Alfred W. Cooper

Correlation of stress‐wave profiles and the dynamics of the plasma produced by laser irradiation of plane solid targets

The interaction of 20‐nsec 300‐MW pulses of 1.06‐μm laser radiation with thick aluminum targets in vacuum has been studied. The time history of the target impulse has been measured with a Sandia quartz gauge. A time sequence of plasma density maps constructed from floating double‐probe data has been used with measured expansion velocities to estimate the plasma momentum. The results show that the stress wave is predominantly produced by about 10% of the evaporated target material which is ionized and expands from the surface in the form of a hot plasma during and shortly after the laser pulse. The estimated momentum of the plasma and neutral emitted particles is 5.6 g cm/sec for a typical case compared with the measured target impulse of 6.1 g cm/sec.

Infrared polarization measurements of ship signatures and background contrast

In previous papers we have presented measurements of degree of polarization in the mid and far infrared both in near-horizon sun-glint and in the adjacent sea background radiance. The polarization has been related to the Fresnel reflection and emission coefficients and compared with predictions. These measurements have been extended to include target-to-background polarization contrast measurements. Target polarization was small, but sea surface emission showed moderate degree of polarization in the p-plane (vertical). Considerable ship- background contrast improvement was achieved by polarization filtering in the horizontal plane. This effect was greater in the far infrared. Values are given for the degree of polarization of target and background and the contrast improvement factor due to polarization filtering.

Infrared polarization ship images and contrast in the MAPTIP experiment

Shore-based polarized infrared images of ship target and background were obtained in the MAPTIP measurement series, using vertical and horizontal polarization filters. These images have been analyzed to yield degree of polarization and polarization contrast enhancement for targets with sea and sky background Strong vertical polarization is observed in the sea surface emission near the Brewster Angle in the 8 - 12 micrometer (LWIR) band, whereas horizontal polarization due to reflection is usually dominant in the 3 - 5 micrometer (MWIR) band. Ship targets at broadside aspect show a degree of polarization less than 5%. An enhancement of contrast by up to 30% in the LWIR (less in the MWIR) is obtained by polarization suppression of sea background. The contrast enhancement in the MWIR is counteracted by the horizontal polarization of reflected sunlight.

Experiments on the Origin of Moving Striations

The origin of moving striations has been investigated by measurement of striation propagation at a sharp change in tube diameter, by study of the relationship between anode spot oscillations and moving striations, by observation of anode spot oscillations at the cutoff current for moving striations, by use of an auxiliary discharge to suppress anode potential oscillations, and by study of a striation system isolated from electrodes. The experiments indicate that: (a) the striation frequency, velocity, and wavelength all change discontinuously at a change in tube diameter; (b) visible spot oscillations are accompanied by oscillations in discharge potential, and exist unchanged above the critical current for disappearance of moving striations; (c) anode oscillations and anode spots may be suppressed by operation of an auxiliary discharge to the anode; (d) the striation frequency synchronizes to an applied oscillation at the anode, provided the applied frequency is close to a small integral multiple of the n...

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.

Atmospheric and sea state dependence of polarized infrared contrast

Sea radiance in the mid and far infrared shows a considerable degree of polarization which affects observed target-to-background contrast. An improvement in contrast is achieved with horizontal (s-plane) polarization filtering to suppress sea surface emission. Visibility and range affect the contrast in both polarizations. Scenes recorded during the MAPTIP measurement series off the coast of the Netherlands with the oceanographic ship HrMs Tydeman show decrease in contrast with range and better contrast for horizontal polarization against sea background. A simple mathematical model is presented relating contrast to extinction and path radiance which increase with increasing path length or worsening visibility.

Dual-band infrared polarization measurements of sun glint from the sea surface

This paper presents a sea radiance polarization model and experimental measurements of near- horizon sea glint polarization in the 3 - 5 micrometers and 8 - 12 micrometers spectral bands. The experimental measurements include the effects of polarization on the glint statistics, the degree of linear polarization and the polarization signal-to-noise ratio (SNR) improvement factor for both spectral bands in the presence of sea glint. The results indicate that the polarization in the 3 - 5 micrometers spectral band is dominated by the reflected solar and sky radiance and is polarized in the s plane. The polarization of intense sea glint in the 8 - 12 micrometers region is low and s polarized due to the weak solar spectrum in this band. In little or no glint, the radiation is weakly p polarized. Experimental data indicate that a polarizing filter can produce a significantly larger SNR improvement for the 3 - 5 micrometers spectral band than for the 8 - 12 micrometers band. Theoretical calculations using the polarization model show good agreement with the experimental data.

PASSIVE RANGE ESTIMATION USING DUAL-BASELINE TRIANGULATION

Modern combat systems based on active radar sensing suffer disadvantages against low-flying targets in cluttered backgrounds. Use of passive infrared sensors with these systems, either in cooperation or as an alternative, shows potential for improving target detection and declaration range for targets crossing the horizon. Realization of this potential requires fusion of target position data from dissimilar sensors, or passive sensor measurement of target range. The availability of passive sensors that can supply both range and bearing data on such targets would significantly extend the robustness of an integrated ship selfdefense system. We consider a new method of range determination with passive sensors based on the principle of triangulation, extending the principle to two orthogonal baselines. The performance of single or double baseline triangulation depends on sensor bearing precision and direction to target. An expression for maximum triangulation range at a required accuracy is derived as a function of polar angle relative to the center of the dual-baseline system. Limitations in the dual-baseline model due to the geometrically assessed horizon are also considered.

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.

Sensitivity analysis of Navy tactical decision-aid FLIR performance codes

UFLR is one of an evolving set of FLIR performance prediction programs used at sea to predict the ranges for detection, classification, and identification of target ships. One aid in the validation of such a program is a sensitivity analysis of the program parameters. Sensitivity analyses indicate that the ranges for detection, classification, and identification are strongly sensitive to target areas, target-to-background, temperature difference and atmospheric conditions such as windspeed, visibility, humidity, and vertical temperature, humidity, and pressure profiles. One uncontrollable parameter is the noncontiguity in space and time of the radiosonde and FLIR measurements. This problem was investigated by dithering the radiosonde data, input to UFLR, with a random number generator to generate variations in the pressure, temperature, and relative humidity in the atmospheric profile. Results indicate that noncontiguity of measurements can lead to 50% error in range predictions.