Scanning infrared radiometer for measuring the air-sea temperature difference.

We describe a vertically scanning infrared radiometer for measuring the air-sea temperature difference without disturbing the water skin layer. The radiometer operates with a single wavelength channel that is 1.1 mum wide, centered on 14.2 mum, on the short-wavelength edge of a CO(2) atmospheric absorption band. The resulting high atmospheric absorption enables calibration of the horizontal-viewing signal with an in situ air-temperature sensor. The signal at all other scan angles is measured relative to that at the horizontal, providing a differential air-sea temperature measurement that is nearly independent of calibration offsets that can be a problem with independent air- and water-temperature sensors. We show data measured on a ship in the Tropical Western Pacific Ocean during July 1999, which exhibit important discrepancies from in situ data using bulk air-and water-temperature sensors. These discrepancies illustrate important differences between bulk versus skin water temperature.

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.

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.

Lidar profiles of fish schools.

A lidar system was used in a seawater tank to measure the average diffuse reflectivity of live sardines. Diffuse reflectivity was measured to be 10 % for a copolarized laser return and 3 % for a cross-polarized return. We used these calibration measurements to infer the density of sardines in areas of the Southern California Bight from vertical profiles obtained with the lidar mounted on a ship. Within schools densities up to ~0.01 kg m-3 were observed. During hourly survey periods total habitat densities up to ~2 x10-3 kg m-2 were observed.

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.

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.

Image jitter, blur, and scintillation regarding the retinal hazards of lasers.

The statistics of position and size of the image of a laser source observed through refractive turbulence in the atmosphere were measured using a simulated human retina. Both 3-mm and 7-mm apertures were used with a variety of propagation conditions. No significant effects of image broadening or motion were observed. Consequently, such effects cannot be depended on to mitigate scintillation hazards.

Effect of penetration depth and swell-generated tilt on delta-k lidar performance.

Delta-k lidar can be used to measure the amplitudes of small waves on the surface of the ocean as long as the tilt of the surface is known. This tilt can be inferred from Δk measurements at orthogonal polarizations. The wavelength of the lidar must be chosen so that the penetration depth of the light is much less than the length of the surface wave being measured; otherwise, the Δk signal will be reduced.

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 detector's 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.

Deltak Lidar sensing of surface waves in a wave tank.

The Deltak lidar remote-sensing method is used in a laboratory wave tank demonstration to measure the frequency of surface waves as a function of their wavelength. The results clearly demonstrate the ability of the Deltak lidar method to detect a single surface wave among an ensemble of waves present on the surface with a signal-to-noise ratio that agrees with the theory.

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.

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.

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.

Optical properties of several Pacific fishes.

Reflectivity and depolarization of six species of fishes were measured using blue and green light. In general, the fish were between 15% and 25% reflective.

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.

Aperture size and bandwidth requirements for measuring strong scintillation in the atmosphere.

Irradiance statistics were simultaneously measured with five apertures of four different sizes and also with five different bandwidths under conditions of strong path-integrated turbulence to determine aperture size and bandwidth requirements. The probability density function and the second and third moments are considered. Good measurements of these statistics can be made with detector apertures near the wave coherence length and with bandwidths near the ratio of the transverse wind velocity to the wave coherence length under these conditions.

Localized measurements of refractive turbulence using spatial filtering of scintillations.

Remote measurements of refractive turbulence strength with high spatial resolution are demonstrated. The technique uses spatial and temporal filtering of scintillation from a spatially filtered incoherent optical source. Spatial resolution as fine as 4.5 m was observed at the center of a 110-m propagation path. An analytic approximation to the theory agrees very well with the data. This theory predicts the spatial resolution of a system of this type to be in the vicinity of the path length divided by the total number of cycles in the transmitter and receiver spatial filters.

Refractive turbulence profiling using stellar scintillation and radar wind profiles.

The fluctuations of spatially filtered starlight contain information about refractive turbulence strength (C(2)(n)) at the spatial filter wavenumber. If the turbulence at different heights in the atmosphere is moving at different speeds, the contribution to the fluctuations from those heights will occur at different frequencies. Therefore, the C(2)(n) profile can be inferred from the power spectrum of the fluctuations and the wind velocity profile. Vertical resolution is expected to be in the range of several hundred meters to about a kilometer. Turbulence strength measurements to better than 50% should be easily obtainable.

Measured statistics of laser beam scintillation in strong refractive turbulence relevant to eye safety.

The time series of irradiance from a diverged He-Ne laser were measured using receiver aperture sizes that simulate the day-adapted and night-adapted human pupil. The data are within the saturation-of-scintillation regime wherein the irradiance variance decreases with further increases of propagation distance and refractive turbulence strength. Irradiance probability distributions, as well as the joint statistics of irradiance and duration of large irradiance values, are presented. The ocular hazard contributed by scintillation is found to decrease with increases of range and/or refractive turbulence strength; this effect is independent of the decrease of mean irradiance associated with such increases of range and refractive turbulence strength.

Refractive turbulence profiling using synthetic aperture spatial filtering of scintillation.

High spatial resolution vertical profiles of refractive turbulence C(2)(n) can be obtained using a translating airborne light source. From spatially filtered observations of the optical scintillation pattern on the ground, caused by atmospheric density fluctuations, it is possible to infer both vertical profiles of C(2)(n) and the shape of the refractive turbulence spectrum. Profiles of key parameters such as the turbulence microscale are thereby accessible from ground-based measurements. The required signal-to-noise ratio and resultant spatial and spectral resolutions are determined.