Cloud Background Research Articles

云层背景目标识别的匹配滤波器设计

An improved matched filter design method based on the maximum average correlation height(MACH) principle is proposed to recognize the targets in cloud backgrounds. In the process of MACH filter design, a theoretical model of power spectrum density(PSD) is established by curve fitting after statistically analyzing the real cloud backgrounds’ PSD distribution to replace the traditional white noise model. Various features of flight targets are extracted and processed by threshold to get the training set images. Correlation results of targets in various cloud backgrounds show that the average peak to correlation energy ratio is 0.71%, and the peak to clutter ratio is 0.92 of improved matched filter, the cloud backgrounds are well restrained, and the filter is also distortion-invariant to feat

TRAVERSABLE WORMHOLES IN A STRING CLOUD

We study spherically symmetric thin shell wormholes in a string cloud background in (3 + 1)-dimensional space–time. The amount of exotic matter required for the construction, the traversability and the stability of such wormholes under radial perturbations are analyzed as functions of the parameters of the model. In addition, in the appendices a nonperturbative approach to the dynamics and a possible extension of the analysis to a related model are briefly discussed.

An improved algorithm for calculating cloud radiation

Clouds radiation characteristic is very important in cloud scene simulation, weather forecasting, pattern recognition, and other fields. In order to detect missiles against cloud backgrounds, to enhance the fidelity of simulation, it is critical to understand a cloud's thermal radiation model. Firstly, the definition of cloud layer infrared emittance is given. Secondly, the discrimination conditions of judging a pixel of focal plane on a satellite in daytime or night time are shown and equations are given. Radiance such as reflected solar radiance, solar scattering, diffuse solar radiance, solar and thermal sky shine, solar and thermal path radiance, cloud blackbody and background radiance are taken into account. Thirdly, the computing methods of background radiance for daytime and night time are given. Through simulations and comparison, this algorithm is proved to be an effective calculating algorithm for cloud radiation.

Statistical distribution of the spatial and temporal characteristics of the radiation of the earth?s cloud background

This paper discusses data published earlier on full-scale measurements made in this country of the spatial and temporal inhomogeneities of various types of clouds-their geometrical sizes and lifetimes. By statistically processing the available uncoordinated data, it is shown that the general probability distribution law of the geometrical sizes and lifetimes of each of the four types of clouds is log-normal, while the total distribution density for all four types of clouds has a tetramodal character.

Distant Green Thunderstorms—Fraser’s Theory Revisited

Abstract The theoretical development presented by Fraser can produce a spectrum of light that would be perceived as a faint green. The theory assumed a perfectly black background thunderstorm. Severe thunderstorms are certainly not black when observed from a distance of 30–40 km. Thus it is useful to compare the theory with some observed examples of severe thunderstorms that should have been green by the Fraser theory but were not. Therefore, some elementary modifications of the Fraser model such as using a nonblack background are suggested. The use of a nonblack cloud background tends to shift the resulting dominant wavelengths away from the green portion of the spectrum, suggesting a better match with observations.

Modeling the performance of direct-detection Doppler lidar systems including cloud and solar background variability

Previous modeling of the performance of spaceborne direct-detection Doppler lidar systems assumed extremely idealized atmospheric models. Here we develop a technique for modeling the performance of these systems in a more realistic atmosphere, based on actual airborne lidar observations. The resulting atmospheric model contains cloud and aerosol variability that is absent in other simulations of spaceborne Doppler lidar instruments. To produce a realistic simulation of daytime performance, we include solar radiance values that are based on actual measurements and are allowed to vary as the viewing scene changes. Simulations are performed for two types of direct-detection Doppler lidar system: the double-edge and the multichannel techniques. Both systems were optimized to measure winds from Rayleigh backscatter at 355 nm. Simulations show that the measurement uncertainty during daytime is degraded by only approximately 10-20% compared with nighttime performance, provided that a proper solar filter is included in the instrument design.

Theoretical and experimental studies of CO2 laser evaporation of clouds

A study of the effects of laser radiation on cloud drops and of the possibility of producing a clear optical channel in a cloud is presented. In order to produce a model that is appropriate to a realistic cloud with a distribution of drop sizes it is first necessary to study what happens to a single water drop subjected to laser radiation of different intensities. Various heating regimes are mapped out as a function of laser flux and fluence at the 10.6 μm wavelength. It is found that typical cloud drops can superheat until they become unstable and explode from the center. For a long laser pulse the boundary for this to occur is found to be 50(5/r)2 kW/cm2, where r is the drop radius in microns. Using these results a model that is spatially one-dimensional through the cloud is constructed for a distribution of drop sizes. Laser beam intensity as the light penetrates a cloud is calculated from Mie scattering and absorption cross sections for a beam diameter that is small in the sense that light scattered once is assumed lost. The internal temperature distribution of the drops is calculated and a phenomenological drop explosion model is given for drops that reach the unstable 305 °C spinodal temperature at their center. Energy and water mass content are conserved as the cloud background is modified in an average sense by drop evaporation or recondensation. Recondensation is treated in the diffusion regime according to the Kohler model, with vapor pressure over a drop modified by surface tension and dissolved nonwater content. Comparison with experimental data for a laboratory produced cloud is given and good agreement, particularly with respect to the predicted onset of drop explosion, is found. Results are also presented for hypothetical cloud conditions and laser intensities. The possibility of clearing a thin cloud with low fluence to the 3.8 μm is considered, as well as the passive evaporation of melted ice crystal clouds.

Pulsed Helium-Neon Gas Laser Applications

The pulsed helium-neon gas laser has provided pulse power more than three orders of magnitude above the average power afforded by CW operation. Analyses of the power-limiting factors show that still greater increases may be expected. By increasing the size of the laser tube several times, by optimizing the gas ratio and pressure, by optimizing reflectivity and transmissivity of the reflecting mirrors, and by controlling the shape and application of the exciting voltage pulse, peak power outputs in the kilowatt range are theoretically possible. A power of 100 watts has been achieved in the laboratory at pulse rates up to 250 cps. Such a result brings the gas laser out of the low power category and into the intermediate power range. The results of range calculations show that 100 w pulse power will provide a radar range of approximately 10 km against a target having a diffuse reflectivity of 0.1, and a range of over 100 km with a cooperative target. Against a bright cloud background only the cooperative target can be used. The range will then be reduced to approximately 20 km. An automatic tracking radar system has been synthesized utilizing the pulsed gas laser as a transmitting component. A brief parametric analysis has beem made and some of the advantages of the optical radar over its microwave counterpart have been outlined.

Infrared Automatic Acquisition and Tracking System

A technique for accumulating airborne visible and infrared spectroradiometric data from missile plumes and re-entry objects establishes the requirements for a precision tracking front surface mirror. The processing of target radiation, reflected by the tracking mirror into an infrared tracker telescope, to derive target coordinate information, is described. The derivation of transformed target error signals to command the motion of the hydraulically driven tracking mirror is outlined. A description of the servo-electronics system, with selected circuitry, is given. The dynamic characteristics of the system, as determined from preliminary field tests, indicate that the system has a tracking rate capability of 10°/sec with an accuracy better than 60 seconds of arc. The detection sensitivity of the infrared tracker telescope is 1.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> w/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in the 2-to 6-α region, and is sufficient to provide 10-miile tracking ranges against a jet target in a sunlit cumulus cloud background. In a typical installation, aiming of the tracking mirror is provided by a 7×35, 10° field of view sighting telescope which is automatically slaved by the tracking mirror when acquisition and target tracking occurs, thereby providing continuous observation of the target. As an alternative to visual aiming, an automatic search program is generated by an accurate electro-mechanical programmer, which is described. Target acquisition, in an automatic mode of operation, is accomplished through the use of a simple logic circuit which provides electronic background discrimination by virtue of signal pulse width.

White Plumage of Sea-Birds

As Pirenne and Crombie have pointed out above, scattered light can, in the long ranges at which aircraft are spotted, have an effect on contrast which is not present at short ranges, but calculations for short ranges show that one might expect a considerable difference in spotting range for black and white birds. A considerable part of the brightness of a roughly hemispherical object such as a bird, seen from below with overcast sky, is contributed by skylight and not by light reflected from the sea. Estimating the reflexion factor of the bird's plumage at 0·9, this would make the mean brightness of the underside about 0·15 of that of the cloud background. This should have a definite effect on maximum spotting range, for at the human threshold, in clear air, the visibility of an object of high contrast is determined mainly by the total reduction in incident light or 'subtractive energy', that is, the contrast multiplied by the angular area. Thus an object of half the background brightness will require to have about twice the area, or to be brought to about l/2 of the distance, to be visible. The same is likely to apply to fish (though their absolute visual acuity is poorer); and for objects blurred, as Pirenne and Crombie point out the image of the bird will be, by surface ripples. On this basis, a contrast of 0·85 should produce a 7 per cent reduction in spotting range, which is of the same order as that to be expected with aircraft. Further, the conditions of cloudless blue sky, under which the bird will be brighter than the sky, are rather rare in temperate climates and there will be other conditions, such as sun shining through breaks in cloud, in which the brightness of the bird may exactly equal that of the background.