We present the statistical results of the optical-turbulence profiles at the Observatorio del Roque de los Muchachos over a period of six consecutive months. The data were obtained using the generalized SCIDAR technique at the 1m Jacobous Kaptein Telescope. In general, most of the turbulence is concentrated close to the observatory level (2400 m above sea level) with no more than two turbulent layers at higher altitudes. The temporal evolution along six consecutive months indicates that the turbulence is concentrated at lower altitude layers during winter. Large isoplanatic angles are also reached in winter compared to the values in spring. For the turbulence profiles measured in February, March and April we have analized the statistical position of demorfable mirrors in an ideal Multi-Conjugate Adaptive Optics system (with two or three deformable mirrors) and the improvements in the isoplanatic angles.

Research activities at the German Aerospace Center (DLR) concerning optical free-space communications have focussed on coherent communication systems for inter-satellite link (ISL) applications for a long time. Under DLR contract Tesat Spacecom has developed the DLR-LCT (laser communications terminal) which relies on coherent technology. This terminal will be verified in space as secondary payload onboard the earth observation satellite TerraSAR-X, to be launched in 2006. In a first step, downlink experiments will be carried out. The DLR Institute of Communications and Navigation is involved in this ambitious project by assessing the feasibility of the downlink experiment through atmospheric turbulence and by conducting channel measurements. An initial feasibility study shall theoretically investigate the influence of atmospheric turbulence on coherent optical transmission and assess the success probabilities of the particular experiment with regard to the specific ground station conditions. Since theory is always based on arbitrary assumptions on the composition and structure of the atmosphere, measurements at the specific ground station shall be carried out. Measurement results shall enable a refinement of disturbance models in order to predict the condition during the downlink experiments. Relevant atmospheric parameters, such as scintillations, phase-front distortions, atmospheric seeing, angle-of-arrival fluctuations, attenuation, Cn2- and wind profiles will have to be recorded. To carry out these measurements, DLR will develop an "Atmospheric Turbulence Monitor" (ATM). The ATM mainly consists of a 16-inch telescope and a number of instruments for various measurements. These instruments are based on astronomical devices for use with stars, however have to be modified to be suited for measurements with close objects such as LEO or GEO satellites. The ATM will as well comprise a tracking system, that allows for measurements with LEO satellites such as TerraSAR-X. This paper presents the outline of the DLR atmospheric turbulence monitoring measurement campaign and describes the preliminary design of the "Atmospheric Turbulence Monitor".

In this work we present results of turbulent effects on the propagation of laser beam through turbulent jets. Several jets were examined, using CFD computations and then the density fluctuations were calculated using several models. The effect of the jet on imaging and on the optical resolution is presented.

There are well-known factors which provoke image distortions during the propagation of light in the atmosphere. We used the solution of the linearized unsteady 3D Navier-Stokes equations in order to model turbulent fluctuations of the refractive index.This fact may be used for the construction of point spread function (PSF) for turbulent distortions. It was found that there exists a range of natural frequency for turbulent fluctuations. Structure function and correlation function were determined as well as their temporal evolution. A large amount of numerical results is presented.

In the theory of light wave propagation in turbulence the refractive index structure constant is taken as a measure of the strength of the turbulence. A lot of efforts have been made to measure this parameter at different regions and seasons. However, this parameter doesn't indicate the fluctuation strength directly and for a non-Kolmogorov turbulence its unit is not fixed. Such a unit-varying parameter shouldn't be a suitable measure of a physical quantity. It is found that the strength of an optical turbulence is the variance of the refractive index. In the inertial subrange the structure function of refractive index generally depends on both the variance and the turbulence outer scale. The light propagation effects could be interpreted more clearly through these turbulence parameters. Instead of the structure constant the variance, the outer scale, the scaling power and the inner scale should be measured in the study of light propagation.

Space Fourier components of the intensity distribution in the images of remote objects, obtained in the process of Fourier-telescopy imaging in strongly inhomogeneous atmosphere, have large amplitude and phase distortions, which lead to strong distortions of these images. Applying the well-known phase closure algorithm can eliminate phase distortions. In order to compensate amplitude distortions, it was proposed to divide the space Fourier components by the modules of direction diagrams of the laser beams illuminating the object under study. However, if these beams propagate in strongly inhomogeneous atmosphere, their wave fronts become considerably disturbed. This leads to strong fluctuations in the direction diagrams of the laser beams and can result in very small values of their modules. Then, in the case of high additive noise, the probability of detecting the object can become very low. To increase this probability, it is proposed to use a matrix of laser sources in each laser transmitter. As a result, laser transmitters will form beams with weakly fluctuating direction diagrams, which will ensure a high probability of the object detection. A compact symmetric device for Fourier-telescopy imaging using a matrix of laser sources in each laser transmitter is presented. It is based on a receiving-transmitting aperture that contains a number of similar receiving sections and a transmitting aperture consisting of two orthogonal linear arrays of laser transmitters. The transmitting aperture is placed in the intervals between the receiving sections. Parameters of the proposed design (resolution and contrast in the speckle pattern of a Fourier-telescopic image and the total dimensions and configurations of the transmitting and receiving apertures) are compared to those typical for the existing design the Geo light imaging national testbed (GLINT). The proposed design ensures formation of a high-quality undistorted image of an object at a large distance (up to 40 000 km) with high resolution (about 0.4 m) along mutually perpendicular directions in the image plane.

In the spectral band typically lower than 290 nm, solar radiation doesn't reach the Earth surface due to ozone absorption. UV radiation of artificial sources can then be recorded with a high contrast, by day or night. Because of this property, this spectral domain is called "Solar Blind". Furthermore, many UV detectors work at ambient temperature and have high sensitivity. One can envisage to use UV sources as beacons, particularly to find one's way around in case of haze. Light scattering by atmospheric particulates and molecules gives rise to an aureole surrounding the source image which tends to reduce the contrast of the source with respect to the background. However, scattering phase functions of the haze droplets present a very important forward peak and spreading of detected signal is not as important as in case of a clear atmosphere where Rayleigh scattering predominates. Moreover, the range of UV radiation propagation is limited by the high ozone absorption cross section. All these physical phenomena have to be taken into account in order to evaluate UV radiation potential interest for landing aid under low visibility conditions. We present here different results on characterization of UV runway light, propagation of UV radiation in the atmosphere and on the use of different kinds of sensors that are necessary to assess this point.

Climate diagnostic studies combine data from different sources (radiosoundes, satellites, meteorological masts, etc) and meteorological models to predict the climate conditions and evolution at a particular site or globally. Products from climate diagnosis are archived in long-term databases that may constitute a useful tool for site characterization. However, a rigorous control of data quality, analysis and cross-comparison to in-situ meteorological measurements need to be performed before the method becomes extensively used for site characterization. We present a statistical analysis for wind vertical profiles, an important parameter for site characterization, using data from a climate diagnostic archive and in-situ measurements (ground-level and balloon data).

A series of airborne imaging experiments have been conducted on the island of Maui and at North Oscura Peak in New Mexico. Two platform altitudes were considered 3000 meters and 600 meters, both with a slant range to the target up to 10000 meters. The airborne imaging platform was a Twin Otter aircraft, which circled ground target sites. The second was a fixed platform on a mountain peak overlooking a valley 600 meters below. The experiments were performed during the day using solar illuminated target buildings. Imaging system performance predictions were calculated using standard atmospheric turbulence models, and aircraft boundary layer models. Several different measurement approaches were then used to estimate the actual system performance, and make comparisons with the calculations.

A new generation of instrument using a launching laser is been developed to correct the atmospheric image blurring and to establish optical communication with space. Then, light pollution generated by laser will be a serious operational problem in next years. This laser could affect astronomical works of adjacent telescopes when the laser lay across the field of view of the observing telescope, this is a kind of light pollution. This could be avoided with an adequate operational politic to detect possible interference between the laser and the astronomical telescopes. In this paper is analysed the mathematical probability of a cross-event happen.

The performance of electro-optical systems can be substantially affected by aerosol particles that scatter and absorb electromagnetic radiation. A few years ago, an empirical model was developed describing the aerosol size distributions in the Mediterranean coastal atmosphere near Toulon (France). This model has been coupled with Mie theory to yield the code MEDEX (MEDiterranean EXtinction) for the aerosol extinction. This contribution deals with the evaluation of MEDEX for aerosol data recorded near the Black Sea coast. For this site, MEDEX correctly predicts the aerosol extinction as function of wavelength, albeit with minor discrepancies below one micron. These differences are attributed to the uncertainty in predicting the concentrations of submicron particles. The comparison shows that MEDEX may be more generally applicable than to the Toulon area.

The application of long-range infrared observation systems is challenging, especially with the currently available high spatial resolution infrared camera systems with resolutions comparable with their visual counterparts. As a result of these developments, the obtained infrared images are no longer limited by the quality of system but by atmospheric effects instead. For instance, atmospheric transmission losses and path radiance reduce the contrast of objects in the background and optical turbulence limits the spatial resolution in the images. Furthermore, severe image distortion can occur due to atmospheric refraction, which limits the detection and identification of objects at larger range. EOSTAR is a computer program under development to estimate these atmospheric effects using standard meteorological parameters and the properties of the sensor. Tools are provided to design targets and to calculate their infrared signature as a function of range, aspect angle, and weather condition. Possible applications of EOSTAR include mission planning, sensor evaluation and selection, and education. The user interface of EOSTAR is fully mouse-controlled, and the code runs on a standard Windows-based PC. Many features of EOSTAR execute almost instantaneous, which results in a user friendly code. Its modular setup allows its configuration to specific user needs and provides a flexible output structure.

An attempt is made to evaluate three different models correlating sea slope variance with wind speed, which are a crucial component of the statistical approach to calculating the sea surface BRDF (Bidirectional Reflectance Distribution Function). The models are those of Cox & Munk, Wu and Mermelstein et al. This is done with the help of publicly available upwelling radiance data taken at the COVE rigid costal platform with a scanning spectral photometer at wavelengths around 444 nm, 501 nm, 677 nm and 864 nm for a wide range of azimuth and elevation angles. The three sea slop variance models are compared with variances inferred from the data by inverting a BRDF models that includes facet hiding and shadowing as well as facet projection weighing. The validity of the models is discussed in the context of varying wind speed and direction. Limitations when dealing with near-horizon BRDF modeling using these statistical models are discussed along with potential improvements.

For automatic target detection in maritime environments using optical imaging systems, it may be critical to have a valid a priori knowledge of the elevation of the border between sea and sky backgrounds. In operational conditions, this border, called the horizon, is predicted to move up and down depending on the refractivity conditions. Predictions of horizon elevation can be made from ray-tracing using bulk estimates of refractivity profiles based on the Monin-Obhukov similarity theory. In this paper, predictions of horizon elevations obtained with IRBLEM, a DRDC Valcartier computer model, are compared with observations made in the North Sea in two different meteorological environments: in Katwijk (The Netherlands, October 1993) and Sylt (Germany, June 1992). Good agreement is shown between observations and model calculations. The expected variation of horizon elevation with changing refractivity conditions is discussed.

Two field trials have been performed on the west coast of Norway to study propagation effects (in particular refraction and turbulence effects) close to the sea surface. A complete meteorological station and a temperature profile buoy were used to characterize the propagation environment, while sensor height was logged continuously. Land and ship mounted sources were studied using infrared (midwave IR and longwave IR FPAs) and visual cameras at about 4 m above mean sea level (MSL). The land-based sources were mounted about 2-13 m above MSL, while the ship mounted sources were 10 m above sea level. Both sub- and superrefractive conditions were studied during the trials. The sensors were mounted on a programmable motion controller, which allowed extraction of absolute apparent pitch angles of the imaged sources. Apparent horizon distances have been determined for the ship sources, while mirror plane positions and apparent elevation (pitch) angles have been determined for the land sources. In addition, normalized variance of intensity (scintillation index) has been calculated for a number of cases. The scintillation index can easily be converted to refractive index structure parameters (C_n²), one of the key parameters characterizing optical turbulence. Measurement results are compared to results from the IR Boundary Layer Effects Model (IRBLEM *). *) IRBLEM is proprietory to the Department for National Defence of Canada as represented by DRDC-Valcartier

When images are made of distant targets over long atmospheric paths, local variations of the refractive index of the air may lead to severe optical distortions and aberrations. An extreme example of such variations is the stream of hot exhaust gases on ships. The performance of infrared and optical sensors, mounted on elevated platforms on board, is strongly reduced when targets are observed through this highly inhomogeneous medium. By means of a high precision ray-tracing model, the effects are quantitatively predicted and compared with measured data. In another application the model is used to predict distortions such as caused by atmospheric layers of variable wind speed, pressure and temperature. In this kind of atmospheric conditions, aircrafts tend to make irregular motions, which can be dangerous in landing operations. The distortions provide information on the presence of this kind of dangerous conditions: the magnitude and the location of the so-called "air pockets". The installation of a suitable distortion measurement system together with an advanced meteorological station (including LIDAR) provides great opportunities to act as a warning device in the neighbourhood of airports. Sample calculations are presented as illustration of the prediction method.

On September 1, 2003, Nukove Scientific Consulting, together with partner New Mexico State University, began work on a Phase 1 Small Business Technology TRansfer (STTR) grant from the United States Air Force Office of Scientific Research (AFOSR). The purpose of the grant was to show the feasibility of taking Nukove's pointing estimation technique from a post-processing tool for estimation of laser system characteristics to a real-time tool usable in the field. Nukove's techniques for pointing, shape, and OCS estimation do not require an imaging sensor nor a target board, thus estimates may be made very quickly. To prove feasibility, Nukove developed an analysis tool RHINO (Real-time Histogram Interpretation of Numerical Observations) and successfully demonstrated the emulation of real-time, frame-by-frame estimation of laser system characteristics, with data streamed into the tool and the estimates displayed as they are made. The eventual objective will be to use the frame-by-frame estimates to allow for feedback to a fielded system. Closely associated with this, NMSU developed a laboratory testbed to illuminate test objects, collect the received photons, and stream the data into RHINO. The two coupled efforts clearly demonstrate the feasibility of real-time pointing control of a laser system.

Adaptive combining of experimentally obtained heterodyned pulse position modulated (PPM) signals with pulse-to-pulse coherence, in the presence of simulated spatial distortions resembling atmospheric turbulence, is demonstrated. The adaptively combined PPM signals are phased up via an LMS algorithm suitably optimized to operate with PPM in the presence of additive shot-noise. A convergence analysis of the algorithm is presented, and results with both computer simulated and experimentally obtained PPM signals are analyzed.

The well defined spin and orbital angular momentum states of photons offers a practical realization of quantum digits and a means of secure single-photon optical communication. The orbital angular momentum is associated with the spatial distribution of the wavefunction and the number of orbital angular momentum eigenstates is unlimited, giving the possibility of arbitrary base-N digits. In this paper, free-space optical communications using angular momentum states of single photons is investigated and in particular the effect of atmospheric turbulence on the angular momentum of the photons is modelled. The refractive index fluctuations in the atmosphere perturb the complex amplitude of a propagating beam so that the photons that were launched in an eigenstate of orbital angular momentum are no longer guaranteed to be in the original eigenstate after propagation. By considering the resulting wave as a superposition of angular momentum states, the probability of obtaining correct or incorrect measurements of the transmitted digit is calculated. The effect on a free-space optical communication using orbital angular momentum and the use of adaptive optics is discussed. The information capacity per photon is quantified and compared to that using polarization states for binary digits.

Atmospheric refraction bends optical beams and lets objects appear in positions they not really are. This phenomenon is very important in astronomy. However, astronomical refraction formulas can only be used if the light source is at a very far distance from the observer. Then, atmospheric refraction mainly depends on meteorological conditions on ground. In case of optical free-space communications, the distances are comparatively short and well-known formulas for astronomical refraction are no longer sufficient. An exact knowledge of the structure of the whole atmosphere is required to assess refraction in this case. Due to the complexity of the atmosphere, analytical solutions are not possible. Hence, a numerical simulation model based on spherical symmetry, atmospheric shell modeling and standard atmosphere models was used instead. Three different categories of refraction were examined: 1) the observer is situated on ground and the object is at an altitude of at least 25 km, 2) both object and observer are below 25 km and the link path is mainly vertical and 3) both object and observer are in the stratosphere and ray paths are mainly horizontal. The results presented in this work are useful for applications like laser beam pointing and satellite tracking, UAV (Unmanned Aerial Vehicle) and HAP (High Altitude Platform) downlinks or long-haul cross-links through the atmosphere, e.g. HAP-HAP or UAV-satellite.

The European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, after reaching its final position in the geostationary orbit. Since then, ESA and the Instituto de Astrofísica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The main purpose of such experimental campaigns is to characterise and model the optical links performance from the propagation and communication points of view, under different atmospheric turbulence conditions. The statistical results presented in this paper cover the uplink and downlink performance, including scintillation, fade and surge statistics, intensity distributions and spectral analysis. The effect of using different number of transmitted beams and different divergences is also considered. Additionally, the results are correlated with the atmospheric turbulence conditions, in terms of profiles of the index of refraction structure constant, isoplanatic angle, seeing and wind profiles, measured in most of the cases simultaneously with the laser communication experiments

Anisoplanatism refers to the angular decorrelation of the waves. We are concerned with the impact of anisoplanatism in the accuracy of Shack-Hartmann wavefront sensing using extended sources possibly larger than the isoplanatic patch. We have used an end-to-end numerical simulation linking propagation through turbulence and Shack-Hartmann wavefront sensing. For an extended source, the wavefront error is found to be larger than the sum of the individual errors due to scintillation and phase effects, as a consequence of the correlation between the two. An analytical formulation is proposed to predict the Shack-Hartmann wavefront measurement error on a double star. We are albe to successfully estimate the wavefront slope error despite the negligible correlation between scintillation and phase effects.

Earlier research reported a comparison of the wavefronts recorded simultaneously by a Shack-Hartmann and a Distorted Grating Wavefront Sensor (DGWFS). In this paper we present the results of a continuation of this earlier work where we have now closed an adaptive optics loop under simulated propagation conditions using the Advanced Concept Laboratory (ACL) at Lincoln Laboratory. For these measurements only one wavefront sensor controlled the deformable mirror at a time. To make direct comparisons between the sensors we took advantage of the ACL's ability to exactly replicate a time varying propagation simulation. Time varying and static comparisons of the two sensors controlling the ACL adaptive system under conditions that ranged from a benign path, D/r₀ = 2, to a propagation condition with significant scintillation, D/r₀ =9, will be shown using the corrected far field spot as a measure of performance. The paper includes a description of the DGWFS used for these tests and describes the procedure used to align and calibrate the sensor.

The main aim of this paper is to demonstrate potential materials and fibres for 589 nm and 569 nm high-power laser sources, which can be developed by using rare-earth ions (Eu³⁺, Sm³⁺) as dopants in glass fibres. In this paper, we also discuss the advantages of using high-power excitation sources: for example the NIR semiconductors at 980 nm and 800 nm and high-power Yb-fibre laser based systems for high-power upconverted lasers. In this context the spectroscopic properties of Eu³⁺-Er³⁺ and Sm³⁺- based glass and fibre systems are also discussed in bulk oxide glasses and in fibre geometries for designing lasers at 589 nm and 569 nm for adaptive optics.

We present in this paper the status and preliminary results of an experiment to create a calibrated profile of turbulent layers, both in position and strength, using a calibrated hot air turbulence generator (turbulator), in so-called multi-pass mode. C_n² profile is retrieved using the SLODAR technique. First results, with only one layer, show the validity of our approach, and give us confidence that a multiple pass scheme is doable and measurable with a few modifications of the current set-up.

We have developed a Shack-Hartmann sensor simulation, moving the complex amplitude of the electromagnetic field using Fast Fourier Transforms. The Shack-Hartmann sensor takes as input the atmospheric wavefront frames generated by the Roddier algorithm, and provides, as output, the subpupil images. The centroids and the wavefront phase maps are computed combining GPU and CPU. The algorithms used on the GPU are written using nVidia language C for Graphics (Cg) and run on a CineFx graphical engine. Such a graphical engine provides a computational power several times greater than usual CPU-FPU combination, with a reduced cost. Any algorithm implemented on these engines must be previously adapted from their original form to fit the pipeline capabilities. To achieve an optimal performance, we compare the results with the same algorithm implemented on GPU and CPU. We present here, for the first time, preliminary results on wavefront phase recovery using GPU. We have chose a zonal algorithm that fits better on the stream paradigm of the GPU's. The result shows a 10x speedup in the GPU centroid algorithm implementation and a 2x speedup in the phase recovery one compared with the same on CPU.

A key element in any adaptive optics system is the deformable mirror used to introduce the conjugate correction. In this paper we will present the results from characterizing a pair of custom 20 element, 38 mm diameter, bimorph deformable mirrors that were specifically designed to provide unusually large stroke to allow correction of significant focus and astigmatism terms in a human fundus adaptive optics imager. Data on the measured correction capability and inherent hysteresis of the mirror shown that the mirrors have 40 μm waves of defocus correction and 20 μm waves of astigmatism correction at 760 nm, with a typical hysteresis at full deflection of 15%. This technology is patented under Patent # 6,331,059 B1.

In the planning stage of extremely large telescopes, site testing and study of high performance adaptive optics systems plays very important roles. Site testing is a very time consuming task, therefore, we have built a fully automatic device - the CUTE SCIDAR instrument with a user-friendly interface and real time processing. This instrument is already in operation and now has been installed in the Jacobus Kapteyn Telescope of Roque de los Muchachos Observatory at La Palma for periodical turbulence profiling. A second version with an additional phase sensor bench contains a motorized field stop, a field lens, a collimator lens, and a Shack-Hartmann sensor. This instrument measures the turbulence from both amplitude and phase variations of the same distorted wave at high frequency bandwidth, with a high resolution and dynamic range. On the one hand, this will solve the calibration problem between different turbulence sensors. On the other hand, it allows investigating the performance of multi-conjugated wavefront sensing using real time information from SCIDAR data and proving validity of the near field assumption. From preliminary Shack-Hartmann measurements we conclude that the instrument should be flexible to change optical layout and detection parameters according to the turbulence conditions. Therefore, the phase sensor branch includes automatically controlled moveable devices, and in the future, fast communication facilities between control computers of both SCIDAR and wavefront sensing are previewed. In this paper, we will present our objectives of building such an instrument, give a detailed state of art design, and considerate the preparation of first observational campaigns, that are the first scientific tasks to do.

In previous work we demonstrated a nematic liquid crystal MEMS adaptive optics system for observation of low earth orbit satellites. However the closed loop bandwidth was limited to 40 Hz due to latency in the interface electronics between the control computer and the device driver. This bandwidth is marginal for compensation of atmospheric turbulence effects, where the Greenwood frequency is often in excess of 100 Hz. Recently the interface has been redesigned and as a result we have been able to nearly double the bandwidth. In this paper we describe laboratory experiments with the faster system.

There are exists several sources of errors in an adaptive optics system. The main source associated with the violation of the optical reciprocity principle in algorithm of phase conjugation, namely, with substitute of beacon amplitude distribution by distribution of a Gaussian beam generated by a laser. Absolute correction of turbulent aberration is possible only in case of strict maintenance of a reciprocity principle, i.e. in case of phase reversal algorithm application. In paper the possibility is considered to realize phase reversal algorithm in a linear system with the use of phase control of the beam. The system should include two mirrors separated by the vacuum gap of a finite size. Comparison of efficiency of correction on the base of phase conjugation and with the use of two mirror adaptive system were obtained.

The performance of radar sensors operating within the marine boundary layer is severely influenced by the actual atmospheric conditions, the sea surface and the geometry between radar and reflection point. Propagation models are in existence, which cope with the varying environment and allow a performance prediction for sensors in different radar bands. To assess the propagation within different layers simultaneously at X-, Ka- and W-band an experiment was performed using the experimental three frequency radar MEMPHIS operating against point targets at different heights above sea, carried on a naval vessel, which moved on outbound and inbound courses to ranges well beyond horizon. In-situ measurements included recordings of atmospheric properties and sea surface parameters. Based upon the environmental measurements, refractivity profiles were calculated. With the help of the vertical refractivity gradient and the air sea temperature difference, conditions for the radar propagation were determined. The experimental results were used to validate respective simulations with the parabolic equation model TERPEM. In conclusion, the experimental results and calculations underline the importance of the environmental characterization of the marine boundary layer with high temporal and spatial resolution. This paper describes the experimental approach and gives representative results for measurement and simulation.

Small-scale inhomogeneities of the atmospheric temperature field are caused by air turbulence and result in refractive index fluctuations, which in turn influence the propagation of optical beams. Understanding small density fluctuations in the atmosphere is important for the free-space laser communication and for high-resolution imaging through the atmosphere. The ultra-fast aircraft resistance thermometer constructed in the Institute of Geophysics, Warsaw University, measures the temperature of cloudless air and of warm clouds with 10 kHz sampling frequency. During a flight at the speed of 100 m/s, at low altitudes up to 2 km, this corresponds to the spatial resolution of the order of one centimeter. This resolution is sufficient for studying small density fluctuations in the atmospheric boundary layer. A streamlined shield protects the sensing wire of the thermometer from cloud droplets and other small particles suspended in the air but introduce aerodynamic disturbances in the form of vortices. The thermometer records the resulting fluctuations of temperature as noise. The shield sucks air and water collected on its surface through the suction slits. This suction also suppresses the disturbances. In this paper we analyze how the temperature measurements are influenced by: (i) turbulence generated behind the shield placed in front of the sensing wire; (ii) suction of air through the shield slits; (iii) cloud droplets of various space distributions, masses and velocities. We have carried out the 2D numerical simulations of the time-dependent, incompressible, viscous flow (the Navier-Stokes equation) around the shield placed in a uniform stream. We solved the particle path equations for an ensamble of droplets in the Stokes approximation. All the simulations are oriented toward optimization of the shield shape in order to (i) reduce noise in measurements at low and high altitudes and (ii) protect the sensing wire against ice crystals in flights at high altitudes.

We present a study and preliminary experimental results on the possibility of using an adaptive optics system for reduction of geometrical fluctuations of input laser beam in long baseline interferometric detectors of gravitational waves. Presently used completely passive systems are expected to reduce fluctuations only at a level that, due to coupling of geometrical fluctuations with interferometer asymmetries, impose requirements on interferometer operation which are at the limit of present technology. Active pre-stabilization could reduce fluctuations and relax these requirements, allowing a safer and more robust interferometer operation on the planned time-scale of years of continuous data acquisition. The system and the methodologies we have developed are going to be adapted to the Mode Cleaner of the IDGW-3P, a prototype of three-meter suspended Michelson Interferometer expressely developed for Seismic Noise measurement, now becoming operational in Napoli.

The European Space Agency has built an optical ground station sited at the Observatorio del Teide operated by the Instituto de Astrofísica de Canarias. This station, equipped with a 1m telescope, has a multipurpose configuration for in-orbit commissioning and checkout of laser communication payloads. Since November 2001, the bidirectional link with satellite ARTEMIS has been established in more than 80 successful sessions. In this paper, we analyze the influence of turbulence parameters on the performance of communications in the bidirectional ground to space laser communication experiments. The link performance observed in the satellite-to-ground channel showed average bit error rates of 1E-6 over long durations (20 minutes), however in some occasions BER's of at least 10^-9 -10^-10 over durations of 5 to 30 minutes were observed. The behavior of the Bit Error Rate measurements performed in different turbulence conditions is characterized.

The infrared sky quality is an important parameter to take into account for the evaluation of astronomical sites. The traditional idea of considering higher altitude sites as better for infrared astronomical observations than sites at lower altitudes is not in agreement with observational data. It has been shown that the observational infrared spectrum at the Observatorio del Teide (OT) at an altitude of 2400m on the island of Tenerife (Spain) is similar to that expected for a site at the altitude of Mauna Kea (4100m) in Hawaii (USA). This result suggests that other parameters besides site altitude is playing an important role in determining the quality of a particular location for infrared astronomical observations. In this paper, we propose the troposphere thickness as one of the parameters that determine the suitability and quality of an astronomical site for infrared observations. The tropopause altitude defines the tropospheric thickness and hence, we present in this paper a statistical study of the tropopause layer altitude for four different astronomical sites. The results presented in this work suggest that the infrared quality at La Palma, La Silla and Mauna Kea could be similar in some epochs of the year, although they are located at different altitudes above the sea level. Mauna Kea presents the thinnest troposphere during Summer and Autumn among the four studied sites, whereas La Palma exhibits the lowest altitude of the tropopause in Winter and Spring. Paranal presents most of the time the thickest troposphere, suggesting worse infrared conditions for astronomical observations (based only in the thickness parameter) at this site, when compared to the other three in study.

The experimental study of sodium layer is relevant to fields such as adaptive optics, in particular for laser guide star generation. We have developed an instrument to make systematic measures of the most relevant parameters of the sodium layer above canarian observatories. The measurement provides information about sodium layer: the medium altitude, the columnal abundance, the density profile and temporal evolution. These observations are important to design the new generation of adaptive optic instruments. We present the experimental set-up, the first results of the experiment and the plans to conduct a statistical study in the future.

The areas of adaptive optics application have increasingly expanded beyond astronomy over past ten years. One of the most striking examples is visual science. Fundus camera equipped with adaptive optics has been extensively investigated over past few years and employed with great success in obtaining fine images of the human retina in real time. But even if the aberrations of the human eye are corrected with adaptive optics the quality of retinal images is still degraded by anisoplanatism effect. We can obtain high-resolution image only if decorellation of the phase that is incident from the beacon on the retina and the point being imaged is small. The wavefront compensation is effective only within a finite area - the isoplanatic patch. On the basis of Zernike decompositions of the aberrated wavefront for different retinal angles we have been able to calculate the residual mean-square error for the corrected wavefront. We estimated the isoplanatic angle in human eye as the angular distance between the two sources where the mean-square error is equal to 1 square rad . Computer simulations illustrating the degrading effects of anisoplanatism on retinal imaging performance of adaptive optics system are presented. In the paper we discuss the limitations of isoplanatic patch enlargement by examining an ideal adaptive corrector that provides compensation of all Zernike modes. We simulated the blur of the retinal image induced by the eye's aberrations and the compensation of these aberrations by the corrector thus illustrating the performance of anisoplanatism-limited adaptive optics systems.

The problem of correct eye aberrations measurement is very important with the rising widespread of a surgical procedure for reducing refractive error in the eye, so called, LASIK (laser-assisted in situ keratomileusis). The double-pass technique commonly used for measuring aberrations of a human eye involves some uncertainties. One of them is loosing the information about odd human eye aberrations. We report about investigations of the applicability limit of the double-pass measurements depending upon the aberrations status introduced by human eye and actual size of the entrance pupil. We evaluate the double-pass effects for various aberrations and different pupil diameters. It is shown that for small pupils the double-pass effects are negligible. The testing and alignment of aberrometer was performed using the schematic eye, developed in our lab. We also introduced a model of human eye based on bimorph flexible mirror. We perform calculations to demonstrate that our schematic eye is capable of reproducing spatial-temporal statistics of aberrations of living eye with normal vision or even myopic or hypermetropic or with high aberrations ones.

The development of an adaptive system for the transformation of optical vortex is undertaken. Content of work consists of generation of a vortex with preset properties, measurement of its wave front by a Shack-Hartmann sensor and its correction by means of an adaptive mirror. The first stage of work has been realized on formation a Laguerre-Gauss beam with the help of the amplitude diffraction gratings and special phase plates fabricated on kinoform technology.

Radar propagation near the sea surface depends on meteorological conditions and sea surface roughness. Often strong gradients of humidity and temperature can be observed close to the air-water interface leading to abnormal propagation effects such as ducting. For shipborne radars operating at frequencies above L-band, the evaporation duct is the dominant propagation mechanism affecting the maximum detection range of horizon-search radars. Ducting can also increase sea clutter return within and beyond the normal horizon, and surface-based ducts can enhance land clutter return from extended ranges. During sea trials in the Skagerrak and the Baltic Sea in 2003 and 2004, FWG was responsible for environmental characterization of the boundary layer. In-situ measurements included recordings of atmospheric and sea surface parameters. Investigations with multi-sensor buoys and with radiosondes were performed on board the German research vessel PLANET respectively on FGS HELMSAND. The drift buoys developed by FWG provided unperturbed, time resolved information on air-sea interaction processes. In addition to meteorological parameters sea state, sea surface roughness, and sea surface temperature were measured. Refractivity profiles were determined based on data sets gathered by measurements of pressure, humidity and temperature from the sea surface up to 1 km altitude. Simultaneously to atmospheric measurements radar propagation investigations were performed by FGAN-FHR (Research Institute for High Frequency Physics and Radar Techniques). PLANET, FGS STOLLERGRUND were illuminated by a radar operating at X-, Ka- and W-band. The radar system was located at the land-based test site Hirtshals, Denmark during the trials in 2003 and at the land-based test site Surendorf, Germany during the experiment in 2004. Radar propagation characteristics at X-band were measured on board the ships with two omnidirectional antennas mounted in two different altitudes above sea surface. Results of refractivity variability in the marine boundary layer are presented in conjunction with radar propagation data and model outputs.

Scattering by atmospheric aerosols is one of the environmental parameters determining the range performance of optical and infrared sensors. Extinction of the target contrast along the path due to scattering is difficult to estimate in real operational conditions due to uncertainties in the size distribution of the particles, their constitution and concentration along the path. Knowledge on their behaviour allows calculation of the atmospheric transmission by means of standard scattering formulae. In-situ measurement of the characteristics of the particles by means of counters provides data of limited value due to the possible impact of the direct environment. The data may also be not representative for the particles at other locations along the path. Similarly the measurement of the particle characteristics by means of LIDAR provide an asymmetric view, while the backscatter by the particles is difficult to translate into extinction coefficients in forward direction. Multi-band transmissometry along the path of interest provides however direct information on the real atmospheric propagation characteristics. Furthermore the multi-band data allow the validation of the aerosol model, to be used in transmission models such as MODTRAN. The VAMPIRA trials, organised in March/April 2004 by Germany in the Baltic Sea near Echernforde, provided an opportunity to test the usefulness of a 7-channel optical/IR transmissometer, developed at TNO-FEL. In this paper the set-up of the system is desribed and samples of data are presented. The multi-band transmission data, collected over an 8.6 km path over water, are compared with extinction values obtained from in-situ particle measurements. The data show clearly that the aerosols have rural characteristics during most of the time.

The concept of a curvature-based wavefront sensor using a distorted grating as the imaging element to capture images of two spatially separated planes onto a single detector has been reported previously. This presentation reports on simulations comparing a Shack-Hartmann (S-H) sensor with a distorted grating wavefront sensor (DGWFS) for a generic adaptive optics (AO) system using a Clear-1 atmospheric model. Using WaveTrain^TM simulation software a model of the DGWFS has been developed and integrated into the software. A simulation of a complete AO system including a tip/tilt system, high order correction system, atmospheric model, and a back-propagating laser system has been constructed. The model has then been exercised using various seeing conditions, noise levels, WFS sensitivities, camera systems, and other parameters. A comparison between the performance of the AO system using the S-H sensor and the DGWFS is presented, both in terms of wavefront measurement accuracy, image quality, and as a beam delivery system.