James H. Churnside

Aperture averaging of optical scintillations in the turbulent atmosphere

We have developed approximate expressions for the aperture-averaging factor of optical scintillation in the turbulent atmosphere. For large apertures and weak path-integrated turbulence with a small inner scale, the variance of signal fluctuations is proportional to the -7/3 power of the ratio of the aperture diameter to the Fresnel zone size. If the inner scale is large, the variance is proportional to the -7/3 power of the ratio of the aperture diameter to the inner scale. In strong path-integrated turbulence, two scales develop. That portion of the variance associated with the smaller scale is proportional to the -2 power of the ratio of the aperture diameter to the phase coherence length. That portion of the variance associated with the larger scale is proportional to the -7/3 power of the ratio of the aperture diameter to the scattering disk. These simple approximations are within a factor of 2 of the measurements.

Scanning-laser glint measurements of sea-surface slope statistics

A scanning-laser glint meter designed for field measurements of sea-surface slope statistics is described. A narrow laser beam is scanned in a line, and specular reflections (glints) are counted in bins according to their slope angle. From normalized glint histograms, moments to the fourth order are calculated, and slope probability density functions are approximated with a Gram-Charlier expansion. Field measurements with this instrument show good agreement with previous results when the stability (essentially air-sea temperature difference) is near neutral (zero). Under conditions of negative stability (warm ocean), both the mean-square slope and the probability density function kurtosis increase.

Polarization lidar measurements of honey bees in flight for locating land mines.

A scanning polarized lidar was used to detect flying honey bees trained to locate buried land mines through odor detection. A lidar map of bee density shows good correlation with maps of chemical plume strength and bee density determined by visual and video counts. The co-polarized lidar backscatter signal was found to be more effective than the crosspolarized signal for detecting honey bees in flight. Laboratory measurements show that the depolarization ratio of scattered light is near zero for bee wings and up to 30% for bee bodies.

Wander of an optical beam in the turbulent atmosphere

A simple, analytic, geometrical optics expression for the variance of the beam displacements caused by propagation through weak refractive turbulence described by the Kolmogorov spectrum is presented. The analytical formula includes the effect of the divergence or convergence of the initial beam. The formula is compared with numerical results obtained from a more complicated expression including effects of diffraction and strong path-integrated turbulence. The simple geometrical optics expression holds for apertures larger than the Fresnel zone size and larger than the ratio of the square of the Fresnel zone to the phase coherence length.

Infrared spectral radiance measurements in the tropical Pacific atmosphere

Downwelling thermal infrared emission from the tropical atmosphere is affected strongly by the typically large amounts of water vapor. In two experiments within the last 2 years we have used a Fourier transform spectroradiometer to measure tropical atmospheric emission, concentrating on the “window” region between about 800 and 1200 cm−1. Shortly after the first of these experiments, substantial differences between measured and calculated radiances led to the development of a new water vapor continuum model. This model subsequently has been incorporated into several widely distributed radiative transfer codes (LBLRTM, MODTRAN, FASCODE). Measurements from the second tropical experiment, which occurred during March and April 1996, validate this new continuum model. This is an important comparison because the new measurements were taken with an improved instrument under better defined clear-sky conditions than the original tropical data on which the continuum correction was based. Model residuals are of the order of the uncertainty in measurements, especially of the atmospheric water vapor and temperature profiles.

Experimental evaluation of log-normally modulated Rician and IK models of optical scintillation in the atmosphere

Approximating the probability-density function of optical irradiance fluctuations in the turbulent atmosphere under all propagation conditions requires a model with at least two parameters. Two phenomenological two-parameter models that have been proposed, the IK and the log-normally modulated Rician probability-density functions, are compared with measured probability-density functions of the irradiance of laser light in a turbulent atmosphere under a variety of propagation conditions. The parameters for each model are obtained from measured second and third moments. It is concluded that the log-normally modulated Rician model is the better approximation to the data. However, the effects of the intermittency of turbulence must be included in the model for short propagation paths.

Angle-of-arrival fluctuations of a reflected beam in atmospheric turbulence

The statistics of the angle-of-arrival fluctuations are studied for the case of a laser beam reflected from a curved surface in a uniformly turbulent atmosphere. The variance for the direct beam and for the reflected beam and the covariance for the two beams are calculated for arbitrary divergence of the illumination and radius of curvature of the reflector. Experimental results support the conclusions that the reflected-beam angle-of-arrival fluctuations are very sensitive to small deviations from perfect collimation and that the correlation of the fluctuations in the direct and reflected beams is high.

Review of profiling oceanographic lidar

Abstract. This paper provides a review of the development of profiling oceanographic lidars. These can provide quantitative profiles of the optical properties of the water column to depths of 20 to 30 m in productive coastal waters and to depths of 100 m for a blue lidar in the open ocean. The properties that can be measured include beam attenuation, diffuse attenuation, absorption, volume scattering at the scattering angle of 180 deg, and total backscattering. Lidar can be used to infer the relative vertical distributions of fish, plankton, bubbles, and other scattering particles. Using scattering as a tracer, lidar can provide information on the dynamics of the upper ocean, including mixed-layer depth, internal waves, and turbulence. Information in the polarization of the lidar return has been critical to the success of many of these investigations. Future progress in the field is likely through a better understanding of the variability of the lidar ratio and the application of high-spectral-resolution lidar to the ocean. Somewhat farther into the future, capabilities are likely to include lidar profiling of temperature in the ocean and an oceanographic lidar in space.

Transient radiative transfer equation applied to oceanographic lidar

We estimate the optical signal for an oceanographic lidar from the one-dimensional transient (time-dependent) radiative transfer equation using the discrete ordinates method. An oceanographic lidar directs a pulsed blue or green laser into the ocean and measures the time-dependent backscattered light. A large number of parameters affect the performance of such a system. Here the optical signal that is available to the receiver is calculated, rather than the receiver output, to reduce the number of parameters. The effects of albedo of a uniform water column are investigated. The effects of a school of fish in the water are also investigated for various school depths, thicknesses, and densities. The attenuation of a lidar signal is found to be greater than the diffuse attenuation coefficient at low albedo and close to it at higher albedo. The presence of fish in the water is found to have a significant effect on the signal at low to moderate albedo, but not at high albedo.

Polarization effects on oceanographic lidar

A simplified radiative transfer equation yields a simple analytic expression for the co- and cross-polarized return in a linearly polarized oceanographic lidar. This equation agrees well with the lidar data over a wide range of oceanographic conditions. The relationship between depolarization and lidar attenuation shows three distinct relationships corresponding to water within the Columbia River plume, near-shore water outside of the plume, and off-shore water.