Random phase screens are essential elements of simulating light propagation through turbulent media. In order to be effective, they must accurately reflect theory and be implementable by the user. This document explains and evaluates three methods of generating random phase screens: using a Fourier series upon a polar frequency grid with logarithmic spacing; using the fast Fourier transform, with its cartesian frequency grid; and using Zernike polynomials. It provides a comparison of the polar Fourier series technique with the two more common techniques (fast Fourier transform and Zernike), with the end result of giving the users enough information to choose which method best fits their needs. The evaluation criteria used are generation time (usability) and phase structure function (accuracy).

Intensity fluctuations at the receiver in free space optical (FSO) communication links lead to a received power variance that depends on the size of the receiver aperture. Increasing the size of the receiver aperture reduces the power variance. This effect of the receiver size on power variance is called aperture averaging. If there were no aperture size limitation at the receiver, then there would be no turbulence-induced scintillation. In practice, there is always a tradeoff between aperture size, transceiver weight, and potential transceiver agility for pointing, acquisition and tracking (PAT) of FSO communication links. We have developed a geometrical simulation model to predict the aperture averaging factor. This model is used to simulate the aperture averaging effect at given range by using a large number of rays, Gaussian as well as uniformly distributed, propagating through simulated turbulence into a circular receiver of varying aperture size. Turbulence is simulated by filling the propagation path with spherical bubbles of varying sizes and refractive index discontinuities statistically distributed according to various models. For each statistical representation of the atmosphere, the three-dimensional trajectory of each ray is analyzed using geometrical optics. These Monte Carlo techniques have proved capable of assessing the aperture averaging effect, in particular, the quantitative expected reduction in intensity fluctuations with increasing aperture diameter. In addition, beam wander results have demonstrated the range-cubed dependence of mean-squared beam wander. An effective turbulence parameter can also be determined by correlating beam wander behavior with the path length.

In remote sensing, atmospheric turbulence and aerosols limit the image quality. For many practical cases turbulence is shown to be dominant, especially for horizontal close-to-earth imaging in hot environments. In a horizontal long-range imaging it is usually impractical to measure path-averaged refractive index structure constant C_n² (which characterizes the turbulence strength) with conventional equipment. In this paper we propose a method for estimation of C_n² based just on the available recorded turbulence-degraded image sequence. The method exploits the turbulence-induced image "dancing". C_n² is extracted from the estimated image shifts variance. Experimental comparison with C_n² measurements using a scintillometer shows reliable estimation results. We also estimate image motion with sub-pixel accuracy for the purpose of obtaining a high-resolution image by applying a simple super-resolution procedure. Results of super-resolution for real imagery are presented.

Image data collected near the ground, in the boundary layer, or from low altitude planes must contend with the detrimental effects of atmospheric turbulence on the image quality. So it is useful to predict operating regimes (wavelength, height of target, height of detector, total path distance, day vs. night viewing, etc.) where atmospheric turbulence is expected to play a significant role in image degradation. In these regimes, image enhancement techniques such as speckle processing, deconvolution and Wiener filtering methods can be utilized to recover near instrument-limited resolution in degraded images. We conducted a literature survey of various boundary layer and lower troposphere models for the structure coefficient of the index of refraction (C_n²). Using these models, we constructed a spreadsheet tool to estimate the Fried parameter (r₀) for different scenarios, including slant and horizontal path trajectories. We also created a tool for scenarios where the height along the path crudely accounted for the topology of the path. This would be of particular interest in mountain-based viewing platforms surveying ground targets. The tools that we developed utilized Visual Basic© programming in an Excel© spreadsheet environment for accessibility and ease of use. In this paper, we will discuss the C_n² profile models used, describe the tools developed and compare the results obtained for the Fried parameter with those estimated from experimental data.

Optical turbulence is important because it can significantly degrade the performance of electro-optical and infrared sensors, such as free-space laser communications and infrared imaging systems. Changes in the refractive index of air along the transmission path of an optical system in free space can influence traveling light waves temporally and spatially causing blurring, scintillation, and bean wander. If left uncompensated, these effects could cause fades and surges in transmitted signals and result in high bit errors in communicated data. An earlier paper discussed the growing need for increasingly accurate and reliable numerical models to predict optical turbulence conditions, especially in complex (non-uniform) signal propagation environments. Hence, we present a finite-difference computer model to predict the microphysical (microclimate) influences on optical turbulence (C_n²) around the ARL A_LOT Facility and its surroundings, e.g., forests and multiple buildings. Our multi-dimensional prototypical model begins to address optical turbulence conditions along more complex optical lines-of-site and account for inhomogeneities in C_n² brought about by horizontal changes in landscape, wind flow, temperature, and humidity. We anticipate that this kind of computational research will be an important vehicle for investigating C_n² and related laser-optic propagation effects in complex areas.

We report on a set of measurements made in December 2005 by researchers from the University of Central Florida, SPAWAR's Innovative Science and Technology Experiment Facility (ISTEF), Harris Corporation, NASA Kennedy Space Center, and Northrop Grumman. The experiments were conducted on the Shuttle Landing Facility (SLF) at Kennedy Space Center (KSC) over terrestrial paths of 1, 2, and 5 km. The purpose of the experiments was to determine the atmospheric-induced beam spreading and beam wander at various ranges. Two lasers were used in the experiments. Both were a pulsed 1.06 μm laser; however, one was single-mode and the other was multi-mode. Beam profiles were recorded near the target position. Simultaneous measurements of Cn2, wind speed and direction, humidity, visibility, temperature, and surface temperature profiles were all recorded.

Turbulence can be a dominant factor in image and laser beam degradation for optical systems operating in the near-surface maritime environment. A long-term propagation field experiment was conducted at Zuniga Shoal (near San Diego) to study the impact of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various points of the year, during which scintillometer measurements were obtained along a 7.2 km over-water path and a 'flux' research buoy deployed along the propagation path collected concurrent mean meteorological, atmospheric turbulence, and wave data. We use the refractive index structure parameter (C_n²) as the critical parameter for quantifying the effects of atmospheric turbulence on laser system performance, including received power fluctuations, beam spread and beam wander. Bulk estimates of C_n² were derived from the buoy mean meteorological measurements using the Navy Surface Layer Optical Turbulence (NSLOT) model. C_n² was also determined from atmospheric turbulence measurements obtained from a sonic anemometer on the buoy. These independent C_n² values derived from the buoy data are compared with C_n² values computed from the infrared propagation measurements to determine how the NSLOT model performs under different environmental conditions. In addition, the optical measurements and bulk estimates of C_n² are used to study the effects of the atmospheric turbulence on operational optical systems.

Intensity fluctuations are formulated for source arrays in weakly turbulent horizontal atmospheric links. Source array is composed of point sources separated by variable distances in the transverse source directions. Formula yielding the on-axis scintillation index for the source array is derived by employing the Rytov solution for the structure and correlation functions in the extended Huygens Fresnel principle. Through numerical results, variations of the scintillations versus the array parameters such as the size of the array, spacing between the array elements, amplitudes and phases of the individual sources in the array are investigated. Numerically evaluated intensity fluctuations for such array parameters are compared with the well known single point source scintillations. We are interested to understand whether the use of a source array will give favorable intensity fluctuations in atmospheric communication links.

In this paper, we discuss the real-time compensation of air turbulence in imaging through long atmospheric paths. We propose the use of a reconfigurable hardware platform, specifically field-programmable gate arrays (FPGAs), to reduce costs and development time, as well as increase flexibility and reusability. We present the results of our acceleration efforts to date (40x speedup) and our strategy to achieve a real-time, atmospheric compensation solver for high-definition video signals.

Shack-Hartmann wavefront sensor is widely used for the measurement of phase perturbations induced by turbulence. Such a wavefront sensor relies on the measurement of the image displacements in the lenslet array focal plane. Different algorithms can be used to estimate this displacement. This paper is dedicated to the analysis and comparison of their performances. Special attention will be paid to correlation techniques which are well suited to extended sources.

Cn2 profile monitoring usually relies on the exploitation of wavefront slopes correlations or of scintillation pattern correlations in the pupil. Scintillation is rather sensitive to high turbulence layers whereas wavefront correlations are mainly due to layers close to the receiving plane. Reaping the benefits of both phenomenon could ease C_n² profile restoration. In the small perturbation approximation, correlations of wavefront slopes and scintillation indices recorded with a Shack Hartmann Wavefront Sensor (SHWFS) have been described analytically. We propose to apply this analytical formalism to C_n² profile retrieval. Two measurement techniques are exposed. In CO-SLIDAR (Coupled SLODAR SCIDAR), SHWFS data recorded on a binary source are exploited to compute both autocorrelations and cross-correlations between the two components of the binary of wavefront slopes, scintillation indices and of their coupling. C_n² profile is retrieved from the experimentally estimated correlations. SCO-SLIDAR (Single CO-SLIDAR) relies on the same principle as CO-SLIDAR but SHWFS data are recorded on a single point source. Only autocorrelations of wavefront slopes, scintillation indices and of their coupling are exploited. The analytical background of CO-SLIDAR and SCO-SLIDAR C_n² profile monitoring is reviewed and both techniques are tested on end to end simulation data.

A new instrument has been developed for measurement of visibility and scattering characteristics in the visible spectrum over extended paths. The instrument, a Multispectral Scattering Imager, is designed to acquire calibrated radiance images in several wavelengths over extended paths. From measurements of the horizon radiance and the radiance of dark targets, combined with measurements of the inherent properties of the dark targets, visibility and effective scattering coefficient over the integrated path can be determined. The instrument has acquired several months of data over a 7.2 km path in San Diego, along with several transmissometers and nephelometers. Initial data comparisons show quite good comparisons with the other systems. The instrument has the advantage that measurements may be taken over extended paths in the wavelengths of choice, yet it is passive, and does not require an active two-ended transmitter/receiver system. We will show the results from the MSI in blue, green, red, and NIR wavebands, and show comparisons with other instruments.

US Navy and Marine Corps pilots receive Night Vision Goggle (NVG) training as part of their overall training to maintain the superiority of our forces. This training must incorporate realistic targets; backgrounds; and representative atmospheric and weather effects they may encounter under operational conditions. An approach for pilot NVG training is to use the Night Imaging and Threat Evaluation Laboratory (NITE Lab) concept. The NITE Labs utilize a 10' by 10' static terrain model equipped with both natural and cultural lighting that are used to demonstrate various illumination conditions, and visual phenomena which might be experienced when utilizing night vision goggles. With this technology, the military can safely, systematically, and reliably expose pilots to the large number of potentially dangerous environmental conditions that will be experienced in their NVG training flights. This paper describes work that is being performed for NAVAIR to add realistic atmospheric and weather effects to the NVG NITE Lab training facility using the NVG-WDT (Weather Dipiction Technology) system. NVG-WDT consist of a high end multiprocessor server with weather simulation software, and several fixed and goggle mounted Heads Up Displays (HUDs). Atmospheric and weather effects are simulated using state-of-the-art computer codes such as the NCAR/Penn State Mesoscale Model (MM5); and the US Air Force Research Laboratory MODTRAN radiative transport model. Imagery for a variety of natural and man-made obscurations (e.g. rain, clouds, snow, dust, smoke, chemical releases) is being calculated and injected into the scene observed through the NVG via the fixed and goggle mounted HUDs.

The effective field-of-view of an electro-optical sensor in a given meteorological scenario can be evaluated using a ray-tracer. The resulting ray trace diagram also provides information pertinent to the quality (distortion, mirages) of the image being viewed by the sensor. The EOSTAR (Electro Optical Signal Transmission And Ranging) model suite contains a ray tracer that has been upgraded to take into account horizontal inhomogeneities in the atmosphere, such as temperature gradients as observed in coastal areas where (e.g.) cold air flows out over warm waters. Initial results for horizontally inhomogeneous atmospheres are presented and compared to calculations for horizontally homogeneous atmospheres. It is shown that the horizontal inhomogeneity of temperature should be taken into account when assessing sensor performance.

BAE SYSTEMS reports on a program to develop a high-fidelity model and simulation to predict the performance of angle-angle-range 3D flash LADAR Imaging Sensor systems. Accurate methods to model and simulate performance from 3D LADAR systems have been lacking, relying upon either single pixel LADAR performance or extrapolating from passive detection FPA performance. The model and simulation here is developed expressly for 3D angle-angle-range imaging LADAR systems. To represent an accurate "real world" type environment this model and simulation accounts for: 1) laser pulse shape; 2) detector array size; 3) detector noise figure; 4) detector gain; 5) target attributes; 6) atmospheric transmission; 7) atmospheric backscatter; 8) atmospheric turbulence; 9) obscurants; 10) obscurant path length, and; 11) platform motion. The angle-angle-range 3D flash LADAR model and simulation accounts for all pixels in the detector array by modeling and accounting for the non-uniformity of each individual pixel. Here, noise sources and gain are modeled based upon their pixel-to-pixel statistical variation. A cumulative probability function is determined by integrating the normal distribution with respect to detector gain, and, for each pixel, a random number is compared with the cumulative probability function resulting in a different gain for each pixel within the array. In this manner very accurate performance is determined pixel-by-pixel for the entire array. Model outputs are 3D images of the far-field distribution across the array as intercepted by the target, gain distribution, power distribution, average signal-to-noise, and probability of detection across the array.

The results of applying the empirical orthogonal functions (EOF) method to decomposition and approximation of aerosol size distributions are presented. A comparison was made for two aerosol data sets, representing coastal and oceanic environments. The first data set includes measurements collected at the Irish Atlantic coast in 1994 and 1995, the second one data collected during the Rough Evaporation Duct (RED) experiment that took place off Oahu, Hawaii in 2001. The main finding is that aerosol size distributions can be represented by a superposition of the mean size distribution and the first eigenvector multiplied by an amplitude function. For the two aerosol data sets the mean size distribution is very similar in the range of small particles sizes (radius < 1μm) but the main difference appears for larger aerosols (radius > 1μm). It is also reflected by the spectral shape of the eigenvector. The differences can be related to the type of aerosols present at both locations, and the amplitude function can be associated to meteorological conditions. The amplitude function also indicates the episodes with the maximum/minimum continental influence. The results of this analysis will be used in upgrades of the ANAM model.

Various experiments have been carried out recently in the Middle East urban (Beer Sheva, Israel) environment for prediction of aerosol particle concentration and size distribution. During these experiments aerosol particle concentrations for different weather conditions were measured and analyzed. This work proposes a new model for urban aerosol size distribution prediction based on an extensive series of measurements. The model introduces coefficients and characteristics of processes of absorption and scattering by aerosol particles in the urban inhomogeneous region for semi-arid and rainy atmospheric conditions. Several parts of the results are compared with those obtained through measurements in different geographic and climatic environments, as well as with different aerosol distribution models. It is shown that, in Middle East urban regions, larger differences in aerosol particle concentration are observed. Effects of different atmospheric conditions for urban aerosol modeling are better described by the model proposed in this work using parameters obtained empirically.

The presence of atmospheric aerosols along the line of sight of infrared and electro-optical sensors greatly determines the range performance of these devices. On the one hand the aerosol particles scatter background (including sun) radiance into the field of view of the sensor, on the other hand they contribute to the atmospheric contrast reduction of the target. Proper knowledge of aerosol characteristics such as composition, concentration and size distribution is of vital importance for the prediction of their scattering and extinction characteristics. It is however found to be very difficult to collect accurate information on the particle size distribution (PSD) of aerosols. One of the reasons is the variation of the PSD along the path, which is likely to occur in a coastal area such as the San Diego Bay. One way to overcome these problems is the use of a multi-band transmissometer, as was done in previous measurement campaigns in the Baltic Sea [1] and in the Persian Gulf area [2]. The TNO seven-band optical/IR transmissometer system, providing path averaged transmission data for the intervening atmosphere, is operating at wavelengths between 0.4 and 14 μm,. In this spectral band, scattering in light hazy conditions is dominated by particles with a diameter of less than 4 μm. In order to simulate the transmission losses by scattering in various spectral bands a special calculation tool has been developed. This tool, described in this paper, allows detailed investigation of the possibilities of the retrieval of the PSD from multi-band transmission data. The slope in the plots of the transmission versus wavelength is directly related to the slope of the (log-normal) PSD plots (known as Junge exponent). The average transmission in a selected number of spectral bands is directly correlated to the average particle concentration (known as Junge coefficient). The principle of the methodology is illustrated with data collected during a measurement campaign, carried out over the San Diego Bay in August 2005. In this campaign we used six of the seven spectral bands, providing data over a 7.2 km over water path. It is shown that the retrieval method is very successful and the data correspond well with those, simultaneously collected with in-situ Particle Measurement Systems (PMS), located on both sides of the path. In addition to the path averaging, another advantage of the transmissometer PSD's, is the accuracy, being an order of magnitude higher than that of the PMS probes due to the fact that the measurement volume is more than a million times larger. A detailed analysis is given of the transmission data, showing peculiar effects were in the 2.3μm band around 19.00 UTC during most of the days. These effects are especially well illustrated in plots where the transmission in one spectral band is plotted against the transmission in another band for various times of the day.

The Navy Aerosol Model (NAM) is widely used as an engineering tool to provide a quick estimate of the aerosol extinction in the marine environment. Since its introduction, several shortcomings of NAM have been identified that are being addressed by the development of the Advanced Navy Aerosol Model (ANAM). At present, the Advanced Navy Aerosol Model has been reviewed as concerns its production mode. The two separate production modes (3rd and 4th modes in ANAM4) have been replaced by a single production mode in ANAM5. The shape of the new production mode is given by two sea spray source functions taken from literature, Vignati et al. and Smith and Harrison. The intensity of the new production mode in ANAM5 at a particular height above the surface is governed by a transfer function that depends on radius and wind speed. The production mode in ANAM5 has several tuning parameters that have been optimized by comparing ANAM5 concentration predictions to experimental aerosol data. ANAM5 performs better than ANAM4 in predicting the concentrations of large aerosols in open ocean conditions, but the performance is reduced in the coastal zone. This may be due to the presence of a strong advection mode that is currently not well taken into account by the ANAM.

The performance of electro-optical systems can be substantially affected by aerosol particles that scatter and absorb electromagnetic radiation. The performance assessment of EO systems by propagation prediction codes then requires accurate aerosol models. However, concentrations and optical properties of aerosol particles in the atmosphere are very variable both in time and space. In particular, coastal areas induce specific physical processes. To include coastal effects in the model for the prediction of aerosol concentrations, Piazzola et al.¹ proposed an extension of the Navy Aerosol Model (NAM2) to coastal areas. This work has been coupled with Mie theory to develop the aerosol extinction code MEDEX,³ which is based on an extensive series of measurements in the Mediterranean. The present paper deals with the extension of the predictions of MEDEX to a regional scale. To achieve it, MEDEX has been coupled with a regional meso-scale meteorological model (RAMS). This allows taking into account the details of the orography of the coast and a better modelling of the unsteadiness for both meteorological and oceanic conditions. The results show the feasibility of extending the predictions of MEDEX to any coastal site.

An infrared (IR) signal propagating along a 'line-of-sight' horizontal or slant path within the marine surface layer can encounter substantial perturbations. These perturbations include signal extinction due to molecules or aerosol particles; refractive modulations that can amplify or reduce a signal; and scintillation, which is a higher frequency fluctuation in signal intensity. In an effort to elucidate these issues an infrared transmission link was included in a long term propagation field experiment conducted at Zuniga Shoal to study the effects of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various times of the year. The transmission path was 7.2 km long connecting an IR broadbeam transmitter (at about 6.5 m ASL) at the Naval Amphibious Base at Coronado, and an IR telescope receiver at Zuniga Shoal (at about 11.5 m ASL). Both locations are in the general San Diego area. The transmission measurements were made at two wavelengths: near-IR, centered at 1.061 μm and short-wave IR, centered at 1.622 μm. In this paper we discuss prominent features of the long-term measurements including diurnal variations and the effects of the marine layer. We also compare the field measurements with the extinction predictions generated by the Advanced Navy Aerosol Model (ANAM), and we discuss how long-term field measurements can be used to tune and correct the ANAM.

Free Space Optics (FSO) has gained considerable importance in this decade of demand for high bandwidth transmission capabilities. FSO can provide the last mile solution, but the availability and reliability issues concerned with it can not be ignored, and requires thorough investigations. In this work, we present our results about light attenuation at 950 and 850 nm wavelengths in continental city fog conditions with peak values up to 130 dB/km and compare them with attenuation under dense maritime conditions with peak values up to 480 dB/km. Dense fog is the most severe limiting factor in terrestrial optical wireless applications and light propagation in fog has properties in the spatial, spectral and the time domain, which are of importance to free-space optic data communication. In 2004 (within a short term scientific mission of COST 270) measurements of very dense maritime fog and low clouds were made in the mountains of La Turbie, close to the coast of southern France. Using the same equipment, the measurements were continued for the conditions of the continental city of Graz, Austria. This campaign was done in the winter months from 2004 to 2005 and 2005 to 2006 and allows us to compare fog properties for different environments, and the impact of snow fall. We provide detail analysis of a fog and a snow event for better understanding of their attenuation behavior.

Airborne infrared limb-viewing detectors may be used as surveillance sensors in order to detect dim military targets. These systems' performances are limited by the inhomogeneous background in the sensor field of view which impacts strongly on target detection probability. This background clutter, which results from small-scale fluctuations of temperature, density or pressure must therefore be analyzed and modeled. Few existing codes are able to model atmospheric structures and their impact on limb-observed radiance. SAMM-2 (SHARC-4 and MODTRAN4 Merged), the Air Force Research Laboratory (AFRL) background radiance code can be used to in order to predict the radiance fluctuation as a result of a normalized temperature fluctuation, as a function of the line-of-sight. Various realizations of cluttered backgrounds can then be computed, based on these transfer functions and on a stochastic temperature field. The existing SIG (SHARC Image Generator) code was designed to compute the cluttered background which would be observed from a space-based sensor. Unfortunately, this code was not able to compute accurate scenes as seen by an airborne sensor especially for lines-of-sight close to the horizon. Recently, we developed a new code called BRUTE3D and adapted to our configuration. This approach is based on a method originally developed in the SIG model. This BRUTE3D code makes use of a three-dimensional grid of temperature fluctuations and of the SAMM-2 transfer functions to synthesize an image of radiance fluctuations according to sensor characteristics. This paper details the working principles of the code and presents some output results. The effects of the small-scale temperature fluctuations on infrared limb radiance as seen by an airborne sensor are highlighted.

The estimation of FSO link availability is based on synthesis of two models: the steady model of given link and the statistical model of installation site. The steady model consists in the knowledge of FSO transceiver parameters so that we can compute the power budget for a given transceiver distance. The result is the dependence of link margin on the transceiver distance. The statistical model consists in the knowledge of statistical parameters of atmosphere at given installation site. The installation site is described by the exceedance probability of atmospheric attenuation. By means of synthesis of both models we can obtain estimation of link availability. In this contribution, estimation of FSO link availability in chosen Central - European localities is presented.

The determination algorithm of aerosol microparticles' size distribution by iteration process is described. In the described method the particle, registered by optoelectronic devices is characterized by parameters of amplitude and duration of impulse. Distribution of particles' size is determined from the measured functional dependence of number of registered particles from amplitude and duration of the proper electric impulses on the output photo-detector. Given dependence, within limits of statistical errors, is repeated when performing measurement series in the medium with identical optical parameters. It is linked by functional dependence to relative particles' fraction of different sizes, which is expressed by integral first kind Fredholm equation.

We introduce an innovative concept for inferring altitude profiles of horizontal wind in planetary atmospheres by measuring the Doppler shift of multiple emission lines versus altitude. Instruments using this approach will be especially well suited for interplanetary missions because they will be compact, rugged, and lightweight while minimizing power consumption and maximizing sensitivity, all without moving parts.

Aerosols have large impacts on radiative transfer through scattering and extinction, which distort the satellite signals in earth observation application. In the paper, we apply lidar for measuring aerosol signals simultaneously when earth observation satellite flyover. In this way, the aerosol influence could be removed precisely by the combination of real-time lidar data and atmospheric radiance transmission model. Here, the aerosol optical depth retrieved by lidar data is integrated into the 6S model for satellite image atmospheric correction. The primary results are shown in this paper.

The scattering (Mueller) matrices for hexagonal ice plates of various aspect ratios are calculated by the facet tracing method developed earlier. For horizontally oriented plates, the Mueller matrices depending on the incident angle have been obtained. The peculiarities inherent to the scattered light along the parhelic circle have been studied. A generalization of the data for small flatter of these plates relative to the horizontal plane is discussed.