James J. Wilson

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.

Oceanographic lidar attenuation coefficients and signal fluctuations measured from a ship in the Southern California Bight

We measured the attenuation coefficient of the National Oceanic and Atmospheric Administration lidar from a ship in the Southern California Bight in September 1995. The region from approximately 5 to 30 m in depth was covered. The laser was linearly polarized, and the receiver was operated with the same polarization and the orthogonal polarization. The measured values were between 0.08 and 0.12 m(-1) and were highly correlated with in situ measurements of the beam attenuation coefficient. Fluctuations of the lidar signal were found to be induced primarily by surface waves whose wavelengths are approximately three times the lidar spot size at the surface.

Airborne lidar imaging of salmon.

Lidar images of adult salmon are presented. The lidar system is built around a pulsed green laser and a gated intensified CCD camera. The camera gating is timed to collect light scattered from the turbid water below the fish to produce shadows in the images. Image processing increases the estimated contrast-to-noise ratio from 3.4 in the original image to 16.4 by means of a matched filter.

Enhanced backscatter of a reflected beam in atmospheric turbulence

We measure the mean and the variance of the irradiance of a diverging laser beam after reflection from a retroreflector and from a plane mirror in a turbulent atmosphere. Increases in both the mean irradiance and the normalized variance are observed in the direct backscatter direction because of correlation of turbulence on the outgoing path and the return path. The backscattered irradiance is enhanced by a factor of ~ 2 and the variance by somewhat less.

Power spectrum and fractal dimension of laser backscattering from the ocean

We flew an airborne lidar perpendicular to the coastline along straight-line transects that varied in length between 230 and 280 km. The sample spacing was approximately 3 m, so we sampled almost five decades of spatial scales. Except for the return from right at the surface, the power spectra of backscattered power had a power-law dependence on spatial frequency, with a slope of approximately 1.49. This corresponds to a fractal dimension of 1.76. This implies that the distribution is not as patchy as that of a purely turbulent process.

Two-color correlation of atmospheric scintillation

We present the results of measurements of the correlation of scintillations of two colors of light made in the turbulent atmosphere. In strong path-integrated turbulence the correlation is below that predicted by the weak-turbulence theory. A phenomological theoretical approach is used to account for saturation effects. This simple theory provides a reasonable approximation to the correlation data. Thus, we conclude that saturation effects reduce the two-color correlation of atmospheric scintillation.

Determination of ocean wave spectra from images of backscattered incoherent light

The application of imaging of sea surfaces has been investigated with respect to determination of sea wave spectra. Incoherent light is projected toward the sea surface, and the backscattered light is imaged with a camera. The primary scattering mechanism is assumed to be from particles suspended in the sea, so the backscattered intensity is determined primarily by the Fresnel coefficients. The ratio of the images detected at two orthogonal polarizations contains the desired information on the local slope of the sea surface, pixel by pixel, in one dimension. By integration, one can obtain the surface-height profile.

Remote sensing of wind velocity and strength of refractive turbulence using a two-spatial-filter receiver

Wind velocity across an optical path and refractive turbulence strength can be measured by observing a light source through the atmosphere with a receiver that contains two spatial filters. The frequency of the detected signal gives the transverse velocity of the turbulent structure, whereas signal intensity is proportional to refractive turbulence strength. The size of turbulent eddies that produce signals is determined by the optical setup. The position along the detectors field of view at which the measurement is made depends on the separation of the filters, and profiles can be made by varying the separation and using a telescope. The system requires longer integration times than one which uses a spatial filter at each end of the optical path, but it has the advantage of being able to use a natural source such as the Sun or a planet. An analysis of the system is presented along with numerical simulations and results from a short-range (several meters) laboratory experiment. The analysis assumes a single layer of refractive turbulence. Scales of the refractive turbulence in the inertial subrange from 5 to 20 cm will be of primary interest for this method.