This paper describes current instrumentation employed in obscuration research and identifies instrumentation needs in areas where progress is impeded by lack of adequate optical or E-0 instrumentation. Discussions are given of the current state of instrumentation research being funded to inform the audience of the Army's needs in this area and stimulate future advances.

Fourier transform analysis of data obtained during Smoke Week III indicates that the mass extinction coefficient for phosphorus smokes appears to be similar to a stochastic variable with a nonstationary mean. In the present work, this analysis is extended to examine the temporal dependence of the mass extinction coefficient for 1) selected wavelengths in the 6 - 12 micrometre band over a 5m path, and 2) for 3.4 micrometres over a path approaching 100m. The nonstationary mean values obtained with the Fourier transform analysis are then compared to those obtained using particle size distribution data. The comparison shows reasonably good consistency between the two sets of data and suggests that, in at least some cases, the mass extinction coefficient should not be treated as an "optical constant" of the smoke.

Results of recent research efforts to quantify effects of battlefield countermeasure smokes on electro-optical sensor performance are presented. The approach is through modeling and testing with emphasis on comparison of modeling theories with results from major smoke effectiveness field tests. The scope of the modeling includes effects of extinction, scattering and (thermal) emission for scenarios from the visible through the infrared. The work represents preliminary results of a long range effort to couple all factors which ultimately effect sophisticated electro-optical sensors. Special emphasis is given to modeling path radiance.

Extinction by a cloud consisting of many aerosol particles has been found to fall into two distinct wavelength regions which are adequately described by the geometric optics theory and the Rayleigh theory. Because the cloud consists of many different particles, the narrow extinction resonance structure of individual particles is lost and extinction is governed by shape and size as predicted by geometric optics. At longer wavelengths particles are in the Rayleigh region and extinction for a large variety of particles is predicted by the Rayleigh ellipsoidal theory. The transition region lying between the applicability of these two simple theories occupies only about one wavelength decade.

Experimental measurements of winds, vorticity and convergence in large Project Flambeau fires were made in conjunction. with propagation measurements of radar, IR and visible electromagnetic radiation. Visibility was reduced to tens of feet, IR attenuation. coefficient were on the order of 1 km^-1, while radar was fore-shortened by one percent with from 3 to 9 minutes of beam bending.

The optical effects of a hot fire Plume can only be calculated in a real-time situation if the dynamics and thermodynamic properties of the plume can be calculated in real-time. A method is presented which can be anplied to the above Problem and many other physical problems. The procedure is to break the Physical nrocess into a sequence of processes to be performed in a cyclic manner, one complete cycle ner time step, with each cycle ending on a physically acceptable state. Calculations are shown first for a steady radially symmetric Plume over a uniform source. Calculations are nresented which show agreement with the much slower conventional approach. The method is then extended by relaxing the assumntion of instant mixing. Radially varying sources, chemical constituents from the flame and atmospheric humidity, are also included.

The development of simulation models to assess the ability of modern day sensors operating under adverse weather conditions and in the presence of man-made obscurants, to detect, recognize, and identify objects of interest is an ambitious goal made difficult by the large number of independent variables involved. A first step toward the achievement of this goal is the development of the Electro-Optical Systems Atmospheric Effects Library (EOSAEL). With this concept (library of models) the effects of stratus, stratocumulus, cumulus congestus cloud types; fog/haze and rain; atmospheric gases; vehicular dust; chemical and artillery smoke/dust can be examined separately or in conjunction to determine their effects (degradation) on the , propagation of radiation through the aforementioned media. The atmospheric constituents can be varied to determine their effects on contrast and transmission. The change in transmission due to the effects of windspeed on the man-made obscurants as a function of time can also be examined. Using EOSAEL a sensitivity study investigating the effects of obscurants on the contrast transmission has been performed for the specific scenario of a sensor (human eye) looking at an object over a fixed geometry under daytime illumination conditions.

Naturally occurring or windblown dust causes periods of low visibility, particularly in arid regions, and can be a source of obscuration of electro-optical devices operating in the visible or infrared spectral regions. Occurrence of windblown dust and factors influencing the injection of soil into the atmosphere by the wind are described. Characteristics of the dust affecting atmospheric optical properties discussed include particle number density, particle size distributions, and complex index of refraction. The ways that variations of these characteristics affect the transmission of visible and infrared energy are described.

An assessment is presented of obscuration by battlefield dust and smoke of downward viewing infrared sensors in the 3.5 - 4.2 μm atmospheric window for altitudes of 1000 m or less. For artillery-produced contaminants, dependences of target acquisition probabilities on intensity of incoming rounds (i.e., rate of aerosol production) and on optically weighted aerosol mass loadings are established.

Cirrus cloud optical properties are largely unknown because of the var-iety of ice crystal shapes frequently occurring in these clouds, which make Mie calculations difficult. In addition, the index properties of the ice surface are functions of temperature as well as wavelength. Because of the large size of the particles in cirrus, scattering is even more predominantly in the forward direction than in liquid water clouds. Consequently, cirrus appear more transparent than is the fact and are seriously under reported. The distribution of cirrus optical thickness is strongly skewed to small values. Unreported thin,(τ∠1) cirrus may occur over as much as twice as much area or time as does reported cirrus. An average (and marginally reported) cirrus cloud might have a beam transmission of 0.5, diffuse transmission of 0.8 and emissivity of 0.2 at 10 μm. Currently measurements of cirrus are being conducted at Boulder and Maui. This paper reviews past and current measurements of cirrus properties and presents simple models relating these properties.

An overview of the relationship between target and background signatures, atmospheric effects, sensor characteristics and their combined effect on thermal imaging sensor performance is given. Specifically addressed are target background temperature differences, atmospheric transmission and path radiance, sensor detector noise, signal processing techniques (AC and DC coupling) and image display. Recently collected case history data relating the above variables to 8-12 micrometer image quality under different weather conditions are presented. A review of current Air Force efforts to further understand their combined effect and to model sensor performance is given.

A primary objective of the SNOW-ONE Field Experiment conducted in Vermont, during January through March 1981 by the US Army Cold Regions Research and Engineering Laboratory, was to initiate the establishment of a comprehensive data base for electromagnetic energy propagation through falling and blowing snow. In addition to transmission, measurements were also made to characterize (a) the meteorological conditions during the snowfall, and (b) snow particle size distributions, shape characteristics and concentration. We present an analysis of visible and infrared transmission measurements made during various snow conditions. Comparisons of the wavelength dependent extinction are also reported.

As part of a program to develop improved models for infrared and visible extinction in adverse weather, a one-kilometer transmission range has been established near Houghton, Michigan, where annual rainfall is about twenty inches, fog is common, and annual snowfall often exceeds 200 inches. A Barnes Engineering Transmissometer is used to record transmission in the 3-5 and 8-14 micrometer bands, and a Helium-Neon laser transmissometer is used to record transmission at 0.63 micrometers. During transmission measurements, an automated weather station periodically records standard weather parameters. Results are presented as plots of the logarithm of the infrared extinction coefficient versus the logarithm of the visible extinction coefficient (the GAP model format). Extinction coefficients for rain tend to be along straight lines, in fairly good agreement with the GAP model, when the atmosphere contains rain drops but not suspended droplets. Measured extinction coefficients in snow tend to lie along straight lines on the log-log plots, with slight differences from one snow storm to another, which are attributed to differences in the types of snowflakes. Measurements in fog, recorded at one minute intervals, trace the evolution of fogs as they form, mature, and dissipate. These processes tend to follow straight lines on the log-log plots, in contrast to the widely scattered data recorded at larger time intervals in other measurement programs, and this suggests that a better model for fog extinction can be developed by considering the evolution of the fog drop size distribution.

The phase functions of several types of natural particles have been measured between 5 and 172 degrees using a polar nephelometer with a Helium-Neon laser as the source of radiation. By cooling the laboratory to the outside temperature and using a vertical duct in the roof, natural snowflakes may be brought into the scattering volume. The phase functions of both falling and blowing snow exhibit the diffraction peak characteristic of particles whose size is considerably greater than the wavelength of the incident radiation as well as significant backscattering. The phase function associated with NaC1 cubes 335±85 micrometers on a side has been studied for various polarizations of both incident and scattered light. Comparison with the phase function calculated for an NaCl sphere with a diameter of 335 micrometers indicates that the cubes scatter more light between the angles of 30 and 150 degrees. This is attributed to external reflections by randomly oriented cubes. Because the phase function is normalized over a 47 solid angle by definition, the diffraction peak must be smaller for cubes than spheres. The effect of surface roughness on the phase function was studied by using particles of volcanic ash. Surface roughness increases the scattering above the Mie theory predictions for angles between 40 and 120 degrees, and also considerably broadens the back scattered peak.

Approximate solutions appropriate for real time calculations of directional path radiance and atmospheric contrast transmittance were developed and tested using an extensive data base gathered with a specially instrumented aircraft. The high-resolution, simultaneous optical and meteorological measurements were made in a broad range of environmental conditions and diverse geographical areas in the United States and western Europe. Modelling techniques based upon these data yield computationally fast and consistent results of reasonable accuracy. The effects of the natural variations in environmental physical parameters upon the calculations of spectral contrast transmittance are examined through systematic application of the modelling techniques. Comparative analyses are made of the changes in the visible spectrum contrast transmittance associated with typical changes in some of the relevant parameters such as the depth and density of the low-level haze layer, aerosol absorption, surface spectral reflectance, solar zenith angle and viewing path.

This report presents a summary of major design features of and some early results from a program of measurements of optical and physical properties of fog and haze recently conducted for a period of six weeks in wintertime conditions in West Germany. Measurements included vertical soundings of fog and haze density in the lower few hundred meters of the atmosphere, nearly continuous aerosol particle size distributions in the 0.115 to 77.5 μm radius range, visibility and transmissometer data, liquid water content measurements, and quantitative LIDAR observations. Samples of measurements and results of some preliminary analysis are included.

Contrast transmittance is an important quantity used in the analysis of electro-optical sensors. It depends upon the geometry of the sensor relative to the target and background as well as the optical properties of the obscuring medium. Earlier models of contrast transmittance treated either broadband systems or uniform sky conditions. The models dealt with in this paper are concerned with contrast transmittance for finite clouds, overcast skies, and precipitation conditions for any wavelength in the visible and infrared spectral regions where scattering processes are dominant. Solutions to the problem are presented for single scattering, multiple scattering, and thermal emission for various positions of the observer and target. The models are applicable to nighttime as well as daytime conditions and are useful for the analysis of the performance of electro-optical systems in adverse situations such as dust clouds, smoke clouds, heavy overcast skies, and rainfall. In addition to the contrast transmittance, the models also allow one to calculate the transmittance and path radiance for the same geometry and values of the optical parameters. The basic input parameters for these models consist of the volume scattering and volume extinction coefficients for specific wavelengths, the scattering phase functions, the surface albedo, the coordinates of the observer relative to the target and the source of radiation, and the background spectral radiance. Three spectral regions were examined in detail; the visible at a wavelength of 0.55 pm, the mid infrared at 4.0 μm, and the thermal infrared at 10.0 μm. In the visible region scattering processes are dominant; in the mid infrared, scattering and thermal emission are equally important; and at a wavelength of 10.0 μm only the thermal emission is of significance.

Model calculations of the time development of the ontical properties of maritime, urban and rural aerosols growing into fogs are given The calculations are carried out for several different ultimate temperatures and rates of change of temperature which lead to different degrees of supersaturation and conseouent optical properties. Different initial temperatures and relative humidities are examined. In the cases of lowest ultimate temperature the fog development and its optical properties are carried to the point at which a single mode distribution oceanic origin maritime aerosol 'becomes bimodal with a well separated heavy fog distribution. The time development of the aerosol near and through the supersaturation phase is obtained using the non-eouilibriun growth equations. The growth of sample point of each component distribution mode of the aerosol are tracked with time. Extinction coefficients are given for A = 0.55μm, 3.7 μm and 10.7μm.

Aerosols play an important part in determining optical properties of the atmosphere over the ocean. Problems of measuring aerosol properties and the need to forecast them, have led to the development of model simulations. At present, the models rely on standard meteorological variables as input and produce aerosol size distribution. These models are generally dependent on the knowledge of wind, relative humidity, visibility and altitude of the observation site, however they do not take into account the history of the airmass and the associated generation and transport processes of aerosol. As a result such predicted aerosol size distributions have a high variability when compared to individual measurements. Their value is therefore limited to climatological applications. Current studies consider the generation, transport and decay processes. In the generation process bursting bubbles during white cap conditions are considered. This process is important for particles larger than about 1 micron diameter. Smaller particles are generated mainly through gas-to-particle conversion processes over continents. Because of their relatively long life time in the atmosphere, these particles cannot meteorologically be traced to their origin. Transport processes are fairly well described through dynamic boundary layer models, bulk formulas for vertical transport, and general circulation models for horizontal air trajectories. Dissipation processes include coagulation for small particles, gravitational settling for particles larger than 5 micron diameter and turbulent transport through the upper boundary of the mixed layer. These studies serve to delineate appropriate meteorological variables which can serve as inputs to a dynamic aerosol model. For a practical use such dynamic models are still too complex and simplifications are needed. A compromise is dictated by the type and accuracy of available meteorological input data. At present white cap coverage, depth of mixed layer, airmass type and travel time of the air over water are considered as additional appropriate model inputs besides wind and relative humidity.

As part of a number of broader-based studies, Calspan acquired a considerable quantity of data describing bcundary-layer aerosol characteristics both at sea and in maritime continental locations. This paper focuses on aerosol composition data obtained in the North Atlantic, in the Mediterranean, in the northern Gulf of Mexico, off the coasts of California, Nova Scotia and Portugal, and on shore on Cape Cod and on the North Sea Coast of West Germany. The data show that the marine aerosol population varies considerably in composition, particularly in coastal areas, and does not necessarily comprise primarily sea salt aerosols. A continental/anthropogenically-derived component to the marine aerosol population is generally always observed, even in remote marine areas. In coastal regions, dramatic changes in aerosol composition occur as a result of wind shifts or airmass changes. As a result of these compositional differences, response of the aerosol to fluctuations in relative humidity is expected to differ from one locale or airmass to another. Therefore, aerosol size spectra alone are not sufficient for the prediction of visibility or the potential performance of EO systems under changing humidity conditions.

The propagation of radiation through a cloud depends on the concentration and size distribution of drops. A series of aerosol measurements in various types of fogs and hazes shows a direct relationship between a subset of the dry aerosol and fog drops. These measurements allow identification of the specific aerosol component (fog condensation nuclei, FCN) upon which fog droplets condense. The extensive variability of these particle concentrations can be seen in the preliminary FCN climatology which is presented. It is shown that the variability in fog drop concentration is largely due to differences in FCN concentrations which are characteristic of various air masses.

Observational experiments on turbulent intensities and aerosol distributions in the marine boundary layer (MBL) have been performed over several years. Observations have been made with ship-mounted and airplane-mounted sensors. Objectives have been to relate optical properties to meteorological descriptions which utilize scaling laws for the MBL. The approach has been to incorporate in the descriptions the surface fluxes of momentum, heat, and moisture, the processes at the inversion and the profiles within the intervening convectively mixed layer. We have found that optical turbulence parameters (C_N² and 10) can be readily estimated using measured mean values of wind, temperature, and humidity with recent bulk formulae to derive the surface fluxes. These estimates appear to be more reliable than values obtained from direct (but difficult to perform) turbulence measurements. The model for obtaining the estimates was evaluated on the basis of optical C_N² values with good agreement. Good comparisons have been observed between extinction values obtained from transmission measurements and those obtained from calculations on measured aerosol distributions. Existing empirical formulations which related the latter to wind speed and relative humidity appear to be inadequate except for climatological purposes. This is because other influences on equilibrium aerosol distribution are not included. Reformulation of these expressions is being performed to include the height of the inversion (mixing volume) and surface fluxes (aerosol generation and transport).

A statistical set of aerosols size distributions obtained by aircraft near San Nicolas Island are used to determine two of the constants of a Deirmendjian modified gamma aerosol size distribution. The computational procedures for relating the calculated volume angular scattering coefficients at 45° and the cross-sectional areas of the distributions to the constants of the gamma distribution are discussed. A limited set of angular scattering coefficients at 45° available from polar nephelometer measurements of San Nicolas Island are used to calculate infrared extinction coefficients (3.5 - 4.0 μm) from the model with Mie theory. The calculated extinction coefficients are compared with those measured by a Barnes transmissometer which operated over a 4.1 km sea water path at SNI at the same times as the nephelometer measurements. With the molecular contributions removed from the Barnes data using LOWTRAN 3B the calculated and measured aerosol extinction coefficients differed by less than 30 percent with an average percent of different 16 percent for the data set.

The ability to measure the vertical optical thickness of aerosols over ocean surfaces has been demonstrated using several different satellite sensors. Landsat data originally showed that a linear relationship exists between the upwelling visible radiance and the aerosol optical thickness (about 90% of this thickness is generally in the lowest 3 km of the atmosphere). Similar relationships have also been found for sensors on GOES, NOAA-5 and NOAA-6 satellites. The linear relationship has been shown theoretically to vary with the aerosol properties, such as size distribution and refractive index, although the Land-sat data obtained at San Diego showed little variability in the relationship. The differ-ences between the results found for the various satellite sensors are attributed mainly to uncertainties in the calibration of the sensors, although varying aerosol properties may be partially responsible. To investigate the general applicability of the technique to dif-ferent locations, a global-scale ground truth experiment was conducted in 1980 with the AVHRR sensor on NOAA-6 to determine the relationship at ten ocean sites around the globe. The preliminary results of this experiment at San Diego show excellent agreement with the previous Landsat data. In addition, analysis of the AVHRR Channel 1 and 2 radiances suggests that information on the aerosol size distribution may be obtained.

Marine aerosol models have historically relied on visibility as the primary descriptor of aerosol influence on visible and infrared E/O systems. A widely used model of this type is the original Air Force Geophysics Laboratory (AFGL) LOWTRAN 3B maritime model which is based entirely on the supplied visibility. Open ocean and shore line measurements of aerosol distributions and atmospheric transmission in the .5 to 12 μm spectral band have both initiated and partially verified models employing important windspeed and relative humidity considerations. This report explores some of these considerations as seen both on aerosol extinction models in present use by the Navy community and on experimental results taken at San Nicholas Island (SNI) and on ships in the North Atlantic. The importance of properly incorporating both relative humidity and windspeed into aerosol models is shown through comparisons of calculated E/O systems detecting performance against resolved and unresolved targets.

The development of sophisticated Electro-Optical systems for use in a shipboard environment is somewhat hampered by the lack of reliable models of the optical turbulence effects in close proximity to the ship, caused by the ship itself. To this end, preliminary microthermal fluctuations were made on an operating aircraft carrier during July 1979, along with other optical and meteorological measurements. These microthermal fluctuation measurements are being studied to ascertain their value in the development of a suitable model. Three CONTEL model MT-2 Microthermal Probes were specially modified for shipboard measurements and remote data acquisition, and deployed at three deck level sites on the USS LEXINGTON. These sites included the forward port and starboard sponscns, and an aft starboard sponson. Data was logged once each second for nine to fourteen hour periods on three consecutive days. The measurements include periods of flight operations during which the launching of aircraft can be clearly observed as strong perturbations in the data. Analysis of the data, measurement techniques and other evidence, raises several cautions as to the application of the standard optical turbulence models. In this light, the value of the data set is discussed and the character of future measurements are suggested.

Since the early 1960s, the Ballistic Research Laboratory (BRL) has been involved in the characterization of millimetre-wave propagation as it applies to military systems. Measurements have been made with radars and radiometers from 30 to 600 GHz of near earth propagation through both natural and cultural obscurants. Results of measurements made in rain, fog, snow, high humidity, dust and some smokes are presented for the major millimetre-wave windows. Some modeling performed by other agencies are presented for comparison.

A moisture sounding radiometer at millimeter wavelengths has been developed for high altitude measurements onboard NASA's WB-57F aircraft. Three channels about the 183.3 GHz water vapor line permit measurement of the atmospheric water vapor profile. A single window channel at 94 GHz provides correction for clouds over the ocean and for surface emissivity variations over land. The instrument is an imaging radiometer operating under microprocessor control throughout each data flight. The system is contained within two packages integrated into the WB-57F pallet. A ground support system was used to perform a quick-look analysis of the data collected immediately following each flight.

The Atmospheric Sciences Laboratory developed model MMTRN for millimeter wave extinction and backscatter is presented, with emphasis on the snow extinction model. The literature on millimeter wave snow extinction and backscatter is reviewed including significant results by Oomori et a1⁵, Nishitsuji⁴, Imai et al¹⁰, Babkin et al⁹, and Malinkin et al⁷. From selected data and empirical model is described which employs a K=ar^b relationship, where R, in mm/hr, is the rain equivalent snowrate. Three snow types are included: dry, moist, and wet. Comparison with similar relationship for rain is made showing analogous behavior. The snow backscatter model is presented, with its current deficiencies. The model predictions are compared with independent snow extinction data, and show reasonable agreement. Model deficiencies are identified and include: (1) lack of snow backscatter measurements, (2) insufficient snow extinction data above 50 GHz.

A study is made of the modeling of atmospheric resonant transmission in the millimeter wave region. The model consists of an exponential function whose argument is defined as the product of a spectral parameter, and the pressure, the temperature and the absorber amount to some powers. All the model functional parameters were determined through least-squares analysis of line-by-line data, computed with the use of the Van Vleck-Weisskopf line shape, at conditions typical of the lower S km of the atmosphere. The development was extended at 1 GHz intervals throughout the entire millimeter wave region (30 through 300 GHz). Individual models were developed for water vapor and oxygen, with a resulting reproducibility of 0.08 and 0.66%, respectively.

Increasing emphasis is being placed on the study of the effects of atmospheric turbulence on the propagation of millimeter and submillimeter waves because of the potential usefulness of these frequency bands in both military and civilian applications. The characterization of millimeter wave turbulence effects is more complicated than that of the optical propagation case because of a strong dependence on the humidity structure parameter q22 as well as on the temperature structure parameter OT². In addition, there is a dependence on the cross-correlation of these two parameters, denoted by G. Potential problems caused by turbulence include fluctuations of both intensity and angle-of-arrival, but the latter problem is of special interest because it leads to aimpoint wander in military fire control systems. This paper discusses methods of measuring both intensity and anglevf-arrival fluctuations at millimeter wavelengths as well as the measurement of CT ²' C_Q² , C_TQ, and other pertinent atmospheric parameters.

Results of an investigation of propagation in snow at 96, 140, and 225 GHz are reported. Measurements were made with the Harry Diamond Laboratories near-millimeter wave mobile measurement facility in snow with mass concentrations up to 0.9 gm/m³. For each frequency a correlation was found to exist between attenuation coefficients and snow mass concentration. Snow backscatter cross sections per unit volume at 96 GHz were found to range between 1 and 7 x 10^-5 m⁴/m³ for snow concentrations between 0.1 and 0.6 gm/m³.

The object of this paper is to discuss the fundamental limitations and the change of limiting resolutions of electro-optical systems operating in an atmospheric turbulent field. A fast-scanning interferometer was built to measure the time-averaged MTF's and the turbu-lence structure function coefficient CN² of a simulated turbulent medium. A computer-aided analysis system was used to measure the difference in contrast functions of the imaging system between the system (internal) noise and the atmospheric (external) noise. Psycho-physical experiments were also conducted to study subjectively the system performance under the influence of the atmospheric turbulence. The correction probability of the variables was analyzed and finally the change of limiting resolution of the electro-optical system was dicussed.

Lasers can be used for a number of applications involving atmospheric propagation. In these applications it is of interest to know the maximum range at which the laser beam can propagate without significant degradation due to atmospheric turbulence. The purpose of the measurements was to determine laser beam spread and wander as a function of the infrared wavelength, the optical turbulence level, and the propagation distance. Experiments were performed at White Sands Missile Range under conditions of low over-land turbulence levels, using 1.06μm, 3.8007μm, and 10.591μm lasers at ranges of 2, 6.4, and 10.5 km. The tests were conducted under a range of turbulence conditions, with C_N² most frequently of the order of 10-15 m-2/3. The NRL Infrared Mobile Optical Radiation Laboratory (IMORL) was used to generate the nearly diffraction limited beams produced by the lasers, to magnify the beam to a 90 cm diameter and to focus the beams onto a 120 cm collector. The intensity distribution at the collector mirror was recorded by a scanning infrared camera which dissects the image into 810 elements. The intensity distributions were recorded at a scan rate of up to 500 frames per second. Two dimensional scanning at the high scan rates provided excellent spatial and temporal resolution of the turbulence-degraded focal spot distributions. The data has been partially reduced and shows that 1.06μm is severly spread and broken up, that 3.8μm is slightly spread, and that 10.6μm shows little spread relative to its diffraction limited spot size.