مجله علوم و فناوری های پدافند نوین
سال نهم شماره 1 (پیاپی 31، بهار 1397)

Compensating a single phase nonlinear load in many of critical applications is worth to pay attention. Non-sinusoidal current consumption in these loads would cause power quality deterioration in PCC. This should be compensated using an active filter. In an active filter, instantaneous current would be decomposed to active and reactive components. Active filter capacity deeply depends on the peak value of its supplied current. This is usually reactive component of load current. Classical theories are just for steady state analysis and modern theories are used for determining active filter reference current. This paper firstly introduces power analysis theories in the path of history and in classical and modern category. Then, the paper presents a new theory which determines reference current for single phase active filters and leads to optimization of reactive current and so filter capacity, in constant current waveform. This theory is applied to the waveforms of a deeply nonlinear, single phase load, and results are compared with existing theories. Outputs show suggested theory supremacy by %21.3 in active filter capacity.

In recent years, electric grids have been one of the main targets of terrorist attacks all over the world. These attacks are not considered in the design of electric grids. Subsequent to several blackouts in recent years, many researchers have studied the vulnerability of electric grids against intentional attacks, in both the operation and planning contexts. No review and classification of these studies is provided so far. In this paper, different approaches to vulnerability analysis of electric grids are presented and classified. Then, different objectives of antagonistic groups in attacking the electric grids, mathematical models and solution methodologies, challenges and solutions to deal with these challenges are represented in detail. Proper classifications that are presented in this paper help the network planners to incorporate several aspects of vulnerability analysis in their decisions about network planning and operation.

The algorithm for Detection of number of transmitting antennas plays an important role in design of intelligent receiver for interception of MIMO communications systems, because foremost it is necessary to know the number of transmitters. Since MDL is a standard solution for this problem, its performance analysis an important task. Previous works provide analysis in which the eigenvalues of covariance matrix of received signal are assumed to be known. Thus the impact of channel impairments on these eigenvalues and consequently the MDL performance is not studied. In this paper, the eigenvalues of covariance matrix of received signal are assumed to be unknown and the error probability of MDL is given based on the spatially correlated MIMO channel model using an estimation of channel eigenvalues. The accuracy of derived equations is evaluated using several simulations which show that the proposed analysis provides an acceptable prediction of MDL performance in different conditions.

Security, robustness, and capacity are the most important evaluation criteria in steganography. The aim of security is reducing the probability of message detection in an image and the aim of robustness is stabilizing the embedded message against removal and distortion due to intentional and unintentional attacks, such as noise and filtering. In this paper, an efficient adaptive image steganographic algorithm is designed in spatial domain which is also usable in frequency domain. In this algorithm, roughness (pixel changes) of each non-overlapping 3×3 window in the image is calculated using entropy of corresponding weighted graph. Each window threshold roughness is calculated based on length of message and overall image texture. Hiding message is done with specified density in LSB2 of 4 pixels of rough windows of an image by considering xor operator and randomization. Simulation on 5000 natural images and applying a modern steganalytic algorithm, SRM, shows that in comparison with other popular novel method, the proposed method has increased the security (Reducing the probability of message detection) about 2% in addition to robustness improvement.

In this paper, a linear two-stage adaptive filter is designed in which single-frequency signal interference (Jammer single frequency) from messages in throughput wideband signals is removed. As a result, in the proposed system which includes the Doppler Effect, unknown frequency interference is automatically earned at the entrance and is removed. The proposed system is implemented using two Normalized least mean square (NLMS) linear adaptive filter. The specific innovation of the proposed filter is that even if the jammer with a reference signal frequency is different, at first it calculates the difference between them. Then the second filter is detected the interference that discover by the first filter which is considered as a reference signal. So, when there is Doppler Effect, the proposed system by combination two filters is accurately detect and easily removed jammer. The simulation results for the proposed interference cancellation system for the broadband QPSK signal which is impregnated with the strong interaction, is achieved. Results show that for the QPSK signal jammer with single-frequency wideband message is effected on them as a strong interference. The proposed system shows up to 80 dB improvement in the Signal-to-Jammer ratio (SJR). In other words, the proposed system can be effectively used for the defense systems which are installed in front of jammers on a variety of birds.

In this study, the nanocomposite of carbon nanotube/iron-nickel was synthesized. Phase studies and morphology of the synthesized sample were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Also, the electromagnetic parameters of the synthesized samples were evaluated using analytical network vector (VNA) in the X-band radar range. In iron-nickel nanoalloy the bandwidth of RL

In this paper, stray lights in optical system of remote sensing satellite camera is simulated. For this purpose, stray lights, sources of production and their destructive effects on the image quality is studied. Moreover, mechanisms to deal with these unwanted lights are presented. Then, the effect of optical housing and surface coatings on the performance of stray light in satellite camera, has been studied by the results of the simulation. Finally an optimal design includes a set of different techniques for improving the performance of the considered camera was selected. Simulation results show that by this method, at large angles, stray lights can be completely eliminated whereas at angles near the FOV, stray lights intensities can be very much reduced namely up to 100000 times of their original intensity without almost no serious disturbance in the camera’s field of view.

Todays, ultrashort pulse lasers are employed in many branches of science and technology. Surface coating destruction, three dimensional micro or nano-structuring, terahertz wave generation and various spectroscopic techniques such as: ultrashort pulse laser induced breakdown spectroscopy, filament induced breakdown spectroscopy, and also different and several applications in nuclear physics are a portion of ultrashort pulse lasers utilizations. In this work, frequency modulation is firstly overviewed and then active mode-locking using the modulation is investigated in details. The modulation is in fact employed for ultrashort pulse generation by cavity loss modulation or phase changes modulation in a round trip. This can be done using an acousto-optical modulator or an electro-optical modulator. Later the mode-locking in a Nd:YLF laser cavity implementing self-consistent method is simulated. Ultrashort pulses shorter than 40ps with 120nJ energy and a rate of 250MHz are output of the designed oscillator.

Nowadays, with the astounding progress and development of hyperspectral imaging systems, the use of spectral knowledge of materials for building camouflage coatings is inevitable. In order to design the optical camouflage coatings possessing appropriate spectral similarity with the intended background, this study determines the optimal thickness of the multi-layer pigment structure in the visible and infrared ranges using a multi-layer interference transmittance filter. In the performed process, the transmittance matrix method and the genetic algorithm have been used. As a case study, the design of a multi-layer structure of optical camouflage in vegetation backgrounds is executed using germanium and zinc sulfide. Results suggest that the multi-layer structure is similar to that of the vegetation, based on the different spectral similarity meseaures and the correlation coefficient.

The specification of underground structures, and investigation of earth-penetrating weapons, and ultimately the major and minor impacts resulted from an invasion on these structures is one of the significant issues that each country may encounter. In other word, understanding the materials and applicable methods in the construction of buried components, to defend against the potential threats, has an important role in terms of the management construction’s principals, like time management, expenditure, and quality. Moreover, according to questionnaires and the experts’ opinions, utilization of the quantitative methods such as AHP, and the EXPERT CHIOCE software provide the opportunity to identify such indexes and alternatives. This study shows that the fiber concrete materials, reinforced concrete, nano materials, and FRP, introduce the best efficiency and function compare to other alternatives. As such, these materials are highly applicable not only in the construction of reinforced or fiber concrete, and Lattice Panel, but also in the process of retrofitting.

Having profound knowledge about the behavior of systems subjected to the external loads such as earthquake, wind, explosion and etc. can assist us to design more resisting structures. Braces are one of the most common lateral resisting systems and their major weakness in compression is buckling which result in a reduction to their loading capacity. Buckling restrained braces (BRB) are one of the strategies to deal with this problem. In this paper, using ABAQUS, the nonlinear dynamic response of the buckling restrained brace subjected to the blast loads with different positive phase durations is studied. Results show that the positive phase duration has no effect on the pattern of stress diagram while by increasing the positive phase duration, the maximum value of the compressive stress of the BRB is increased and the maximum value of the tensile stress is decreased. According to the results, when the blast loads are applied, for all positive phase durations, the total displacement is almost the same while the value of the plastic displacement is more for the blast load with larger positive phase duration. However, if the blast load is considered to be as an impact load, the value of the plastic displacement is less than the other types of loading. For these kinds of loading, more time is spent to reach a persistent plastic displacement. For such impact load and at the beginning of the loading, the total displacement falls sharply. This period of time is less for velocity and acceleration time history diagrams and does not depend on the positive phase duration. In addition, for the blast load with larger positive phase duration, the value of absorbed energy is more in the compression zone.

The air transport industry, especially in the military sector, as one of these critical centers for carrying out operations to help and repel enemy attacks is considered as an essential facility for crisis management. One of these facilities is the proper performance of airport runway pavement for timely landing and takeoff of airplanes. Since the destruction of the airport runway pavement to disrupt the flights is one of the primary goals of the enemy for the next attacks, in this research, sulfur is used as a substitute for bitumen in asphalt with the aim of quickly repairing of asphalt pavement. The performance of the mixture against rutting damage has been evaluated in a certain short time after making the mixture. The evaluations show that sulfur, due to having special characteristics, causes the rapid prepration and hardening of the mixture by temperature drop. It increases the rutting resistance sharply.