This paper establishes a method for evaluating the visible light stealth effect of small surface targets for visual reconnaissance from the perspective of image similarity. The method is used to quantify and analyze the effect of visible light stealth and monochromatic coating on a typical small surface target. The results show that the proposed method can generate a reasonable response to the structural similarity, texture similarity, color similarity and second-order statistical moment, etc. for small surface target images with different visible light stealth states, and obtain a reasonable magnitude. Therefore, the comprehensive similarity considering the above factors is finally determined as the basis for determining the stealth effect.
With the increasing threat of attacks and reconnaissance on surface targets, in addition to radar and infrared measures, visible light camouflage measures for surface targets are receiving more and more attention. Camouflage measures such as camouflage color have been widely used in small surface targets in recent years. However, since the meteorological conditions on the sea are different from those on the land and in the sky, how to determine the design distance of visible light camouflage on surface targets through scientific analysis still needs to be solved. To this end, this paper establishes a calculation method of visible light transmittance on the sea through Modtran, combined with human visual characteristics and Johnson’s criteria, to calculate the camouflage standard design distance of different target sizes under different contrasts on the sea, and solve the problem of camouflage design distance selection under human eye observation conditions.
At present, water spray cooling is widely used in the fields of fire prevention and cooling as an economical and convenient method. In recent years, surface ships have been using water curtains for surface cooling and infrared stealth, which is becoming more and more common. The LECHLER nozzle used in the infrared stealth field of ships adopts the built-in type, featuring a mounting surface flush with the shell plate, which is suitable for ship stealth. It has a water pressure of 0.5Mpa and a water volume of 4m3/h, and the water spray covers a diameter of 7m. However, through observing the inside, the author found that there is a total pressure loss existing in many structures in the nozzle, which generally results in a loss of flow energy inside the nozzle, thereby reducing the spray coverage of the nozzle. In this paper, the structural optimization research of the nozzle is carried out, and the dual control conditions of inlet water pressure and water supply are proposed. The pressure loss and the coverage of water spray are analyzed by numerical simulation. The calculation shows that the total pressure loss is reduced from the original 47.5% to 10% with the optimized design. Under the premise of controlling the inlet water pressure and flow, the spray coverage is increased by 86.6%.
In this paper, a new method to suppress both barrage jamming and deceptive jamming is proposed based on spaceborne azimuth multichannel synthetic aperture radar (AMSAR). The relationship between signals received over arbitrary channel and reference channel is obtained by analyzing the signal models of the jammed AMSAR. Based on this relationship, a system of equations, whose solution contains SAR echo and all jamming signals, is established. In order to obtain high-precision solutions, the system noise has to be removed and accurate direction of arrival (DOA) estimation of jammers is required. For this purposes, a singular value decomposition (SVD) based method for noise reduction and a least square method for the two-dimensional locations of jammers are put forward. By solving the equations, SAR echoes can be recovered. Finally, several simulation experiments are provided to illustrate the effectiveness of the proposed method.
The symplectic multi-resolution time-domain(SMTD) scheme with cylindrical grids is developed for electromagnetic simulation. Based on Daubechies' scaling functions and sympletic propagation technique, equations are implemented. In cylindrical coordinates, some singular areas are analyzing and solving. In different stencil sizes, the stability and dispersion property is investigated and it turns out that SMTD is better than FDTD for stability restriction and medium discretization. Perfectly matched layer(PML) absorbing boundary conditions (ABC's) are derived for cylindrical SMTD grids. The numerical simulations validate that SMTD and PML are effective, so this paper may provide a new solution for electromagnetic simulation of cylindrical objects.
Optical camouflage painting, as a basic measure against optical reconnaissance and optical sighting weapon attack, is a general method of optical camouflage of small vessels. By utilizing this method, we can design the camouflage pattern according to basic features of vessel background, paint the camouflage pattern to equipment target surface with camouflage coating, and imitate the natural background or the outline of segmentation targets in color and texture, thus reducing the exposure signs of equipment targets. Due to the fact that most of existing qualitative evaluations on camouflage effectiveness are quantitative evaluation or evaluation based on experts' judgment, this paper put forward a new evaluation method of optical camouflage effectiveness of marine target based on target identification probability and similarity between the target and the background, carried out practical measurement test on optical camouflage effectiveness of marine targets, and calculated quantitative camouflage evaluation results of 50 collected images according to actual contribution of feature indexes. A mathematic relation model of optical identification probability and similarity of marine targets was built after comparison with manual interpretation results of identification probability. The results indicate that: the higher the similarity, the lower the identification probability. If the target and background are totally different, the corresponding target identification probability is 1, which indicates that the lower the similarity, the more the exposure features of the target, and the easier the target to be detected; if the target and background are similar, the corresponding target identification probability is 0, which indicates that the higher the similarity, the less the exposure features of the target, and the more difficult the target to be detected.
In recent years, the attenuation characteristics of log-normal water spray in the infrared region of atmospheric window have been studied in depth. However, there is no report on the comparison between calculation of the infrared transmittance of water spray based on Mie scattering using the LNMCM method and experiment, and the calculation error has not been publicly discussed. In this paper, we used Fluent to calculate the droplet concentration of water spray formed by a square arrangement of four FF-12 nozzles. After the water droplet number density distribution was obtained, the LNMCM method was used to calculate the infrared transmittance of water spray formed by the four-mounted FF-12 nozzles. On the other hand, an infrared attenuating test platform was built with four FF-12 nozzles, and an infrared imaging measurement test was conducted on the water spray formed by the four-mounted nozzles and the test panel that was shielded by it. The article compares the calculated results with the experimental results. Error analysis shows that the calculated value of the infrared radiation intensity of 7.5~13μm deviates from the experimental value by 3.1%. This paper verifies the accuracy of LNMCM and Fluent to calculate the infrared transmittance of log-normal distribution water spray through test measurement method, and forms a relatively complete calculation method of infrared radiation and attenuation of water spray.
The calculation and theoretical analysis of water spray infrared attenuation have been partially solved, where multi-scattering, distribution of droplet diameter, droplet density and water optical character in the infrared atmosphere window, etc. are considered. However, under the premise that droplet density does not change, is there a stair phenomenon in water spray monochromatic infrared transmission? If yes, what are the reasons? These questions are still to be answered. In the calculation, with the unchanged general droplet density, through changing the droplet size of water spray, the corresponding water spray spectrum 3~12μm infrared transmission is calculated with MIE scatter theory and Single Diameter Monte Carlo Method. The results show that there are two conditions for the occurrence of infrared transmission stair phenomenon of water spray: 1) with the increase of droplet diameter, the droplet extinction index decreasing speed is consistent with the square speed of droplet diameter increase; 2) the corresponding infrared albedo change small. Meanwhile, results also show that when the infrared wave length is less than 3.3μm or more than 5.7μm, there is no stair-like phenomenon in water spray transmission curve. While between 3.3 and 5.7μm, the spectrum infrared transmission stair phenomenon will occur in certain droplet diameter range.
Water spray is widely used in fire prevention and heat radiation protection due to its high infrared (IR) attenuation.
Recently, the calculation and theoretical analysis of water spray IR attenuation have been partially solved, where
multi-scattering in the water spray, distribution of droplets, concentration of droplets and water optical constant in the
infrared atmosphere windows are considered. However, is there a minimum for spectrum transmission of water spray in
the IR atmosphere windows band? If yes, what are the water spray parameters? These questions are still to be solved.
This article combines Lognormal Monte-Carlo Method (LNMCM) and Single Diameter Monte Carlo Method (SDMCM)
to calculate water spray infrared transmission. The droplet mean diameter is studied with SDMCM when the water spray
infrared transmission reaches the minimum. The geometric deviation of water spray is calculated with LNMCM at the
same conditions. The infrared transmission is calculated and analyzed with changing droplet diameters, on the stipulation
that total water content of air keeps unchanged. In this condition, droplet concentration increases with droplet diameter
decrease. Calculation results show that there is a minimum IR transmission of water spray in the atmospheric windows
band, which means there is a maximum IR attenuation. Infrared transmission of water spray reaches the minimum when
the droplet mean diameter is 12 micron and geometric deviation is less than 1.5. According to these, it is possible to
achieve maximum infrared attenuation with limited water supplied, as long as the spray nozzles parameters are in
accordance with the results.
Because of high infrared attenuation, water mist (or fog) are widely used in fire prevention and heat radiation
protection etc. However, the theoretical analysis and calculation of water mist about infrared attenuation are not easy due
to parameters that complicate the transport process, such as multi-scattering in the water mist, distribution of droplets,
concentration of droplets and water optical constant in the infrared atmosphere windows, etc. There are several methods
to calculate the water mist infrared attenuation, for example, Monte-Carlo method (MCM), independent scattering
method (ISM) and solo mean diameter method (SMDM). These methods have different accuracy and complicacy.
Theoretically, MCM should be more precision and complicated than the other methods but no report on the comparisons
between these methods is found by the authors. The present article studies the spectral transmittance of water mist with
different concentration and different thickness using MCM and Mie theory, since spectral transmittance of water mist in
the infrared atmosphere windows with lognormal distribution is important data for infrared attenuation analysis. At the
same time, the transmittance of the same water mist was calculated with ISM and SMDM and the results of MCM were
compared with those of ISM and SMDM. It is clearly indicates that the ISM is accurate when the droplet concentration is
less than 320 droplets/cm3, the SMDM is accurate when the droplet concentration was less than 300 droplets/cm3, with
variance 2 and mean diameter 8μm lognormal distribution. When the mean diameter of water mist is below 8μm, ISM
and SMDM are applicable with higher mist concentration. The ISM and SMDM is valuable for engineering calculation
about water mist infrared attenuation, when the mist concentration is not high enough.