A summary of the Air Force Geophysics Laboratory Atmospheric Transmission Modelling Effort is presented. A description and status of the HITRAN and LOWTRAN models is provided with emphasis on recent developments and modifications. Several outstanding issues requiring additional work are also discussed.

Infrared solar spectra, with . 02 cm^-1 resolution, were collected during sunset from a balloon at 40 km on October 27, 1978. Portions of the data obtained during the flight have been compared with theoretical calculations made using the data on the AFGL tape. The results of these comparisons reveal a number of areas of disagreement between theory and experimental results. The areas of disagreement are discussed in detail.

Water vapor absorption measurements and comparisons with predictions are discussed along with ongoing and planned research efforts. Hydrogen fluoride (2.7-3.0 μm) laser line measurements as functions of temperature and pressure are presented. Comparisons of these results with predictions suggest the need for using an improved set of absorption line strengths, and a super Lorentz/Voigt line shape. Results thus far have been carried to -18°C although ongoing efforts plan to extend these to -50°C. Deuterium fluoride (3.5-4.0 μm) laser line measurements have been made under mid-latitude summer conditions and as a function of temperature and pressure. The measured absorption in this region is about twice as large as current predictions and ex-hibits a larger self-broadening term. These results can be modeled by assuming both a far wing and a water-aggregate contribution. Other ongoing efforts include determination of pressure and temperature dependencies of water vapor absorption in the 8-12μm region using a carbon dioxide laser including considerations of possible dimer contributions as well as similar measurements in the 0.3-2mm region using a far infrared/submillimeter laser. Possible physical models for the water vapor absorption (continuum or anomalous absorption) in the 3-5 μm and 8-12μm regions are also discussed.

Calculations of atmospheric radiances are compared with observations made in the 15 pm and 4.3 pm spectral regions by the HIRS/2 on TIROS-N. Differences between calculation and observation lead us to make empirical corrections in calculations of atmospheric transmittances. The largest correction is required in the HIRS/2 interval at 690 cm-1. Corrections arealso required in the intervals centered at 703, 2210, and 2270 cm^-1. At NOAA's National Environmental Satellite Service, several studies of laboratory spectra show good agreement between observation and calculation in these spectral regions. However, in the 15 μm CO₂ Q-branch, classical theory underestimates the observed temperature dependence of widths of spectral lines.

A discussion of a series of airborne measurements of atmospheric optical properties in Europe. Visible spectrum measurements made using an instrumented C-130 aircraft between 0.5 and approximately 6 km above ground level are described with respect to experimental procedures, results and application of the data to the determination of atmospheric contrast transmission. Data locations are illustrated as are the primary radiometer systems utilized to make the measurements. Sample data from flights conducted during 1976 and 1977 are presented illustrating irradiance levels beneath scattered to broken cloud decks, and total volume scattering coefficient variations as a function of altitude above ground level. Comparisons between scattering coefficient and relative humidity profiles are illustrated and the data base availability is reviewed.

The contributions by aerosols to the total extinction as measured by a BARNES transmissometer at San Nicolas Island were determined by subtracting the molecular contributions as calculated using LOWTRAN. These aerosol extinction coefficients are compared to those calculated using (a) aerosol size distributions measured at one end point of the transmissometer and airborne over the optical path, and (b) analytical models (LOWTRAN 3B maritime model and the Munn-Katz model which includes surface wind speed and relative humidity dependencies). The extinction coefficient variation with altitude calculated using the analytical models are also compared with those calculated from measurements at sea of vertical aerosol size distributions.

From February to May 1977, extensive atmospheric transmission measurements were carried out over a 5.08 km, over-water path at Cape Canaveral Air Force Station using NRL's Infrared Mobil Optical Radiation Laboratory. This paper will report transmission measurements of 80 CO₂ laser lines for three measurement periods. These periods were typified by high visibilities, (> 20 km), and 7.4, 15.7, and 18.4 torr atmospheric water vapor contents, respectively. The set of laser lines include the following transitions: 00°1 - 10°0 P4 to P44 and R4 to R42 00°1 - 02°0 P4 to P42 and R4 to R40 The Hitran calculations of atmospheric extinction were made using a computer code adapted by NRL for our DEC 11/40. The December 1975 AFCRL line compilation was augmented by a current H2O continuum model. A comparison points out possible problems with some H₂O absorption lines as well as the shape of the water continuum absorption profile.

A study is presented of the variance of the monochromatic absorption coefficient and of the corresponding slant-path molecular transmittance due to errors in the tabulated line-parameter data and in the meteorological profiles used. General mathematical expressions for the variances are derived, which are simple and convenient to use in calculations involving specific atmospheric profiles and spectral line data. The results of calculations at eight representative frequencies for an assumed atmospheric profile and for typical uncertainties in the data, indicate that deviations larger than 0.01 should be reasonably expected in transmittance calculations along vertical atmospheric paths.

The atmospheric transmittance and radiance code LOWTRAN 4 has been modified to include first order scattering of sunlight into the line of sight. This scattering becomes important for wavelengths shorter than 5μm under daytime conditions. The radiance due to both aerosol and molecular scattering is combined with atmospheric extinction and thermal emission already contained in the model. A mean phase function is defined by averaging over a given aerosol size distribution, composition, and wavelength in order to simplify the calculations. A comparison of the model with observational data is presented.

This paper will summarize the current NASA Tropospheric Environmental Quality Remote Sensing Program for studying the global and regional troposphere from space, airborne and ground-based platforms. As part of the program to develop remote sensors for utilization from space, NASA has developed a series of passive and active remote sensors which have undergone field test measurements from airborne and ground platforms. Recent measurements with active lidar and passive gas filter correlation and infrared heterodyne techniques will be summarized for measurements of atmospheric aerosols, CO, SO₂, 0₃, and NH₃. These measurements provide the data base required to assess the sensitivity of remote sensors for applications to urban and regional field measurement programs. Studies of Earth Observa-tion Satellite Systems are currently being performed by the scientific community to assess the capability of satellite imagery to detect regions of elevated pollution in the troposphere. The status of NASA sponsored research efforts in interpreting satellite imagery for determining aerosol loadings over land and inland bodies of water will be presented, and comments on the potential of these measurements to supplement in situ and airborne remote sensors in detecting regional haze will be made.

Remote sensing methods offer various advantages over contact measurement methods both for characterizing the gaseous and particulate air pollutants emitted by different types of sources and for verifying that established emission standards are being met by regulated industries. Two such instrumentation systems are in routine use for characterization studies: a mobile pulsed ruby lidar system measures stack plume opacity with an accuracy comperable to an in-stack transmissometer; and a mobile high resolution (0.1 cm^-1) infrared spectrometer system measures multiple gaseous species concentrations in a longpath absorption mode or in a single-ended emission mode with near-laboratory accuracy. A laser-Doppler velocimeter system for measuring the velocity of stack plumes and winds aloft has recently been obtained. Several systems particularly aimed at meeting the measurement needs of enforcement personnel are under evaluation. Tuneable laser systems for use in the longpath absorption mode and in the differential absorption lidar mode are in various stages of development. Research programs are underway to determine the feasibility of remotely measuring particulate size distributions and pollutant (gases and particles) mass emission rates. This paper presents results obtained with the instruments currently in use and summarizes the current state of development of the various other systems.

We present a methodology for calculating the uncertainty in particulate backscattering derived from lidar aerosol measurements. Algebraic expressions are presented that include the effects of errors in (i) lidar signal, (ii) molecular density, (iii) atmospheric transmission, and (iv) lidar calibration. We also describe a simulation procedure that can be used to check the algebraic results by injecting random errors into simulated lidar measurements and retreival calculations. The algebraic and simulation results are demonstrated by applying them to stratospheric aerosol measurements by a new airborne lidar. A large set of balloonborne aerosol-counter measurements is analyzed to assess the probable error incurred by calibrating a lidar with the return from a "clean" (nearly dust-free) layer. The results show that in most latitude bands the upper troposphere is the preferable region for calibration (other factors being equal), and that the calibration errors are acceptably small.

Landsat data were originally used to demonstrate that a linear relationship exists between the upwelling visible radiance and the aerosol optical thickness (essentially all of this thickness is in the troposphere) over oceans. Since that time similar relationships have been shown to exist for sensors on the GOES and NOAA-5 satellites. A global scale ground truth experiment using Tiros-N data is planned and will investigate the variability of the linear relationship at different sites around the globe. A comparison of the results for the different satellites is presented, together with a discussion of the requirements for routine satellite monitoring of tropospheric aerosols on a global scale.

The attenuation of solar radiation by airborne Saharan dust was examined using hemispheric pyranometric data and a numerical radiative transfer routine. The analyzed data were taken as part of the GARP Atlantic Tropical Experiment (GATE) during several days in 1974. Changes in solar heating rates and atmospheric reflection and absorption due to the presence of dust were inferred from differences in the values computed from observations and those computed for a dust-free atmosphere. Measurements from dust-free days were analyzed to test the inference technique. It was determined that the Saharan dust increased the atmospheric reflection of shortwave radiation in the region by an average of 50% over clear sky values. Atmospheric absorption increased by an average of 9% in the presence of dust. These results yielded an average extinction of 61 Wm^-2 of the incoming solar flux due to Saharan dust. Observed heating rates in the dust layer were consistently higher than could be accounted for by gaseous absorption alone. Increases in heating rates did not exceed 0.10°C hr^-1.

The optical properties of carbon monoxide gas uniformly distributed in the atmosphere have been simulated in a 20.48 cm long cell in the laboratory. The altitude of the peak of the weighting function for several concentrations was found using a Pressure Modulator Radiometer (PMR). The effect of a fluctuating background radiance and gaseous nitrous oxide on the carbon monoxide signals were examined.

Landsat II imagery data and aircraft nephelometer measurements were analyzed to determine the quantitative properties of a stack plume emitted from a moderate-sized pulpmill. Aircraft measurements were obtained at several heights across the plume at 1.0, 3.0, and 6.5 km downwind from the stack, and for comparative purposes, the Landsat data were also analyzed at these same three locations and at 0.5 and 10.0 km. Overlapping, consecutive-day MSS data provided plume/no-plume radiances upwelling from the stack site. Imagery data from a 10- by 10km region in the vicinity of the mill were normalized to correct for atmospheric, solar, and viewing angle differences for the 2 observation days, and cloud-shadow data were used to evaluate sky radiance. Particle concentrations, vertical and lateral dispersions, and plume heights determined from both the aircraft and spacecraft measurement techniques are in good agreement.

This paper presents an overview of the effects of the atmosphere on remote sensing of terrestrial and man-made objects, which includes brief discussions of the present capability to represent such effects analytically. Absorption, scattering, and emission by molecules and aerosols, cloud interference, and atmospheric turbulence are among the topics discussed. Examples of analytical and measured results are presented, and the relationship is drawn between remote sensing effects and the other papers of this session.

This paper reports the results of calculations on the wavelength sensitivity of the irradiance of a laser beam due to the combined effects of diffraction and atmospheric turbulence, for propagation through the earth's atmosphere to an orbiting satellite. In particular, the issue of the optimum wavelength selection which minimizes the effective beam spread is determined for the following effects: wavelength, from 0.5 μm to 10 μm; long time turbulence; short time turbulence (perfect tilt correction); non-isoplanaticity, since the satellite is in motion; perfect higher order corrections, various zenith angles; and large aperture diameters. The new results differ from the results of previous studies in several aspects. First, the transmitter is located on a mountain as opposed to sea level to reduce the effects of atmospheric turbulence and extinction. Numerical evaluations are presented of the mean-square tilt angular pointing error for non-isoplanatic effects for both zenith and off-zenith angles. The resulting mean-square pointing error scales very closely to the -2.11 power of aperture diameter, and -1.75 power of the cosine of the zenith angle for a lead angle of 52 microradians, based on the profile of Barletti, et al. The results, for 3 to 4 meter diameter apertures, indicate that the minimum beam spread is always in the infrared, for all zenith angles. In the infrared, the beam spread angle can be decreased by 50% with tilt correction, with the spread at 10.6 μm wavelength smaller than that for 3.8 μm, although the beam spread angle is relatively insensitive to the infrared wavelength. Still smaller beam spreads can be achieved at the short wavelength end of the infrared spectrum only by using higher order adaptive optics to compensate for atmospheric turbulence; while the longer wavelengths can benefit from still larger diameters without the necessity for the use of higher order turbulence corrections.

A methodology has been developed for calculating the probability of cloud-free intervals of any size from conventional meteorological data. The methodology contains two important features. First, the probability calculations are done analytically. Previous cloud-free line-of-sight models were largely empirical, and putting them on an analytic basis has removed some inconsistencies caused by observer biases. An additional benefit was the quantification of the relationship between ground-observed sky cover and cloud amount observed from a satellite. Secondly, the notion of cloud-free line of sight (CFLOS) has been generalized to the much more useful cloud-free interval, which includes cloud-tree line of sight as a special case. The methodology can be used to calculate the probability of a cloud-free interval of various lengths within a straight line path of any length. The probabilities can be obtained for any time, season, geographic region, observation angle, or cloud altitude layer. Model parameters have been determined with the use of NOAA digital satellite imagery.

A series of cloud height measurements using a pulsed N₂-laser ceilometer were made at a location 5.6 kilometers from the Logan Airport tower (Boston, Mass.) The results were compared with data obtained from the Logan Airport rotating beam ceilometer and reports by the pilots of aircraft passing through the cloud layers. Wherever simultaneous data was available, the comparison was favorable. The system has the capability for 10 meter resolution and data rates to 500 complete vertical profiles per second. The system demonstrated its capability for performing well under all sorts of adverse weather conditions including rain, snow and fog. Cloud thickness could be inferred, the presence of multiple layers was easily detected, and clouds to altitudes of 9 kilometers were observed. The maximum observed altitude was not limited by signal-to-noise, but because higher clouds were not available during the test period. In addition to a conventional A-scope presentation, the round trip time-of-flight to the base of the nearest cloud was digitized, and presented visually as a numerical readout. A three minute movie, taken during conditions of light rain, will be shown, demonstrating the rapid fluctuations which can occur in the cloud structure on a time scale of a few seconds.

For quantitative analysis of ocean color from high altitude or orbital platforms, it is necessary to correct the apparent signal for losses and gains due to atmospheric scattering and absorption. However, comprehensive knowledge of the transmittance and path radiance of the atmosphere over the oceans necessary for these corrections is limited. In order to support studies of ocean color utilizing the Coastal Zone Color Scanner aboard Nimbus-7, measurements of atmospheric transmittance and path radiance have been made at a number of maritime stations. The measurements were made at ten wavelengths between 400 and 750 nanometers and have provided new insight into atmospheric scattering and absorption over the ocean. A description of the solar transmissometer used in the program is provided along with some of the measurements. Comparisons of the measurements with predictions from the atmospheric transmittance program LOWTRAN have been made and are discussed. The significance of the measurements to the remote sensing of the ocean color is also reviewed.

An overview is given of the wide variety of linear and nonlinear effects that influence the propagation of high energy laser radiation in the atmosphere. Thermal blooming and air breakdown are shown to be the most important nonlinear effects since they impose the most severe restrictions on the atmospheric propagation of high energy laser beams. Subsequent papers in this volume consider more detailed aspects of both of these problem areas. Special emphasis is given here to the review of thermal blooming to help place into proper perspective these subsequent papers.

Gas breakdown, or the ionization of the air in the path of a high power laser, is a limit on the maximum intensity which can be propagated through the atmosphere. When the threshold for breakdown is exceeded, a high density, high temperature plasma is produced which is opaque to visible and infrared wavelengths and thus absorbs the laser radiation. The threshold in the atmosphere is significantly lower than in pure gases because of laser interaction and vaporization of aerosols. This aspect of air breakdown is discussed in detail. Parametric studies have revealed the scaling laws of breakdown as to wavelength, laser pulse duration and these will be discussed and compared with existing models. A problem closely related to breakdown is the plasma production when a high intensity laser interacts with a surface. In this case, the plasma can be beneficial for coupling laser energy into shiny surfaces. The plasma absorbs the laser radiation and reradiates the energy at shorter wavelengths; this shorter wavelength radiation is absorbed by the surface thus increasing the coupling of energy into the surface. The conditions for the enhancement of laser coupling into surfaces will be discussed, particularly for cw laser beams, an area of recent experimental investigation.

High energy laser propagation in an absorbing medium is described by the Fresnel equation with a nonlinear term. While perturbative solutions exist for some special simple cases, the most useful approach has required development of efficient computer algorithms to satisfactorily solve problems of interest. This paper reviews the numerical techniques which iteratively solve nonlinear laser propagation problems and indicates the agreement obtained in comparison with laboratory experiments. The wide range of physical effects in addition to the nonlinear phenomena which are also included in the computer codes to predict the propagation of the laser beam from the laser cavity, through the optical train and into the atmosphere are also discussed.

Systems analysis studies frequently require simplified predictive methodology for determining nonlinear thermal blooming effects on a high energy laser beam propagating through the atmosphere. Because of the many variables, tens of thousands of propagation runs may be required in the course of a systems study. As a result, the wave optics codes are generally not practical due to excessive computation times. A methodology highly suitable for systems analysis has been constructed by observing that a phase integral can be defined which has the properties of a similarity variable. For a particular aperture plane beam shape, a high energy laser beam propagating through a convective medium experiences beam spread due to thermal blooming which can accurately be correlated as a simple function of the phase integral. A few well chosen wave optics calculations provide the data base for the correlation which then becomes a "scaling law". This scaling law expresses the dependence of the non-linear beam spread as a function of beam properties such as power, pulse repetition frequency, wavelength, Fresnel number and atmospheric properties such as absorption, transmission and wind speed. This correlation is combined with formulations for jitter and turbulence providing the basis for a simple yet highly accurate predictive methodology.

Propagation of high energy laser beams to space has previously been evaluated on the basis of constant, seasonally averaged standard atmospheres, time constant levels of turbulence with the appropriate altitude profile, and time constant winds in direction and velocity, independent of altitude. In actuality, atmospheric conditions are highly variable resulting in a large spread in propagation performance as a function of time. The purpose of this paper is to develop a methodology for determining the propagation performance on an annual probability basis, and to give an example of the use of this methodology.

This paper discusses the reduction of thermal blooming in pulsed laser beams. Pulsed lasers are of interest because under many conditions they suffer less blooming degradation than cw systems owing to partial clear out of atmospheric density perturbations between pulses. Unfortunately, scenarios do exist where the pulsed laser system remains limited by thermal blooming. Therefore, techniques to counteract the blooming of pulsed beams merit investigation. Thermal distortion effects in pulsed beams are typically classified in two categories: single-pulse blooming (transient degradation) and multiple-pulse blooming (steady-state degradation). The different perturbing phenomena exhibit dramatically different time scales, a fact which impacts on the selection of correction techniques. Potential enhancement techniques for alleviating pulsed thermal distortion include basic parameter optimization; correction by a transmitter with adaptive phase compensation; and beam guiding by active, remote control of atmospheric density gradients with auxiliary beams. UTRC is investigating the capabilities of several compensation approaches. The results of laboratory scale experiments will be presented demonstrating improved system performance from open-loop and closed-loop phase compensation with adaptive optics and intensity enhancement with beam guiding. Finally, the experimental results will be compared with theoretical code calculations.

A multidither adaptive optical technique using dither frequencies below that of the atmosphere's characteristic response has been proposed for thermal blooming compensation. When such low dither frequencies are used, the atmosphere, which is coupled to the high-energy laser (HEL) beam through the absorption processes that create blooming, is thermally driven at the dither frequency. The modulated thermal radiation returning from the hot spot (created at the target by the high-energy beam) is optimized by using a hill-climbing servo system, and through reciprocity the HEL target irradiance is correspondingly optimized. Computer analyses of the slow dither concept predict convergence to an optimum target irradiance even when strong thermal blooming conditions are present. We have established a laboratory facility for the evaluation of adaptive optical compensation. A low-speed wind tunnel containing a small concentration of Freon 12 is used to simulate a wide variety of realistic blooming scenarios. In this paper we report some results of the first experimental investigation of the slow dither concept.