Measurement of atmospheric turbulence progressed though several stages in the last decades but has of recent seen little advance. Uses of lidars, ground based radar and intrusive techniques have all had limitations in their ability to measure the more fundamental atmospheric properties. Be it poor spatial or temporal resolution, difficulty in maintaining the sensor, or the requirement to use a preconceived atmospheric model, all have had shortcomings. Of the several physical atmospheric properties that can be quantified, the inner and outer scale sizes associated with the index of refraction, and hence the other atmospheric properties, are of high interest in the prediction of the performance of various adaptive optical sensors. In this paper, we will discuss a method based on a thin beam optical system to measure the inner and outer scales size that overcomes some of the limitations and assumptions in previous techniques. Based on research originally conducted at the University of Florence, we have extended the theory to optically thin layers that can account for real world design effects. Using this theory the paper will discuss the feasibility of using the technique to measure turbulence scale sizes in the upper atmosphere. Data from laboratory measurements will be shown.

Mechanical transmitter vibration is an important reason for errors in scintillation measurements, especially when using laser sources. The effect is due to the movement of the emitted beam caused by translational and rotational vibrations of the transmitter in combination with the finite beam width. The movement of the beam across the receiver results in intensity fluctuations which are not caused but often erroneously interpreted by turbulence. This paper presents a technique to identify and quantify the contribution of vibration to the measured intensity fluctuations. The technique uses detectors with a spacing larger than the Fresnel zone. Correlations of the intensity fluctuations measured at these spaced detectors are mainly caused by transmitter vibrations and are used for correcting the measured intensity fluctuations at the individual detectors to infer the pure turbulence effect. In an experiment, vibrations of different magnitude are simulated using a rotating wedge prism in front of a scintillometer transmitter. The measured data are compared with the data of a second scintillometer, with and without the vibration correction applied. Structure function constant C_n² and inner scale l₀ of refractive index fluctuations are derived from corrected and uncorrected data. This demonstrates the effectiveness and importance of the vibration correction.

The performance of operational military E-O systems including imaging FLIRs, target designators, and laser rangefinders (LRF) is limited by atmospheric refractive- index turbulence. In locations subject to intense daytime heating and significant nighttime cooling, typically an arid desert-like environment, the diurnal change in C_n² can range over three to four orders of magnitude or larger in some cases. Elevation of the path above the desert floor even at one end can significantly reduce the performance- degrading effects of atmospheric turbulence on FLIRs, designators, and LRFs. In case where operation of these systems at longer wavelengths is possible, performance limitations can, to some extent, be mitigated. This paper discusses the use of multi-wavelength scintillation measurements as a diagnostic, and LRFs. In cases where operation of these systems at longer wavelengths is possible, performance limitations can, to some extent, be mitigated. This paper discusses the use of multi-wavelength scintillation measurements as a diagnostic to infer a path- integrated value for C_n² which can be related to the performance of various E-O systems. An experimental design utilizing IR wavelengths and several slant-paths ranging in length from 2.8 km to 10 km and elevated approximately 730 m above a desert floor is discussed. The multi-wavelength scintillometer design used is based on the 11.15 micrometers scintillometer described in a paper previously presented at an earlier conference.

The bi-directional reflectance distribution function (BRDF) plays a major role to evaluate or analyze signals reflected by Earth in the solar spectrum. A BRDF measurement device that covers a large spectral and directional domain was recently developed by ONERA/DOTA. It was designed to allow both laboratory and outside measurements. Its main characteristics are a spectral domain: 0.42-0.95 micrometers ; a geometrical domain: 0-60 degrees for zenith angle, 0-180 degrees for azimuth; a maximum target size for nadir measurements: 22 cm. For a given zenith angle of the source, the BRDF device needs about seven minutes to take measurements for a viewing zenith angle varying from 0-60 degrees and relative azimuth angle varying from 0-180 degrees. The performances, imperfections and properties of each component of the measurement chain are studied. A part of the work was devoted to characterize precisely the source, and particularly the spatial variability of the irradiance at the target level, the temporal stability and the spectral profile of the lamp. Some of these imperfections are modeled and taken into account in corrections of BRDF measurements. Concerning the sensor, a calibration in wavelength was done. Measurements of bi- directional reflectance of which is well known. A software was developed to convert all the raw data acquired automatically into BRDF values. To illustrate measurements taken by this device, some results are also presented here. They are taken over sand and short grass, for different wavelengths and geometrical conditions.

An algorithm based on the Monte Carlo principle is developed to solve the radiative transfer problem in the reflective domain of the solar spectrum and is used to precisely evaluate ground irradiance on a rugged terrain. This method allows to simulate paths of photons inside the earth- atmosphere system without any assumption and calculate the different identified irradiance components and particularly those coming from environment. To establish the relative contribution of each of these terms, several typical relief and atmosphere configurations are considered. In a first step, two ground types simulations assuming lambertian reflectances are computed. Over vegetation-covered hills in the near IR, in the portion badly exposed to the direct solar beam, the environment irradiance contributes more than 20 percent of the total signal received at ground level. When severe slopes and higher reflectance values are considered, this contribution can exceed 60 percent in shadowed areas. These simulations demonstrate the necessity to take into account the high order terms when the region of interest presents important slopes and/or high reflectance ground. the case of non-lambertian reflectances is also dealt and it is shown that in the present configuration lambertian reflectances can be assumed to calculate the environment terms without significant errors on total irradiance, even in the shadow.

This paper discusses adverse and favorable effects of multiple scattering in lidar measurements. Adverse effects arise because the conventional lidar equation ignores multiple scattering radiation. Hence, the parameters retrieved under this approximation are in error by an amount that depends on the multiple scattering strength. This is illustrated by numerical simulations of space-based DIAL measurements of atmospheric ozone profiles in the presence of cirrus clouds and ground layer aerosols. On the other hand, multiple scattering contributes additional information on the aerosol extinction coefficient and particle size. The paper shows that this can be exploited to derive simultaneous solutions for the extinction coefficient and mean particle diameter independently of external data as required in conventional solution methods.

This paper deals with the analysis of airborne targets close to the horizon. During the LAPTEX in Crete, 1996, FGAN-FfO took IR-sequences of several aircraft in different altitudes. The atmospheric conditions often changed within one day, leading to different transmissivity and refractivity. The influence of these effects on the observation range is analyzed. Measurements are compared with model predictions. Especially sub-refractive conditions with the tendency to create double images are discussed in detail.

The authors present a novel simulation for studying the interaction of coherent illumination with cirrus clouds. The software traces the propagation and E field vectors through a 3D volume of ice crystal in the shape of columns, plates, bullets, and bullet rosettes with random positions, sizes, and orientations. The magnetic (B) field vectors can be found from a cross product of the two. Back-scattered depolarization results are compared to published studies. The use of this simulation for detailed studies of the impact of cirrus clouds on the wavefront of an illuminating beam is discussed.

In this paper we present the result of calculations of refraction angle for laser beam propagating in the surface layer in dependence on thermal stability of the atmosphere. It is shown that under unstable atmospheric conditions a laser beam can hit to the same point at the receiving plane along two trajectories. It occurs two different refraction angles due to focusing atmospheric lens, which is caused by unstable stratification of temperature. The difference between these trajectories decreases with increasing the turbulent flux of heat. Beginning with the definite critical value of heat flux a laser beam on horizontal path does not hit to the given point. The estimates of maximal length of path between two points where steady optical connection can occur at different atmospheric stability are presented in the paper.

We describe here a preliminary set of experiments to demonstrate the feasibility of laser illuminated imagery for remote surveillance from an airborne platform. Such an imaging sensor enhances the presently available sensor suite in that it provides high resolution day and night capability.

Turbulence effects close to the air-ground interface may be expected to be non-Kolmogorov, even if that model is an adequate description of free-air turbulence effects. Direct measurements of the optical effects of propagation through the boundary layer are therefore required and are being undertaken as part of a program in which various potential applications of adaptive optics are being examined. The measurements are intended to characterize the spatio- temporal characteristics of optical wavefronts after propagation through the air-ground boundary layer. The objective in these measurements is to describe the level of performance that will be required in an adaptive system intended to mitigate the deleterious effects of atmospheric propagation on image formation and on other optical measurements. The principles of measurements and the preliminary results are presented.

Strategic laser system are subject to residual pointing errors arising from such sources as mechanical vibrations, telescope dome turbulence, and atmospheric turbulence. Systems such as those fielded by the Imaging Branch of the Air Force Research Laboratory during the Floodbeam Experiments at Starfire Optical Range, Albuquerque, NM, have energy requirements that dictate that the beam profile at the target must be small. This result in a system where the pointing jitter and boresight represent a sizeable fraction of the beam width. Analysis of result from the first FLoodbeam Experiment suggested a relationship between the simple statistics of the total photon returns and the expected system pointing jitter. Monte Carlo analysis has identified a linear relationship between the simple statistics of total returns and the pointing jitter, and an ideal analytic solution has confirmed the result. The simple relationship requires a substantial number of shots and has a bias in the presence of boresight. A second approach advancing the linear relationship, and based on statistical (chi) ² techniques, is presented which simultaneously estimates jitter and boresight. Data from both Floodbeam experiments is analyzed. The results are consistent with the consensus view of system pointing performance during the experiments.

This communication presents a new method to overcome some limitations in the simulation of the propagation of waves originating from a point source through a very long path in a turbulent medium. Existing propagation simulation algorithms suffer from either windowing or lack of resolution when applied to long paths. If Cartesian coordinates are used, the limited size of the numerical mesh originates undesired windowing errors in the long run. Casting the classical split-step Fourier algorithm in a spherically diverging coordinate system can solve this problem. In this way an angular mesh that adapts the source and the propagation algorithm to the geometry of the problem is used. But in long path propagation, this spherical divergent mesh causes a loss of resolution that can become a serious problem in the evaluation of the field statistical moments.

The results of the laboratory experimental study of reflected laser beam intensity distribution behind the receiving lens under conditions of strong optical turbulence are discussed in the paper. As is shown in a number of preceding research in case of strong optical turbulence the part of reflected beam field remains highly coherent in central area of a beam cross section due to long-range correlation. It leads to appearance of a narrow peak around the axis of the lens focal plane intensity distribution at the background of wide spot broadened by turbulence. We measured the size of first scale, its amplitude and energy as well as its localization along the axis receiving system relative to lens focal plane.

Measurements of atmospheric turbulence, mainly for adaptive optical correction, are carried out today by using a reference source near the astronomical object. Laser-created guide stars are being tested as alternatives to scarce natural stars. It might be easier to obtain radio-created guide stars at high elevations. By interference of radio beams, visible plasma is created or modified in fine fringes, and their observation at multiple angles is used for tomography of the turbulence. Such guide stars might also be used for phasing radio-telescopes.

Authors analyze a representation of random wave phase in different bases: the orthogonal Karhunen-Loeve-Obukhov (KLO) functions, Zernike polynomials, discrete Walsh functions, and Haar wavelets. To calculate functions of the optimal KLO basis for the Kolmogorov atmospheric turbulence the effective approach developed by the authors is used. The statistical criteria of the optical system performance, as the phase error variance, Shtrehl radio, and others, are calculated in numerical experiment.

There are two main ways to mitigate the effects of atmospheric turbulence on an imaging system. A post factor approach, where data are opportunely acquired and processed in order to increase the overall resolution attainable by the optical system, speckle imaging is an example of such technique. The other approach is to use an adaptive optics system that will compensate for atmospheric effects before the data are recorded. Of course, the situation is not sharply distinct. Hybrid approaches have been proposed and demonstrated. Other approaches that are a mid-way between the two are also possible. The basic idea of static and dynamic pupil masking will be presented. Experimental results based on point sources and extended objects will be presented. Advantages and limitations of such technique will be discussed. Finally some new ideas involving fiber optics and liquid crystals will be presented.

We discuss the use of Liquid Crystal Phase Modulators (LCPM) as a repeatable test source for use with adaptive optics systems. LCPMs have the potential to induce controlled, repeatable, dynamic aberrations into optical system at low cost, low complexity, and high flexibility. Since they are programmable, and can be operate as transmissive elements, they can easily be inserted into the optical path of an adaptive optics system and used to generate a disturbance test source. Laboratory experiments with a Meadowlark liquid crystal phase modulator are presented.

The purpose of Propagation Delay technique is to retrieve high frequency portion of absolute tip-tilt from a laser guide star by observing the fired artificial beacon just from the laser projector. This technique can relax the requirements on absolute tilt retrieving from other schemes, such as, for example, a fainter natural guide star in the same isoplanatic field, thus increasing the sky coverage factor. Simulations made under Matlab-Simulink and IDL software tools show some improvement in absolute tilt retrieving that can justify the efforts in carrying out this technique for an increase in Adaptive Optics efficiency.

Adaptive Optics produces diffraction-limited images but does not fully compensate for the atmospheric degradation of the incoming signal. Post processing is important to fully restore the image. The results of applying a physically constrained iterative deconvolution algorithm to adaptive optics data are presented here for different types of simulated data with different signal-to-noise ratios.

When a laser beam is used to produce a sodium guide star, a Rayleigh guide star can be got as a 'by-product' with the temporal-gating method. In the case of the atmospheric phase distortion is confined to two thin turbulent layers, the sodium guide star is used to measure both the low-altitude and the high-altitude phase distortion (phi) _b while the Rayleigh guide star provides a suitable probe signal for the contribution (phi) _l of low-altitude turbulence layer. The wavefront distortion (phi) _l of low-altitude turbulence layer. The wavefront distortion (phi) _h causing by the high-altitude turbulence layer alone is then easy obtainable from (phi) _l and (phi) _b. The multiconjugate correction system could be founded upon these.

A laser beam propagating in the atmosphere are influenced simultaneously by thermal blooming and turbulence that result in aberrations of a focal spot. Turbulent aberrations prevail at large speed of scanning, from this point of view they are more important. On the contrary, effectiveness of correction for turbulent aberrations decreases when the object velocity increases. Turbulent aberrations do not depend on sped of angular scanning but in the case of a moving object the requirements to the adaptive system bandwidth are higher than that for a motionless target. The stated above is also true for the low atmosphere paths when a source is placed on the earth and an object altitude is a few kilometers. In this case the influence of thermal blooming is less comparing with turbulence because the coefficient of atmospheric absorption decreases more abruptly than intensity of turbulent aberrations. Additional factor of thermal blooming decreasing is angular scanning. Even when the turbulent and nonlinear aberrations are of the same order, correction for thermal blooming is easier because scales of thermal aberrations are greater and frequencies lower. The only exception is a homogeneous path in the absence of scanning. In this case the strong thermal lens which appears near the beam focus may induce instability and decrease of control efficiency. For a moving object this effect can be disregarded.

The time and place of hazardous tectonic phenomena can be predicted most efficiently with the use of spaceborne systems. In Russia a network of small satellites equipped with detectors of different types is being developed to detect places of future earthquakes. Feasibility of application of optical detectors is based on the following interesting fact. The number density of atmospheric aerosol particles with sizes from 0.1 to 1.0 micrometers tends to increase with time above geological faults several hours or tens of hours ahead of a volcanic eruption or an earthquake. As a rule, epicenters of possible hazardous tectonic phenomena are in the regions of the Earth where only remote means can be used to detect anomalous aerosol number density. The first lidars intended for remote cloud sensing have already been tested on board the Shuttle and Mir space stations that orbited at altitudes of 350-400 km. To evaluate the feasibility of spaceborne detection of anomalous surface aerosol emissions, we did calculations for the model of the aerosol atmosphere developed at the Institute of Atmospheric Optics using the experimental data obtained at the TRINITI and the Sankt-Petersburg State University. The light scattering theory demonstrates that wavelengths of 1.06, 0.532, and 0.355 micrometers are most suitable for sensing of anomalous aerosol emissions. The garnet lasers with diode pumping have already been manufactured commercially. They nave suitable energetic parameters, weight, and overall dimensions. A receiving telescope on the basis of metal- coated carbon plastic mirrors can be used to receive signals from anomalous aerosol emissions in the photon counting mode at night and to detect regions with enhanced number density of finely dispersed aerosol fraction. Technical and technological peculiarities of spaceborne lidar detection of anomalous aerosols are discussed in the present report.

Speckle backscatter form actively illuminated satellites contains information on object size, shape and orientation and on the optical properties of the object materials. This speckle information is useful for object discrimination and classification tasks. In particular, the polarization properties of these materials may provide unique signatures that enhance the discrimination between certain objects. This simulation investigation explores the use of polarization information in speckle backscatter to form useful polarization signatures. The simulation uses detailed polarization renderings of objects and coherent field propagation techniques to form pupil plane speckle polarization components fields characteristic of the object materials' polarization properties. The fields are then converted to four speckle intensity fields using a four- channel polarimeter model and are spatially resolved with an array of detectors. Polarization signature information is obtained from Stokes parameters and Mueller matrices formed from these speckle measurements. The object polarization signatures considered include depolarization, diattenuation and retardance. These parameters are calculated for several object models to illustrate their possible use in object discrimination and to investigate their sensitivity to perturbing effects. The perturbing effects of detection noise, passive unpolarized background and atmospheric turbulence are considered. Spatial and temporal averaging of the Stokes parameter fields is shown to reduce the effect of these errors on the polarization parameters yielding improved signature measurements. Estimation and removal of unpolarized components from the speckle measurements is also shown to improve signature accuracy.

This paper is presented to give a general description of the ORACLE project and of the technology development results obtained to date. ORACLE is a feasibility study of a fully automated differential absorption lidar for global measurements of tropospheric and stratospheric ozone and aerosols with high vertical and horizontal resolution. The proposed program includes both novel technology demonstrations and obtaining scientific data from spacecraft. These data are needed to address key issues in atmospheric research including the depletion of stratospheric ozone, global warming, atmospheric transport and dynamics, tropospheric ozone budgets, atmospheric chemistry, and the atmospheric impact of hazards. Only a space-based lidar system can provide the required spatial resolution for ozone and aerosols in both the stratosphere and the troposphere on a global scale at all required altitudes. To deliver these data, the most novel technologies such as all-solid-state lasers, photon-counting detectors and ultra-lightweight deployable telescopes must be employed in the mission payload.

Up to now, retrieval of the atmospheric extinction and backscatter has mainly relied on standard straightforward non-memory procedures such as slope-method, exponential- curve fitting and Klett's method. Yet, their performance becomes ultimately limited by the inherent lack of adaptability as they only work with present returns and neither past estimations, nor the statistics of the signals or a prior uncertainties are taken into account. In this work, a first inversion of the backscatter and extinction- to-backscatter ratio from pulsed elastic-backscatter lidar returns is tackled by means of an extended Kalman filter (EKF), which overcomes these limitations. Thus, as long as different return signals income,the filter updates itself weighted by the unbalance between the a priori estimates of the optical parameters and the new ones based on a minimum variance criterion. Calibration errors or initialization uncertainties can be assimilated also. The study begins with the formulation of the inversion problem and an appropriate stochastic model. Based on extensive simulation and realistic conditions, it is shown that the EKF approach enables to retrieve the sought-after optical parameters as time-range-dependent functions and hence, to track the atmospheric evolution, its performance being only limited by the quality and availability of the 'a priori' information and the accuracy of the atmospheric model assumed. The study ends with an encouraging practical inversion of a live-scene measured with the Nd:YAG elastic-backscatter lidar station at our premises in Barcelona.

The Mission Demonstration Satellite lidar (MDS) is being developed by the National Space Development Agency of Japan for launch in early 2000's. The MDS-lidar will provide a dataset of global distribution of aerosols and clouds. The data will be useful for studies of radiation effects of clouds and aerosols, and validation and assimilation of climate models. This paper discusses the lidar signal inversion algorithms, for the MDS-lidar to quantitatively retrieve the optical properties of aerosols and clouds. Since the effects of multiple scattering and cloud spatial inhomogeneity are significant in the space-lidar measurements, these effects are considered in this paper. Also, an iterative algorithm which is based on the backward inversion algorithm and uses available near-end boundary condition is presented and applied to the inversion of dense cloud signals for the MDS-lidar. A high-spectral-resolution- lidar has been used as a ground-based test tool for the algorithm study.

NASA recently approved a mission to fly a Doppler Wind Lidar on a US Space Shuttle. SPARCLE, managed by Marshall Space Flight Center in Huntsville, AL, is targeted for launch in March 2001. This mission is viewed as a necessary demonstration of a solid state lidar using coherent detection before committing resources to a 3-5 year research or operational mission. While, to many, this shuttle mission is seen as the first step in a series leading to a fully operational wind observing system, to others, it is a chance to validate predictions of performance based upon theoretical models, analyses of airborne and ground-based data and sophisticated observing system simulation experiments. The SPARCLE instrument is a 100 mJ, 6 Hz, diode pumped 2 micron laser with a .25 m telescope using heterodyne mixing in a fiber and an InGaAs detector. A 25 cm silicon wedge scanner will be used in step-stare modes with dwells ranging from 60 seconds to .5 seconds. Pointing knowledge is achieved with a dedicated GPS/INS mounted close to the lidar. NASA's hitchhiker program is providing the instrument enclosures and mission logistics support. An on- board data system in sized to record 80 Gbytes of raw signal from two 400 MHz A/D converters. On-board signal processing will be used to control the frequency of the Master Oscillator. SPARCLE is predicted to have a singleshot backscatter sensitivity near 5 by 10-6 m-1 sr-1. To achieve higher sensitivity, shot accumulation will be employed. Ground-based, 2 micron DWLs have been used to assess the benefits of shot accumulation. Airborne programs like MACAWS have provided good data st for evaluating various sampling strategies and signal processing algorithms. Using these real data to calibrate out simulation models, we can describe when and how well SPARCLE is expected to perform.

Atmospheric wind speed profiling from an Earth-orbiting satellite with an active lidar probe is considered. 'Direct detection' of the Doppler shift, i.e., with optical interferometry, can employ the UV, and thus can be based on Rayleigh backscatter, permitting measurements without dependence on atmospheric aerosol content. Two direct detection methods, 'edge technique' and 'fringe imaging', are compared here for measurement sensitivity in the signal shot noise limit. There is no significant difference; both are capable, at best, of measurement statistical uncertainty about 2 to 4 times the Cramer-Rao limit of a perfect, lossless receiver.

ESA is planning to perform the Atmospheric Dynamic Mission from the International Space Station. The Space Station is a multi purpose platform, many experiments will be carried out during the same time. Therefore, the Doppler lidar instrument ALADIN, the main sensor to get wind information in the troposphere is not on an ideal platform. There is no full coverage and there is limited observation time caused by other orientation of the space station. To answer the question of the usefulness of a Doppler lidar on the space station the so-called targeted observation was mentioned. Is it possible to get result for the weather forecast improvement if one focuses the observation in sensitive areas of the globe. Both sensor specialists and numerical weather prediction scientists work together to answer this question in the Aladin Impact Study. First results will be presented.

The poor signal to noise ratio of Doppler lidar data, given by the small amount of back scattered radiation and the large distance to the measurement volume, enforces the need of some averaging technique. The major question at which state of signal processing the averaging should take place have been investigated. The major problems with incoherent averaging, spurious estimates and as a consequence a bias on the averaged estimate, can be overcome using accumulation technique before the estimate is determined. Computer simulation have shown that this technique shows the same efficiency as one would expect using coherent averaging.

We investigate the gain in ensemble average and the variance reduction of the total power received by a telescope array compared to a single-aperture system in a typical lidar scenario. For these parameters, we give both limiting expressions and curves taking into account the speckle- related statistical dependences of the power coupled into different subapertures as a function of the number of subapertures, the subaperture spacing, and the ratio of transmit aperture size to receive subaperture size. The improvement of the array with respect to average coupled power is found to be nearly 3dB at the most, the reduction of the coupled power's variance depends on the array geometry. As coupling light into singlemode fibers is closely related to optical heterodyning, our results also represent a generalization for the carrier-to-noise ratio improvement of an optical phased array employing a 'maximal ratio' heterodyne receiver.

A solid state airborne lidar for profiling cloud and aerosol scattering has been demonstrated. The transmitter of the lidar is a laser diode pumped Q-switched Nd:YLF laser. The wavelength of the laser is 1,053 nm, output energy from a transmitter optics is 16.5 mJ and repetition rate is 50pps. Eye safety is obtained through beam expansion, whose divergence is 2.6mrad. The diameter of the telescope made of beryllium is 200mm, the field of view is 0.3mrad. The receiver employs a photon counting solid state Geiger mode avalanche photodiode. The vertical resolution is 75m. The received signals were integrated 300 times to the horizontal direction to improve the signal to noise ratio. The measurements for airmolecule, aerosol and cloud were performed using the lidar in November, 1997. The airplane made a round trip from Nagoya airport to Kashima nada via Tsukuba in Japan. The altitude of the airplane was 6,150 m. The measurements indicate that the lidar is capable of detecting and profiling cloud and aerosol scattering through the atmosphere.

The FGAN-FfO participated in the LAPTEX trial in Crete in July 1996. Irradiance fluctuations or scintillation of visible point targets were measured with a digital camera at frame rates up to 1000 Hz during different atmospheric turbulence conditions. One optical path with a length of 2560 m was directed parallel to the coast line crossing water as well as land. The strength of atmospheric turbulence was determined by measuring the structure constant of refractive index, C_n², and the inner scale of turbulence, l₀, with a laser scintillometer over irregular coastal rock structures. The analysis of the recorded image sequences was performed with the interactive data language IDL. The irradiance fluctuations of the target-to-background differences show a log-normal frequency of occurrence distribution. The corresponding variances in angle-of-arrival fluctuation ((alpha) ²) were determined. Calculations of the variance in angle-of-arrival fluctuation were carried out using the path-weighted structure constant of refractive index, C_n². Theoretical values of ((alpha) ²) indicate a variation, which is an order-of- magnitude larger than the variation in the experimental values.