In the early 70's, the scientists in Italy (A.Consortini, M.Bertolotti, L.Ronchi), USA (R.Buser, Ochs, S.Clifford) and USSR (V.Pokasov, V.Lukin) almost simultaneously discovered the phenomenon of deviation from the power law and the effect of saturation for the structure phase function. During a period of 35 years we have performed successively the investigations of the effect of low-frequency spectral range of atmospheric turbulence on the optical characteristics. The influence of the turbulence models as well as a outer scale of turbulence on the characteristics of telescopes and systems of laser beam formations has been determined too.
The ultrafast aircraft thermometer is an airborne device designed for measuring temperature in clouds with centimeter spatial resolution. Its sensor consists of 5mm long and 2.5μm thick thermo-resistive wire protected against impact of cloud droplets by a shield in the form of a suitably shaped rod, placed upstream. However the disturbances of airflow around this rod result in noise in the temperature record. Suction applied through slits located on both sides of the rod reduces the noise generated by vortices shed from the rod and lowers the probability of droplet-wire collisions. Our recent theoretical analysis and numerical simulations led to optimization of this device and additionally clarified the role of the sampling method in processing of the analogue output of the thermometer. In this paper we try to deepen our understanding of the nature of the noise as well as to improve calculations of the corrections connected with the dynamic heating. For this purpose we have done extensive three-dimensional numerical simulations of the airflow around the protective rod and the sensing wire, which permitted precise computation of dynamic heating and showed how applying the suction removes the thermal boundary layer from the rod and damps the sources of the noise.
McCART is a numerical procedure to solve the radiative transfer equation for light propagation through the atmosphere especially developed to study the effect of the atmosphere on the response of hyperspectral sensors for remote sensing of the earth's surface. McCART is based on a single Monte Carlo simulation run for a reference layered plane non-absorbing atmosphere and a plane ground with uniform reflectance. The spectral response of the sensor for a given distribution of ground reflectance and for a specific profile of scattering and absorption properties of the atmosphere is obtained in a short time from the results of the Monte Carlo simulation, making use of scaling relationships and of symmetry properties. The response includes the effects of adjacent pixels. The results can be used to establish the limits of applicability of approximate algorithms for the processing and analysis of hyperspectral images. The algorithm can be also used to develop procedures of atmospheric compensation.
The intermittency in optical scintillation was theoretically studied and experimentally observed by laser propagation through the turbulent atmosphere over sea surface. It is found that the intermittent characteristics of the atmospheric turbulence repeat itself in optical irradiance fluctuations, then a method of the singularity measurement analysis was introduced and the intermittency index was presented to quantitatively evaluate the intermittency strength. Experiment results show that the intermittency index in laser scintillation is around 0.1 and varies a little compared with the significant variation of the turbulence strength, which had been approximately estimated at two orders of magnitude in a whole day.
The dependence of the intermittency on the wavelength was further studied and the relationship was obtained using a multi-wavelength scintillometer. Generally, the intermittency indices display some differences at the different optical wavelengths, especially in strong turbulence. However, the wavelength dependence is not so notable that the intermittency indices are assumed to be equal within less than 10%, even the maximum error is no more than 20%. Moreover, some statistical results of the intermittency strength are also obtained from a long-term plan of laser propagating in coastal surroundings.
The member nations of AC/323 SET-RTG056/RTG32 on Integration of Radar and Infrared for Ship Self Defence have performed the Validation Measurements for Propagation in the Infrared and Radar (VAMPIRA). The objective was to get insight into the radar and infrared synergy concentrated on propagation in a coastal environment including horizontal inhomogeneity and to validate radar and infrared propagation models. The trial was held in the period 25 March-5 April 2004 near Surendorf Germany. As part of the trial TNO made RF 1-way transmission measurements, 24 hours/day during the whole trial period. The transmission path over the Eckernforder Bucht in Northern Germany had a length of 8.2 km. The transmitted signal was a sweep consisting of 6 frequencies i.e. 3.36, 5.32, 8.015, 9.7, 13.45, and 15.71 GHz. The transmitter height was 11.5 m, the receiver height 6.4 m above 'normal null'. At each end of the path a meteorological station was installed measuring every 30s the air temperature, relative humidity, air pressure, wind speed and wind direction. About halfway the path the TNO meteo buoy was anchored measuring air temperature and relative humidity at 5 heights between 0.65 and 5.15m above the sea surface. Also the sea water temperature was measured by the buoy on a depth of 1m below the sea surface. The effects of evaporation ducting at the propagation at the various frequencies were clearly demonstrated. Some times very deep fadings were present at 13.45 and 15.71 GHz where at the same time almost no effect at 3.36 and 5.32 GHz was observed. The measured propagation at 15.71 GHz was more enhanced than at 13.45 GHz due to the ducting conditions and the elevation angle of the transmitter and receiver antenna. In several sample cases the 1-way propagation factors are computed for every 5 minutes using the propagation model TERPEM (Signal Science) and the vertical refractivity profiles computed by the TNO model TARMOS. The 1-way computed propagation factors compared very well to the measured data at all frequencies, although the computed fadings were not always as deep as the measured ones. A first promising result has been obtained computing the observed height of the RF source under various atmospheric conditions using the transmission phases computed by TERPEM.
The performance of Electro-Optical systems operating within the marine boundary layer is severely influenced by atmospheric conditions. Vertical temperature and humidity gradients lead to refraction and turbulence effects. Refraction causes focusing and defocusing of rays, detection range limitations, mirage formation and angular deviation. Associated with the VAMPIRA (Validation Measurement on Propagation in the IR and Radar) experiment, we made investigations on the apparent elevations of point targets. The location of the trial was at the coast line of the Baltic Sea, overlooking a bay. An IR camera system was fixed on a pier at an altitude of 6.5 m. The camera continuously recorded a series of lights 8.2 km away at the other side of the bay. With these recordings we obtained 24-hours-observations of refraction effects due to changing weather conditions. In addition to the VAMPIRA measurements a similar experiment was performed in tropical waters. First results of this trial are given here. Aim of this paper is to discuss and analyse the measured elevations and compare them to the propagation model IRBLEM (IR Boundary Layer Effects Model by DRDC, Canada).
Ray-bending resulting from atmospheric refraction in the maritime environment has been shown to potentially produce significant effects on electro-optical target detection and imaging. Positive bending makes possible detection beyond the horizon while negative bending reduces the maximum inter-vision range (MIVR) and is likely to produce severe image distortion or mirages. It has been shown by many authors that these phenomena can be efficiently described using ray-tracing in conjunction with bulk estimations of the refractivity profiles based on the Monin-Obhukov theory. In this paper, the accuracy of bulk methods to describe ray bending is assessed by examining angular deviations of apparent target elevations with respect to the meteorological conditions. Prediction accuracy is shown for a large spectrum of conditions, characterized by the air-sea temperature difference, by combining measurements collected in the North Sea and in the Baltic Sea. Moreover, the use of bulk profiles as opposed to profiles measured at sea by using a buoy is discussed.
The propagation of IR radiation through the marine boundary layer is very much dependent on atmospheric conditions. Especially the vertical temperature gradient has a strong influence on the maximum detection ranges of an imaging system. Due to the Air-Sea Temperature Difference (ASTD) the distance to the visible horizon can be shorter (ASTD < 0°C) or larger (ASTD > 0°C) than the distance to the geometric horizon. Propagation models exist which predict maximum ranges for varying environmental conditions. FGAN-FOM conducted measurements in the mid and long wave IR (MWIR and LWIR) at different locations. Analysis was made on the signal of IR point sources on board a boat sailing towards the horizon. In this paper measured detection ranges and decreasing and increasing signals with range are compared to model predictions.
A study is carried out to classify possible combinations of refractivity conditions for RF and IR over a wide range of meteorological conditions using different micrometeorological bulk models. The calculated refractivity profiles are analyzed for evaporation duct height (EDH), mainly relevant for RF propagation, and for gradients of the modified refractivity at different heights, relevant for both RF and IR propagation. These refractivity gradients are a direct indicator for the occurrence of sub- or super refraction at the height of interest. The present study reveals that under humid and unstable conditions evaporation ducts are found at approximately 3±2 m above cold (5°C) waters and at approximately 8±5 m over warm waters (25°C). Under dry conditions, these duct heights are approximately 9±5 m and 20±10 m, respectively. Duct heights decrease with increasing wind speed. Under humid and near-neutral conditions, duct heights range from 1 to 25 m, and decrease with increasing air temperature and/or wind speed. On the other hand, for dry and near-neutral conditions, and also for neutral conditions, the duct height is not well defined. Values between 1 m and 100 m are found, with an irregular dependence on air temperature and wind speed. Reliable modeling of duct height under these conditions remains questionable due to a lack of vertical mixing in the surface layer. The paper also shows that all four combinations of RF and IR sub- and super-refraction can occur in the atmosphere. The occurrence of a specific combination depends predominantly on temperature and humidity, and to a relatively minor part on wind speed. The magnitude of refraction effects in the two spectral bands is not necessarily coupled but varies with environmental conditions and height. Sub-sub refraction is generally weak and occurs under neutral conditions or at large heights. Super-super refraction occurs under warm and dry conditions and can reach medium strengths. RF-super refraction in combination with IR-sub refraction occurs under strong unstable conditions (e.g., surface temperature higher than air temperature) and can reach medium strengths. RF-sub refraction in combination with IR-super refraction occurs under stable and warm conditions. The magnitude of refraction can be very large, especially at low altitude.
We recently developed a method1 to measure the gradient of the refractive index of the atmosphere, by using the lateral fluctuations of thin laser beams along a given path. We showed that the horizontal and vertical instantaneous fluctuations of a ray on a screen, at the end of a path of length L, are related to the horizontal and vertical components, averaged along the path, of the refractive index gradient, respectively.
Our method, which is based on the geometrical optics approximation, requires that the beams are "thin", that is that their transverse dimension be not larger than the dimensions of the smallest inhomogeneities of the atmosphere (inner scale of turbulence). In practice, it allows "local" measurements, that is measurements over short horizontal paths, even few meters' paths. Experiments were made in the open air.
Here we describe the method and present results of measurements made in different locations and conditions, at near ground levels. Temperature and transverse wind velocity were also measured and some correlation of these quantities to the refractive index gradient will be presented. Some cases of strong vertical anisotropy are also shown, which can be related to the local air convective movements from the ground.
This paper details a generalised SCIDAR system developed for characterising atmospheric parameters using single star targets. The instrument, which is based on a commercially available 250 mm diameter telescope, offers the potential for characterising atmospheric parameters for wide areas of the sky. Here, we describe the system and results of a proof of principle study performed at an observing site in Galway, Ireland with the instrument. The paper also outlines the approach adopted in the data reduction and in solving the altitude dependence of refractive index structure constant given the raw data from the instrument.
The atmospheric conditions in the Persian Gulf region are significantly different from other places in the world. The particle size distribution may vary daily and during the day. The aerosols can contribute to the amount of rainfall over land, important for the nations around the Gulf. In 2004 NASNGSFC and NRL (Naval Research Laboratory) introduced a proposal to improve the modelling of aerosol transport for the Persian Gulf area. The proposal included a measurement campaign in the UAE (United Arabian Emirates), held in the summer/fall of 2004, sponsored by the DWRS (Department of Water Resources Studies) in Abu Dhabi: UAEz (Unified Aerosol Experiment in the UAE). In this campaign NASA installed a number of multi-spectral sun-photometers at various locations in the UAE (http://aeronet.gsfc.nasa.gov). NRL installed ground based and airborne particle samplers. In addition, TNO (the Netherlands) installed its multi-band opticaUIR transmissometer, in order to collect horizontal, path-integrated transmission data. This device provides additional information on the scattering behaviour of the aerosols compared to the other instruments, which either integrate scattering over the full vertical path (the NASA sun-photometers, providing the Aerosol Optical Depth (AOD)) or sample the particles in-situ (the NRL particle samplers, providing size distribution and composition). This paper deals with our transmission measurement set-up, which was located in a coastal area near Abu Dhabi. This
location allowed the investigation of the local variability of the atmospheric conditions: from desert dust to pollution, such as fossil fuel and biomass burning, depending on the wind direction. For logistic reasons a set-up was chosen with a retro-reflector. This choice implies consequences for the calibration procedure and measurement accuracy, which are discussed in detail. Also the effects of path-inhomogeneity and scintillation for such a two-way set-up are considered. Results are presented for the measurement period of two weeks in September, showing interesting transmission effects
due to temporal changes in aerosol particle composition. These phenomena cannot be explained by scattering theory for spherical particles. More knowledge is required on the shape and composition of the particles. Comparison of the transmission data with the data from other instruments will be done in a next phase.
We investigated Gaussian beam propagation through the turbulent atmosphere. The solutions of the linear and nonlinear unsteady 3D Navier -- Stokes equations have been used for modeling of turbulent fluctuations. Parabolic equation method has been applied for study of the intensity variations of the beam. Numerous examples of modeling are presented. Results may be applied to the study of turbulent fluctuations as well as of distortions of object images.
Applications of adaptive optics to terrestrial imaging involve anisoplanatic imaging conditions in which the turbulence-distorted wavefront may be highly scintillated and have present phase discontinuities. We will describe experiments designed to assess these properties of the wavefront, and discuss the observational strategy for measurement of atmospheric properties under a range of atmospheric conditions and propagation distances. By reconstructing the wavefront and comparing the calculated and measured images we will also aim to investigate the effect of strong scintillation on phase diversity wavefront reconstruction techniques. Laboratory tests of the equipment and preliminary measurements will be described, as well as some theory and modeling.
We present design considerations for a mobile adaptive optics (AO) system intended for ground-level or near ground-level applications in the near infrared spectral range. Starting from the expected atmospheric parameters we arrive at a preliminary system design. Since atmospheric conditions at ground level are considerably worse than those encountered for vertical AO systems, we conducted extensive end-to-end simulations of our system design for three anticipated scenarios in order to evaluate expected performance; these simulations concentrated on anisoplanacy and the effect of scintillations on wave-front control.
Multi-frame iterative blind deconvolution algorithms for image enhancement have been widely used for over ten years. Originally developed for enhancing astronomical images from large ground based telescopes, the algorithms were adapted for ground based satellite observations. Most algorithms involve some type of multi-frame iterative Bayesian optimization assuming either Poisson or Gaussian statistics. Many algorithms use an iterative conjugate gradient search technique, however it has been our experience that an algorithm based on Gaussian statistics, combined with projection onto convex sets adaptation leads to a simple algorithm that quickly converges to a result. Recently our thrust has been to transition these algorithms to the airborne imaging problem. We present a number of examples. First, results from observation of low earth orbit satellites with uncompensated data taken at the focal plane of a large telescope. Finally we move to the problem of air-to-ground imaging. Such scene based imaging scenarios require an algorithm that can operate in the presence of anisoplanatic effects. For this case we have developed an algorithm that calculates a position varying point-spread function.
This paper presents a speckle image restoration algorithm using wavelet transform. Based on the wavelet theory, a new gauss PSF accurate estimation is put forward. Firstly, wavelet with varied scales is transformed, after which the local maxima of the modulus of the wavelet are computed respectively. Secondly, on the basis of the relation deduced among the local maxima of the modulus of the wavelet at different scales, Lipschitz exponent and variance, the variance of a Gaussian point spread function is computed. According to Fried theory, the Fried parameter can be deduced from the variance. From the Fried parameter we could estimate the optical transfer function of the turbulence. Row action projection Method is applied to restore the image through atmosphere over a distance of 1 km. Primary results are obtained.
Active imaging polarimetry is a unique imaging technique in which a particular scene of interest is illuminated by a laser source with a known polarization state. Changes in the state of polarization of the received light yields information beyond what is available in conventional intensity imaging. This approach has an advantage over passive polarimetry in that one has control over the polarization state of the illumination with the potential of determining all sixteen elements of the associated Mueller matrix. While determining the entire Mueller matrix is the most comprehensive method for describing the polarization changing properties of the scene, for most cases it does not yield significantly more information than simply determining the 4 diagonal elements of the Mueller matrix. The Active Wollaston Polarimeter is based around the ability of the Wollaston prism to split orthogonal polarization states into two beams propagating at slightly different angles allowing two images to be formed on a single camera. The Wollaston prism, combined with a series of liquid crystal variable retarders allows monopulse determination of any polarization contrast image (PCI), which is directly related to a specific Muller matrix element. This technique results in a fast, compact polarization measurement system. This paper presents the continued investigation and analysis of the performance of the polarimeter and possible viability as a practical polarization measurement system.
We present the optical design of a laboratory demonstrator for a low- order adaptive optics system with possible application to improving the performance of a free-space optical communication system. The initial design includes a Shack-Hartmann wavefront sensor with high-speed CMOS camera and a 37-element membrane mirror.
The speckles produced in the image of a laser-illuminated object introduce errors in the wavefrontmeasurement by a Shack
Hartmann (SH) Wave-Front Sensor (WFS). The speckles are similar to a spatial noise onto the image and contribute to the error in the computation of the image position for each lenslet of the sensor. This speckle error impacts directly the performance of an adaptive optics system. We express this error term for different cases without and with turbulence and show that it is an additive white noise. Based on end to end numerical simulations, we quantify the speckle error and compare it to the turbulent wavefront variance in different turbulence regimes.
The paper addresses the problem of extending conventional Monte Carlo procedures to cases of multiple scattering in media with suspensions of non-spherical or chiral particles. Extinction coefficients of the media depend on polarization of radiation. Along the propagation path polarization of radiation changes, unless the field is polarized according to one of two particular modes. The relationship between these modes and the elements of the amplitude scattering matrix for the type of particle is shown by means of a simple formalism, tested with reference to simple shapes and orientation of the particles. Some possibilities for extending Monte Carlo procedures are suggested. A case of small chiral spheres is considered.
In order to get a better understanding of the optical turbulence at the free atmosphere we present a statistical analysis of the wind and temperature profiles at different heights measured by balloon-born sonde during the years 2003,2004 and until mid 2005, and the stellar image twinkling measurements from a DIMM. The data from the balloon-borne measurements shows the variation of the strength of the jet stream and temperature gradient that are related to formation of turbulent layers in the upper air which contribute to the degradation of the optical images or to laser propagation through the atmosphere. The use of standard radiosonde data compared with the seeing can be used as a tool to define the vertical distribution of the strength of the turbulence in the atmosphere and that will contribute to better optimization of the performance of Adaptative Optics systems.
In problems of optical location, communication, and energy transmission one frequently runs up against the problem of transporting radiant energy in the form of a light beam to an object located in a random inhomogeneous medium. Here, as a rule, it is necessary to maximize the amount of energy delivered to the object. As is well known, scattering of radiation by refractive index inhomogeneities of the medium leads to a decrease of the average intensity in the near-axial region of the light beam and to the appearance of intensity fluctuations, which taken together substantially degrade the energetic characteristics of the indicated systems.