The effective field-of-view of an electro-optical sensor in a given meteorological scenario can be evaluated using a ray-tracer. The resulting ray trace diagram also provides information pertinent to the quality (distortion, mirages) of the image being viewed by the sensor. The EOSTAR (Electro Optical Signal Transmission And Ranging) model suite contains a ray tracer that has been upgraded to take into account horizontal inhomogeneities in the atmosphere, such as temperature gradients as observed in coastal areas where (e.g.) cold air flows out over warm waters. Initial results for horizontally inhomogeneous atmospheres are presented and compared to calculations for horizontally homogeneous atmospheres. It is shown that the horizontal inhomogeneity of temperature should be taken into account when assessing sensor performance.
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.
EOSTAR, a PC based Windows application, integrates the required modules necessary to calculate the electro-optical sensor performance on the basis of standard meteorological data. The primary output of EOSTAR consists of the synthetic sensor image ("what does the sensor see?") and a coverage diagram ("detection probability versus range"). As part of the EOSTAR validation effort, the refraction and turbulence modules are being evaluated against literature data, similar models and experimental results. It is shown that the EOSTAR model can predict with reasonable success the occurrence of optical turbulence and refraction phenomena such as mirages. The major cause for discrepancies between the various models is attributed to the underlying micrometeorological bulk modules, whereas the sensitivity of the predictions on the values of the meteorological input parameters is held responsible for the discrepancies between model predictions and measurements.
Small maritime targets, e.g., periscope tubes, jet skies, swimmers and small boats, are potential threats for naval ships under many conditions, but are difficult to detect with current radar systems due to their limited radar cross section and the presence of sea clutter. On the other hand, applications of lidar systems have shown that the reflections from small targets are significantly stronger than reflections from the sea surface. As a result, dedicated lidar systems are potential tools for the detection of small maritime targets. A geometric approach is used to compare the diffuse reflection properties of cylinders and spheres with flat surfaces, which is used to estimate the maximum detectable range of such objects for a given lidar system. Experimental results using lasers operating at 1.06 μm and 1.57 μm confirm this theory and are discussed. Small buoys near Scheveningen harbor could be detected under adverse weather over more than 9 km. Extrapolation of these results indicates that small targets can be detected out to ranges of approximately 20 km.
The quality of long range infrared (IR) imaging depends on the effects of atmospheric refraction and other pathintegrated effects (e.g., transmission losses, scintillation and blurring), which are strongly related to the prevailing meteorological conditions. EOSTAR is a PC based computer program to quantify these strong nonlinear effects in the marine atmospheric surface layer and to present a spectrally resolved target image influenced by atmospheric effects using ray tracing techniques for the individual camera pixels. Presently, the propagation is predicted with bulk atmospheric models and the sea surface is idealized by steady regular periodic Stokes' waves. Dynamical wind-waves interactions are not taken into account in this approach, although they may strongly modify the refractive index in the near-surface layer. Nonetheless, the inclusion of the sea surface in the ray tracer module already has a great impact on the near-surface grazing rays and thus influences the images especially in situations of super refraction and mirage. This work aims at improving the description of the sea surface in EOSTAR taking into account the non-uniformity of spatially resolved wind-generated waves and swell. A new surface module is developed to model surface wind-waves and swell in EOSTAR on the basis of meteorological observations and spectral wave modeling. Effects due to these new surfaces will be analyzed and presented.
IR decoys can be an effective countermeasure against IR guided anti ship missiles. However, it's not so easy to determine how the decoys should be deployed to get maximum effectiveness. A limitation of trials is that results are obtained for the specific trial condition only. Software tools have been developed to solve these problems. One solution uses recorded IR imagery from a decoy deployment trial, while the other solution generates IR imagery and is thus independent of trials. In the first solution, a combination of hardware and software is used that allows recording of a scene with an infrared camera, and simulating a missile seeker. A pre-processing algorithm corrects the recorded images before they are fed into the seeker algorithm of the simulated missile. To perform this correction the pre-processing uses the speed, distance to the target and field of view of the IR camera as fixed parameters and the speed and starting distance of the simulated missile as variable parameters. Modtran and the Navy Aerosol Model are used to calculate the atmospheric transmission effects in the pre-processing. The second solution generates artificial IR images that are subsequently fed into the seeker algorithm. This solution also allows variation of those parameters that are fixed when recorded IR imagery is used. Examples are among others: the signature of the target ship, the orientation, size and speed of the target ship, the type of decoy, the timing of the decoy sequence, atmospheric conditions etc. With these tools the effectiveness of decoy deployment in various scenarios can be evaluated.
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 infrared signal or a laser beam propagating along a horizontal near-surface path will encounter substantial perturbations. The fluxes of momentum and heat near the surface are relatively large, and these in turn cause large changes in the propagated intensity, direction, and coherence. It is important to be able to accurately
model the separate effects that generate changes in a propagated beam, and it is also important to combine the different factors accurately. We will present some evidence from field experiments to demonstrate how refractivity changes on a ten-minute scale are manifested in a recorded infrared transmission signal. The EOSTAR (Electro-Optical Signal Transmission and Ranging) model is used to provide performance predictions for the experimental work. The EOSTAR model is built upon a geometrical optics approach to infrared propagation: a ray is traced through the propagation environment, and path-dependent perturbations to the signal can be determined. The primary computational tool for analysis of refractive effects in the EOSTAR model is a geometrical optics module that produces a ray-trace calculation for a given refractive environment. Based on the vertical profiles of temperature, humidity, refractive index structure parameter, and the calculated ray trajectories, EOSTAR calculates the path-integrated and spectrally-resolved transmission, background-radiation and path-radiation, as well as the scintillation and blur for a point source at any range and height position.
A first version of the integrated model EOSTAR (Electro-Optical Signal Transmission and Ranging) to predict the performance of electro-optical (EO) sensor systems in the marine atmospheric surface layer has been developed. The model allows the user to define camera systems, atmospheric conditions and target characteristics, and it uses standard (shipboard) meteorological data to calculate atmospheric effects such as refraction, turbulence, spectrally resolved transmission, path- and background radiation. Alternatively, the user may specify vertical profiles of meteorological parameters and/or profiles of atmospheric refraction, either interactively or in data files with a flexible format. Atmospheric effects can be presented both numerically and graphically as distorted images of synthetically generated targets with spatially distributed emission properties. EOSTAR is a completely mouse-driven PC Windows program with a user-friendly interface and extended help files. Most calculations are performed in real-time, although spectral transmission and background radiation calculations take up to a few seconds for each new meteorological condition. The program can be used in a wide range of applications, e.g., for operational planning and instruction.
The Advanced Ray Tracing with Earth Atmospheric Models (ARTEAM) aims at a description of the electro-optical propagation environment in the marine atmospheric surface layer. For given meteorological conditions, the model evaluates height- and range-resolved transmission losses, refraction and turbulence effects. These results are subsequently applied to an electro-optical sensor with given specifications to evaluate the effective range and performance of the sensor under the prevailing meteorological conditions. Finally, for specified sensor-target geometry, the model calculates characteristic parameters for geometrical and spectral intensity, scintillation, blur and image distortion. ARTEAM is developed with an extensive Graphical User Interface (GUI) for Windows environments.
A scanning backscatter lidar, operating at a wavelength of 1.06 micrometers , was used to measure the volume backscatter coefficient of coastal aerosol at the Mace Head Research Station on the west coast of Ireland during the PARFORCE experiments in September 1998 and June 1999. Lidar measurements under a fixed elevation angle at fixed time intervals provided the depth and evolution of the boundary layer throughout the day. Horizontal scans over the bay near the station indicate that waves breaking on the small islands and rocks in the bay near the station generate coherent plumes of a few hundred meters wide that propagate over distances of more than 5 km. Vertical scans have shown that the initial height of these plumes is a few tens of meters, rising to several hundreds of meters while advected over several kilometers. The backscatter coefficient in the plumes was between a factor of 2 and 10 higher than outside the plume. Large series of consecutive vertical cross sections showed that patches of aerosol plumes are taken aloft in the boundary layer to altitudes of more than one kilometer. So far, no relation was found between the horizontal and/or the vertical extent of the aerosol plumes and the air-to-sea temperature difference. The lidar measurements indicate that in situations that the wind is from over one of the islands the ambient aerosol concentrations can be enhanced by more than a factor 2 due to local aerosol sources.
A lidar-transmissometer intercomparison was made during an international experiment held in the German Alps to characterize the vertical structure of aerosols and clouds. The transmission path was 2325-m long and inclined at 30 degrees along the slope of a steep mountain ridge. the transmissometer consisted of a Nd:YAG and a CO2 laser located in the valley and a large-mirror receiver that captured the full beams on the mountain top. Two lidars, one at 1.06 micrometer and one at 1.054 micrometer, were operated with their axes approximately parallel to the transmissometer axis but separated by a horizontal distance on the order of 20 - 40 m. The first one was operated in retroreflector mode and the relative transmittance was determined from the reflection off the mountain ridge above the cloud layer. The second one had a special receiver designed to make simultaneous recordings at four fields of view. The range-resolved scattering coefficient and effective cloud droplet radius are calculated from these four-field-of-view measurements by solving a simplified model (Appl. Opt. 34, 6959-6975, 1995) of the multiply scattered returns. The two simultaneous solutions for the scattering coefficient and effective droplet size make possible extrapolation at wavelengths other than the lidar wavelength of 1.054 micrometer. The main measurement event analyzed in this paper lasted 1.5 hours and produced transmittances ranging from less than 5% to more then 90%. The comparisons show good correlation between the transmissometer data and all lidar solutions including extrapolation at 10.59 micrometer.
During the MAPTIP experiment, that was organized by NATO AC/243 (Panel 4)/RSG.8, the marine atmosphere was characterized, among others, with lidar (optical radar). The investigations were carried out both in horizontal planes (PPI scans), in vertical planes (RHI scans) and combinations of those two. In addition, the temporal variability of the atmosphere along one line of site was characterized by operating the lidar in a fixed direction at a sufficiently high repetition rate to follow the eddy structures. The marine aerosol layer was monitored within the surf zone and out to ranges of about 10 km. Slant path measurements provided information on the depth and structure of the mixed layer. Results obtained during this experiment are presented.
A model was developed for the prediction of turbulence in the marine surface layer. The model requires standard meteorological values of air temperature, air humidity, wind speed each from any given height from within the surface layer and the sea surface temperature. Internally, the model is controlled by the exchange coefficients for momentum, heat and water vapor. A variant using the surface roughness length instead of the drag coefficient has also been implemented. The micrometeorological output parameters of the model are used to predict vertical profiles of the refractive index -- to predict refractivity effects -- and profiles of the refractive index structure function parameter Cn2(z). The latter is the controlling parameters to calculate optical turbulence effects such as scintillation and blurring. Experimental data were obtained from images taken of a point source over a 19 km path over the North Sea at a frame rate of 25 Hz using a 3 - 5 micrometer infrared camera system. The images were analyzed for scintillation, blur and image dancing. Predicted and measured turbulence effects are compared.
Time and range resolved multiple-scattered radiation from the aureole around a traveling radiation, using an additional off axis scanning lidar-receiver, on top of an existing lidar. This system can be considered as an analogue equivalent of Bissonnette's fixed multi-field-of-view lidar. Because multiple-scattering is effective over much larger volumes than the illuminating volume by the laser pulse itself; the variations in the signals due to atmospheric structures are reduced to a certain extent. A simple second-order scattering model has been developed to understand this volume averaging effect. It has been shown that this model can be used to estimate the extinction coefficient of clouds and the average diameters of the cloud droplets. Lidar waveforms, obtained during a field experiment, have been inverted with the proposed model. The results appear to be in reasonable agreement with the in situ measured extinction using a nephelometer mounted on a cable car which was moving up and down a mountain slope through the clouds.
Atmospheric eddies, which have slightly different properties than their environment and are believed to be transported by the wind (Taylor's hypothesis), are used as tracers for remote wind measurements with a fast incoherent lidar. Horizontal measurements, parallel with the wind, have shown that the atmospheric structures can be traced in space and time and that the horizontal wind speed can be determined from a set of subsequent measurements. Also, the characteristic size and life time of the strucutres were inverted from the lidar system with their axes pointing in slightly different horizontal directions. Using the cross-correlation technique, the wind vector was derived from the geometry of the sysetm and the transient times of the structures crossing the two lidar field-of-views. The measured wind vectors are comparable with the in-situ measured wind vector. The same technique was applied to measure the vertical profile of the wind vector using a single lidar in the triangulation mode using fast adjustable platform pointing subsequently in three different (azimuth and elevation) directions. The wind vector could be measured to altitudes of about 1 km and were in agreement with the in-situ measured data from sensors on a 200 m high meteo mast and from data provided by a Doppler sodar.
During 1994 an experiment took place over the North Sea in order to derive the infrared atmospheric scintillation and beam deformation process. An infrared point source was positioned at a platform in sea, with a height ranging between 1.5 and 7 m above the sea level. Some additional sources, at different height levels, were installed later on in the experiment. The receiver system was placed at 18 km on a pier near the coast of the Netherlands. Recordings took place with the receiver at two heights, 40 and 15 m above the average sea level. A single recording consisted of 10 seconds of measurements at 25 Hz with a 64 X 64 elements Cincinnati IRC-64A camera in the 3 - 5 micrometers band. Examples of these recordings are presented. The data were analyzed for scintillation effects, atmospheric point spread function effects and refraction effects. These data are compared to the atmospheric conditions that were recorded simultaneously, in order to model the infrared scintillation effects with meteorological conditions. Results of these comparisons are shown.
Atmospheric structures can be measured with incoherent optical radars (lidars). Because these structures drift with the wind, they can serve as a tracer for remote sensing of the wind vector. For this purpose, a dual monostatic-scanning lidar system is available to measure the atmosphere simultaneously in two different directions over a maximum range of about 1 km. The transit time of identified patterns between two sensing points in the horizontal plane, in combination with the geometry of the lidar, provides sufficient information to derive the horizontal wind vector. The method is based on cross-correlation techniques. To determine the spatial wind vector as a function of altitude it is sufficient to measure in three different slant directions. This can be realized with a triple lidar or with a single lidar by measuring consecutively in three different directions and using a similar but more extensive inversion method. A selection of experimental results is presented.
NOVAM, the Naval Oceanic Vertical Aerosol Model, has been developed to predict the nonuniform and nonlogarithmic extinction profiles that are often observed in the marine atmospheric boundary layer. The kernel of NOVAM is the Navy Aerosol Model (NAM) that calculates the aerosol size distribution at 10 m ASL from meteorological parameters. The aerosol profile is calculated from the surface layer size distribution with a physical model. Extinction profiles are calculated from the aerosol profiles using a Mie code. NOVAM requires validation in different meteorological scenarios. During the KEY-90 experiment, July 1990 near Marathon, Florida, NOVAM was validated in a tropical marine environment. We measured the surface layer particle size distribution profile at levels from 0.5 to 4 m ASL to evaluate the large-particle end of NAM. The NOVAM prediction of the aerosol profile in the mixed layer was evaluated by lidar measurements of the 1.06 micrometers backscatter profile. The time-serial lidar measurements show the convective plumes and the variability in both the aerosol content at higher levels in the boundary layer and in the boundary layer height itself. Consequences for application of NOVAM for slant path transmission are discussed.
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