The Shuttle Landing Facility runway at the Kennedy Space Center in Cape Canaveral, Florida is almost 5 km long and 100 m wide. Its homogeneous environment makes it a unique and ideal place for testing and evaluating EO systems. An experiment, with the goal of characterizing atmospheric parameters on the runway, was conducted in June 2005. Weather data was collected and the refractive index structure parameter was measured with a commercial scintillometer. The inner scale of turbulence was inferred from wind speed measurements and surface roughness. Values of the crosswind speed obtained from the scintillometer were compared with wind measurements taken by a weather station.

We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 m. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating at 1550 nm through a transmit aperture of 2.54 cm. An identical model of the same instrument was used as a receiver with a single aperture that was varied in size up to 20 cm to measure the effect of receiver aperture averaging on Bit Error Rate. Simultaneous measurements were also made with a scintillometer instrument and local weather station instruments to characterize atmospheric conditions along the propagation path during the experiments.

Astronomical images obtained on large ground based telescopes are blurred due to the effect of the atmospheric turbulence but this can be compensated by means of adaptive optics. A knowledge of the vertical profile of the turbulence might help to optimize the adaptive optics control system, especially when an attempt is made to correct over a wide field of view (MCAO). We present the development of a remote sensing technique called Single-Star SCIDAR (SSS) system for characterizing atmospheric parameters, such as the refractive-index structure function constant C_n²(h), using single star targets. The technique is based on the analysis of stellar scintillation produced by the passage of the light through the atmospheric turbulence. The instrument is intended to be used in generalized mode, i.e. with several measurement planes. The autocorrelation of scintillation images, taken at several measurement planes with a short exposure time using a 25cm diameter telescope, allows us to characterize atmospheric parameters for wide-ranging area in the sky. Computational simulations of a wave propagating through atmospheric turbulence are made using a Kolmogorov model. Retrieving the refractive-index fluctuation profile of the turbulence at different heights from single stars is challenging, contrary to the triangulation inherent to the binary star SCIDAR technique. The problem is an ill-posed one, made easier to solve by the use of multiple conjugated altitudes. A least square method solution with a Tikhonov regularization is used for the resolution. Methods to enforce non-negativity, reflecting the physical property of the quantity, are investigated.

SLODAR (slope detection and ranging) is a technique we have developed to monitor the vertical profile of atmospheric phase distortions, for application to astronomical adaptive optics systems. The technique uses the correlation between slope measurements made using a Shack-Hartmann wavefront sensor observing a binary star. In this paper we describe the principle of SLODAR and then describe our work on using a system for the measurement of horizontal turbulence profiles for application to free space optical communications.

An infrared (IR) signal propagating along a 'line-of-sight' horizontal or slant path near the earth's surface can encounter substantial perturbations. These perturbations result in refractive distortions (low-frequency modulations that can amplify or reduce a signal) and scintillation (a higher frequency fluctuation in signal intensity). Scintillation impacts the ability of IR systems to detect high-speed, sea-skimming missiles that are a serious threat to Navy assets. Scintillation is quantified by the refractive index structure parameter C_n². An opportunity to quantify scintillation occurred during the Validation Measurement for Propagation in the Infrared and Radar (VAMPIRA) field test at Surendorf, Germany, during March - April 2004. Scintillation measurements were made along a slant, near-sea surface path of about 8200 m with a transmissometer that operated in the mid-IR regime. At the same time an AMBER camera system was used to obtain images of stationary lights at the mid-IR. The purpose of this paper is to present several ways of obtaining C_n² by using either the SSC transmissometer or the AMBER imagery.

Optical turbulence information is important because it describes an atmospheric effect that can significantly degrade the performance of electromagnetic systems and sensors, e.g., free-space optical communications and infrared imaging. However, analysis of selected past research indicates that there are some areas (i.e., data and models) in which optical turbulence information is lacking. For example, optical turbulence data coupled with atmospheric characterization models in hilly terrain, coastal areas, and within cities are few in number or non-existent. In addition, the bulk of existing atmospheric computer models being used to provide estimates of optical turbulence (Cn2) intensity are basically one-dimensional in nature and assume uniform turbulence conditions over large areas. As a result, current program codes may be deficient or in error for non-uniform areas, such as environments with changing topography and energy budgets. By exploring alternate (non-similarity) numerical models for momentum, Reynolds stress, and heat flux we suggest that some very practical computational research can be performed to provide better characterization of optical turbulence (Cn2) and related effects beyond current limitations.

In this work, simulation of beam propagation through atmospheric turbulence is made by means of the split-step method, including spatially separated two-dimensional phase screens, which represent the existing turbulence. These phase screens may be generated mainly by two techniques, namely fractal interpolation or in the spatial frequency domain. We report some important considerations to take into account in order to generate phase screens with enough accuracy to properly reproduce the structure function of the turbulence. It is shown that slight deviations from the theoretical structure function in the set of screens used along the propagation may increase in an appreciable way the statistical error inherent to any particular realization. Some comparisons are made with analytical results based on the second order Rytov approximation. One of the conclusions that may be clearly drawn from these comparisons is that beam wander effects are not included in these theories.

Numerical modeling of optical wave propagation in atmospheric turbulence is traditionally performed by using the so-called "split" - operator method, where the influence of the propagation medium's refractive index inhomogeneities is accounted for only within a set of infinitely narrow phase distorting layers (phase screens). These phase screens are generated on a numerical grid of finite size, which corresponds to a rather narrow slice (spatial area) of atmospheric turbulence. In several important applications including laser target tracking, adaptive optics, and atmospheric imaging optical system performance depends on atmospheric turbulence within an extended area that significantly exceeds the area associated with the numerical grid. In this paper we discuss methods that allow the generation of a family of long (including infinitely long) phase screens representing an extended (in one direction) area of atmospheric turbulence-induced phase distortions. This technique also allows the generation of long phase screens with spatially inhomogeneous statistical characteristics. It can be applied to the numerical analysis of laser tracking and directed energy systems over long target trajectories.

Atmospheric turbulence has a significant impact on the quality of a laser beam propagating through the atmosphere over long distances. Turbulence causes the optical phasefront to become distorted from propagation through turbulent eddies of varying sizes and refractive index. Turbulence also results in intensity scintillation and beam wander, which can severely impair the operation of target designation and free space optical (FSO) communications systems. We have developed a new model to assess the effects of turbulence on laser beam propagation in such applications. We model the atmosphere along the laser beam propagation path as a spatial distribution of spherical bubbles or curved interfaces. The size and refractive index discontinuity represented by each bubble are statistically distributed according to various models. For each statistical representation of the atmosphere, the path of a single ray, or a bundle of rays, is analyzed using geometrical optics. These Monte Carlo techniques allow us to assess beam wander, beam spread, and phase shifts along the path. An effective C_n² can be determined by correlating beam wander behavior with the path length. This model has already proved capable of assessing beam wander, in particular the (Range)³ dependence of mean-squared beam wander, and in estimating lateral phase decorrelations that develop across the laser phasefront as it propagates through turbulence. In addition, we have developed efficient computational techniques for various correlation functions that are important in assessing the effects of turbulence. The Monte Carlo simulations are compared and show good agreement with the predictions of wave theory.

EOSTAR, a PC based Windows application, integrates the required modules necessary to calculate the electro-optical sensor performance on the basis of standard meteorological data. The primary output of EOSTAR consists of the synthetic sensor image ("what does the sensor see?") and a coverage diagram ("detection probability versus range"). As part of the EOSTAR validation effort, the refraction and turbulence modules are being evaluated against literature data, similar models and experimental results. It is shown that the EOSTAR model can predict with reasonable success the occurrence of optical turbulence and refraction phenomena such as mirages. The major cause for discrepancies between the various models is attributed to the underlying micrometeorological bulk modules, whereas the sensitivity of the predictions on the values of the meteorological input parameters is held responsible for the discrepancies between model predictions and measurements.

Results from high-resolution, forced, three-dimensional direct numerical simulations on the vertical variability of shear-stratified turbulence and its outer length scales in nonuniformly stratified tropopause jets are presented. Vertical scales 1m-50m are resolved. Turbulent dynamics leads to the formation of an N²-notch which favors gravity wave emission well within the temperature mixing layer. We demonstrate that, in inhomogeneous shear-stratified turbulence, scaling of various turbulent quantities (such as variances, fluxes, mixing efficiency, turbulence outer scales) with respect to a single parameter (such as the gradient Richardson number) typically exhibit multiple branches. Certain qualitative changes in eddy mixing during transitional regimes towards stronger stratification are highlighted. The behaviour of turbulent eddy mixing parameters found in these studies is consistent with some recent observational results in stably stratified atmospheric shear flows. The implication of this study is that such transitions and multiple scalings need to be accounted in the parameterization of microscale atmospheric optical turbulence.

Infrared scintillation measurements were obtained along a 7.2 km path over San Diego Bay, concurrently with mean meteorological and turbulence measurements obtained from a buoy located along the path. Bulk estimates and turbulence measurements of C_n² were computed from the buoy data and compared with the optical scintillation-derived C_n² values. Similar to the results of previous experiments, the bulk C_n² estimates agreed well with both the scintillation and turbulence measurements in unstable conditions, increasingly underestimated C_n² as conditions approached neutral, and agreed less well with scintillation and turbulence C_n² values in stable conditions. The mean differences between bulk C_n² estimates and both the turbulence and scintillation measurements when conditions were not near-neutral exhibited an air-sea temperature difference and wind speed dependence, possibly indicating that the forms of the empirical stability functions used by the bulk model are incorrect. The turbulent C_n² measurements from the buoy showed excellent agreement with the scintillation values in unstable conditions, but had surprisingly large differences in weakly stable conditions. This disagreement may be related to the fact that humidity fluctuations begin to increasingly influence refractive index fluctuations when the air-sea temperature difference is small and are not properly taken into account by the sonic temperature measurements. As the absolute air-sea temperature difference approaches zero the bulk C_n² estimates decrease much more rapidly and to much smaller values than either the scintillation or turbulence measurements. Fortunately, in such near-neutral conditions scintillation is usually small enough to have little effect on many optical system applications.

The refractive-index structure parameter C_n² is the parameter most commonly used to describe the optically active turbulence. In the past, FGAN-FOM carried out long-term experiments in moderate climate (Central Europe, Germany), arid (summer), and semiarid (winter) climate (Middle East, Israel). Since C_n² usually changes as a function of time of day and of season its influence on electro-optical systems should be expressed in a statistical way. We composed a statistical data base of C_n² values. The cumulative frequency of occurrence was calculated for a time interval of two hours around noon (time of strongest turbulence), at night, and around sunrise (time of weakest turbulence) for an arbitrarily selected period of one month in summer and in winter. In October 2004 we extended our long-term turbulence experiments to subarctic climate (North Europe, Norway). First results of our turbulence measurement over snow-covered terrain indicate C_n² values which are similar or even higher than measured values in Central European winter. The statistical data base was used to calculate the expected turbulence-induced aperture-averaged scintillation index for free-space optical systems (FSO system) in different climates. The calculations were performed for commercially available FSO systems with wavelength of 785 nm and 1.55 µm respectively and with aperture diameters of the receiver of 60 mm and 150 mm for horizontal path at two heights, 2.3 m and 10 m above ground.

The estimation of the performance of electro-optical systems depends on the accuracy of the atmospheric models being used in the propagation prediction codes. On the basis of a large set of imaging LIDAR measurements a Middle East model of refractive turbulence strength (C_n²) vertical profile has been developed. The model is presented in this work. Also, the results of lidar measurements of aerosol size distribution, volume, and number concentration at different heights in the Mediterranean region (Be'er-Sheva, Israel) and comparison with models (AFGL, MODTRAN) are presented. Implications can be important for optical communication, imaging through the atmosphere, and adaptive optics.

The detection of targets at low levels above the sea surface by electro-optical (EO) sensors is affected by the atmosphere. Models have been developed to describe the electro-optical propagation in the marine atmospheric surface layer as a function of meteorological parameters. EOSTAR is an end-to-end model suite for EO sensor performance in which the Advanced Navy Aerosol Model (ANAM) is embedded for computing the aerosol extinction. While ANAM provides favorable results in open ocean conditions where the aerosols predominantly consist of sea salt particles, the model lacks accuracy in coastal zones due to the presence of aerosols from a variety of other sources. In offshore wind conditions continental aerosols of anthropogenic and natural origin mix with marine aerosols produced in the surf zone and by wave breaking further offshore. In principle, ANAM can be extended with the various aerosol types that may occur in the coastal zone, but to correctly handle their effect on EO propagation, information is required on the actual aerosol mixture over the range of interest. In this contribution we explore the potential of satellite instruments to provide this information. Radiometers on satellites can be used to retrieve the spatial variation over an extended area determined by the swath width, with a resolution determined by the radiometer pixel size. Input into this retrieval is a model describing the aerosol mixture in varying ratio, e.g. a mixture of continental and marine aerosol. While the marine component can be constrained by ANAM using local meteorological input parameters, the continental component can be retrieved and used as input to determine the fine particle distribution in ANAM.

The Navy Aerosol Model (NAM, available in MODTRAN) is widely used as a tool to assess the aerosol extinction in the marine atmospheric surface layer. NAM was built as a regression model in the 1980s to represent the aerosol extinction at deck height as a function of the meteorological conditions. The recently developed Advanced Navy Aerosol Model (ANAM) utilizes additional experimental evidence to supersede NAM by correcting the underestimation of the concentration of aerosols larger than a few microns. More importantly, ANAM provides the aerosol extinction as a function of height between the surface and several tens of meters. Present-day naval surveillance and threat scenarios require detection of targets at the horizon, such as seaskimming missiles, or small targets such as rubber boats. In either case, the propagation path from sensor to target is likely to come very close to the wave surface and in order to estimate detection ranges, an assessment of the transmission losses along the path is necessary. To answer the question posed in the title, we assess the two models using two meteorological data sets (784 cases) representative of diverse maritime conditions in regions of interest around the world.

The quality of long range infrared (IR) imaging depends on the effects of atmospheric refraction and other pathintegrated effects (e.g., transmission losses, scintillation and blurring), which are strongly related to the prevailing meteorological conditions. EOSTAR is a PC based computer program to quantify these strong nonlinear effects in the marine atmospheric surface layer and to present a spectrally resolved target image influenced by atmospheric effects using ray tracing techniques for the individual camera pixels. Presently, the propagation is predicted with bulk atmospheric models and the sea surface is idealized by steady regular periodic Stokes' waves. Dynamical wind-waves interactions are not taken into account in this approach, although they may strongly modify the refractive index in the near-surface layer. Nonetheless, the inclusion of the sea surface in the ray tracer module already has a great impact on the near-surface grazing rays and thus influences the images especially in situations of super refraction and mirage. This work aims at improving the description of the sea surface in EOSTAR taking into account the non-uniformity of spatially resolved wind-generated waves and swell. A new surface module is developed to model surface wind-waves and swell in EOSTAR on the basis of meteorological observations and spectral wave modeling. Effects due to these new surfaces will be analyzed and presented.

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 have acquired more attention, and a need for thorough investigations. In this work, we present our results about fog attenuation at the 950 and 850 nm wavelengths in heavy maritime fog with peak values up to 500 dB/km. For the attenuation measurement, optical wavelengths are transmitted over the same path of fog in free air to a receiver, measuring the power of every wavelength. The RF marker technology employed takes advantage of modulating every optical wavelength with an individual carrier frequency, allowing to use one optical front end for the receiver and to separate individual wavelengths by electrical signal filters. The measurement of fog attenuation at different wavelengths was performed at the France Telecom R & D test facility at La Turbie. Maritime or advection fog, which caused the light attenuation consists of water droplets of larger diameter in the order of 20 μm and can cause visibilities as low as 30 meters. The visibility was measured using a transmissiometer at 550 nm. We compare our measurement data with the commonly used light attenuation models of Kruse and Kim, and present some interesting insights. The practical measurements described try to validate the models and therefore should lead to a more accurate availability prediction for FSO links.

A new system is being developed for measurement of visibility and scattering characteristics in the visible and NIR wavelengths over extended paths. This is being developed to better understand transmission properties over long horizontal near-surface propagation paths over the ocean. The instrument, a Multispectral Scattering Imager, is designed to acquire calibrated radiance images in several wavelengths over extended paths. From measurements of the radiance near the horizon 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 technique uses the Koschmeider equations for radiative transfer, and it allows for correction of sensor characteristics such as non-linearity and of non-zero target reflectance. The technique is based in part on the Horizon Scanning Imager, a visibility system developed in the 1990's by the Marine Physical Lab. However, it utilizes better imaging systems than were available at that time, as well as improvements to the visibility and scattering algorithms. Initial experiments are designed to test the strengths and weaknesses of the system, and to provide multispectral visible band results. These results will be used for modeling and statistical studies with respect to simultaneous measurements by a suite of other instruments. This talk will present an overview of the system and algorithms, as well as initial experimental results.

Cloud Free Line of Sight (CFLOS) statistics can be important to a number of applications involving transmittance of light through the atmosphere, including laser propagation, light propagation, and detection of objects by humans and instruments. This paper will discuss Cloud Free Line of Sight (CFLOS) and cloud persistence statistics determined from cloud measurements taken with Whole Sky Imagers (WSI). The WSIs are ground-based digital imaging systems that image the full upper hemisphere down to the horizon in wavebands in the visible and NIR. Digital automated WSI systems were originally developed by the Marine Physical Lab in the 1980's to address the CFLOS application, and then further developed for 24-hour day and night capability. Approximately three million image sets have been acquired with the Day/Night WSI in conjunction with DOE's ARM program. Recent advances in the cloud decision algorithms at Marine Physical Lab have enabled the extraction of processed cloud images of sufficient quality to obtain reliable cloud statistics. A test sample of approximately 4500 image sets has been processed to yield CFLOS statistics down to the horizon, as well as statistics related to the persistence of clouds and cloud holes. This talk will provide a brief overview of the instruments and current algorithm developments. The CFLOS results and sample persistence results will be presented.