We have been conducting research aimed at enabling prediction of desert optical environments from meteorological and satellite observations. To this end we have been collecting aerosol size distributions, visibility and meteorological data continuously for the past year at 2 sites in the Mojave Desert of California. Optical properties of dust are calculated from these data for a great variety of meteorological conditions. The concentration of dust particles is strongly dependent on wind speed for speeds greater than a threshold (7 m/s at Edwards and 15 m/s at China Lake). For individual wind episodes there is a clear relationship between wind speed and dust mass. However, that relationship changes from event to event leading to noisy summary plots; thus, indicating that other factors, such as dust sources, also influence dust loading. The HYPACT program is used to map out the sources, concentration and flow of dust. HYPACT is a pollution transport program that uses RAMS meteorological code output for input. HYPACT can calculate concentration forward in time from an assumed emission source or backward in time from an observation site. This facilitates the location of dust sources and the calculation of dust concentrations along air streams. Once dust concentrations are known and assuming the particle size distributions are the same as at the measurement sites knowledge of IR extinction is no longer confined to just the measurement sites.
Shipboard optical system are used as passive sensors for threat detection. When a threat is at low altitude part of the optical path from it may lie in the marine surface layer where it is distorted by refractivity gradients caused by large vertical changes in temperature and humidity in the first several meters above the sear surface. In addition, the poorly characterized giant sea salt aerosols in this region not only contribute to scattering but add to atmospheric refractivity by an amount equal to the product of water refractivity and aerosol contribution to atmospheric liquid water content. The added refractivity is about 10 percent of that due to water vapor, depending on relative humidity. These aerosols are created from bubble fragments and jet droplets caused by air bubbles bursting at the sea surface and can be hundreds of micrometers in size. Their size and number depend on wind speed and turbulent diffusivity. Because of the droplets' large size the vertical profile of liquid water content decreases more rapidly than exponential leading to correspondingly larger ray bending. As for scattering the large size of the droplets means that Beer's law of extinction does not apply. Part of this work has been presented previously, but the current work will incorporate more up-to-date size distribution data for near sear surface aerosols, obtained from a literature review in progress, into calculations of forward scattering and vertical refractivity profiles using selected wind speeds, relative humidity and air-sea temperature differences.
Shore-based polarized infrared images of ship target and background were obtained in the MAPTIP measurement series, using vertical and horizontal polarization filters. These images have been analyzed to yield degree of polarization and polarization contrast enhancement for targets with sea and sky background Strong vertical polarization is observed in the sea surface emission near the Brewster Angle in the 8 - 12 micrometer (LWIR) band, whereas horizontal polarization due to reflection is usually dominant in the 3 - 5 micrometer (MWIR) band. Ship targets at broadside aspect show a degree of polarization less than 5%. An enhancement of contrast by up to 30% in the LWIR (less in the MWIR) is obtained by polarization suppression of sea background. The contrast enhancement in the MWIR is counteracted by the horizontal polarization of reflected sunlight.
As previously reported aerosol size and composition and meteorological data were collected at three sites (Tehachapi Pass and Rogers and China Lake dry lake beds) in the western Mojave Desert in the summer of 1990. Aerosol size distributions exhibit the usual accumulation and wind speed dependent dust modes. The dust mode aerosols are illite clay. Their composition is wind speed independent for speeds up to 10 m/s, i.e. there is no silicate mode. Dust mass is wind speed independent up to 7 m/s. Beyond that, dust mass is exponentially related to wind speed by m equals 0.55 exp (0.59 u). Dust mass computed from the measured size distributions also exhibits the 7 m/s threshold. These characteristics are significantly different from the dust model used in LOWTRAN7/MODTRAN (based on Sahara data) which uses a large particle silicate mode in placed of the dust mode. It needs to be determined whether the optical properties of the two dust models are different enough to warrant changing the model in LOWTRAN. Their optical properties (extinction, albedo and asymmetry factor) from 2 to 12 (mu) are compared and used in LOWTRAN 7 for a variety of geometries and wind speeds. Perhaps two desert dust models should be used, one for an old desert such as the Sahara and another one for young deserts.
Sea radiance in the mid and far infrared shows a considerable degree of polarization which affects observed target-to-background contrast. An improvement in contrast is achieved with horizontal (s-plane) polarization filtering to suppress sea surface emission. Visibility and range affect the contrast in both polarizations. Scenes recorded during the MAPTIP measurement series off the coast of the Netherlands with the oceanographic ship HrMs Tydeman show decrease in contrast with range and better contrast for horizontal polarization against sea background. A simple mathematical model is presented relating contrast to extinction and path radiance which increase with increasing path length or worsening visibility.
The Visibility Impact Summer Study held from July to September 1990 was an intense, comprehensive study intended to measure aerosol size and chemical composition and to ascertain their optical effects. Size distributions for particle diameters from 0.01 to 10 (mu) were measured at hourly intervals and particle samplers were used to obtain chemical compositions at daily intervals at Tehachapi Pass and Edwards AFB, California. The extracted aerosol characteristics are discussed and compared to the desert aerosol model in LOWTRAN and the size and estimated composition of aerosols at China Lake reported upon earlier. We obtain relationships between aerosol mass and wind speed, diurnal size changes, and meteorological effects. Secondarily, extinction was calculated and used with LOWTRAN and radiosonde data for examination of aerosol effects on narrow band 3 to 5 and 8 to 12 (mu) imaging radiometer performance.
In previous papers we have presented measurements of degree of polarization in the mid and far infrared both in near-horizon sun-glint and in the adjacent sea background radiance. The polarization has been related to the Fresnel reflection and emission coefficients and compared with predictions. These measurements have been extended to include target-to-background polarization contrast measurements. Target polarization was small, but sea surface emission showed moderate degree of polarization in the p-plane (vertical). Considerable ship- background contrast improvement was achieved by polarization filtering in the horizontal plane. This effect was greater in the far infrared. Values are given for the degree of polarization of target and background and the contrast improvement factor due to polarization filtering.
Ground level atmospheric extinction from 0.5 to 12 (mu) was determined as a function of date and time of day by a combination of direct visibility measurements and Mie calculations made using simultaneously measured particle size distributions. The measurements were made over a three month period in the Indian Wells Valley of the Mojave Desert during quiescent weather conditions. From previous work and a review of the literature an estimate was made of the composition of the dust and combustion produced aerosols. Results of Mie computations agree very well with direct visibility measurements. However, extinction in the 8 - 12 (mu) range was frequently greater than in the 3 - 5 (mu) range consistent with unquantified field observations made using HgCdTe detectors. This unexpected result is due to ammonium sulfate absorption bands centered at 7 and 8.8 (mu) indicating that a detector designed to have a narrow spectral response centered at 7 (mu) could experience difficulties. This work surveys detector data available in the open literature and compares the effect of detector spectral response on calculated transmission in the Indian Wells Valley as a function of time of day and frequency of occurrence of aerosol conditions.
Measurements are reported of the infrared sunglint clutter channel caused by the direct solar reflection from the wave-perturbed sea surface at near-grazing angles of incidence. Apparent radiance has been measured over Monterey Bay as a function of azimuth and elevation angles relative to the sun direction using an AGA Thermovision 780 dual-band radiometric imaging system in the wavebands 2 to 5.6 (SW) and 8 to 12 micrometers (LW) with 7 degree(s) FOV. Time averaged profiles from multiframe averages show near-Gaussian angular distributions with half widths in the range 3 to 20 degrees (depending on solar angle) for look-down angles of 1 to 10 degrees below the horizon. The p- and s- polarized components of sea surface radiance have been obtained using an external wire-grid polarizing filter and compared with unpolarized measurements. The degree of polarization within the glint is shown to be horizontal and variable in the range 1% to 30%, depending on solar elevation, the higher degrees of polarization being found in the SW band. Significant vertical sea radiance polarization has been observed outside the solar glint in the 8 to 12 micrometers band, and is attributed to sea surface emission polarization.
Tactical Decision Aid codes provide field prediction of maximum range of FLIR use using simplified local environmental parameter input. A series of experimental comparisons at sea using airborne operational FLIRs with an instrumented ship target have shown poor correlation of observed range with prediction for detection and recognition. Classification and recognition range in UFLR are found to be highly insensitive to radiosonde atmospheric profile data input. Previous work has addressed modeling of the average target to background contrast temperature difference and atmospheric propagation of contrast. This paper addresses the implementation of the MDTD and MRTD algorithms in the code. Comparisons are presented of the prediction accuracy of the UFLR TDA using the standard Moser/Hepfer algorithm and an adaptation of the Johnson criterion used in the NVEOL Ratches code. For the limited data set of the study a reduction of RMS prediction error is achieved using the NVEOL algorithm.
An approach aimed at completely simulating the operation of camera or telescope, including direct calculation of images, in a spatially complex, turbid environment is presented. A Monte Carlo radiative transfer program coupled with an optics design program is used to compute aureoles at the image plane of a camera. The procedure uses the camera MTF so that the operation of the camera in a scattering environment can be completely simulated. The code is validated through a comparison of the analytical solutions for cameras with plane and Gaussian apertures.
In support of a continuing program of evaluation and experimental validation of FLIR Tactical Decision Aid performance codes, a series of measurements has been made of ship radiance temperature distributions together with sea and sky backgrounds. The measurements have been made at ranges from one quarter to one mile off the coastline in Monterey Bay, using a land- mounted Agema 780 dual band Thermovision radiometric sensor, with computer data acquisition and storage. The target ship was the research vessel Point Sur carrying a full suite of meteorological instruments and an array of thermal sensors for ship surface temperature distribution. Rawinsonde balloons were released to obtain vertical temperature and humidity profiles for path correction using LOWTRAN. The normal skin emissivity was measured in a separate experiment. The current data band consists of 898 stored radiometric frames containing ship images including starboard, port, bow and stern aspects, together with sea and sky background frames with varied zenith angle. These files are available for false color display and analysis in a variety of formats.
UFLR is one of an evolving set of FLIR performance prediction programs used at sea to predict the ranges for detection, classification, and identification of target ships. One aid in the validation of such a program is a sensitivity analysis of the program parameters. Sensitivity analyses indicate that the ranges for detection, classification, and identification are strongly sensitive to target areas, target-to-background, temperature difference and atmospheric conditions such as windspeed, visibility, humidity, and vertical temperature, humidity, and pressure profiles. One uncontrollable parameter is the noncontiguity in space and time of the radiosonde and FLIR measurements. This problem was investigated by dithering the radiosonde data, input to UFLR, with a random number generator to generate variations in the pressure, temperature, and relative humidity in the atmospheric profile. Results indicate that noncontiguity of measurements can lead to 50% error in range predictions.
The performance of the Tactical Environmental Support System FUR tactical decision aid program UFLR was reported upon previously. In using this code it was assumed that the temperature difference between ship and sea is fixed. In actuality the temperature difference changes with sea state, aspect, cloud cover, and visibility conditions. In this article a simple predictive temperature contrast taking these effects into account is described and validated against experimental data. A modified ThA code UFLRB including this input is compared with observed ranges. The modified procedure shows up to 36% improvement in detection range prediction at night.