We present some preliminary results and discussion of our ongoing effort to develop a prototype volumetric atmospheric optical refraction simulator which uses 3D nonlinear ray-tracing and state-of-art physics-based rendering techniques. The tool will allow simulation of optical curved-ray propagation through nonlinear refractivity gradient profiles in volumetric atmospheric participating media, and the generation of radiometrically accurate images of the resulting atmospheric refraction phenomena, including inferior and superior mirages, over-the-horizon viewing conditions, looming and sinking, towering and stooping of distant objects. The ability to accurately model and predict atmospheric optical refraction conditions and phenomena is important in both defense and commercial applications. Our nonlinear refractive ray-trace method is currently CPU-parallelized and is well-suited for GPU compute implementation.
The Navy is actively developing diverse optical application areas, including high-energy laser weapons and free- space optical communications, which depend on an accurate and timely knowledge of the state of the atmospheric channel. The Optical Channel Characterization in Maritime Atmospheres (OCCIMA) project is a comprehensive program to coalesce and extend the current capability to characterize the maritime atmosphere for all optical and infrared wavelengths. The program goal is the development of a unified and validated analysis toolbox. The foundational design for this program coordinates the development of sensors, measurement protocols, analytical models, and basic physics necessary to fulfill this goal.
The refractive index structure parameter C 2/n(z) as a function of vertical height z, is a key parameter describing the turbulent intensity of the atmosphere, and is important for modeling and predicting beam propagation behavior. Over the past several decades many vertical C 2/n models have been developed, many based on empirical data from field test campaigns involving difficult in situ measurements by radiosondes, or remote-sensing using scidar/lidar/radar techniques. Each model has its own set of limitations and caveats. We have developed an improved C 2/n parametric model for the maritime environment, which uses the Navy Surface-Layer Optical Turbulence model for the low-altitude surface boundary layer, and merges with a generalized Hufnagel-Valley model for the middle- and upper-altitudes (up to 24 km elevation). It takes inputs of local bulk meteorological measurements and forms an estimate of C 2/n based on Monin-Obukhov similarity theory. We present phase-screen wave-optics propagation simulations comparing our improved model with previous models, in terms of turbulence metrics such as Fried's atmospheric coherence width r0, the scintillation index, and the Strehl ratio for both the weak and strong turbulence regimes, for vertical and slant paths, and for various characteristic regimes of the ratio w0=r0, where w0 is the Gaussian beam waist radius.
Using an 850-nanometer-wavelength free-space optical (FSO)communications system of our own design, we acquired field data for the transmitted and received signals in fog at Point Loma, CA for a range of optical depths within the multiple-scattering regime. Statistical estimators for the atmospheric channel transfer function and the related coherency function were computed directly from the experimental data. We interpret the resulting channel transfer function estimates in terms of the physics of the atmospheric propagation channel and fog aerosol particle distributions. We investigate the behavior of the estimators using both real field-test data and simulated propagation data. We compare the field-data channel transfer function estimates against the outputs from a computationally-intensive radiative-transfer theory model-based approach, which we also developed previously for the FSO multiple-scattering atmospheric channel. Our results show that the data-driven channel transfer function estimates are in close agreement with the radiative transfer modeling, and provide comparable receiver signal detection performance improvements while being significantly less time and computationally-intensive.
The performance of terrestrial free-space optical communications systems is severely impaired by atmospheric aerosol
particle distributions where the particle size is on the order of the operating wavelength. For optical and near-infrared
wavelengths, fog droplets cause multiple-scattering and absorption effects which rapidly degrade received symbol
detection performance as the optical depth parameter increases (visibility decreases). Using a custom free-space optical
communications system we measured field data in fog within the optical multiple-scattering regime. We investigate the
behavior of the estimated channel transfer function using both real field-test data and simulated propagation data based
on field-test conditions. We then compare the channel transfer function estimates against the predictions computed using
a radiative-transfer theory model-based approach which we also developed previously for the free-space optical
atmospheric channel.
Efficient military operations require insight in the capabilities of the available sensor package to reliably assess the
operational theatre, as well as insight in the adversary's capabilities to do the same. This paper presents the EOSTAR
model suite, an end-to-end approach to assess the performance of electro-optical sensor systems in an operational
setting. EOSTAR provides the user with coverage diagrams ("where can I see the threat?") and synthetic sensor images
("how do I perceive the threat?"), and allows assessing similar parameters for threat sensors. The paper discusses the
elements of EOSTAR and outlines a few of the possible applications of the model.
We present some preliminary results from our recent free-space optical communications field test experiments in
the foggy littoral environment along the coast of Point Loma, San Diego, conducted between October 2009 and
June 2010. Our custom-built 850nm lasercomm system uses on-off keyed non-return-to-zero intensity-modulation
and direct-detection to transmit pseudo-random bit sequences (PN-11 codes) at 250 Mbps over a 300m horizontal
atmospheric path. We investigate improvements offered by using the latest Advanced Navy Aerosol Model to
calculate the aerosol size-distribution function, a fundamental input to the radiative transfer code which we use
to generate an estimate of the channel frequency response/impulse-response function. The estimated channel
response function is used to design an equalization filter to correct signal distortion due to multiple-scattering
effects and additive noise. We compare the performance of the Advanced Navy Aerosol Model against the
more simplistic log-normal, Gaussian, and Mooradian Pt. Loma distributions. In this conference proceeding
manuscript we are presenting only preliminary findings of our work in progress. Additional analysis, verification,
and study is required before any final results can be posited from these preliminary findings.
The environment is nowadays one of the most limiting factors for reliable detection, clear imagery and thus a successful
classification of potential threats by electro-optical (EO) sensors. However, the characterization of the environment and
the assessment of its impact on sensor performance remains a difficult issue. Measurements of meteorological
parameters are not always easy and cannot always be reliable. It becomes more and more interesting to extract the
information the environment by new methods. In this paper, the initial steps and the methodology of an inverse scheme
that retrieve valuable information about the EO propagation conditions from infrared (IR) camera images is proposed.
The use of the method under subrefractive conditions shows that features of the medium can be derived through a
thorough analysis of sensor images. By an original use of EO propagation modeling, it is possible to partially reconstruct
sensor images that were deformed by a refractive atmosphere.
We have been engaged in a long-term test to determine the beam extinction effects of aerosols. During four test periods
of one month duration each, we propagated a beam over a 7-km path near the ocean surface, and measured the received
intensity in two near-infrared wavebands (1.061 μm and at 1.622 μm). In each test period, meteorological
measurements were obtained from a meteorological buoy located at the mid-point of the propagation path. These
meteorological data were used as input for the Advanced Navy Aerosol Model (ANAM). In this paper we will describe
the comparison between the ANAM predictions and the measured transmission. We found that there are significant and
sustained discrepancies between the ANAM model predictions and the optical transmission data. We will focus on two
particular problems that have emerged from our comparison: first, the ANAM dependence on local wind speed can
cause errors, and second, the local relative humidity is not optimally coupled in the ANAM. We will present the
analysis we used to support these claims, and we will present recommendations for modifications to the model.
The performance of imaging and laser systems can be severely degraded by atmospheric turbulence, especially
for near-horizon propagation paths. Having the ability to predict turbulence effects from relatively easily obtained
measurements can be useful for system design and feasibility studies, and for real-time optimization of optical systems
for the current environment. For this reason, so-called 'bulk' models have been developed that can estimate turbulence
effects through the refractive index structure parameter (Cn2) from mean near-surface meteorological and sea surface
temperature measurements. Bulk Cn2 models are directly dependent upon empirically determined dimensionless
functions, known as the dimensionless structure parameter functions for temperature and humidity. In this paper we
attempt to improve bulk optical turbulence model performance by determining new over-ocean forms for the
dimensionless temperature structure parameter (T).
During 2005-2006 atmospheric propagation experiments were conducted in the Zuniga Shoals area near San
Diego to examine the impact of environmental conditions on low-altitude electro-optical propagation above the ocean
surface. As part of this experiment the Naval Postgraduate School (NPS) deployed its flux research buoy along the
propagation path. The measurements obtained on the NPS buoy enabled T values to be obtained and new functions to
be determined. These new functions differ greatly from those presented in the past, in that the new T values asymptote
towards very high values as the stability approaches neutrality. The dependence of the new T function on the stability
parameter in stable conditions was also different from that previously proposed. When these new functions were
inserted into the NPS bulk Cn2 model, the resulting values agreed much better with directly measured turbulent Cn2
values in unstable conditions, but in stable conditions the new function actually made the agreement worse.
An infrared (IR) signal propagating along a 'line-of-sight' horizontal or slant path within the marine surface layer can encounter substantial perturbations. These perturbations include signal extinction due to molecules or aerosol particles; refractive modulations that can amplify or reduce a signal; and scintillation, which is a higher frequency fluctuation in signal intensity. In an effort to elucidate these issues an infrared transmission link was included in a long term propagation field experiment conducted at Zuniga Shoal to study the effects of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various times of the year. The transmission path was 7.2 km long connecting an IR broadbeam transmitter (at about 6.5 m ASL) at the Naval Amphibious Base at Coronado, and an IR telescope receiver at Zuniga Shoal (at about 11.5 m ASL). Both locations are in the general San Diego area. The transmission measurements were made at two wavelengths: near-IR, centered at 1.061 μm and short-wave IR, centered at 1.622 μm. In this paper we discuss prominent features of the long-term measurements including diurnal variations and the effects of the marine layer. We also compare the field measurements with the extinction predictions generated by the Advanced Navy Aerosol Model (ANAM), and we discuss how long-term field measurements can be used to tune and correct the ANAM.
Turbulence can be a dominant factor in image and laser beam degradation for optical systems operating in the near-surface maritime environment. A long-term propagation field experiment was conducted at Zuniga Shoal (near San Diego) to study the impact of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various points of the year, during which scintillometer measurements were obtained along a 7.2 km over-water path and a 'flux' research buoy deployed along the propagation path collected concurrent mean meteorological, atmospheric turbulence, and wave data.
We use the refractive index structure parameter (Cn2) as the critical parameter for quantifying the effects of atmospheric turbulence on laser system performance, including received power fluctuations, beam spread and beam wander. Bulk estimates of Cn2 were derived from the buoy mean meteorological measurements using the Navy Surface Layer Optical Turbulence (NSLOT) model. Cn2 was also determined from atmospheric turbulence measurements obtained from a sonic anemometer on the buoy. These independent Cn2 values derived from the buoy data are compared with Cn2 values computed from the infrared propagation measurements to determine how the NSLOT model performs under different environmental conditions. In addition, the optical measurements and bulk estimates of Cn2 are used to study the effects of the atmospheric turbulence on operational optical systems.
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.
The Navy Aerosol Model (NAM, available in MODTRAN) is widely used as a tool to assess the aerosol extinction in the marine atmospheric surface layer. NAM was built as a regression model in the 1980s to represent the aerosol extinction at deck height as a function of the meteorological conditions. The recently developed Advanced Navy Aerosol Model (ANAM) utilizes additional experimental evidence to supersede NAM by correcting the underestimation of the concentration of aerosols larger than a few microns. More importantly, ANAM provides the aerosol extinction as a function of height between the surface and several tens of meters. Present-day naval surveillance and threat scenarios require detection of targets at the horizon, such as seaskimming missiles, or small targets such as rubber boats. In either case, the propagation path from sensor to target is likely to come very close to the wave surface and in order to estimate detection ranges, an assessment of the transmission losses along the path is necessary. To answer the question posed in the title, we assess the two models using two meteorological data sets (784 cases) representative of diverse maritime conditions in regions of interest around the world.
Infrared scintillation measurements were obtained along a 7.2 km path over San Diego Bay, concurrently with mean meteorological and turbulence measurements obtained from a buoy located along the path. Bulk estimates and turbulence measurements of Cn2 were computed from the buoy data and compared with the optical scintillation-derived Cn2 values. Similar to the results of previous experiments, the bulk Cn2 estimates agreed well with both the scintillation and turbulence measurements in unstable conditions, increasingly underestimated Cn2 as conditions approached neutral, and agreed less well with scintillation and turbulence Cn2 values in stable conditions. The mean differences between bulk Cn2 estimates and both the turbulence and scintillation measurements when conditions were not near-neutral exhibited an air-sea temperature difference and wind speed dependence, possibly indicating that the forms of the empirical stability functions used by the bulk model are incorrect. The turbulent Cn2 measurements from the buoy showed excellent agreement with the scintillation values in unstable conditions, but had surprisingly large differences in weakly stable conditions. This disagreement may be related to the fact that humidity fluctuations begin to increasingly influence refractive index fluctuations when the air-sea temperature difference is small and are not properly taken into account by the sonic temperature measurements. As the absolute air-sea temperature difference approaches zero the bulk Cn2 estimates decrease much more rapidly and to much smaller values than either the scintillation or turbulence measurements. Fortunately, in such near-neutral conditions scintillation is usually small enough to have little effect on many optical system applications.
An infrared (IR) signal propagating along a 'line-of-sight' horizontal or slant path near the earth's surface can encounter substantial perturbations. These perturbations result in refractive distortions (low-frequency modulations that can amplify or reduce a signal) and scintillation (a higher frequency fluctuation in signal intensity). Scintillation impacts the ability of IR systems to detect high-speed, sea-skimming missiles that are a serious threat to Navy assets. Scintillation is quantified by the refractive index structure parameter Cn2. An opportunity to quantify scintillation occurred during the Validation Measurement for Propagation in the Infrared and Radar (VAMPIRA) field test at Surendorf, Germany, during March - April 2004. Scintillation measurements were made along a slant, near-sea surface path of about 8200 m with a transmissometer that operated in the mid-IR regime. At the same time an AMBER camera system was used to obtain images of stationary lights at the mid-IR. The purpose of this paper is to present several ways of obtaining Cn2 by using either the SSC transmissometer or the AMBER imagery.
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.
The U.S. Navy has an interest in the use of laser systems for surface ships. Such systems must operate within a thin near-surface environment called the marine atmospheric surface layer. There exist substantial gradients in temperature and momentum within this layer which make turbulence a strong function of height. We are interested in robust and simple optical turbulence models that can be used to predict turbulence along near-horizontal paths. We discuss several different models that are based upon similarity theory, and we compare the models with field transmission data taken from both over-water and over-land propagation paths.
The U.S. Navy has a renewed interest in the use of high energy laser (HEL) systems for ship defense scenarios. Surface ships must track and engage targets within a thin near-surface environment called the marine atmospheric surface layer. Within this layer exist substantial gradients in temperature and momentum, thus making extinction and turbulence strong functions of height. In such an environment, a primary cause of beam degradation is the atmospheric composition along the beam path, and this problem is compounded by the vertical variations in extinction and turbulence.
The three primary effects that must be predicted for a successful system model are extinction, turbulence, and thermal blooming. Although these factors are present for any HEL scenario within the atmosphere, they are particularly prominent for the marine near-surface environment. Aerosol extinction can be a strong function of the near-surface path height when there are windy conditions, and this vertical dependence must be reflected in the model. The occurrence of turbulence along the path also degrades the on-target beam intensity, and this effect is also strongly height-dependent, with paths nearest the ocean-air interface encountering the greatest scintillation.
We will discuss our efforts to provide a useful irradiance-on-target envelope using existing models and meteorological databases to analyze the efficacy of an HEL system. An irradiance prediction for a HEL weapons system must be accurate and reliable since it will be impossible to perform appropriate field tests across the full spectrum of possible operational environments to be encountered by such a system.
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 Rough Evaporation Duct Experiment (RED) assessed the effects of the air-sea boundary layer on microwave and infrared (IR) signal propagation near the sea surface. The experiment was designed around the Floating Instrument Platform (FLIP) research platform, which was moored 10 kilometers off the northeast shore of Oahu, Hawaii. A 10-kilometer infrared propagation path was created from FLIP to a shore-based receiver and both scintillation and transmission measurements were made around the clock for a two-week period.
An accurate model for the propagation of infrared signals in the marine atmospheric surface layer remains an elusive goal. Within the first tens of meters of elevation above the sea surface there are substantial vertical gradients of mass and temperature, and this has a strong effect on the prediction of extinction of the infrared signal. The effectiveness of the propagation models will be investigated and the results from the infrared signal propagation study during RED will be shown.
An infrared (IR) or optical signal propagating along a 'line-of-sight' horizontal or slant path near the earth's surface can encounter substantial perturbations. These perturbations result in refractive distortions (low-frequency modulations that can amplify or reduce a signal) and scintillation (a higher frequency fluctuation in signal intensity). In an effort to elucidate the above issues a field test was conducted at the Naval Air Warfare Center (NAWC) at China Lake, CA, during July 2001. Transmission and scintillation measurements were made along slant, near-surface paths over land at 1280 m and 3850 m with a SSC (SPAWAR Systems Center) San Diego transmissometer that operated in the IR regime and in an almost aerosol-free environment. The field test has revealed that slow-scale refractive effects can create pronounced changes in the recorded one-minute average intensity of the IR source. Scintillation can also generate signal changes by a factor of 5 to 10 over very short time scales. In this paper we explore the relation between the refractive changes and scintillation, as well as models developed to describe and predict transmission and scintillation effects. Models include the exploitation of the propagation factor (a multiplicative factor defined from the local refractive field and geometry of the measuring system) and the use of wavelets as IR signal processors.
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