The MAPTIP (marine aerosol properties and thermal imager performance) experiment was organized as part of a project to assess atmospheric effects on the detection and identification of targets using thermal imagers in coastal areas. The experiment took place at the North Sea from 11 October - 5 November 1993 and was centered around Meetpost Noordwijk, a research tower at 9 km from the Dutch coast. Platforms included a beach station, ship, research airplane, P3 Orion, helicopter and three buoys. The objectives were centered around the characterization of the atmosphere (aerosols, extinction, turbulence, refractivity, and the vertical and horizontal variations of the relevant meteorological parameters), in combination with detailed measurements of optical and IR effects using thermal imagers, visual cameras, transmissiometers and visibility meters. Detection ranges of various targets were determined and IR properties of extended targets were continuously monitored using radiometers. Extensive studies were made on polarization effects, backgrounds and effects of sun glint. An overview of the experimental efforts and the ensuing analysis and modeling studies are presented. MAPTIP was the first validation of some recently developed atmospheric propagation models, including aerosol models, in a coastal environment. MAPTIP has yielded a wealth of data for the development of advanced aerosol models, description of horizontal variability, improvement of point target detection algorithms, validation of detection range models, and EOTDA validation.

This paper addresses the question of what is the structure of the aerosol concentration profile close to the sea surface during high wind and whitecapping conditions. EO energy must pass through this layer on long horizontal trajectories because of the curvature of the earth. This is also a region of the marine atmosphere where very little is known during high wind conditions. The paper discusses the profile of large aerosol size distribution data obtained from rotorod devices during the MAPTIP experiment. These devices were lowered from an overhang on the Dutch tower, Meetpost Noordwijk, MPN (located about 9 kilometers off the Dutch shore). Altitudes of from 10 meters to just above the wave tops were investigated in calm and high wind conditions. In addition to aerosol size data, relative humidity profiles and other pertinent meteorological measurements were made throughout the MAPTIP experiment and utilized in the analysis. The wind speed dependence of the structure of very large aerosol in the surface layer above the sea is derived from this data. This analysis shows that the turbulence produced by the high wind speed (required to build up the sea state) also mixes the large aerosol in the lowest 10 meters of the atmosphere. This causes any vertical structure in the aerosol spectrum to disappear while the net amount of large aerosol increases non-linearly with the wind speed.

Aerosol particle size distributions and meteorological parameters were measured during the MAPTIP experiment in the fall of 1993 in the vicinity of MeetPost Noordwijk (MPN), a platform at 9 km from the Dutch coast. One data set was collected at MPN, the other on board of a ship that sailed down and cross wind trajectories. from MPN to a distance of 200 km. Both the aerosol concentration and extinction respond to variations in the meteorological conditions. For down wind trajectories, the responses at ship and platform are observed with a delay in accordance with the time it takes for an air mass to travel from one to the other. For cross wind trajectories, the responses are observed simultaneously at ship and platform. The comparison of ship and platform also shows that the meteorological conditions change as a function of fetch in off-shore winds. This fetch effect does not show up clearly in the aerosol concentrations, which suggests that in off-shore winds the influence of the land (natural, industrial and urban aerosol) extends up to more than 200 km from the coast.

During the MAPTIP experiment, that was organized by NATO AC/243 (Panel 4)/RSG.8, the marine atmosphere was characterized, among others, with lidar (optical radar). The investigations were carried out both in horizontal planes (PPI scans), in vertical planes (RHI scans) and combinations of those two. In addition, the temporal variability of the atmosphere along one line of site was characterized by operating the lidar in a fixed direction at a sufficiently high repetition rate to follow the eddy structures. The marine aerosol layer was monitored within the surf zone and out to ranges of about 10 km. Slant path measurements provided information on the depth and structure of the mixed layer. Results obtained during this experiment are presented.

During the first MAPTIP workshop in Oslo, Norway (May 94) after the trial (Oct. 93), a number of working groups were created. These groups reported some of their joint findings at the second workshop in Quebec City, Canada (May 95). This presentation summarizes the results obtained by the 'refractive effects in the visible and the IR group.' The experimental setups of the three different groups (DEV, FGAN-FfO & CELAR) and the experimental procedures used during the trial are presented along with a summary of the available datasets. Due to the large number of datasets only a select number were chosen for further in-depth analysis. These case studies were chosen using essentially two criteria: (1) interesting meteorological condition and (2) measurements were obtained by more then one group. The meteorological conditions for these case studies are presented along with comparisons of the measurements with the theoretical results obtained using both the PIRAM (CELAR) and L(W)WKD (DREV) models. Differences between the model predictions and some of the problems encountered are also discussed.

During the MAPTIP experiments in the Dutch coastal waters, 11 October - 5 November 1993, transmission curves were determined from the intensities of the image of a point source suspended from a helicopter at ranges between 0.5 and 6 NMi. The images were recorded with a 10 micrometer USFA 9092 camera from the MeetPost Noordwijk, a research tower in the North Sea at 9 km from the Dutch coast. The transmission determined from the point source intensities at several ranges is compared with calculated values. The transmission is determined by extinction due to aerosols and molecular species in the propagation path. Both contributions can be determined using code using measured size distributions. Also effects of path radiance and background on the image intensity are considered. In this coastal area, and the off- shore winds that were usually encountered during MAPTIP, the aerosol size distributions are known to be a complicated mixture of continental and marine aerosols. Hence the common aerosol models, that usually work well over the open ocean, are often not so reliable in a coastal environment. An attempt is made to assess the influence of marine and anthropogenic contributions to the aerosol on the detection range of point targets in a coastal atmosphere.

The 1993 marine aerosol properties and thermal imager performance (MAPTIP) exercise conducted in the North Sea off the Dutch coast provided data for evaluating the electro- optical tactical decision aid Mark III (EOTDA) in a littoral environment. The EOTDA is a strike-warfare planning tool that is installed in the Navy's tactical environmental support system [TESS(3)] and tactical aircraft mission planning system (TAMPS 6). The objective of this report is to compare predicted detection ranges from the EOTDA with actual reported detection ranges collected during the MAPTIP trials. During MAPTIP, TNO Physics and Electronics Laboratory the Netherlands employed a Safire infrared FLIR system aboard a P-3 Orion aircraft and used the ship, the Hr. Ms. Tydeman, as a target. Ten sorties were flown and meteorological conditions were continuously recorded aboard the Hr. Ms. Tydeman. Additional weather observations were made at an oceanographic platform and at NAS Valkenburg. The weather information was compiled and converted to terminal aerodrome forecast (TAF) code for input to the EOTDA. The Safire FLIR was installed as a user-defined sensor into the EOTDA using the minimum resolvable temperature (MRT) curves of the manufacturer. The EOTDA was then run using the standard frigate model included in the EOTDA target menu. When the results are compared with the reported detection ranges, the data was scattered and showed a tendency to over-predict detection ranges. The average error was 51% on first pass. After correcting the FLIR operator observations using the video recordings, the error was reduced to 41%. Clearly, improvements are needed in the EOTDA, such as, a more accurate target model, a ship course tracking capability, and improvements to the background and transmission models.

Shore-based polarized infrared images of ship target and background were obtained in the MAPTIP measurement series, using vertical and horizontal polarization filters. These images have been analyzed to yield degree of polarization and polarization contrast enhancement for targets with sea and sky background Strong vertical polarization is observed in the sea surface emission near the Brewster Angle in the 8 - 12 micrometer (LWIR) band, whereas horizontal polarization due to reflection is usually dominant in the 3 - 5 micrometer (MWIR) band. Ship targets at broadside aspect show a degree of polarization less than 5%. An enhancement of contrast by up to 30% in the LWIR (less in the MWIR) is obtained by polarization suppression of sea background. The contrast enhancement in the MWIR is counteracted by the horizontal polarization of reflected sunlight.

Turbulent wind and temperature fluctuations were measured with an ultrasonic anemometer deployed on a buoy near the Meetpost Nordwijk platform during MAPTIP. These measurements were designed to provide MAPTIP investigators with knowledge of over-water turbulence properties and the suitability of existing bulk models for describing them. The friction velocity, u*, and the temperature structure function parameter, C_t², were determined using both bulk and inertial-dissipation methods. Inertial-dissipation measured C_T² values agreed well with bulk estimates for values greater than about 0.02. Below this value the inertial-dissipation C_T² measurements were larger than the bulk estimates, probably due to noise contamination of the weak temperature fluctuation signal in the temperature variance spectrum. Inertial-dissipation u* values were larger than the bulk determinations for higher wind speeds. The bulk formulation used in this study was determined for large fetches with west winds at the MPN location, while only east winds with shorter fetches occurred during MAPTIP. Since fetch limited seas are rougher for a given wind speed, this would cause the inertial- dissipation measured u* values to be higher than the bulk estimates.

A model was developed for the prediction of turbulence in the marine surface layer. The model requires standard meteorological values of air temperature, air humidity, wind speed each from any given height from within the surface layer and the sea surface temperature. Internally, the model is controlled by the exchange coefficients for momentum, heat and water vapor. A variant using the surface roughness length instead of the drag coefficient has also been implemented. The micrometeorological output parameters of the model are used to predict vertical profiles of the refractive index -- to predict refractivity effects -- and profiles of the refractive index structure function parameter C_n²(z). The latter is the controlling parameters to calculate optical turbulence effects such as scintillation and blurring. Experimental data were obtained from images taken of a point source over a 19 km path over the North Sea at a frame rate of 25 Hz using a 3 - 5 micrometer infrared camera system. The images were analyzed for scintillation, blur and image dancing. Predicted and measured turbulence effects are compared.

Calculation of the microwave refractivity traditionally depends on the vertical gradients of temperature and humidity. It is shown herein that if there are horizontal gradients of the momentum, heat, and moisture flux between the air and sea (or alternatively the horizontal gradients of the roughness lengths for these fluxes), the refractivity exhibits a strong dependence on fetch. The equations were demonstrated using typical environmental meteorological data, typical wave spectra, and differences between the refractivity and duct strength are calculated both with and without the terms involving horizontal gradients. In general, for offshore flow the ducts are deeper and radius of curvatures are smaller as one moves closer to the shore from distances far offshore.

Several marine boundary layer (MBL) models have been developed over the last few years to predict the propagation behavior of electromagnetic radiation near the sea surface. Originally, they were developed to model the refractive effects of the MBL on radar systems, but in recent years they have been extended for use at visible and infrared wavelengths. Three of the more advanced models are DREV's L(W)WKD model (Canada), CELAR's PIRAM model (France) and the LKB model developed in the U.S. Only very limited comparative studies have been performed between them. This study discusses the differences between these models for a large number of realistic atmospheric conditions. In particular we present the differences between the calculated roughness heights, scaling constants, and vertical profiles with respect to various meteorological parameters.

The ability for passive electro-optical sensors to provide simultaneous target angle and range data could substantially enhance system performance in many applications. This paper evaluates a technique where the sensor monitors two or more wavelengths of the target signal and the differential attenuation through the intervening atmosphere is used to calculate target range. In particular, effects of wavelength, bandwidths, and molecular absorption characteristics on the accuracy of target range determination are assessed.

The degree of linear polarization of the thermal radiation depends on the surface roughness. A simple facet-surface model is used to describe the rough surface and to evaluate the far-field Stokes parameters are evaluated. The dependence of the polarized emission on the roughness parameter, fractal dimension, and material type is investigated.

A lidar-transmissometer intercomparison was made during an international experiment held in the German Alps to characterize the vertical structure of aerosols and clouds. The transmission path was 2325-m long and inclined at 30 degrees along the slope of a steep mountain ridge. the transmissometer consisted of a Nd:YAG and a CO₂ laser located in the valley and a large-mirror receiver that captured the full beams on the mountain top. Two lidars, one at 1.06 micrometer and one at 1.054 micrometer, were operated with their axes approximately parallel to the transmissometer axis but separated by a horizontal distance on the order of 20 - 40 m. The first one was operated in retroreflector mode and the relative transmittance was determined from the reflection off the mountain ridge above the cloud layer. The second one had a special receiver designed to make simultaneous recordings at four fields of view. The range-resolved scattering coefficient and effective cloud droplet radius are calculated from these four-field-of-view measurements by solving a simplified model (Appl. Opt. 34, 6959-6975, 1995) of the multiply scattered returns. The two simultaneous solutions for the scattering coefficient and effective droplet size make possible extrapolation at wavelengths other than the lidar wavelength of 1.054 micrometer. The main measurement event analyzed in this paper lasted 1.5 hours and produced transmittances ranging from less than 5% to more then 90%. The comparisons show good correlation between the transmissometer data and all lidar solutions including extrapolation at 10.59 micrometer.

It is generally thought that the maximum signal averaging time for coherent (heterodyne) optical receivers is limited by the aerosol speckle phase decorrelation time. Fortunately, for many applications this is not true. It is demonstrated analytically and confirmed by experimental data that for many applications the signal averaging time is limited by the shorter of the laser transmitter or atmospheric turbulence phase decorrelation times. This can have a significant impact on lowering the threshold for signal detection when the fast Fourier transform (maximum likelihood estimator) is used to detect the signal.

Enhanced backscatter effects associated with the mean intensity and scintillation index for a monostatic propagation channel through atmospheric turbulence are analyzed for two spectral models of refractive-index fluctuations. The analysis presented here shows that using a more physically realistic modified spectrum with inner and outer scale parameters as well as a high-wave-number rise (bump) can lead to significantly different results than those predicted by the idealized Kolmogorov power law spectrum. The target models used in this analysis include a smooth target finite mirror, glint target finite retroreflector, and a rough surface target (Lambert surface). The general method of analysis is based on weak fluctuation theory.

Estimates of the fractional fade time, expected number of fades, and mean duration of fade time associated with a transmitted Gaussian-beam wave are developed for uplink laser satellite-communication channels and compared with similar results based on a spherical wave model. Weak fluctuation theory using the lognormal model is applicable for zenith angles less than 60 degrees. Because spot size and off-axis scintillations are significant in the Gaussian- beam wave model but not for the spherical wave model, pointing errors become an important consideration in a reliable laser communication link. Off-axis scintillations increase even more rapidly for large diameter beams and can in some cases lead to a scintillation saturation for pointing errors greater than 1 (mu) rad off the optical beam axis.

The limitations placed on optical imaging through the earth's atmosphere are well understood. A large portion of the body of work on this subject deals with vertical optical paths. The transition to the study of horizontal propagation of light is currently being made. Various methods exist that quantify the disturbances introduced on optical signals by turbulent air. Small perturbations of the wave front phase can be measured using a Hartmann wave front sensor (H-WFS). For long optical paths through turbulence spatial and temporal variations in intensity called scintillation arise. Using scintillation statistics and theoretical expressions for structure functions of H-WFS slope values, turbulence strength is studied. Data sets taken from two mountains in Hawaii during an optical communications experiment are studied for comparison with slope structure function and scintillation statistics theory. This experiment was performed at an altitude of 10.000 feet over a horizontal pathlength of 150 km. Results indicate that very strong scintillation exists under these conditions, and that very little faith can be placed in the H-WFS phase-dependent results. Scintillation may provide a more dependable method for optical characterization of these conditions.

Turbulence has long been recognized as one of the most significant factors limiting the performance of optical systems operating in the presence of atmosphere. Atmospheric turbulence over vertical paths has been well characterized, both theoretically and experimentally. Much less is known about turbulence over long, horizontal paths. Perturbations of the wavefront phase can be measured using a Hartmann wavefront sensor (H-WFS). These measurements can be used to characterize atmospheric turbulence directly. Theoretical expressions for the slope structure function of the H-WFS measurements are derived and evaluated using numerical quadrature. By concentrating on the slope structure function, we avoid the phase reconstruction step and use the slope measurements in a more direct fashion. The theoretical slope structure function is compared to estimated slope structure functions computed from H-WFS measurements collected in a series of experiments conducted by researchers at the U.S. Air Force's Phillips Laboratory. These experiments involved H-WFS measurements over high altitude (airborne) horizontal paths 20 - 200 km in length.

We have investigated the feasibility of building an innovative optical remote sensing instrument to monitor the vertical profile of the refractive index structure characteristic C_n². There is currently no active optical remote sensing instrument which is capable of doing this. Calculations have been performed for a system designed specifically to resolve a site survey question at the South Pole, where recent balloon soundings suggest that excellent astronomical seeing conditions could be obtained by mounting telescopes above a thin layer of atmospheric refractive turbulence near the surface. The new sensor considered here is essentially an imaging lidar which measures range- dependent laser beam wander, from which the vertical profile of C_n² can be derived. Calculations based on atmospheric characteristics and preliminary design parameters have been carried out for a practical system based on commercially available components. Design parameters include the choice of operating wavelength, elevation angle, transmitter and receiver diameters, and image scale. The calculations indicate that it is feasible to develop an optical remote sensor for monitoring vertical profiles of C_n² at the South Pole.

We demonstrate that by using the remote turbulence profiling method of scintillation detection and ranging (SCIDAR) one is able to determine both the optical strength of turbulence and the velocity of the turbulent layers, as a function of altitude. The technique of generalized SCIDAR, which allows low level turbulence to be characterized, is also outlined and demonstrated. Preliminary results obtained from SCIDAR observing runs at La Palma, Canary Is., are discussed.

The effect of atmospheric turbulence on the imaging of scenes, for a horizontal propagation of the light over a distance of 20 km, 15 meters above the sea surface, was analyzed at visible wavelength using a 20 cm telescope. Point-sources images were recorded during the night, and the fried parameter r₀ was derived leading to values ranging from 1.5 to 3.6 cm. A very high level of scintillation was observed. Studies of correlations between close-by sources lead to a very small domain of isoplanatism. Daytime observations of an extended source area are also performed; an image motion of small spatial coherent length seems to be drawn by a horizontal wind producing wave-like distortion of horizontal lines and boiling-like of vertical ones.

The time-evolution of optical degradation in the near nozzle region of a heated axisymmetric jet is measured using conditional sampling techniques. A novel linearized stability experiment is performed in order to identify the flowfield states most applicable for conditional sampling techniques. The results of the conditional sampling experiment exhibit a condition where two distinct flowfield states are evident. Potential explanations for the observance of these two distinct states are proposed, with the most probable explanation being due to pi-jumps that can arise between the phase of the excitation signal and the phase of the flowfield events.

Two high-speed cameras were used to generate a set of measurements of laser light propagating through a high- Reynolds-number mixing layer. A channel-flow mix of helium gas at 4.5 m/s and nitrogen gas at 1.0 m/s induced optical perturbations which were sensed using shadowgraph and wave front slope measurements. Phase surfaces were reconstructed from the wave front slopes. An ensemble of 255 frame- registered measurements were obtained for each sensing technique at eight aperture locations in the flow, spaced between 0 cm and 15 cm from the exit nozzle of the turbulence generator. A technique is introduced which computes an array of correlation coefficients over a two- dimensional sliding window for various temporal separations as a means to quantify frozen flow properties. For the wave front phase, the peaks of the correlation coefficient arrays degraded by 30 to 60 percent for 1 ms separation, indicative of limited validity in a frozen flow assumption for this type of flow.

A high-frequency crossed-beam correlation (CBC) experiment was performed to determine the mean-squared fluctuating density, convection speed, and characteristic turbulent coherence length of a supersonic turbulent mixing layer. Aero-optical conditions were representative of actual flight. Orthogonal helium-neon laser beams intersected to interrogate a 100 micrometer -- diameter volume. Beam motion was sensed by two quadrant detectors, whose output signals were recorded after being digitally sampled at a 5 MHz rate. Cross-correlation of angular beam deviations was computed, and from this, the mean squared fluctuating density was determined. By offsetting the beams in the streamwise direction, convection speeds were determined, enabling turbulent cell sizes to be estimated. RMS densities reached approximately 15% of the local mean density in the mixing layer, and correlation length estimates ranged from 1.5 to 2 mm. Fluctuating densities were lower, and correlation lengths were higher than predicted by a simple model. This paper summarizes experimental design and procedures, and provides a theoretical treatment of the results.

A full aperture tilt sensing technique which does not require transmitting laser irradiance through the main optical train is described. This technique is based on measurements of differential motion of the images of the two laser guide stars (LGSs) formed in the atmospheric sodium layer by the two parallel collimated laser beams separated at a distance equal to the telescope diameter. It is shown that a one-axis wavefront tilt component might be determined by measuring the LGS's differential motions simultaneously with the main telescope and with an auxiliary telescope. To sense two tilt components, four laser beam and two auxiliary telescopes are needed. A tilt angular anisoplanatism is studied, and parameter of the tilt measurement scheme are estimated.

An experimental model of an adaptive imaging system for compensation of large-scale phase distortions under conditions of strongly anisoplanatic imaging is studied. We investigate adaptive compensation of phase-distorted extended source images using new types of image quality criteria. These image quality criteria are dependent on the Fourier spectrum of the image, and can be obtained using a coherent optical system. For adaptive control of a ten electrode continuously deformable bimorph mirror, simple gradient algorithms for image quality criteria were applied. We experimentally demonstrate the efficiency of large-scale phase distortion compensation for extended and complex targets.

Within the last decade, the declassification of adaptive optics techniques and systems developed for defense purposes opened new opportunities to the astronomical community. Since the military resolution requirements are not qualitatively different from the astronomical ones, astronomers may profit from the quite sizable investments already made. On the other hand, the astronomical observations are much more demanding than the military ones with respect to the required accuracy, stability and sensitivity. In 1994, after contacts made during an adaptive optics meeting in Munich, we started a joint project to observe the ejected matter from the luminous blue variable (LBV) P-Cygni with the AMOS compensated imaging system (CIS). In this paper we describe the problems encountered and the experience gained in more than two years of operations with CIS. The satisfactory results obtained so far prompted us to plan a more ambitious program. We aim to profit from the acquired know-how for preparing a proposal of astronomical observations designed in such a way of taking the utmost advantage of the capabilities of the new USAF AEOS adaptive optics system.

A 37 element adaptive optics system has been built in the Institute of Optics and Electronics for experimental research of atmospheric effect compensation. In this system a 37 subaperture Shack-Hartmann sensor with frame rate 380 Hz is used as wavefront sensor, a 37/55 element deformable mirror and a fast steering mirror as wavefront correctors, a digital signal processor with peak operation speed 100 Mops as wavefront processor and controller, and 39 channels of high voltage amplifier for controlling the wavefront correctors. This system has been integrated with a turbulence cell developed by the Anhui Institute of Optics and Fine Mechanics and experiments for compensating for wavefront errors induced by the turbulence of the cell with different strength have been conducted. Another Shack- Hartmann wavefront sensor is used for measuring the strength of turbulence. The integrated probability density functions of both the coherence length r₀ and the Strehl ratio of S of the focused spot are used to describe the strength of turbulence and the sharpness of focused spots with and without correction. In this paper the adaptive optics system and the experimental results are briefly reported.

A 21-element adaptive optical system has been built and installed at the 2.16 m telescope of Beijing Astronomical Observatory (BAO). It consists of a pair of photon counting shearing interferometers, a 21-element deformable mirror, a fast steering mirror, a digital processor and an imaging CCD camera. The wavefront error sensing is in the visible wavelength and the compensated images are detected in infrared band. In this paper, the system and its preliminary observation results are briefly reported. Moreover, the closed-loop error of the wavefront sensing noise is analyzed and the measured results in different photon counting rate levels is presented. Based on it and the residual error induced by atmosphere turbulence, the effectiveness of the system and its relationship between the photon counting rate and the closed-loop bandwidth in different improvement are also analyzed. Both the closed-loop total error and the effectiveness of the system are functions of the photon counting rate and the closed-loop bandwidth. The analysis results show the optimal closed-loop bandwidth with which the total error of the system is minimum is proportional to the 3/8 power of the photon counting rate, and the limiting photon counting rate is about 1200 cps per subaperture, with lower than this photon counting rate the compensation of the system is ineffective.

Imaging quality of optical systems in a turbid environment is influenced not only by the contents of the turbid layer between the object and the optical receiver but also by the inhomogeneity of that medium. This is important particularly when imaging is performed through clouds, non homogeneous layers of dust, or over vertical or slant paths through the atmosphere. Forward small angle scattering influences more severely image quality and blur when the scattering layer is closer to the receiver. In this study the influence of the position of the scattering layer along the optical axis on the image quality and modulation transfer function (MTF) is investigated. The scattering layer was in controlled laboratory experiments consisted of calibrated polystyrene particles of known size and quantity. A point source was imaged by a computerized imaging system through a layer containing polystyrene particles and the point spread function (PSF) was recorded. The scattering MTF was calculated using the measured PSF. The MTF was measured as a function of the relative distance of the layer from the receiver. The experimental results were compared to theoretical models based on the solution of the radiative transfer theory under the small angle approximation.

Transmission characteristics of layers that contain fractal aggregates are investigated. In particular, the modulation transfer functions of these layers are studied in comparison with the case of uniformly dispersed particles. The effects of aggregation type (fractal dimension) and aggregate size are discussed.

Sheared beam imaging (SBI) is a coherent active imaging technique that employs a pupil-plane phase difference measurement approach to overcome the perturbing effects of atmospheric turbulence. The technique has shown promise for applications such as imaging satellites from ground-based sites. However, atmospheric compensation is dependent on the effective atmospheric isoplanatic patch size; which mines that degradations can occur for larger objects. We present theory and simulation results that demonstrate this limitation and we suggest approaches to reduce the residual degradations caused by anisoplanatism. Our simulations use wave optics propagation through phase screens to model atmospheric turbulence effects.

In order for advanced image simulation systems to properly model the image formation phenomenology of remotely sensed imagery, simulations need to account for the effects of transmissive objects in a scene. These transmissive effects are important in simulating imagery of littoral scenes, vegetation canopies, meteorological clouds, and gas plume discharges. This is especially true for imaging scenarios in the long wave infrared region where very complex target- background radiational and thermal interactions are critically affected by transmission. The digital imaging and remote sensing (DIRS) Laboratory's synthetic image generation (DIRSIG) model has recently been improved to include the general simulation of transmissive scene elements. The model emphasizes quantitative prediction of the radiance reaching sensors with bandpass ranges between 0.28 and 20 micrometer. It includes spectral radiation propagation using MODTRAN, thermal modeling based on the environmental history, and extensive target-background interactions. An overview of DIRSIG's capabilities is presented along with the methodology and mechanism of simulating transmissive scene elements. Imagery illustrating the various scenarios and imaging phenomenologies are presented and discussed.

Restoration for actual atmospherically blurred images is performed using a Wiener filter which corrects simultaneously for both turbulence and aerosol blur by enhancing the image spectrum primarily at those high frequencies least affected by the jitter or randomness in turbulence MTF. Correction is based upon predicted rather than measured atmospheric MTF. Both turbulence and aerosol MTFs are predicted using meteorological parameters measured with standard weather stations at the time and location where the image was recorded. A variety of weather conditions are considered. Past results have shown good correlation between measured and predicted atmospheric MTFs. Corrections are shown here for turbulence blur alone, for aerosol blur alone, and for both together. Since recorded images suffer frequently from poor contrast because of atmospheric path radiance, a simple image contrast improvement is also considered for the clarity of the atmospheric deblurring effect.

Bispectrum speckle imaging uses the average of many short exposure frames to eliminate atmospheric effects on images. Unfortunately, objects are often dim, requiring longer exposure times to collect enough photons to reconstruct an image. We investigate this trade-off using a computer simulation to create image frames under various seeing conditions, then determine the exposure time that yields the highest signal-to-noise ratio for the unbiased speckle interferometry estimator and the lowest mean square error for the reconstructed phase. We have found that for low light levels and for high read noise cases the optimum exposure time is greater than one Greenwood coherence time.

The paper discusses the salient features of the impulse response characteristics of IR channels of image transfer through the atmosphere (lambda equals 3.75 and 10.8 micrometer) obtained by solving the radiative transfer equation by the Mont-Carlo method. A distorting effect of horizontal photon diffusion on infrared images of the temperature-inhomogeneous Earth's surface recorded from space under conditions of a turbid atmosphere has been investigated. As an example, two different situations have been considered: remote measurements of the surface temperature near the dividing line between two large regions of the surface with different temperatures (for example, near a coastal line) and spaceborne detection of subpixel high-temperature sources. Results of simulation for a 3.75- micrometer channel have shown that in the first case, band zones are formed on both sides of the dividing line due to aerosol scattering, within which the measurement results may strongly depend on the geometry of observations, the value of the temperature gradient, and the degree of atmospheric turbidity. Aerosol scattering may have a marked effect on the quality of satellite data interpretation when estimating size and intensity of subpixel high-temperature sources.

We have constructed wavefront compensated system that measures wavefront tilt of light propagation through the laboratory-generated turbulence in a convective tank. The measurements of optical characteristic in simulation cell have been measured. The wavefront tilt power spectral density and arrival-angle of fluctuations are also shown simultaneously The spectra show distinct asymptotic slopes, in general, agree with theoretical predictions based on the Kolmogorov model. The distribution variance of arrival-angle fluctuations follows (Chi) ² distribution.

The physical basis of SRS process in transparent droplets caused by intensive pumping radiation is considered. The vector Maxwell equations, where the nonlinear polarization is the source for Stokes field origin, are used as a starting point for theoretical analysis of SRS process in a droplet. In stationary case the expression for SRS threshold intensity is obtained. The values of the energy threshold of the SRS generation in droplets are evaluated depending on the droplets radius. The results of theoretical investigations of spatial structure of SRS field far from the droplet are represented also. The angular distribution of SRS is noted to be more homogeneous than elastic scattering diagram, what is connected with the absence of refracted on particle profile field component at Stokes frequency. In opposition to elastic scattering the angular SRS structure is practically symmetric with approximate period of pi/2. The results obtained are compared with the available experimental data.

A phenomenological model is developed for the strength and spatial width of the backscattered coherent intensity peak produced by reciprocal path scattering through atmospheric turbulence. The model is applied to a ground-based monostatic laser radar tracking a space target under optically saturated atmospheric turbulence conditions. The models for the amplitude and width of the RPS peak are based upon the spatial coherence widths of the propagating fields over the up-link and down-link paths within the atmospheric turbulence as well as the cross-sectional area of the up- link beam.

When optical beam propagates through the atmosphere, the wavefront is distributed by the turbulence. The wavefront error can be measured by Hartmann-Shack wavefront sensor (HSWFS) as arrival angle fluctuations in subapertures. Statistic characteristics of atmospheric turbulence can be calculated from these data. To make these characteristics more accurate, the spatial and temporal resolution should be high, and the measurement time should be long enough. A high resolution measuring and data processing system has been built for this usage. A 37-element HSWFS and a high frame rate (800 Hz) CCD image sensor are used in this system. Image data from CCD are processed by a multi-processor system in real time to extract the wavefront slope data of each subaperture. These slope data are sent into the extension memory of a personal computer, which is the host computer of the system. Data of about 200,000 frames can be stored continuously for one measurement period. After this period, these data are saved in hard disk, and the characteristics of turbulence can be obtained from these data.

A numerical code for simulating light propagation in the atmosphere and adaptive optics compensation for wavefront degradation induced by turbulence is described. Adaptive optics is simulated on the basis of the tilt-direct wavefront reconstruction. The compensation results are given in the sense of Strehl ratio variation with turbulence coherence length. The variation of residual phase deviation with d/r₀ is also discussed.

An unusual dual-aperture 28.5-inch, f/21 Ritchey-Chretien telescope has been completed and will be installed in the recently upgraded University of Denver extreme high altitude observatory facility, atop 14,268 ft. Mount Evans in Colorado. Designed to optimize high spatial resolution imaging, the Meyer binocular telescope incorporates active thermal management of the telescope structure. The secondary mirror support elements are fabricated from INVAR and permit active tip-tilt and focusing capability. The optics were fabricated from Zerodur by Contraves USA, and each system has a measured total wavefront error less than 0.050 lambda at 633 nm. All optical surfaces are coated with a multi- layer dielectric enhanced silver, providing high reflectance from below 350 nm to beyond 26 micrometer. The telescope control system has been designed to allow initial operation from an insulated control room. Long-term plans call for totally remote operation from the University of Denver campus via direct microwave radio link. Instrumentation planned for the telescope at first light includes: (1) a low order 400 nm to 1,000 nm band adaptive optics system (AO5: adaptive optics, 5 mode) equipped with a large format CCD camera; (2) a mid-infrared array camera (TNTCAM: ten and twenty micron camera); and (3) a mid-IR moderate dispersion spectrometer (TGIRS: two grating IR spectrometer). Some of the science problems the dual aperture telescope is uniquely situated to tackle include the study of planetary atmosphere, detection of planetary systems around nearby stars and the analysis of evolutionary changes in stars. The Mount Evans site (at 4,303 meters elevation, the highest operating astronomical facility in the world) is located 70 km west of Denver and can be reached via a paved state highway which extends all the way to the summit. The observatory is currently under construction with installation of the telescope planned for late summer 1996.

In the Institute of Optics and Electronics, Chinese Academy of Sciences, a series of deformable mirrors (DM) and fast steering mirrors (FSM), including 19, 21, 37/55, and 61 element deformable mirrors with continuous face plate and discrete PZT actuators and two dimensional fast steering mirrors with different diameters and tip-tilt angles ranging from ten arc sec. to several arc min., has been developed. These mirrors have been successfully used in different adaptive optical systems. In this paper, the specifications such as influence function, deformation-voltage response, frequency response, step response and their measuring methods are reported. Some aspects related to environment of employment such as temperature stability of surface figure are discussed.

Considering time-delay and finite spatial bandwidth of an adaptive optics system, which is used in astronomical observation, the influence of transverse wind along the light propagation path on the performance of the system has been analyzed in this article, and the related analytical expressions have been obtained. The results have been compared with those obtained under open-loop operation, and those obtained when there is an ideal adaptive optics system, i.e., its spatial bandwidth is infinite. It is shown that the transverse wind has serious influence on a system with large diameter when the time delay is long and its spatial bandwidth is high. In some conditions, the performance of a system with higher spatial bandwidth is worse than that of a system with lower spatial bandwidth. This phenomenon has been analyzed. Its possible cause also has been presented in this article.

A single-ended, non-Doppler, laser wind sensor has been developed to measure path-integrated cross winds by viewing a distant target through a large telescope and observing the motion of a laser speckle pattern. The speed of the moving speckle pattern is determined by a cross-correlation between the signals from two detectors in the telescope focal plane. A prototype laser wind sensor was developed and tested. Results are shown for a laboratory test in a wind tunnel and for an outdoor test in a non-homogeneous wind field. Practical applications of the sensor are discussed, and possible modifications to measure two- or three-dimensional wind fields are described.

Wave front dislocations generated by the vortex flow of the light energy in vicinity of the points where the field intensity equals zero are studied on the basis of obtained relation between the phase and intensity in optical beam (the phase problem solution). Reasons of an ambiguity of the phase problem solution are discussed. An analytic formula for retrieving the potential phase from the known intensity distribution has been obtained. A possibility to use an information on the potential phase for correction of singular phase distortions by the optical adaptive systems is discussed.

We show that an effective scattering radius r_s and aerosol extinction coefficient at a wavelength of 3.8 micrometer (alpha) _e (3.8 micrometer) can be estimated from the simultaneous measurement of visible and 10.6 micrometer extinction in haze and fog. For fog, functions relating r_s and (alpha) _e (3.8 micrometer) to the ratio of 10.6 micrometer to 0.55 micrometer extinction were derived based on accepted size distribution models for clouds and fog. For haze, functions for r_s and (alpha) _e (3.8 micrometer) were based on an analysis of measured aerosol size distributions. We compare values of r_s and (alpha) _e (3.8 micrometer) derived from measurements of local visible and 10.6 micrometer aerosol extinction to values obtained from aerosol size spectra. We conclude that measurements of visible and 10.6 micrometer aerosol extinction provide useful additional information about the aerosol.

Double passage imaging through discrete random medium with large particles is under study in this paper. The effect of correlation of the illuminating and return waves, which occurs due to passage of the waves through the same randomly distributed scatterers, on the object image is analyzed. It is shown that correlation of the opposing waves can lead to significant 'enhancement' of the coherent component of the optical image and can enable us to observe the distant objects in turbid media which are indistinguishable in discrete medium under side illumination when such correlation is absent.

The results of experimental study of the influence of the counter wave correlation on a quality of double passage imaging through random medium. The measurements were carried out with the use of setup modeling artificial convective turbulence. It is established that under conditions of strong optical turbulence the quality of image of objects illuminated and viewed from the same point is improved when using the transmitting and receiving apertures of equal size. Obtained experimental results are in a qualitative agreement with the theoretical calculations.

Atmospheric transmission measurements have been made along a horizontal path through the marine atmospheric boundary layer have been made. Measurements were made in the thermal infrared region at an average spectral resolution of 0.018 micrometer in the 3 - 5 micrometer region and 0.06 micrometer in the 8 - 14 micrometer region. The Vindeby, Denmark site provided conditions of moderate wind speeds and moderate to high humidities. Both land and sea fetch conditions were encountered. Measured transmission is compared to LOWTRAN 7 predictions. Midlatitude Winter atmospheric profiles were used in conjunction with the rural aerosol model for the land fetch conditions and the Navy Maritime aerosol model for the sea fetch conditions. It was found that neither model gave the same spectral content as the measured data although the magnitude of broad band transmission was similar. It is apparent that further transmission measurements must be made in conjunction with gas, aerosol, and other environmental parameters for adequate LOWTRAN 7 model validation in the marine atmospheric boundary layer.

Deconvolution from wavefront sensing is a powerful and relatively low cost high resolution imaging technique compensating for the degradation due to atmospheric turbulence. It is based on a simultaneous recording of short exposure images and wavefront sensing data. Two different deconvolution schemes have been proposed: the self- referenced estimator originally presented by Primot et al. and the post-referenced estimator recently suggested by Roggemann et al. A theoretical study allows us to estimate the bias and signal to noise ratio of these various estimators. Self-referenced deconvolution is shown to have a good signal-to-noise ratio but the estimator is biased, while post-referenced deconvolution is bias-free but has very limited performance for bright sources. A new-self referenced deconvolution scheme accounting for the wavefront sensing noise is proposed. This leads to an optimal data reduction which should overcome the bias problems while providing good signal-to-noise ratio performances. Encouraging numerical results are presented.