the amount of aerosols has no major significance for the ultraviolet extinction at visual ranges above 15 km.

Due to the short period of data collection, mostly fall-like weather conditions, the conclusions that can be drawn from the measurements are limited. Furthermore, the ground-level horizontal measurements make it very hard to draw conclusions about the ultraviolet transmission at higher air layers.

Using seasonally averaged meteorological and spectrally resolved aerosol profiles extracted from a maritime environment, this paper investigated how the resolution of the vertical profiles influences the 3-5μm and 8-12μm average transmittance and integrated path radiance computations conducted by MODTRAN in high-elevation scenarios. First, the minimum altitude to which the atmosphere should be defined in order to accurately determine the transmittance and path radiance along vertical and slant paths was investigated by recursively removing vertical layers until the relative changes in the transmittance and path radiance became smaller than those due to instrument uncertainty. Once this minimum height was found, the vertical resolution in the atmosphere below the minimum altitude was systematically varied. The suitability of several gradient-based criteria has been investigated to determine the optimal discretization of the vertical profiles. The results indicate that, depending on the quantity to be calculated, vertical discretizations based on the gradient in either the pressure, temperature or humidity serve as optimal discretizations in maritime high-elevation scenarios. Moreover, the followed methodology demonstrates how to adaptively implement a vertical resolution in a generic atmosphere, which generates crucial knowledge in supporting signature and sensor performance modelling for high-elevation scenarios.

In software-based turbulence mitigation, reliable no-reference image quality metrics are indispensable. In this research, a straight line detector (SLD)¹ is used to quantify warping due to turbulence. Our metric is based on the average length of detected straight lines, which constitutes a credible representation of the straightness in the image. The performance of the metric was tested on an anisoplanatic warping simulation. With increasing warping strength, the metric shows a decreasing average length of detected straight lines in the images. It is shown that the metric is able to differentiate between a set of simulated data with strong warping (C²_n = 10^-14 m^-2/3) compared to weak warping (C²_n = 10^-15 m^-2/3). Hereby the least and most warped frames found from the metric are considered and checked for credibility. A reference frame generation is developed by averaging the four least warped frames found from the metric. For evaluation of the reliability of the metric, it is tested on several data sets acquired at different distances. A correction of dewarping with optical ow is performed and the results with the reference frame generated based on the straightness metric and with a long exposure image as reference are compared.

Absorption and scattering by gaseous species, aerosol particles and precipitation lead to atmospheric attenuation of laser radiation. High power laser propagation over long horizontal and/or vertical distances is significantly affected by atmospheric attenuation in outdoor locations. Laser transmission experiments can be used to derive experimental extinction coefficients, which can be included in the assessment of the laser safety. However, a full representation of the effect of precipitation types on high power laser propagation is unknown to the authors.

Laser transmission experiments at 1030 nm under rainy and snowy weather conditions were performed on a laser test range to investigate the effect of atmospheric attenuation on high power laser radiation. Based on these laser transmission measurements, experimental extinction coefficients were derived. The experimental extinction coefficients are compared with theoretical values. In addition, the experimental extinction coefficients are correlated with visibility measurements.

Military operations on target practice and wind farms demand on knowledge of the turbulent state and wind speed in the surrounding atmosphere. However, the definition of turbulence is completely different in both cases. In the military context we speak about optical turbulence in the wind industry, kinetic turbulence is essential. In the following we want to present an approach that combines both turbulences. In the vicinity of a military test range and 120 m high wind turbines a new Doppler LIDAR system (WINDCUBETM v200s, Leosphere, France) was operated to investigate the vertical distribution of turbulent kinetic energy and the spatial distribution of the radial wind speed in the atmospheric boundary layer. Deployments in different modes were carried out to get best estimations of turbulence. Vertical profile measurements obtained with two ultrasonic anemometers at discrete levels on an 80 m high mast close to the position of the LIDAR were used for comparison. We present an adapted algorithm to calculate the TKE from a low-frequency signal (vertical profile every 10 s) and a approach to calculate C²_n. The first preliminary results show a good agreement. Results on diurnal cycles of the TKE in different stability regimes and local effects will be discussed.

High frequency fluctuations in the refractive index are the main atmospheric influence on wave propagation in the atmosphere. They are caused mainly from variations in atmospheric temperature. Ultrasonic anemometers have been established as common tools in performing in-situ measurements of turbulence. They were used as fixed point instruments from towers, for the characterization of local turbulence or from mobile platforms. Besides wind velocity fluctuations, high frequency variations of the sonic temperature are analyzed. Time series of temperature data are analyzed to derive the structure function of temperature C_T ², that should be proportional the strength of optical turbulence C_n ². The application of the Fast Fourier transformation, temporal resolution and averaging time are addressed to show the applicability of ultrasonic anemometers. Also the influence of humidity fluctuations on the determination of C_n ² in applications over land and over the sea is discussed. Errors in the applications from fixed points as well as from mobile platforms are estimated for different types of anemometers.

Atmospheric turbulent fluctuations interfere wave propagation in the atmosphere. Therefore they are important to know to describe the atmospheric influence on imaging, laser propagation and electro-optical sensor performance. Regarding in situ point measurements standard instrumentation for monitoring turbulent fluctuations are ultrasonic anemometers. These instruments have a good accuracy in high frequency single temperature and wind speed measurements, but they are very expensive. Often they serve as a reference for a whole measurement area. To characterize a complex or heterogeneous area better, cheaper and yet still reliable all-you-can-measure weather measurement systems would help to increase the amount of measurement stations and gathered data. These would allow a better understanding and description of local environmental conditions and serve for a better validation of high resolution models. We set up a low-budget three-dimensional instrument also on the base of monitoring the velocity of sound in the atmosphere. The new instrument is to be developed, assembled and tested. A 3D printed housing will be used for saving the sensor of environmental conditions. First steps of the development of the new instrument will be presented and discussed.

Probability Density Functions (PDFs) describing intensity fluctuations of a propagated beam are necessary to reliably predict operation of Free-Space-Optical (FSO) systems. This becomes especially important when considering the tails of the distribution. For example, accuracy in the low-intensity tail is necessary in the design of FSO communication systems as it determines the probability of fade. Accuracy in the high-intensity portion of the tail is necessary for eye safety calculations and to prevent equipment damage. In this work, we evaluate PDF models considering the statistics of the fit to empirical data for point and distributed apertures for several values of Rytov variance. Based on p-values and level of significance, we compare the quality of the fit of common PDF models to empirical data. As part of this, we evaluate the transition of the best fit PDF as the aperture area is increased. Both simulated and measured data will be included in this analysis.

The propagation of laser beams through the atmosphere is of interest for problems of communication with Earth satellites. Unfortunately, only paraxial beams are currently being investigated. Non-paraxial beams are considered in [1, 2]. We studied a non-paraxial Gaussian beam. The problem has been considered on the basis of our previous paper [3] which was revised and substantially supplemented. The task was reduced to a calculation of a line integral. Systematic computations were fulfilled, the distance of beam propagation varied from a few meters to one kilometer. First of all, we considered the beam propagation through homogeneous gas. The results are as follows. The distribution of the transverse component of the electric field is similar to that of the paraxial beam at small distances. But there arise additional maxima on the periphery, if the distance exceeds several tens of meters. Those maxima appear due to diffraction. They do not exist for paraxial beams. The axial component of the electric field has its maxima on two curves located on the plane which is normal to the beam axis ( at x₂=const ). The value of the component is linearly dependent on x₁ along the curves.

A procedure was set forth which allows to solve the problem when the beam propagates through the inhomogeneous gas. If the permittivity is close to unity and the permeability is equal to unity we get the explicit solution. The obtained solution may be generalized to the case when the permittivity depends on time.

Retro-reflectors often find use in long-range laser propagation experiments. They double the propagation distance and cancel atmospheric tip/tilt alleviating the hardware requirements on the laser transceiver system. On the other hand, the effects that a retro-reflector can have on collimated or focused laser beams have not been thoroughly investigated. Additionally, for use in combination with beams carrying orbital angular momentum, which become increasingly popular in free-space optical communications, retro-reflector’s effect must also be analysed. Demodulation efficiency of a communications system based on such beams is very sensitive to the fidelity of helical wavefronts and therefore retroreflector’s effect on both amplitude and phase for collimated and focused orbital angular momentum beams should be studied.

In this paper, the determination of the topological charge of the vortex beams by means of shearing interferometry was achieved, for both common and non-common path shearing interferometers, using simple yet effective optical elements. The recording and analysis of interference patterns from different setups was accomplished using: cyclic, rotational and reversal shearing interferometers. The use of cyclic and rotational shearing interferometers resulted in interference patterns with two oppositely oriented forks for both setups. However, with the reversal shearing interferometer, a single forked pattern was obtained and a mathematical approximation was deduced.

Free Space Optics (FSO) is a growing up technology offering a higher bandwidth with fast and cost-effective deployment compared to fiber technology associated to a lack of regulation unlike radio frequencies. Multiple applications are envisioned including but not limited to campus-scaled network, substitution for a fiber network after a disaster (e.g., earthquake, attack, etc.) or connecting a drone as a relay in white spot. Despite that, FSO performances are restricted by atmospheric phenomena (e.g., turbulence, fog or scattering). Therefore, the operating wavelength is an important parameter that has to be chosen wisely so as to reduce the impact of the environmental parameters. In order to improve the FSO availability, performance and range, the investigation of the cross relation between the climatic conditions and the wavelength is highly required. The main scope of this study is to evaluate and compare the availability and the performance of an FSO link of a few kilometers in urban environment for two infrared wavelengths matching the appropriate atmospheric windows. To do so, we computed the transmission rate under various atmospheric conditions, including fog, both at 1.55-µm (standard telecom wavelength) and 4-µm (mid infrared) for which the attenuation of the beam intensity is smaller whatever the extinction coefficient. Using the transmission rate of an FSO link extracted from our radiative transfer software MATISSE the link budget is derived for a simple direct detection system design. The source and detector components characteristics are also considered to compute the reception induced noise and estimate the Bit Error Rate (BER) of the FSO link for the two wavelengths with respect to the visibility. Assuming error correction and the corresponding BER limit, it is possible to derive the minimum of visibility below which the efficiency of the optical link is strongly affected. Thus, for a 50 mW source, the minimum visibility required to get an effective FSO link is respectively 600 m at 1.55 µm and 400 m at 4 µm. A weather visibility database is then used to obtain the theoretical availability of the FSO system during a whole year. As an example the availability at Vélizy-Villacoublay (France) weather station throughout the entire year 2017 will be displayed. This paper presents the results of the transmission simulation, the development of the link budget calculation, and the contrast between laser and background. We describe the different detection noises and discuss about the availability of our FSO link during a year for the two wavelengths in urban environment.

Tracking the moving objects in a video sequence is a critical problem in wide area airborne surveillance systems due to its cyber-physical nature. The large amount of imagery data generated by the unmanned aerial vehicle (UAV) requires high data rate link to handle real-time video streaming to the ground control station. Free space optics (FSO) technology is a promising data link for UAV due to its easy installation process, long range and duplex operation, and high bandwidth with more security. The main issue using FSO communication is the beam attenuation due to the various atmospheric impairment. In this work, the characteristics of FSO communication links with various data rate and different wavelength from UAV to the ground station has been investigated with Polarization Shift Keying (PolSK) and On-Off Keying (OOK) modulation techniques to improve the performance of data communication under various UAE weather conditions. The proposed system’s degradation model considered the rain, foggy and haze and height of UAV from the ground and a case study is experimented on the effects of FSO based UAV communication system in UAE weather conditions. The results show the performance of the mobile optical link in the presence of atmospheric effects is compared in UAE weather conditions and the graphs plotted against bit error rate (BER), attenuation and distance which highlights the benefits of FSO.

Free-space optical (FSO) communications are becoming promising schemes for high-capacity wireless links due to their plentiful characteristics originated from higher carrier frequency. These characteristics also yield a greater security advantage over radio frequency counterparts: the physical ability of a wiretapper is reasonably restricted due to the high directionality of communication beam and the line-of-sight configuration of the link. Secret key agreement over FSO links (FSO-SKA) employs this security advantage as well as the post-processing over an authenticated public channel to establish an information-theoretic secure key which cannot be broken even with unbounded computer resources. In the previous works, the authors demonstrated the full-field implementations of FSO-SKA with a 7.8-km FSO link testbed including a probing station to estimate the possible wiretapping risks from the sidelobe of the communication beam. In the demonstration, however, there is still room to improve the secret key rate by exploiting the optical fading which contains additional information about random states of the FSO links. We here propose a novel protocol for FSO-SKA employing such channel state information. In the protocol, the legitimate receiver decides whether to discard the received symbols or not according to the received optical power at the time. Based on the experimental data from the FSO link testbed, we demonstrate that the proposed protocol improves the secret key rate compared with our previous result. To our best knowledge, this is the first demonstration that exploits the effect of atmospheric turbulences to improve the security performance of communication systems. We anticipate that this idea will be applicable on the broader areas of FSO communications and opens a way toward practical wireless network spanned by FSO links.

Surveillance using aerial systems provides a different angle of monitoring, maintain security or locate wanted targets. The imaging data is live streamed to the ground using different types of datalinks. However, the large amount of imagery and video information transferring from the unmanned aerial vehicle (UAV) require high data rate for communication. A method of optical wireless communication that has a high bandwidth which can be used in the field of UAV is free space optics (FSO). FSO technology offers faster and secure data transmission. In this work, FSO channel model have been investigated pertaining to atmospheric turbulence to analyze the average spectral efficiency (ASE) using various modulation schemes such as Polarization Shift and On-Off Keying, and Coherent Optical Wireless Communication. The atmospheric turbulence channel model depends on various parameters Such as the refractive index structure, strong or weak turbulences. Presented in this work, the refractive index is manipulated to obtain the exact turbulence differences through various operational altitudes while operating the UAV. Furthermore, the misalignment error between the overall system receiver and transmitter is considered when performing the analysis. The results show the performance of the optical link in different altitudes and different modulation schemes to optimize the performance of the link.