E. C. Crittenden

Mid- and far-infrared measurements of sun glint from the sea surface

Measurements are reported of the infrared sunglint clutter channel caused by the direct solar reflection from the wave-perturbed sea surface at near-grazing angles of incidence. Apparent radiance has been measured over Monterey Bay as a function of azimuth and elevation angles relative to the sun direction using an AGA Thermovision 780 dual-band radiometric imaging system in the wavebands 2 to 5.6 (SW) and 8 to 12 micrometers (LW) with 7 degree(s) FOV. Time averaged profiles from multiframe averages show near-Gaussian angular distributions with half widths in the range 3 to 20 degrees (depending on solar angle) for look-down angles of 1 to 10 degrees below the horizon. The p- and s- polarized components of sea surface radiance have been obtained using an external wire-grid polarizing filter and compared with unpolarized measurements. The degree of polarization within the glint is shown to be horizontal and variable in the range 1% to 30%, depending on solar elevation, the higher degrees of polarization being found in the SW band. Significant vertical sea radiance polarization has been observed outside the solar glint in the 8 to 12 micrometers band, and is attributed to sea surface emission polarization.

Effects Of Turbulence On Imaging Through The Atmosphere

The degradation in performance of imaging or beam projection systems, due to turbulence in the atmosphere, can be evaluated or predicted in terms of a path-position weighted value of Cn2, the optical turbulence structure constant. The degradation also depends on the wavelength and range. At visible wavelewth, for paths of a few km, serious degradation occurs for Cn2 of the order of 10-14 m-2/3. Such turbulence is not uncommon for paths over the ocean and occurs frequently over land. The appropriately weighted value of Cn2 can be measured experimentally for a given path, with a slit scanning telescope, imaging a point source at the far end of the range. A portable system will be described that is capable of these measurements. This is coupled directly to an on-line data processing minicomputer to predict the performance of a given system, using Fourier and Abel transform techniques applied to models by Lutomirski and Fried. The results can be presented in a variety of forms, including immediate hard copy plots of the MTF of the atmoshpere, of the overall system being tested, or plots of predicted radial distribution of intensity on target. For non-uniform turbulence, the proper weighting of Cn2 as a function of path-position is crucial. For example, Cn2, obtained from scintillation, weights path-position differently and yields results of marginal value to determination of image resolution or spot size.

Vertical ocean reflectance at low altitudes for narrow laser beams

A narrow-beam laser altimeter was used to measure the reflected signal from the ocean surface as represented by the waters beneath the Golden Gate Bridge. This site allowed precise measurements as a function of angle from the vertical not possible from flying platforms. For short-wavelength water waves superimposed on swell, the signal amplitude probability distribution for the reflected signals showed periods of zero reflection, even for vertical incidence, apparently due to tipping of the water surface. The nonzero signals showed a distribution that could be fitted with an antilog-normal distribution. This is skewed toward higher signals than a normal (Gaussian) distribution. With incidence angle displaced from the vertical, the distribution shape was retained but with more frequent zero reflections. The decrease with angle of the average signal, including the zeroes, is well fitted with a Gram- Charlier distribution, as seen by earlier observers using photographic techniques which masked these details of the structure. For the simpler wave pattern due to a long sustained wind direction, the probability distribution is log-normal with no zero signal periods. At large angles from the vertical the log-normal distribution shifts toward exponential. For surface states intermediate between the above two extremes the distribution is often normal. The larger return signals resulting from the skew toward larger amplitudes from lognormal are more favorable for disposable laser altimeters than previously believed. Also, for an altimeter which may be swinging from a parachute or balloon, the return remains high at angles other than vertical. The presence of occasional zero return signal does somewhat degrade the accuracy of altitude measurement for a descending altimeter, but the signal available assures performance at larger altitudes than previously expected.

Ship signature measurements for tactical decision-aid input

In support of a continuing program of evaluation and experimental validation of FLIR Tactical Decision Aid performance codes, a series of measurements has been made of ship radiance temperature distributions together with sea and sky backgrounds. The measurements have been made at ranges from one quarter to one mile off the coastline in Monterey Bay, using a land- mounted Agema 780 dual band Thermovision radiometric sensor, with computer data acquisition and storage. The target ship was the research vessel Point Sur carrying a full suite of meteorological instruments and an array of thermal sensors for ship surface temperature distribution. Rawinsonde balloons were released to obtain vertical temperature and humidity profiles for path correction using LOWTRAN. The normal skin emissivity was measured in a separate experiment. The current data band consists of 898 stored radiometric frames containing ship images including starboard, port, bow and stern aspects, together with sea and sky background frames with varied zenith angle. These files are available for false color display and analysis in a variety of formats.

Degradation Of Optical Transmission In The Atmospheric Marine Boundary Layer

Optical transmission through the atmosphere is degraded both by the turbulence-induced fluctuation of the optical index, described by Cn², the turbulence structure constant for index, and also by extinction due to molecular absorption and scattering by fog or aerosols. Various techniques for direct optical measurement of Cn² and extinction are compared and the results shown to depend differently on the distribution of turbulence along the path. Tech- niques of field optical measurements of Cn² and extinction coefficient developed at NPS are presented. Results are compared with thermal and humidity fluctuations, and Knollenberg particle size spectra.

Image Resolution Through Atmospheric Turbulence

The effects of atmospheric turbulence on imaging systems, and beam projection systems such as laser designators, can be evaluated and the performance predicted in terms of a properly path-weighted value of the turbulence structure constant, Cn2. An operating system for direct field determination of this weighted value with a portable slit scanning telescope and on-line data reduction minicomputer system will be described. This system utilizes a point laser source located in the vicinity of the target and scans the image of that source from the opposite end of the optical path. The on-line computer system combines data, for the system on test, with the data taken on the atmosphere at the instant of test firing, utilizing Fourier and Abel transform techniques, to provide a prediction of behavior of the system on test, or an evaluation of the effects of the atmosphere on that test. The results can be presented in a variety of forms, including immediate hard copy plots of the MTF of the atmosphere, of the overall system being tested, or plots of predicted radial distribution of intensity on target for the system on test. The slit scanning telescope can track a slowly moving target, with internal optical tracking. A gyro platform is under development which can provide operation from shipboard or other unstable platform.

Image Resolution (OTF) Through Atmospheric Turbulence Over The Ocean

The optical transfer function (OTF) of the atmosphere has been measured over the ocean for four wavelengths from visible to far IR, using laser sources and a slit scanning telescope. The effects of diffraction, finite slit width, and aberrations have been removed by digital Fourier processing. The shapes of the curves of long term average OTF, and the image-centered (tracked) OTF, as well as the magnitude of the wander variance, all agree well with a theoretical model by Fried. Comparisons of the path-integrated values of Cn2, obtained from the OTF, with the path-integrated values of Cn obtained from CT2, indicate that, for nonuniform Cn2, the weighting of Cn2 as a function of position on the path, behaves as predicted. This weighting heavily emphasizes the part of the path nearest the telescope for imaging devices, whereas scintillation emphasizes the path center. The weighting that applies to imaging devices also applies to beam-forming or projection sys-tems, with the heavily emphasized part near the projection optics. Measurement with a scanning telescope thus yields directly the properly weighted value for such systems.