This PDF file contains the front matter associated with SPIE Proceedings Volume 12731, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

This conference presentation was prepared for SPIE Remote Sensing, 2023.

In the framework of two research groups of the Science and Technology Organization (STO) Fraunhofer IOSB and partners conducted a field trial in a humid coastal environment in Florida (USA). The research groups investigate the sensitivity of EO/IR-TDAs (Electro-Optical/Infrared Tactical Decision Aids) to environmental factors and physics-based EO/IR scene simulation tools for decision support systems. Main objective of the trial was the collection of a dataset, that can be used to study/improve the quality of the current EO/IR scene simulation toolsets and to evaluate the impact of different parameters on the reliability of EO/IR-TDA predictions. An overview of the trial and results of a first analysis are given.

Atmospheric optical turbulence strength, parametrized by the atmosphere refractive index C_n², mainly depends on temperature fluctuations. Recently, using ultrasonic anemometers (USAM) for fast temperature measurements has become very attractive since it possesses the advantage of measuring C_n² by a single-point device rather than along a path. Here, we performed a fundamental experiment with multiple devices measuring C_n² in parallel, including a scintillometer and USAM at the center of its path on flat and uniform terrain. This campaign reveals that the main challenge for measuring turbulence strength using USAM occurs in meandering stable conditions, where the changes in wind speed and sonic temperature are lower than USAM sensitivity, and the energy budget deviates from the pure Kolmogorov spectrum. A detailed comparison of the scintillometer and USAM results enable us to optimize the calculation of C_n² from the USAM measurements in meandering stable conditions.

Free-space optical communications systems offer increased data rates and improved channel security in comparison to conventional radio frequency systems for wireless communications. By using the coherent properties of laser light, coherent communication systems have arisen in laser links as a means to increase the data rates. One method of coherent communication is based on the Kramers-Kronig coherent receiver which can retrieve phase information from intensity-only measurements using digital signal processing. As atmospheric turbulence in the optical path will degrade the signal quality in a free-space optical link, the Kramers-Kronig method was tested in an outdoor free-space optical path with data rates on the order of Gbit/s. This free-space optical link was used to record simultaneously the laser spot on a camera, the bit error rate of the received signal, and the average optical power hitting the photodetector after propagation through turbulence. All three measurements were synchronized in an attempt to correlate the effect of atmospheric turbulence on the Kramers-Kronig coherent receiver method.

The research task group NATO SET-304 Modelling, measuring and mitigating optical turbulence (M3T) conducted a field trial at the ONERA Le Fauga-Mauzac center near Toulouse, France, from October 10th to 15th, 2022. Turbulence characterization experiments were performed by imaging an array of light-emitting diodes (LEDs) at a distance of 0.2 km. The refractive index structure parameter 𝐶_𝑛² was calculated from differential angle-of-arrival fluctuations (DAoA) for pairs of LEDs. A large area scintillometer was also used, with good correspondence between the two sensors. The use of multiple pairs of LEDs in the DAoA-method provides error measures for the 𝐶_𝑛²-measurements and allow for the study of spatial variability. Fluctuations of 𝐶_𝑛² occurring on timescales of the order of seconds have been investigated. As the temporal variation was consistent over independently analyzed rows of the LED array, it can be concluded that they exist for the local path. Statistics of the measured parameter and correlations to other parameters, such as atmospheric refraction effects, solar insolation, and temperature variations with height above ground, are reported.

Within the scope of NATO SET-304 research task group, a field trial was conducted in October/2022 at the ONERA campus located in Le Fauga, Toulouse, France. Part of this trial was devoted to the study of the influence of atmospheric turbulence on laser beam propagation. In this paper, measurement results obtained during the trial for a laser beam degraded by optical turbulence are presented along with the results of analytical expressions and a numerical model. The beam of a 50-Hz pulsed 2-µm laser source (owned and operated by FFI, Norway) was focused on a target screen located approximately 1 km away from the laser emitter. Throughout the trial under various atmospheric turbulence strengths (C_n^2 varying between 5×10-16 m-2/3 and 3×10-13 m-2/3), the scattered laser radiation was captured with an infrared camera (owned and operated by TÜBİTAK BİLGEM/İLTAREN, Türkiye) looking at the nearby target screen in an oblique direction. Also, at different times of the trial, a point receiver setup consisting of two photo-diodes was positioned in the propagation path to directly capture intensity fluctuations of the incident laser beam for further investigation of the turbulence effects. The infrared camera was triggered by a 100-Hz trigger signal that enabled successive recordings of the laser beam profiles and instantaneous background. Several post-processing techniques (background subtraction, 2X soft aperture etc.) are performed on the recordings to correctly evaluate the effect of atmospheric turbulence on laser beams such as beam spreading, wandering, and degradation. Calculated beam metrics such as short- and long-term beam radii and beam wander variance are compared with the results obtained from analytical expressions and it is observed that measurement results match well with theoretical expectations. Furthermore, propagation scenarios encountered in the trial are simulated using a wave-optics-based split-step phase screen model with/without considering temporal dynamics. Numerical model results also show good agreement with the measurement results.

We have investigated effects of optical turbulence on laser beams with wavelengths of λ = 1064 nm and 2036 nm. The two laser beams (one diode and one thulium fiber laser) were expanded to 7 cm diameter through a common telescope by means of an off-axis parabolic mirror. The two approximately Gaussian beams were focused along a 1-km-long horizontal path (about 1 m above ground) onto a roughened metal plate. The beam spots were recorded by two separate cameras with band-pass filters. The laser beams were pulsed at 50 Hz and synchronized with the camera shutters. Altogether 76 runs of 20 s length (resulting in 1000 single frames each) were recorded during the days of Oct. 12 to 14, 2022 at ONERAs test site in Fauga-Mauzac in southern France. Turbulence conditions during that time varied from C²_n as low as a few 10⁻¹⁶ m^−2/3 to as high as several 10⁻¹³ m^−2/3. We discuss our findings for both wavelengths in terms of long-term (20 s) beam spot size, short-term (fraction of ms) beam spot size as well as spatial and temporal beam wander. We also find departure from Gaussian beam shape in average at increasing turbulence level. Comparison of experimental data with some selected models is provided.

Long range imaging is limited by the optical aberrations produced by the light propagation through the atmosphere. These aberrations will very often limit the angular resolution of the instrument below its theoretical diffraction limit. Various wavefront sensing techniques have been developed over the years to measure the wavefront distortion and correct it. In this paper we investigate the use of phase diversity phase retrieval for the correction of images aberrated by atmospheric turbulence. We describe the implementation of the system and the image correction. We present the reconstruction results and evaluate the performance of the approach for horizontal path imaging with various turbulence strength in the dataset collected during the NATO SET-304 data collection trial.

Aerosols in the atmosphere scatter and absorb radiation, which reduces the transmission of electro-optical radiation through the atmosphere and impacts on the radiative balance of the planet. To accurately predict the performance of electro-optical systems for naval applications, fine-scale modeling of the atmospheric dynamics of aerosols in the coastal zone is crucial. To this end, the MIO laboratory has coupled the numerical meteorological model MESO-NH in its LES version to the particle extinction model MEDEX (Mediterranean Extinction Model MEDEX, Piazzola et al., 2003). The numerical aerosol concentrations in the Mediterranean coastal zone were compared to experimental data acquired in May 2008 on board a research vessel and on the island of Porquerolles. This provided insight in our capability to model the fine-scale spatiotemporal variation of aerosol and visibility for a coastal area of complex geography.

Fluctuations in the refractive index of air, known as optical turbulence, may have a deleterious effect on system performance in free space laser applications. Optical turbulence is often quantified by the coefficient of the second order structure function of the refractive index, C²_n. While a range of measurement techniques for determining C²_n exist, they may be broadly categorized as those that directly interrogate the optical medium, and those that measure the effects of the medium on some propagation characteristic of an optical wave. In this work, we compare estimates of C²_n obtained from temperature fluctuations via a sonic anemometer to measurements from irradiance fluctuations via an adjacent optical scintillometer. Both instruments are located in a near-maritime environment (coastal estuary) within the atmospheric surface layer, several meters above the surface. Our data set consists of observations that span more than one year, which allows us to examine a wide range of atmospheric conditions and to examine seasonal effects on the measurements. As a single-point measurement, C²_n from sonic anemometers is frequently used in calculations that require path-averaged C²_n values. This work explores the conditions under which this connection is valid and the parameter space where anemometry provides reliable C²_n values.

We present a new analysis of laser propagation experiments carried out with the Laser Propagation Testbed (LPT) developed by TNO. A major goal of these experiments is to validate and improve atmospheric propagation models that are essential to applications such as laser communication, high energy laser weapon systems and remote sensing. The data were obtained during a field campaign with a 1W 1556 nm laser beam deployed over a 3.6 km maritime path in The Netherlands. The measurements consist of intensity profiles of the propagated laser beam and local meteorological and atmospheric conditions (visibility, refractive index structure parameter and aerosol data) obtained during a ten day period under varying weather conditions. We use the locally measured atmospheric conditions and numerical weather prediction to constrain a turbulent laser propagation model developed by TNO, and compare the results with the time series measurements of the laser beam profile.

Extended Rytov Theory (ERT) is often interpreted as describing intensity scintillations as a function of volume turbulence strength. This approach can create difficulties when trying to compare theory to experimental results and wave optics simulations. Recent works in both areas show differences of a factor of two in the focusing regime where peak scintillation occurs. In this work, simulation campaigns are used to better understand the role of propagation geometry in terms of the first Fresnel zone size on peak scintillation.

Results from comparative measurements of turbulence and transmission at laser wavelengths in the shortwave infrared (SWIR) and longwave infrared (LWIR) wavelength bands are presented. The experimental setup uses a dual band transmitter with coaxially aligned laser beams and a receiver based on a common collecting off-axis parabolic mirror to direct the radiation to the detectors. Measurement of turbulence was performed on one occasion and transmission was measured in two events with fog and one event with falling snow. The turbulence data is compared to optical wave simulations, and the transmission data is compared to Mie calculations based on aerosol and hydrometeor data. As expected, the received power scintillations caused by atmospheric turbulence are stronger in the SWIR band compared to the LWIR band. The measured transmission in fog and snow can be explained by the measured characteristics of the scattering medium. In fog, depending on the particle size distribution, higher transmission can be observed in either wavelength bands. In snow, on the other hand, it is shown that the transmission is higher in the SWIR band as an effect of forward scattering.

Optical or microwave measurements from numerous geodetic devices are affected by their path through the atmosphere. Deterministic changes in the atmospheric refractivity index can be modelled and, to some extent, corrected. On the other hand, random fluctuations coming from atmospheric turbulence correlates the observations thus reducing the effective number of available observations. They have to be accounted for to get a realistic description of the measurement error but also to increase the reliability of early warning system within the context of risk management with light detection and ranging (lidar) sensors. We have developed a novel method to investigate the impact of turbulence on long-range laser scanner observations by searching prisms within repetitive scans at different times of the day, during consecutive days in a mountainous region in Austria. The empirical analysis of the power spectral density of the measurements combined with information from meteorological sensors (pressure, temperature, wind velocity) help gaining a better understanding of how and when turbulence affects the range measurements. Our method gives an averaged description of turbulence across the atmospheric layers travelled by the laser light, and paves the way for the development of an improved stochastic model for such observations without using additional equipment such as scintillometers.

Event based image sensors (EBS), also known as neuromorphic cameras, create a stream of spatially tagged events corresponding to localised changes in intensity. From this stream, statistics of the turbulence altering the optical path may be extracted, and image restoration of the target is potentially more feasible because, contrary to a traditional image with a fixed exposure time, the events are causal, so spatial and temporal correlation, filtering, or tracking is possible. We explore this further with a long range surveillance system.

Free space optical communication (FSOC) represents a promising technology to enable a secure transmission with high data rates over long distances. Ensuring proper operation in turbulent atmospheric conditions by adaptive optics (AO) is essential for the general feasibility of FSOC. A bottle neck of the AO performance is the limited speed of control loops based on Shack-Hartmann wavefront measurements. Alternatively, holographic wavefront sensors provide the potential for an increased measurement speed by enabling a direct measurement of individual spatial mode amplitudes of the turbulence. So far, their performance is limited by inter-modal crosstalk. In this work, a design algorithm to build a holographic sensor out of a sequence of holographic plates is developed. To this end, an adapted wavefront-matching mode sorter algorithm is implemented, which is based on the use of Zernike-polynomials. The algorithm allows for designing multiple holographic phase plates in sequence with a built-in optimization of crosstalk reduction. The phase profile of the received beam is converted through interaction with each phase plate before the beam hits a detection plane. The designed computer-generated-holograms are characterized by simulation and the mitigation of crosstalk is shown for a higher number of holographic phase plates. Additionally, a proof-of-concept experiment is performed to demonstrate the desired behavior of the holographic sensor.

The usage of CubeSat platforms has seen a significant increase over the last decade. CubeSats are compact and cost-efficient. With it the need for free-space optical communication (FSOC) between satellites and optical ground stations did increase as well. Achieving a good ratio between the size and the performance of the optical payload is still a challenge. We propose a FSOC terminal to be implemented on a 16U CubeSat platform. A combination of a coarse pointing assembly (CPA) based on a dual Risley prism scanner in front of an all-metal freeform telescope, and a fine pointing assembly (FPA) with a fast-steering mirror (FSM) were developed. The Risley prisms have a smaller mechanical envelope compared to classical gimbal-based mirror mounts or periscopes but can still provide a suitable range of beam deflection. The use of such a Risley prism scanner has been a research topic in aerospace for quite some time. Especially the challenging driving and controlling of the nonlinear beam pointing behavior presents a challenge. To keep the payload on the CubeSat feasibly small, only a microcontroller with limited calculation power is used. Therefore, we propose a combined control scheme for the CPA and FPA based on simplified calculations and the use of classical digital control theory. Coarse and fine pointing are controlled in a closed loop pointing simultaneously.

When information is spatially repeated in self-similar fractal beam patterns, only a portion of the diffracted beam is needed to reconstruct the kernel data. What is unique to a fractal-encoding scheme is that the image demultiplexing process can be, to a first approximation, easily performed optically. In prior work, we experimentally and numerically study fractal-encoded optical beams and their mid- and far-field propagation without added turbulence. Here, we present preliminary simulations of fractal-encoded beams with high turbulence (C²_n ≥ 10⁻¹⁴ m^−2/3 ) where we achieve respectable bit error rates of 10⁻³ . These results are impressive given that: data with low fractal orders is shown, simple threshold-algorithms are used (i.e., no machine learning), and only a third of the beam, off-axis, is needed. More robust channel encoding is associated with increased fractal orders, larger collection areas, and higher kernel singular value decomposition entropy.

Accurate prediction of the vertical distribution of optical turbulence strength (see manuscript PDF for symbol) is essential for several applications, such as ground-to-satellite optical communications and astronomical seeing prediction. We have developed an algorithm that couples the calculation of (see manuscript PDF for symbol) profiles with the results of the Weather Research and Forecast Model (WRF), using the Tatarskii formula for (see manuscript PDF for symbol). The mixing length scale was identified as a critical parameter along with the gradients of potential temperature and wind speed. To determine the effects of the number of vertical levels used in the simulations and the effects of diurnal variations, we conducted a study using 100 to 500 levels in both daytime and nighttime simulations. We analyzed the results to identify differences and to suggest improvements in the calculations, taking into account the trade-off between longer simulation time and accuracy. We also compared our results to vertical profiles of (see manuscript PDF for symbol) derived from experimental data obtained from radiosonde ascents, for the Bergen radiosonde station in Germany. Our results contribute to refining the modelling of (see manuscript PDF for symbol) and improving the accuracy of the prediction, which will benefit various practical applications.

Free-Space Optical Communication (FSOC) links are considered a key technology to support the increasing needs of our connected, data-heavy world, but they are prone to disturbance through atmospheric processes such as optical turbulence. Since turbulence is highly dependent on local topographic and meteorological conditions, modeling optical turbulence strength (see manuscript PDF for symbol) is challenging during the design phase of an optical link or network. Over the past 25 years, (see manuscript PDF for symbol) parameterizations of varying complexities have been combined with various numerical weather prediction models for the spatio-temporal estimation of (see manuscript PDF for symbol). However, the outputs of these models can exhibit substantial variability based on the user-defined configuration that determines how atmospheric processes are represented. To address this concern, we propose to run not a single model configuration but multiple diverse ones to generate an ensemble estimate of (see manuscript PDF for symbol). We employ the Weather Research and Forecasting model (WRF) with ten different Planetary Boundary Layer (PBL) physics schemes forming a diverse ensemble yielding a probabilistic (see manuscript PDF for symbol) estimate. We demonstrate that this ensemble outperforms the individual runs when compared to scintillometer field measurements and show it to be robust against outliers. We believe that FSOC downstream tasks such as link budget estimations should also become more robust if based on a (see manuscript PDF for symbol) ensemble estimate compared to single model runs.

Taylor’s Frozen Turbulence Hypothesis (TFTH) has been used extensively in theoretical studies to model the temporal fluctuations of optical quantities affected by atmospheric turbulence. It has been relied upon to provide temporal-frequency spectra under varying propagation conditions and for different atmospheric refractive index models. However, experimental works have revealed its limitations, such as systematic inaccuracies in estimating cross winds during calm nights in scintillation measurements at astronomical sites, scintillation discrepancies in ground-layer measurements, and broad estimates of the coherence time in phase fluctuation measurement techniques. This highlights the need to recognize the limitations of the TFTH and seek alternatives that can provide a more reliable description of atmospheric turbulence’s temporal fluctuations. Here, we propose a spatio-temporal statistics for refractive index fluctuations through fluid dynamics models and evaluate the complex phase propagation under weak turbulence. Then, we test its ability to reproduce experimental observations under different ground-layer turbulence conditions.

The analysis of budget link and free space optical system performances requires the calculation of several metrics of the atmospheric turbulence-affected collected light. In this paper, assuming the collected light is focused into an optical fiber positioned in the focal plane, we use the ABCD ray-matrix representation to show the impact of Kolmogorov and non-Kolmogorov turbulence on the Power in the Fiber (PIF). Calculation of the PIF requires also the knowledge of the transmitted average power that enters the receiver aperture (power in the bucket) and the long-term beam spread in the focal plane.

Free Space Optics (FSO) technologies are a promising and cost-effective solution to meet the increasing demand for bandwidth. These technologies ensure high transmission security and are compatible with OCDMA (Optical Code Division Multiple Access) systems, providing potential for an all-optical communication network in the future. However, FSO systems suffer from two major limitations that must be overcome: high dependence on atmospheric conditions and Multiple Access Interference (MAI) in FSO-MIMO (Multiple Input Multiple Output) networks. In this study, we simulated an FSO-MIMO-Direct Sequence)-OCDMA transmission link considering a Gamma-Gamma channel. The DS-OCDMA sequences were modulated using OOK (On-Off Keying) and PPM (Pulse Position Modulation), which are commonly used in FSO transmissions. The performance of the transmission link was evaluated in terms of BER (Bit Error Rate) using a Monte Carlo algorithm. Our simulations indicate that PPM modulation provides better resistance to atmospheric scintillation while allowing access to a higher number of users than OOK modulation.

Generally, LiDAR sensors use near-infrared light, so in highly water contented areas such as marshes, the laser reflection intensity is weakened due to the absorption of a lot of light. In addition, during the IMU calibration process of the sensor, it is not possible to obtain a high-precision point cloud due to the shaking of the aircraft, accumulation of errors, and other factors. To post-process the acquired point cloud, it is necessary to separate the data that contains the vibrations of the aircraft, such as acceleration, rotation, and calibration, from the point cloud by aligning the trajectory of the UAV with the point cloud. However, manually separating the trajectory for a wide area of UAV flight can take a lot of time and can affect the consistency of the data. In this study, aim to extract a stable LiDAR point cloud by separating the trajectory of the UAV based on the following criteria for UAVs with a certain pattern of trajectory. First, separate the two trajectories by distinguishing between acceleration and cruising of the UAV. Second, separate two regions where the direction of the UAV's travel changes sharply. Finally, apply a process to separate the IMU calibration process. Through this process, can automatically extract the LiDAR trajectory data and select only the point cloud obtained at the same flight speed and altitude, thereby obtaining a point cloud density of a constant value. This study reduces the time required for separating and post-processing the trajectory of LiDAR data and enables the production of high-resolution terrain data for a wide area that needs to be flown at low altitudes.

Long range horizontal path imaging through atmospheric turbulence is hampered by spatiotemporally randomly varying shifting and blurring of scene points in recorded imagery. Software mitigation strategies have been developed to produce sharp and stable imagery. However, it remains challenging to correct video disturbed by turbulence in real time. Real time implementation of turbulence mitigation will search for compromises between processing performance and corrected image quality. In this paper we compare the quality of real time turbulence mitigation to the results of non-real-time algorithms that should provide a higher image quality. The paper presents the method for the data collection and the results of the comparison on different scenarios relevant in an operational context.