Sharif Digital Repository

Accurate modeling of clutter and detection of low observable targets within clutter are the major problems in radar signal processing applications. Recently, fractal geometry is applied to the analysis of high range resolution radar sea clutters. The intrinsic difference in the fractal dimension between sea clutter and the echoes from targets is extracted and used to detect targets. The box-counting method is widely used to estimate fractal dimension but it has some drawbacks never or rarely considered in literature. We explain the drawbacks and propose a new fractal dimension based detector to increase detection performance in comparison with traditional detectors. Both statistically... 

In comparison to attitude control of a satellite which is widely used in practical missions, orbit control (espescialy autonomous orbit control) has been only recently paid attention. Autonomous, on-board orbit control, also called autonomous stationkeeping, means the automatic maintenance of all of orbital elements by the satellite itself. In this thesis, an autonomous absolute orbit control strategy for a single Low Earth Orbit (LEO) satellite is presented. When the satellite violates the control trigger error limits, then the controller is activated and calculates a sequence of orbital maneuvers that move the satellite towards its desired states. The absolute orbit control of the... 

In this thesis, the problem of failure-tolerant control of the satellite in coupled orbital and rotational motion is investigated. All of the actuators of the satellite are thrusters and only the failure of the actuators is considered. At first, the equations of coupled relative translational-rotational motion of the satellite are derived. Then, the required specifications of the approapriate control method for the problem is explored with the conclution that the controller must be able to predict the behavior or planning the future trajectory of the satellite. In the next step, model predictive control, which has the required specifications, is used to control the coupled... 

The primary goal of this thesis is to develop a Fault Tolerant Control (FTC) system for a flexible satellite in presence of actuators fault and failure. The actuators of the satellite are including three magnetorquers and three reaction wheels on each principal axis. At first, the three-degree-of-freedom rotational motion of a rigid satellite with two flexible solar panels is modeled. The flexible panels are modeled through the assumed mode approach by a finite set of bending modal motion. The ordinary differential equations of their generalized coordinates are found using the Lagrange’s equation. Then, the dynamic model is validated by comparing the simulation results of the model to the NX... 

This dissertation investigates the Multiple-Horizon Multiple-Model Predictive Control method and its application in soft landing problem on an irregular-shaped asteroid. In this way, a predictive framework including a heuristic guidance law named Predictive Path Planning and Multiple-Horizon Multiple-Model Predictive Control (MHMM-PC) as the control scheme is introduced for soft landing on an asteroid. Obviously, reducing the fuel consumption is a priority in space missions. Thus, the employed control framework should minimize the required control effort. Furthermore, the control method should be robust enough to deal with the effects of model uncertainties and disturbances. The unique... 

The observations of the cosmic background radiation and large-scale structures show that the standard cosmological model is in good agreement with observational data. The standard model is based on the cosmological constant, dark matter, and Gaussian, nearly scale-invariant, adiabatic, and isotropic initial conditions. Although this model has been phenomenologically successful, but the problems of dark energy, dark matter, missing baryons and early universe remain unresolved. One of the most important theoretical questions in cosmology is whether the cosmological constant is the reason for accelerated cosmic expansion. In other words, is Einsteinian gravity the correct theory on the cosmic... 

The aim of this thesis was to study and fabricate stable inks of Cu2SnS3 (CTS), Cu2ZnSnS4 (CZTS) and Cu3SbS4 (CAS) copper-based chalcogenide nanoparticles (NPs) and their application in solar cells. Inorganic semiconductors with p-type conductivity, high stability, high hole mobility (compared to organic type), and the ability to synthesize and deposition by simple low temperature solution-based methods have the potential to provide suitable alternative for hole-transporting material (HTM) in perovskite solar cells (PSCs) and the photo-absorbers in thin film solar cells (TFSCs). The ability to adjust energy levels and optoelectrical properties is another advantage of these compounds. In... 

Several nanostructured photocatalysts were prepared by sol-gel, hydrothermal and precipitation methods. The products were characterized by X-ray diffraction (XRD), nitrogen adsorption, UV-VIS diffuse reflectance spectroscopy, fourier transform infrared absorption spectroscopy (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM). Their Photocatalytic activities were evaluated by degradation of organic pollutants in liquid-solid system. In this research work, the influence of metal oxide, copper oxide, on structure of nanocrystalline mesoporous titanium dioxide were evaluated. Different molar percentages of copper oxide on the surface of titanium dioxide were... 

In this thesis, We propose and test a method for controlling the size of nanoparticles, which plays a fundamental role in their electrical, optical, and mechanical properties. The method utilizes turbulent mixing, and is applicable to the fabrication of any type of nanoparticle that uses a solution environment in the preparation process. We show by well-controlled experiments on the CdS nanoparticles, which are semiconducting materials, that the average size < d > of the particles decreases with Reynolds number Re. To gain a deeper understanding of the phenomena reported here, we have carried out molecular dynamics simulations of the processes. In second investigation, We provide a simple... 

In this research, we focus on study and investigation of the role of Tin(IV) Sulfide (SnS2) nanostructured layer as electron transport layer in perovskite and chalcogenide thin film solar cells. For this purpose, SnS2 powder was prepared through hydro/solvo-thermal method, utilizing different ratios of water and ethanol as solvent and various sulfur sources (thioacetamide and thiourea). Afterwards, different solvents were investigated to achieve a stable ink (about one month) with uniform dispersion. After determining the appropriate ink and powder, thin films of SnS2 were prepared employing spin coating, spray pirolysis and laser pulsed deposition (PLD) methods and characterized. With each... 

In this thesis, optimal station keeping of spacecraft on the closed-orbits near the Earth-Moon Lagrange points has been investigated. Lagrange points L1 and L2 are unstable equilibrium points of the circular restricted three-body problem (CRTBP), and periodic orbits near them are the destination of modern missions. However, perturbations resulting from the gravitational forces of the Sun and the planets and the eccentricity of the orbit of the Moon around the Earth make the Lagrange point unstable, even L4 and L5, which are stable in CRTBP. The most suitable choice for spacecraft mission near the collinear Lagrange points is the Halo and other closed-orbits around them, which need... 

In this thesis, autonomous orbit estimation of regional navigation satellites has been investigated. Estimation of satellite orbit is an integral part of satellite navigation, which may be executed via assistance of ground station. As the satellite is exposed to the ground station view, range, direction and rate of range is measured by mean of ranging instrumentation relative to ground reference, hence the precise position is calculated. In this thesis a set of satellites are investigated in a unique structure called constellation with the purpose of navigation. The occurrence which is likely to happen in this condition, is the failure of ground station under certain circumstances or losing... 

In this thesis, the satellite constellation orbit design for regional coverage is investigated. The orbits of satellites in a constellation for Earth observation missions play a crucial role in performance and characteristics of the mission; for example the image resolution, the area of coverage, and the coverage interval. In this thesis the problem is to find the optimal initial orbital elements for meeting the earth observation mission objectives for one or several sites. The cost functions are the total coverage time, the maximum gap time, time average gap, altitude, and number of satellites. The problem is solved in a multi-objective manner for different combination of two of the above... 

This research aims to introduce a new integrated fault tolerant attitude control system of satellites based on the neural networks. First, a linear controller and an optimal controller are designed assuming that no fault occurs in the reaction wheels of the satellite. It is obvious that these controllers are not able to respond properly when a fault occurs. Therefore, artificial neural networks are employed in the fault tolerant controller design. Neural networks are well-known for their adaptiveness and the ability to learn the dynamics of a system. Therefore, they can be used to predict the faulty conditions. In this research two approaches are examined to deal with the faulty conditions.... 

This paper presents an integrated attitude control and estimation of satellite by using adaptive neural fuzzy (ANFIS) logic, indeed a system which using ANFIS controller and estimator is considered then an ANFIS simulator that can control and estimate contemporary is supposed. ANFIS system has been trained by using data obtained from an optimal PID controller. Then ANFIS controller performance has been compared with the performance of a PID controller, the analogy has repeated in different situations such as noise and uncertainty. Results show that system with ANFIS controller is more optimal than PID’s in all these situations. After that ANFIS estimator is studied and results indicate the... 

UF membranes are widely used in various industries now, a major type of these membranes are ceramic membranes which are resistant to high temperatures and corrosive fluids. In this thesis we developed an anisotropic UF ceramic membrane based on 20nm TiO2 nanoparticles on a prous sustrate. We had two major challenges, the first one was to find a suitable prous substrate and the second one was to find an appropriate coating method. Our main task was to optimise the process conditions to achive a crackless and uniform membrane layer with minimal ups and downs. Lots of coating methods and TiO2 inks have been tried, finally we used a coating method which we called it “Soft Blade Coating”, with... 

In this thesis, the optimal trajectories of a spacecraft in the binary asteroids system is studied. The objective functions of optimal trajectories in these system are the maximum coverage of the asteroids surface and minimum time and fuel consumption. For this purpose, the dynamic of binary system is modeled. The binary system of 1999 KW4 is selected as a case study. The ellipsoid-sphere model is a good approximation for the case of this study. In this model, the asteroids orbit in a circular orbit around each other in a spin-spin-orbit resonance configuration, in which the asteroids’ rotational velocity is equal to orbital mean motion. Then, the equations of motion of the spacecraft as the...