Near the sea surface, atmospheric refraction and turbulence affect both IR transmission and image quality. This produces an impact on both the detection and classification/identification of targets. With the financial participation of the U.S. Office of Naval Research (ONR), Canada's Defence Research Establishment Valcartier (DREV) is developing PRIME (Propagation Resources In the Maritime Environment), a computer model aimed at describing the overall atmospheric effects on IR imagery systems in the marine surface layer. PRIME can be used as a complement to MODTRAN to compute the effective transmittance in the marine surface layer, taking into account the lens effects caused by refraction. It also provides information on image degradation caused by both refraction and turbulence. This paper reviews the refraction phenomena that take place in the surface layer and discusses their effects on target detection and identification. We then show how PRIME can benefit detection studies and image degradation simulations.

PIRAM is a french bulk model that computes, within the MSBL, vertical refractivity profiles and refractivity gradients calculated for each optical transmission window as well as for radar bands. PIRAM also computes the C_n² vertical profiles. Under unstable situations, subrefraction occurs and reduces the optical horizon. Near the horizon an intervisibility zone may be observed: any source (target) located in this zone will be seen by an EO sensor under two distinct apparent elevation angles. We developed, a few years ago, a simple ray-tracing algorithm using PIRAM refractivity outputs, to compute optical horizons and intervisibility ranges for a given atmospheric situation. More recently, we have added new capabilities to our ray-tracing program; now, it also computes the refractance parameter and the optical path between a given optical source or target and the considered EO sensor. Atmospheric turbulence effects are quantified by several parameters such as the scintillation variance, the atmospheric coherence length or the standard deviation of the angular displacement. All these parameters are computed by taking into account the exact optical path and the complete C_n² vertical profile. Our first comparisons with the values provided by the Canadian IRBLEM software lead to promising results.

FGAN-FOM participated in a maritime IR experiment carried out in Nettuno, Italy, in July 1998. IR sequences were recorded with a PtSi camera (spectral band: 1.2 to 5.9 micrometers ), positioned on the coast. The system was equipped with a rotating polarization filter. In this paper some selected IR sequences are presented showing the suppression of hot spots due to filter orientation. Moreover examples are given of intensity and spatial fluctuations caused by high turbulence.

Wave optics propagation codes are widely used to simulate the propagation of electromagnetic radiation through a turbulent medium. The basis of these codes is typically the two dimensional Fast Fourier Transform (FFT). Conventional FFTs (i.e. the standard Matlab FFT) do not use parallel processing and for large arrays, the processing time can be cumbersome. This research investigates the use of network- based parallel computing using personal computers. In particular, this study uses the Air Force Institute of Technology (AFIT) Bimodal Cluster a heterogeneous cluster of PCs connected by fast Ethernet for parallel digital signal processing using an FFT algorithm developed for use on this system. The parallel algorithms developed for the Parallel Distributed Computing Laboratory could greatly increase the computational power of current wave optics codes. The objective of this research is to implement current parallel FFT algorithms for use with wave optics propagation codes and quantify performance enhancement. With the parallel version of the FFT implemented into existing wave optics simulation code, high resolution simulations can be run in a fraction of the time currently required using conventional FFT algorithms. We present the results of implementing this parallel FFT algorithm and the enhanced performance achieved over the Matlab FFT2 function.

In the atmosphere, light scattering by molecules, particulates and aerosols causes an aureole around point- like sources. In various meteorological conditions, the radiance field coming from this aureole can be a non- negligible part of the total detected signal by large Field Of View sensors. In the framework of aureole's study, we have developed a method based on Monte Carlo calculation. The corresponding code permits to deal with monochromatic point sources in the 240 to 300 nm spectral range. The source's intensity angular dependency is axisymmetric and the atmosphere is described as a plane-parallel medium composed of a user defined constituents profiles. Systematic tests have been performed in order to evaluate the influence of the different input data on aureole results, such as ozone and SO₂ concentration values or ozone and aerosols concentration profiles. These computations will help us to define the set of meteorological parameters which need to e known accurately in order to compare computations with detected signals recorded during field experiments.

Laser illumination systems regularly employ an estimation model, referred to as the range equation, to determine system requirements to assure that sufficient signal levels exist. Satellite imaging necessarily employs an estimate of the active optical cross section (OCS) of the illuminated object. This normally assumes the object is a Lambertian reflector. The OCS is typically the least well characterized element of the range equation. In this paper, we use pointing estimates and statistical methods to obtain field estimates of satellite OCS. If an object were illuminated perfectly, a simple inversion of the range equation would provide the OCS. However, for a laser system with a narrow divergence beam, mechanical and atmospheric effects cause frequent off-center illuminations. Because of the random off-center illumination, two statistical approaches, referred to as the peak and mean methods, are described that estimate the OCS and detect non-Lambertian returns known as glints. The peak method relies on the fact that while an object may not be fully illuminated, statistically as the number of shots increases the peak return will approximate that from a full illumination. The mean method establishes that the average of multiple illuminations must statistically represent a fraction of the full illumination. Both methods provide a statistical distribution of OCS estimates and neither requires an imaging system.

In this paper we discuss the evolution of water vapor profiles over a complex valley located in Calabria Peninsula, in souther of Italy. This evolution is studied by means of water vapor profiles measurements, based on a CO₂ LIDAR/DIAl station and by numerical simulations of the water vapor atmospheric content. The simulations have been performed by the CSU-RAMS mesoscale model. As shown in a previous study, the water vapor profiles evolution over Calabria are strongly affected by the main topographic features even in calm large scale condition, when the interaction between the orography and the large scale winds weakens. Indeed, in calm or nearly calm synoptic scale conditions, a strong sea-breeze and intense mountain valley flow develop over this peninsula determining convergence and updraft over the mountain peaks. In this paper we discuss interesting features in three days of July 2000. These days were characterized by both light winds, fair weather conditions and stronger wind, sky covered conditions.

This paper presents the results of experimental studies of nonlinear amplitude and phase distortions of sounding beams with clearing of small volume of artificial fog by CO₂ laser radiation of microsecond and millisecond duration. The conclusion about clearing efficiency is drawn on the basis of measurements of the nonlinear transmission of radiation at the wavelengths 10.6 micrometers and 0.63 micrometers and photoacousitc signal immediately characterizing dissipate processes in the cleared channel. It has been shown that thermal losses at explosive drop evaporation under condition of complete clearing of fog are much less than the evaporation heat. And the clearing of artificial fog by longer pulses (1 ms) at energies, close to a threshold of explosion, but not reaching it, is of higher thermodynamic and optical efficiency, than at an explosive evaporation of the fog by microsecond pulses, because of lacking the heterogeneous secondary condensation of particles. The phase was measured using a dual beam interferometer with amplitude division. A theoretically predicted effect of the medium density increase at explosive particle evaporation has been detected for both types of pulses, which results in radiation focusing, and the lifetime of this effect is determined. The experimental data on dynamics of the refractive index in the clearing process have been obtained.

Use of fractional moments of low order, recently proposed by Consortini and Rigal for investigating probability density functions (PDF's) in laser scintillation statistics through atmospheric turbulence, is here proposed for overcoming the saturation effects of the electronics due to low dynamics acquisition systems.

We demonstrate the utility of laser illuminated imaging for clandestine night time surveillance from a simulated airborne platform at standoff ranges in excess 20 km. In order to reduce the necessary laser per pulse energy required for illumination at such long ranges, and to mitigate atmospheric turbulence effects on image resolution, we have investigated a unique multi-frame post-processing technique. It is shown that in the presence of atmospheric turbulence and coherent speckle effects, this approach can produce superior results to conventional scene flood illumination.

Image reconstruction techniques for atmospheric applications often work best with an initial estimate of the object support. This paper examines the ability of a non-imaging laser pointing system to obtain an estimate of target size and shape based on the statistics of the return signal. Fundamental limits on system pointing, such as the tracking errors, corrupt a simple raster scan that would provide gross object shape form the convolution of the far-field pattern with the target. Using techniques developed previously for the estimation of pointing performance, it is possible to distinguish between simple shapes such as bars, circles and T's based on the statistics of the received time signal. Simulated space objects, such as those illuminated during field experiments, may also be distinguished.

One-, half- and ten-micron wavelength radiation was used to study laser beam propagation through turbojet aircraft engine exhaust. A feature of the methods was that instantaneous distributions of the beam intensity were recorded during the experiment. Analysis of experimental data has shown that turbulent stream has a dramatic impact on spatial characteristics of a laser beam. For example, the averaged angle divergence for 30-mm one-micron beam becomes about ten times higher than its diffraction divergence. Results of different experiments showed that the average angle divergence of the narrow one-micron beam disturbed by the jet plume is several times less than that of the ten-micron beam which is characterized by a large diffraction divergence, and that of the half-micron beam stronger subjected to disturbances. Experiments in which the beam crossed the plume close to a nozzle at (phi) = 90 degree(s), 45 degree(s) and 10 degree(s) have shown that angular divergence increases with decreasing cross-angle, practically doubling the value when coming from the maximal angle of (phi) = 90 degree(s) to the minimal (phi) = 10 degree(s). Mathematical models have been derived, based on the experimental studies. The value of the structural characteristic in a turbulent stream is in the range of Cn₂~10^-9m^-2/3.

Phase reconstruction under the conditions of strong scintillation is limited by the presence of phase discontinuities (singularities) that accompany the intensity nulls in speckle field. The singularities are hidden for the wave front sensors of conventional adaptive optics systems. Rigorous theory has been developed in the paper to reconstruct the singular phase with allowance for the vortex nature of the phase gradient vector field. Singular functions of the wavefront slope (phase gradient) are exposed to an appropriate regularization; the divergence and rotor of the regularized phase gradient are determined. It has been established that the divergence of the phase gradients contains singularities of the same type as gradient and, therefore, proper phase reconstruction can be executed using the regularized slopes only. Topological transformation of the wave front in the process of appearance and annihilation of the discontinuities is manifested as changing in relief of the regularized rotor, the relief characteristics correspond to the discontinuity parameters. With the use of Pompeiu integral the phase gradient is presented as a sum of divergent and rotor parts added by the term depending on the receiving aperture boundary conditions. The obtained wavefront slopes are unwrapped into the singular phase ignoring the noise and singularities of the phase gradient divergence and rotor. Derived analytical expression connecting the phase reconstruction from the measurements of wavefront slopes. Examples of such a reconstruction are given. The developed approach can be base for creating the discrete phase reconstruction algorithms.

Adaptive optics is limited toady to correction of turbulence inside a cone extending from the reference source to the telescope aperture. Even when the reference source is a natural star, the measured- and corrected- cylinder does not allow observation of most extended astronomical objects. Hence the search is on for a method to measure turbulence in a conical volume which opens up from the telescope upwards. Various schemes were proposed to widen the field of view by using more artificial or natural guide stars, and by processing the measured data in different ways. It has been shown experimentally that the existence of three natural stars around the rim of the required cone is sufficient. Using multiple laser guide stars, schemes varying from separation of measured volumes and stitching of their edges, to integrated methods were suggested. It was also proposed to infer the turbulence form the shape of the beams as they propagate up in the atmosphere. Structured light above the turbulence is another option that was raised. Such a grid is created by interference of laser beams or by interference of powerful radio beams that break down the air into visible plasma. It can be shown that these fringes, either from a laser of from radio, can be analyzed optically, reducing the power requirements significantly. This field of atmospheric tomography is likely to produce soon corrected images of extended astronomical objects. In addition, being able to separate the contribution of the atmospheric layers, we will acquire better knowledge of atmospheric turbulence.

The idea of using liquid crystal as adaptive optics components has been proposed by several authors. In recent years a vigorous research effort has been carried out, and it si still flourishing, in several countries. Mainly the research and experimental work has been concentrated in US, U.K. and Russia. There are several reasons why liquid crystal may represent a valid alternative to the traditional deformable mirror technology that has been used for the past two decades or so. The main attractiveness of LC resides in the cost. Current deformable mirror technology has a range of price going from 2K to 15K per channel. LC technology promises to be at least a couple of orders of magnitude cheaper. Other reasons are connected with reliability, low power consumption and with a huge technological momentum based on a wide variety of industrial applications. IN this paper I present some of the experimental results of a 5 years, on going, research effort at the Air Force Research Lab. Most of the work has been on the development of suitable devices with extremely high optical quality, individually addressable pixels, fast switching time. The bulk of the work has been concentrated in the arena of the untwisted nematic material. However new devices are now under development using dual-frequency nematic material and high tilt angle ferroelectric material.

We present here results of laboratory experiments using a dual frequency nematic liquid crystal. These devices have the advantage of low cost, low power consumption, and compact size. Possible applications of the devices are astronomical adaptive optics, laser beam control, laser cavity mode control, and real time holography.

We present, in this article, an analytical study of the phase reconstruction error for MCAO systems. Two approaches are considered; a classical estimator based on a Least Square (LS) approach and a Maximum A Posteriori (MAP) estimator which uses the prior knowledge we have both on the measurement noise and the turbulence volume statistics. The effects of these modes both on the phase reconstruction and on the phase correction error are studied and quantified. T is shown that, using a MAP approach, a large part of the unseen modes can be extrapolated using correlations between unseen eigenmode coefficients and well-measured coefficients. We observe that the use of a MAP estimator allows a significant gain in terms of correction quality in the whole field of view.

Phase correction of a plane wave, propagating through a turbulent layer, is considered. The required adaptive corrector element size and the system bandwidth were found by numerical simulation. These requirements were determined to be the same as for weak intensity scintillation approximation.

Fourier telescopy is not used for reconstructing the remote rough objects images in turbulent atmosphere by means of the illumination of the objects by arrays of laser emitters. In result the object surface is modulated by sinusoidal patterns with various periods and orientations, forming interfering laser beams, and the energy of scattered light contains different space Fourier components of object image intensity distribution. These components critically distorted by multipliers in the form of direction diagrams of laser emitters int eh case of strongly heterogeneous atmospheric turbulence when laser emitters sizes exceeds correlation radius of phase distortions of laser beams. For this case the method of atmospheric turbulence compensation, based on the combining the operation of division by direction diagrams amplitudes of distorted Fourier components with famous compensation method, based on the phase closure algorithm, is proposed. Because of considerable broadening of laser beams in a strongly heterogeneous atmosphere, only a small portion of light falls on the object and, therefore, the probability of detecting such weak signals from the object in the presence of additive noises is small. The surface roughness scattering also decreases the detection probability. The methods of increasing the probability and of calculating it in dependence on surface roughness parameters and the parameters of phase distortions caused by the turbulent atmosphere are proposed.

The atmospheric turbulence limits the performance of high resolution instruments. Adaptive Optics (AO) is a real time technique which compensates for the turbulent phase using a Deformable Mirror (DM) located in the instrument pupil. When a significant amount of turbulence is far away from the pupil, the AO performance is however limited by anisoplanatism and scintillation effects. For astronomical applications anisoplanatism effect is dominant, and can be corrected with several DMs conjugated with different turbulent layers ahead of the pupil. Recent studies have shown that such a concept of Multi-Conjugate Adaptive Optics (MCAO) can provide high resolution images in a large field of view. The goal of this study is to show that, in more severe turbulence conditions encountered in endoatmospheric applications, MCAO can also correct for scintillation effects, whereas classical AO is ineffective in this case. It appears that outside the weak perturbation turbulence domain, the perturbations of the nearest turbulent layers have to be corrected first in order to counteract the turbulence in the whole volume. This implies the use of relay optics for each turbulent layer in practical MCAO system designs. A simplified MCAO configuration is considered to study the number of DMs required to obtain a significant reduction of the scintillation.

The ESA's SMOS (Soil Moisture and Ocean Salinity) Earth Explorer Opportunity Mission will be launched by 2005. Its baseline payload is a microwave L-band (21 cm, 1.4 GHz) 2D interferometric radiometer, Y shaped, with three arms 4.5 m long. This frequency allows the measurement of brightness temperature (Tb) under the best conditions to retrieve soil moisture and sea surface salinity (SSS). Unlike other oceanographic variables, until now it has not been possible to measure salinity from space. However, large ocean areas lack significant salinity measurements. The 2D interferometer will measure Tb at large and different incidence angles, for two polarizations. It is possible to obtain SSS from L-band passive microwave measurements if the other factors influencing Tb (SST, surface roughness, foam, sun glint, rain, ionospheric effects and galactic/cosmic background radiation) can be accounted for. Since the radiometric sensitivity is low, SSS cannot be recovered to the required accuracy from a single measurement as the error is about 1-2 psu. If the errors contributing to the uncertainty in Tb are random, averaging the independent data and views along the track, and considering a 200 km square, allow the error to be reduced to 0.1-0.2 pus, assuming all ancillary errors are budgeted.

It is usual in the problems of adaptive optics that the phase of optical wave is expanded into an orthogonal basis for a convenience of analysis. Zernike polynomials are popular for this expansion because they have simple analytical expression and their first modes coincide with the classical aberrations. However, if the power spectrum of distortions is known, its egienfunctions named Karhunen- Loeve-Obukhov (KLO) functions are a natural choice for such basis. Authors have derived the KLO functions represented through the Zernike polynomials and developed the effective method to expand distorted phase through the orthonormal bases. Usually, in investigations the Kolmogorov model of turbulence is used. However, the nature experimental data do not agree always with this model. But the range of validity for the Kolmogorov model can be extended introducing in it the outer scale of turbulence. Authors developed the algorithm to obtain analytically the KLO functions allowing for the outer scale of turbulence (von Karman model). The results of numerical experiment for representation of a random phase in different bases for various atmospheric conditions are presented.

High data rate communication system at millimeter and optical frequencies used for both mobile and satellite links may be influenced by pulse spreading due to scattering by turbulent inhomogeneities and hydrometeors in the troposphere and by the electron concentration in the ionosphere. Theoretical analysis of pulsed signal propagation is usually based on the analysis of the two- frequency mutual coherence function. In this work we propose a new reference wave method which enable us to present an analytic solution for the equation of the two frequency mutual coherence function. It his shown that when approximating the transverse structure function of the medium by a quadratic form, the solution reduces to the exact result derived previously. Extension to the more general types of media and pulsed signal propagation is considered.

This paper discussed a simulation fo the imaging of a space- based object through cirrus clouds. The wavefront reflected by the object is propagated to the top of the cloud using Huygens-Fresnel propagation theory. At the top of the cloud, the wavefront is divided into an array of input rays, which are in turn transmitted through the cloud model using the CIRIS-C software. At the bottom of the cloud, the output ray distribution is used to reconstruct a wavefront that continues propagating to the ground receiver. Images of the object as seen through cirrus clouds with different optical depths are compared to a diffraction-limited image. Turbulence effects from the atmospheric propagation are not included.

A novel configuration for a reference-beam, continuous-wave, heterodyne low-power radar prototype is presented. It measures both magnitude and sign of the radial component of the displacement velocity. The basic set-up includes a low power (~10 mW) commercial HeNe laser, a beam-splitter, an acousto-optic modulator, and a two-lens system that both focuses the transmitted beam on the target surface and collects the scattered light. Both the reference beam and the radiation collected are focused onto a Si avalanche photo-detector. The self-aligned configuration of the receiver makes possible, theoretically, to perform optimal mixing between the received scattered radiation and the reference beam. The resulting electrical signal is fed to a transimpedance amplifier and displayed on a spectrum analyzer. Laboratory experiments employing as a target the rim of a 50 cm-diameter rotating wheel placed at several distances have been performed. Results concerning detected signal-to-noise ratio, detected- signal spectral width, accuracy of the radial component of the velocity under measurement, system working range, and system tolerance in focus-adjustment distance will be presented and discussed. Compared to a previous homodyne prototype presented by the authors, the present system shows a shorter working range (~12 m compared to nearly ~16 m in the homodyne prototype). We attribute this smaller range to the additional losses in the acousto-optic modulator.

A field experiment to investigate an imaging technique based on reflection tomography has been performed at the Air Force Research Laboratory. The experiment, called HILT (Heterodyne Imaging Laser Testbed), involves the illumination of objects with short pulses of laser radiation, and the measurement of the temporal characteristics of return pulses scattered by the object. Return pulses, referred to as projections, provide range resolved information characteristic of the object surface shape and viewing angle. By obtaining multiple projections at different viewing angles, a tomographic reconstruction of the object's surface can be obtained. Recent testing has produced images of targets at a range of 990 meters using 1.4 ns pulses from a 10.6 (mu) CO₂ laser. A heterodyne detection technique was utilized to record the weak return signals. The results obtained from this system are believed to be the first LADAR range resolved reflection tomographic images of diffuse objects in a field environment, and the first use of a heterodyne detection system for LADAR reflection tomography. A description of the system is provided and experimental results are presented.

The National Aeronautics and Space Administration's (NASA's) Marshall Space Flight Center (MSFC) has ben developing and testing video-based sensors for automated spacecraft guidance for several years. The video sensor currently under development is to have a tracking rate of 50 Hz while delivering full 3-dimensional relative information (X,Y,Z, Pitch, Yaw, and Roll). Prior systems have been developed and tested in both open- loop and closed-loop simulations. The prototype Video Guidance Sensor (VGS) was built for a flight experiment and performed well on two separate Space Shuttle flights. The VGS uses two wavelengths of light to illuminate a target that has a pattern of filtered retro-reflectors. The filters pass only one wavelength of light and absorb the other. Two fast, successive pictures are taken of the target, each picture illuminated by a different wavelength. When the background picture is subtracted from the foreground,a high signal to noise ratio is achieved, and the target spots are easy to track. The next generation VGS will be using a CMOS imaging chip for higher-speed target tracking and a Texas Instruments DSP for higher-speed image processing. The system is being designed to have lower weight and power requirements than the previous generation, and it will be suitable for other applications.

The traffic density on the roads is increasing every day. Therefore, the development of equipment contributing to higher traffic safety deserves particular attention. The perceptibility of automotive lighting and light signals under poor visibility conditions is one of the safety aspects involved. The planned system has the task to ensure that the rear lights are visible to the driver of the vehicle driving behind. Spray, however, as being whirled up by the tires at rain-covered roads also affects the visibility very strongly. Depending on moisture on the road and driving speed, this spray is dragged like a flag of more than 20m behind the vehicle. That is why a measuring principle employing spot-like scanning of only a small measuring volume would not be suitable for this purpose. Thus, a method is needed that can detect the turbidity in a long-extending measuring volume.

Hardware development for remote sensing costs a lot of time and money. A virtual instrument based on software modules can be developed faster and is flexible for changes. Such a virtual instrument can be used to optimize the sensor in advance. Adaptation of the different user aspects is possible. Finally, tests of the signal quality with existing sensors can be used to understand misalignments and internal sensor problems.

The effect of aerosol microstructure on the estimation error of wind velocity measured by cw CO₂ Doppler lidar is analyzed in the paper. Based on the numerical simulation of a lidar return is has been shown that aerosol particles, due to their difference in size, make essentially different contribution to a measured lidar return power that leads to deviation of the lidar return statistics form the Gaussian one and results in the increase of the lidar estimation error of wind velocity, especially with small lidar probe volumes. With the increase of lidar probe volume the error of wind velocity estimation decreases. The values of the error variance (sigma) _e² calculated with the use of developed by authors algorithm of simulation of lidar return are in agreement with the experimental values of (sigma) _e² obtained under different longitudinal size of probe volume.

The objective of Russian space project Tectonica-A is creation of automated lidar for studying aerosol anomalies, mainly, in the low atmosphere. By such anomaly is meant heterogeneity both of the aerosol concentration and its microphysical properties. Therefore, this project assumes that a small spacecraft will be equipped with multifrequency (wavelength range from 0.35 to 1.06 micrometers ) lidar. The lidar is aimed to investigate the aerosol fields formed during tectonic (including seismic) processes and catastrophes. Using the lidar high potentiality providing detection of local emissions in the low atmosphere. The project execution is in the stage A-B (draft design). The lidar installation is oriented to the platform MKA-200. Simultaneously with the lidar creation, the methods are developed on the basis of its mini-prototype. In October, 1999, the spatial, temporal boundaries of aerosol anomalies near Rostov-on-Don break (West Caucasus) were investigated during artificial and natural earthquakes. Simultaneously, gases show (radon, first of all) and fluctuations of atmospheric electric fields were estimated. Some correlation between aerosol, gas, and electric anomalies has been found, which underlines once again a priority of aerosol from the standpoint of remote optical monitoring of tectonic processes. Based on the obtained results, the methodological aspects of space laser sensing of aerosol anomalies are discussed in the report. The requirements to the lidar Tectonica-A and space experiments conducting are refined as well.

An examination on how to divide the optical power between the local oscillator radiation and radiation sent towards the target for maximized signal-to-noise ratio, has been carried out. From the self-mixing equations, by substituting electrical fields with optical power and including relevant noise sources, an expression for the signal-to-noise ratio (SNR) for the photodiode has been obtained. From this equation the conditions for maximum SNR has been derived.