The refractive index structure parameter (C_n²), a statistical measure of refractive index fluctuation in the atmosphere, is useful in predicting the intensity of atmospheric effects upon optical propagation. It can be derived from point measurements of high-speed temperature probes, but in many cases it is better to have a path-averaged measurement. Earlier optical techniques needed corrections for the inner scale of turbulence and were severely limited by the saturation of scintillation. This paper discusses an optical system developed at NOAA that is free of these problems, and makes path-averaged measurements of C_n² over path lengths from 100 to 500 meters.

The atmospheric modulation transfer function (IMF) depends upon an integral of the index of refraction variations (C_n²) along the optical path. C_n² undergoes large changes as a function of time and position within the 1-3km thick planetary boundary layer. Diurnal and seasonal variations of C_n² near the surface of south-central New Mexico are discussed.

We examine the seasonal, day-to-day, and diurnal fluctuations of C_n² using data from the MST radar at Poker Flat, Alaska. The radar has been operated in a nearly continuous mode since early 1979, with profiles recorded every 2-4 minutes. The results for C_n² are pre-sented at about 2 km height intervals from 4 to 20 km. It is found that the mean values of C_n² are about 10 dB larger in summer than in winter at all levels. The diurnal range is about 3 dB in the troposphere during all seasons, and varies in the stratosphere from about 10 dB during winter to 16 dB during summer. The relationship of mean C_n² with meteorologi-cal variables is discussed.

We review ray-optical methods of analyzing short-wavelength propagation in random media. The advantages and limitations of ray methods are discussed, and results of the statistical theory of ray segment fluctuations pertinent to ray tracing are summarized. The standard method of Monte Carlo ray tracing is compared to a new method which takes into account recent results on the statistics of ray segment fluctuations.

The continuum absorption by H₂O has several characteristics that are common throughout the windows in the infrared and millimeter-wave regions. Values of the continuum absorption coefficient calculated on the basis of simple, widely used line shapes may differ greatly from observed values in the windows between strong absorption lines. The temperature dependence of this absorption is also not predictable from present day understanding of line shapes or of dieters, which may also contribute. The shapes of self-broadened H₂O lines are quite different from those of N₂-broadened lines, and the difference increases with increasing distance from the centers of the lines. Data obtained from laboratory samples and from atmospheric paths are presented to compare the various windows in the infrared and millimeter regions.

The CO₂ laser photoacoustic technique has been used to measure 002 laser absorption spectra of water vapor/air mixtures containing various water vapor partial pressures at selected temperatures down to 10°C. At CO2 laser wavelengths where water vapor continuum absorption contributions are dominant, the water absorp-tion possesses a temperature coefficient of -2.2%/°C between 27°C and 10°C. At the fixed temperatures em-ployed, the water continuum possesses a dependence on water vapor partial pressure that includes both linear and quadratic terms. A similar temperature coefficient and water partial pressure dependence has been observed in previous studies of water continuum absorption between room temperature and 100°C.

Accurate field characterization of aerosol number density, extinction cross-section, mass extinction, and aerosol mass concentration is of basic importance in the development of improved smoke screens and electro-optical weapons systems. During a recent field test involving smoke screen tests, data were obtained from 1) a spectral transmissometer which measured transmittance once per second for 200 wavelengths between 2.5 and 14 micrometres, 2) optical particle size analyzers, 3) an electric cascade impactor, 4) a quartz crystal microbalance, and 5) mechanical mass samplers. The aerosol parameters of interest (number density, extinction cross-section, etc.) were computed through combinations of independently measured parameters in order to determine data consistency and the spread in values which might be encountered from such measurements. Results obtained from some of these instruments are described which show how the mass extinction coefficient can be expected to vary with time. It is shown that this time dependence is a major reason for wide variability in field measurements of the extinction coefficient.

To realize the full potential of spectroscopic measurement techniques for atmospheric measurements, accurate frequency calibration is vital to obtaining species specificity, and good intensity data are necessary to quantify the number of molecules being observed. This paper will discuss the various problems and solutions found in infrared laboratory measure-ments on atmospheric molecules. The advantages and limitations of different measurement techniques and equipment will be discussed. Calibration techniques and measurement accuracy and precision will be emphasized. Frequency calibration aids will be presented including current and planned tables of absorption frequency standards being developed by the Bureau of Standards for the calibration of high resolution devices with an accuracy of ± 3 MHz. Intensity measurements are very susceptible to systematic errors and great care most be exercised in order to obtain accuracies better than ±;10%. It is even more difficult to know what is the accuracy of a given measurement. Recent efforts at providing accurate intensity data will be described.

We will describe precision lineshape measurements of CH₄, both self- and air-broadened, and of HF in rare gas buffers using a tunable difference-frequency laser spectrometer. We find that the lineshape cannot always be characterized by the simple Lorentzian or Voigt profiles usually employed in atmospheric modelling. The effect of collisional narrowing of the Doppler distribution is quite evident for certain light molecules of atmospheric interest.

Long path atmospheric infrared spectra at ~0.02 cm^-1 resolution, as obtained by the University of Denver from balloon experiments, will be presented. The spectra will be analysed in terms of quantifying atmospheric trace constituents. Identification of new atmospheric spectral lines will be demonstrated. Spectral regions which require further experimental and theoretical work for more accurate modeling of atmospheric transmission will be discussed.

Atmospheric spectroscopy, employing the sun as a source, has been routinely used for a number of years as a means for identification and quantitative measurements of atmospheric species. Recent improvements in Pb-salt semiconductor laser technology when combined with infrared heterodyne technology provide the capability of obtaining solar spectra with sub-Doppler spectral resolution. In this paper we will discuss ultra-high resolution (0.007cm^-1) atmospheric solar absorption spectra that have been obtained from a Tunable Infrared Heterodyne Radiometer. The radiometer was developed for ground based observations in the 8 to 12 pm region and tunability is achieved through the use of Pb-salt semiconductor laser local oscillators (LO). Spectra have been obtained in a piece-wise fashion from 9.1 to 11.1 pm using laser emission modes that exhibit characteristics suita-ble for LO operation. Spectra showing absorption features of HNO₃, 0₃, CO₂, and H₂O will be presented along with comparisons of experimental and synthetic spectra calculated using a line-by-line atmospheric transmission model.

Studies will be described that define the capability of a spectrometer system to re-motely detect and characterize trace gases in a localized cloud; e.g., a stationary source effluent. The detection method utilizes all of the information contained in the observed spectral radiance contrast between cloud and background. It consists essentially of deter-mining the degree to which this spectrum is correlated with a computed (FASCOD1) reference spectrum. It is shown that trace gases can be reliably detected even when spectrum fea-tures are well below the noise level. The minimum detectable quantities (MDQ's) for various trace gases at one atmosphere total pressure are given. The MDQ's determine the combinations of gas column thickness and gas-background temperature difference that corres-pond to 95 percent detection probability and one percent false detection probability for a given system noise equivalent spectral radiance. The capability of the method to remotely infer the gas column thickness and temperature will also be discussed.

It is generally recognized that current and future atmospheric measurements, as well as the evaluation of the relative merits of the various instruments, depends heavily upon the availability and accuracy of infrared spectroscopic data for all atmospheric species. A review on the current spectroscopic data base will be given, as well as what is needed for present and future atmospheric measurements.

Molecules and suspended particulates in the atmosphere scatter and absorb visible and infrared radiation as it passes through the atmosphere. Gaseous absorption bands exist primarily in the infrared part of the spectrum in which the scattering of radiation is less important than in the visible. Nevertheless, if the scattering optical thickness of the medium is greater than about one for uncollimated radiation or greater than about ten for highly collimated radiation, then a photon may be scattered many times as it propagates from source to detector. In addition to multiple scattering, the radiation is scattered predom-inantly into the forward direction for the case of radiation incident upon relatively large particles. These conditions of multiple scattering and a highly anisotropic scattering pattern occur frequently in hazes, fogs, clouds, smoke, and dust. In order to account for the radiant energy at some point within the medium, it is necessary to consider multiple scattering radiation models. A number of detailed mathematical models and computational procedures of varying complexity have been developed in the last twenty years. In this review we consider some of the more practical radiative-transfer models used in the calculation of multiply scattered radiation in optically thick media.

Many of the available low-resolution models for calculating atmospheric IR spectral transmittance, including LOWTRAN, represent the transmittance as an empirically-derived function of a single composite variable. As a result, the calculated transmittances will be accurate only If "strong-line" conditions prevail. Accuracy is also compromised in the empirical models when too few of the function parameters are allowed to depend on species and wavelength, or when a single empirical function is used for many species and at all wavelengths. A generalized band model will be described that uses more accurate trans-mission functions having two composite variables (like the classical band models) and four to six wavelength-dependent parameters. Calculations based on this model have been used to estimate the magnitude of errors in some CO₂ and 0₃ transmittances and radiances computed by the empirical models.

A computer code, LOWTRAN 5, has been developed for calculating the transmission 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 and aerosol models is provided to the user for any atmospheric slant path. The LOWTRAN model is described and application of the code is given by examples of atmospheric transmittance and radiance spectra. Comments on the validity of the code are presented.

REALTRAN is the generic name given to a series of computer codes developed at NRL for predicting atmospheric transmittance in real time. The REALTRAN algorithms, tailored to meet the requirements of specific applications, are derived both from appropriately modified variants of codes such as HITRAN and LOWTRAN and from recent results of NRL's own atmospheric-propagation field experiments. REALTRAN codes have been developed for both laser and broadband applications. These codes are presently implemented on a Hewlett-Packard model 9825A calculator which has been configured to scan inputs from external sensors. REALTRAN provides cost-effective capabilities for using locally monitored meteorological conditions to forecast the operational performance of specific electrooptical systems.

There are presently in existence several well-established models for the calculation of transmittance through atmospheric gases. One is the low-resolution model called LOWTRAN, developed by AFGL and another is the monochromatic (or laser) model called LZTRAN developed by Science Applications Incorporated. In previously reported work, the present authors developed an analytical method using double exponentials to replace the low resolution gaseous transmittance tables in LOWTRAN. In the present work, the authors propose to replace the polynomial functions in LZTRAN with double exponentials, accounting also for atmospheric pressure varia-tions not originally included in this code. The proposed method was tested at 6 laser frequencies for a mixture of absorbers within the bottom 5 km of the atmosphere with a resulting transmittance accuracy of about 3%. The calculations use line-by-line data obtained with the use of the AFGL line-parameter tape.

For the development of long range infrared sensors, it is important to have an accurate assessment of the transmission of IR radiation in the atmosphere. Measured experimental data to date have been obtained over tens of kilometers. In this paper a theoretical analysis of factors affecting clear band atmospheric transmission over ranges exceeding 200 km is presented. The analysis considers altitudes below 10 km (important for detection of low altitude targets) in the IR and uses a Grumman line-by-line code, LINETRAN, which incorporates the AFGL Line Parameter Atlas. Multiple atmospheric layers in LINETRAN account for the effects of refraction and altitude-dependent absorbers. The absorption contribution from each layer is calculated and summed to give the total transmission between the source and the observer. LINETRAN molecular absorption (smoothed to 20 cm^-1 resolution)is compared to the molecular absorption predicted by the band-model code LOWTRAN for several clear bands and several ranges. The relative effects of humidity and visibility on band integrated transmission are then shown. Molecular line absorption, molecular continuum absorption, and aerosol extinction are included for two clear bands. The need for measurement of atmospheric parameters such as humidity and visibility during long range transmission measurements is seen to be essential for further progress in long range atmospheric modeling.

A model and computer code, FASCODE1 (Fast Atmospheric Signature Code) has been developed for the line-by-line calculation of radiance and transmittance with particular applicability to the earth's atmosphere. An algorithm for the accelerated convolution of line shape functions (Lorentz, Voigt and Doppler) with spectral line data is used. The contribution from continuum absorption is included in the model. The method is described and applications of the present code, FASCOD1B, are presented.

For the past several years the Air Force has been involv..ed with the attempt to measure narrowband atmospheric transmission over horizontal paths (up to 8km) in three spectral regions--visible, 3-5 micrometers and 8-12 micrometers. The data is required for use in LOWTRAN model verification, itself a prime tool for predicting imaging sensor performance as it relates to meteorological conditions. Problems associated with the use of commercially available equipment purchased by the Air Force are briefly described, as well as the design of a new transmissometer receiver intended to operate over the full 8km range. Measurements taken with this new receiver are presented along with their LOWTRAN predictions. Preliminary data indicates that the accuracy attainable with this instrument far exceeds that of the original equipment and will serve as a great help in LOWTRAN validation efforts currently underway.

One of the major problems in ensuring the accuracy of IR transmission data lies in the calibration of the measuring systems. As part of the activities of an international group a comparison of the calibration of a-number of similar transmissometers was investigated and clearly indicates the nature of the problems to be faced when trying to calibrate these or similar systems. This paper presents a summary of the effects noted and attempts to indicate the magnitude of the various sources of error and suggest a possible calibration procedure.

The major subject of the paper is advancing atmospheric transmission field measurement systems in response to new requirements. From the viewpoint of a complete field system installation, attention is given to the nature of the measurement and the capabilities and limitations in sensitivity, stability, and the time required for individual measurements. From the same system viewpoint calibration is reviewed with regard to concept, techniques, uncertainties and assumptions. Examples are given of system advances and these include making real-time measurements with automatic high-speed scanning, high sensitivity, wide spectral range and remote control. Special measurement conditions that are described include those encountered on the battlefield, in fog and precipitation, and in the presence of countermeasures.

A workshop comparing various techniques of measuring the light-absorption properties of aerosol particles is described. Preliminary results show that measurement error is a substantial fraction of the variability of those properties seen in the atmosphere. The relationship of absorption and light transmission is discussed, and a brief review of the techniques and measurements of light absorption is given.