The status and updating of the AFGL Atmospheric Absorption Line Parameter Compilation is discussed. This data base and its concomitant Trace Gas Compilation form the basis of HITRAN, the overall program for calculating high resolution transmittance and radiance through atmospheric paths. A new code has been developed which calculates absorption coefficients at equally spaced wavenumber intervals through multilayer atmospheric paths with a great deal of computational advantages realized. Examples of transmittance and radiance calculations are presented.
A computer code, LOWTRAN, has been developed for calculating the transmittance and background radiance of the earth's atmosphere at moderate spectral resolution (20 cm-1). The code was derived using a single parameter band model for molecular absorption and includes the effects of continuum absorption, molecular scattering, and aerosol extinction. A choice of atmospheric models is provided to the user for any atmospheric slant path. The model is described and application of the code is given by comparisons with atmospheric transmittance and radiance measurements.
Recent ground based interferometer measurements of the spectral characteristics of natural sky radiance and the radiance emanating from the effluents of a conventional power station smoke stack are presented. The interferometer measurements were performed at a spectral resolution of 0.5 - 1 cm-1. The meteorological conditions existing at the time of the measurements was recorded as well as the operating conditions of the woke stack. Comparisons are given between calculated sky spectral radiance using meteorological data and measurements at both ground level and at 15,000 ft altitude. The calculations were performed using both a line-by-line approach and a low resolution method (LOWTRAN 4 Radiance Model). The sensitivity of both methods to uncertainties in meteorological conditions is discussed.
Laboratory measurements of the infrared absorption by CO2 and H2O in regions of weak absorption has provided new information on the shapes of the extreme wings of the absorption lines. CO2 lines absorb much less in the wings than Lorentz-shaped lines with the same intensities and half-widths; N2-broadened lines absorb even less than self-broadened lines. Both self-broadened and N2-broadened H2O lines appear to absorb more over a large portion of the wings than Lorentz-shaped lines. Like CO2 lines, the wings of self-broadened H2O lines absorb much more than the N2-broadened H2O lines. Absorption by the wings of all of the lines decreases unpredictably fast with increasing temperature. Much of the H2O absorption frequently attributed to dimers may be due to the extreme wings of self-broadened H2O lines.
During the past several years the Optical Radiation Branch of the Naval Research Laboratory (NRL) has conducted extensive field experiments designed to validate high resolution atmospheric transmission codes for near-and mid-infrared wavelengths. The combined use of laser extinction data with Fourier transform spectrometer measurements over long atmospheric paths has produced several examples of high-quality, precisely calibrated transmission spectra. Instrumentation and procedures used in these experiments will be described and selected measurement results will be presented. Application of this information to current infrared atmospheric transmission problems will be discussed together with comparisons of the experimental data to high-resolution computer code calculations.
The present knowledge of the atmospheric aerosols is summarized briefly with respect to geographical distribution, time variations, optical properties, chemical nature, and natural and artificial sources. The measurement techniques for determining aerosol properties are sumarized with the limitations and results from each. The problems resulting from variations in refractive index, shape, composition, volatile constituents, relative humidity and time from collection until measurement are described. Needed measurements on atmospheric aerosols are outlined with some of the latest methods for obtaining them, including the needs and goals of some world-wide programs.
The objective of the NATO Project OPAQUE, a measurement program on the Optical Atmospheric Quantities in Europe, is to develop a data base of atmospheric optical and IR properties in Europe. An important part of this program is the measurement of the distribution of optical properties of aerosols as a function of time, geography and weather. Six OPAQUE stations are presently operational. The concentration and size distribution of aerosol particles is being measured with PMS and Royco particle counters. To verify comparability of these measurements an intercomparison of all OPAQUE aerosol counters was conducted at the Air Force Geophysics Laboratory. Results emphasized the need for proper maintenance and calibration. Properly maintained, all instruments appear to agree within a factor of two. In a supporting effort AFGL has been conducting airborne measurements of aerosol distributions in several of the OPAQUE areas. Measurements of concentration and size distribution have been conducted at altitudes between 300 and 5000 meters. Vertical profiles show very frequently a well defined low level haze layer of 1.5 to 2 km thickness, and a significant change in size distribution with increasing altitude.
Recent work comparing results of sampling aerosol through ductwork, with impactors, and by other standard methods versus in situ methods are presented for three aerosol environments: Airborne measurements of atmospheric aerosol, at high temperatures inside a smoke stack, and in a laboratory environment at very high concentrations. The airborne work involves the comparison of particle size distributions measured by an in situ sizing instrument inside a wind tunnel with that obtained through ductwork using an isokinetic intake. The results show a sharp decrease in particle numbers measured in the ductwork at sizes above a few microns, in all cases, with negligible losses in the submicron range. A water cooled in-stack particle size spectrometer is being developed for the EPA. In situ measured size distributions will be compared to impactor samples and integrated extinction coefficients with transmissometer opacity values. A high concentration in situ measuring laboratory instrument is described which can perform measurements in the 0.3 to 6.0 µm size range at concentrations up to 107cm-3 without dilution.
Single-wavelength lidar systems can be used to observe distributions of aerosol, cloud, and precipitation elements over extended atmospheric regions with extremely high spatial and temporal resolution. Atmospheric structure revealed by this technique can provide information on atmospheric dynamic and physical behavior that is difficult to obtain by other means. However, quantitative evaluation of optical or physical densities from back-scatter records normally requires information or assumptions on the size, shape, and composition of the scattering particles. Recent research efforts have been directed to the development of lidar techniques for remote density analysis that minimize uncertainties introduced by particle characteristics and that assess the magnitude of these uncertainties. This paper reviews the capability of lidar for remote observation of atmospheric structure and presents recently obtained experimental data pertaining to the remote measurement of aerosol optical and physical properties. Presented are: (1) a comparison of backscattering observed from black and white smoke; (2) two-wavelength (0.7 and 10.6 µm) data obtained from lidar measurement of large-sized dust and small-sized smoke particles; and (3) transmission measurements of generated aerosols of various known size distributions at visible and 3.39-µm wavelengths. These data indicate that an optimum wavelength region exists for single-wavelength measurement of aerosol physical density and that multi-wavelength techniques can be used to derive particle size information.
This paper comprises a review of theoretical concepts needed to describe the effects of atmospheric turbulence on the performance of optical imaging systems and a discussion of techniques by which image resolution can be measured and the theoretical results tested experimentally. A comparison of experimental data with theoretical results will be presented. The close agreement between the measured and calculated image resolution supports the theoretical models.
For most applications, simplified models may be used to describe the power spectra of quantities affected by atmospheric turbulence. The atmospheric model consists of a layered structure of wind velocity and turbulence strength, which is found to adequately approximate the usual measured data. Power spectra of such quantities as log-amplitude, phase, phase-difference, arrival angle, and arrival-angle difference, are derived for a single turbulence layer located at an arbitrary position along the path. The individual spectra are appropriately scaled and added together to form a composite spectrum. In one example, of the arrival-angle spectrum, we see that the high frequency asymptotic dependence is no fixed power-law, but may be almost anything, depending on the wind and turbulence distribution.
For several years, experimental data relative to Atmospheric Turbulence and its effects on electro-magnetic wave propagation has been collected at the AMOS Observatory atop Haleakala on Maui, Hawaii. In general, the results obtained are consistent with theoretical modeling. A number of propagation parameters have been measured. The average value of the Mutual Coherence Function correlation scale, ro, is 9.6 cm at 5000Å. The average log-amplitude variance integrated over a 36 cm aperture is 6 x 10-4. Sufficient data exists to develop a low resolution vertical profile. On the average, this profile is consistent with the measured optical parameters. A strong enhancement of turbulence in the vicinity of the meteorological tropopause is not indicated.
In this paper we discuss how and why turbulence distorts the irradiance distribution and coherence properties of a light beam, with special emphasis on the irradiance scintillations in strong turbulence. This includes a discussion of the probability distribution of the scintillations, and the relationships among the source properties, receiver and turbulence strength in accounting for these scintillations. Finally, we indicate the extent to which adaptive techniques can be used to compensate for the effects produced by strong turbulence.
The effect of speckle after propagation through the turbulent atmosphere is an important consideration in remote sensing, coherent imaging through the atmosphere, optical coherent radar, speckle interferometry and COAT systems. In order to assess the effects of speckle, first and second order statistics and the temporal power spectral density for the received intensity are needed. A review of the current state of knowledge and new work in progress will be presented.
An approximate expression is presented for the joint probability density function of the IF signal magnitudes from an array of optical heterodyne detectors operating in the clear air turbulent atmosphere. The treatment considers local oscillator shot noise, log-normal amplitude fluctuations, and Gaussian phase front perturbations, where arbitrary correlation of phase and of amplitude fading are allowed.
The total atmospheric mutual coherence function is introduced as a tool for analyzing the dependence of the performance of several types of EO systems as the combined effect of molecules, turbulence, and aerosols. Analytic expressions for this function are derived in terms of the usual atmospheric parameters (absorption and scattering coefficients, Cn, etc) and applied to the analysis of several systems.
The degradation in performance of imaging or beam projection systems, due to turbulence in the atmosphere, can be evaluated or predicted in terms of a path-position weighted value of Cn2, the optical turbulence structure constant. The degradation also depends on the wavelength and range. At visible wavelewth, for paths of a few km, serious degradation occurs for Cn2 of the order of 10-14 m-2/3. Such turbulence is not uncommon for paths over the ocean and occurs frequently over land. The appropriately weighted value of Cn2 can be measured experimentally for a given path, with a slit scanning telescope, imaging a point source at the far end of the range. A portable system will be described that is capable of these measurements. This is coupled directly to an on-line data processing minicomputer to predict the performance of a given system, using Fourier and Abel transform techniques applied to models by Lutomirski and Fried. The results can be presented in a variety of forms, including immediate hard copy plots of the MTF of the atmoshpere, of the overall system being tested, or plots of predicted radial distribution of intensity on target. For non-uniform turbulence, the proper weighting of Cn2 as a function of path-position is crucial. For example, Cn2, obtained from scintillation, weights path-position differently and yields results of marginal value to determination of image resolution or spot size.
With recent changes provided by the Clean Air Act Amendment of 1977, the effect of industrial smoke plumes on scenic landscapes assumes heightened importance. The impact of large coal-fired power plants is most easily understood through the use of before-and-after photographs. A technique has been developed to modify a clean "before" scene as dictated by solutions to the radiation transfer problem in a polluted atmosphere. This allows one to produce simulated "after" scenes, which can illustrate the visual effects of pollutants emitted under a variety of circumstances. Application of this technique to very large coal-fired power plants suggests that such facilities may impair scenic vistas under some circumstances, unless stricter pollution controls and standards are enforced.
Optical heterodyne receivers operating in the turbulent atmosphere are subject to severe degradations in performance caused by random fluctuations in the index of refraction of air. These performance degradations can be minimized by understanding the statistical nature of the optical field after propagation over an atmospheric path. In order to gain some insight into these effects, an optical link consisting of a He-Ne laser transmitter operating at a wavelength of 632.8 nanometers, and a four element heterodyne receiver array containing a frequency tracking local oscillator was fabricated and operated over a 1.6 kilometer horizontal path in the turbulent atmosphere. The outputs of each of the array elements was separately demodulated and the probability density function for two or more elements was experimentally determined. The measured density function agree well with theoritical predictions. This work provides a basis for determining optimum receiver processing systems and will lead to bit error probability measurements for these optimum recievers.
The simulation of the operational performance of systems utilizing optical subsystems is a design tool that is evolving with the rising cost of hardware development. The scope of existing individual sensor models and broad scope operational simulations is reviewed in terms of their suitability for use by the design engineer. The philosophy of an interemdiate category of models is presented in terms of an existing, evolutionary small scale operational model, PERTAM. The structure of PERTAM is described in terms of the integration of the individual sensor phenomenological simulations with the operational aspects of utilization. An example of the execution of the model is presented.
A contrast telephotometer, capable of simultaneouly measuring the brightness of two adjacent objects at various wavelengths, was designed and constructed. The instrument was used to measure the contrast of Mormon Mountain (located 30 km southeast of Flagstaff, AZ) against the horizon sky. Measurements were initiated on September 11, 1974, and data taken through August 29, 1975 are presented here. A theoretical model was formulated to examine the dynamics of contrast variations as a function of observation angle, tropospheric and stratospheric aerosol extinction coefficients. Calculations and measurements indicate that while measurements in the blue portion of the visible spectrum are primarily sensitive to aerosol within the mixing layer, contrast measurements in the red portion of the spectrum are sensitive to aerosol above the mixing layer. The relationship between wavelength, contrast and vertical aerosol distributions is quite dependent on observation angle. Because contrast is related to both tropospheric and stratospheric aerosol concentrations, the instrument is well suited to measure contrast changes caused by spatial variations in aerosol concentrations that take place over hundreds of kilometers. Contrast is directly related to visibility and consequently the contrast telephotometer is also a direct measure of visibility as a function of spatially varying aerosol loads.