نشریه علوم و فناوری فضایی
سال یازدهم شماره 3 (پیاپی 36، پاییز 1397)

Modeling and analyzing systems, especially in complex systems with high dynamics, noise and uncertainty in understanding the behavior of systems and decision making is very important problem from long time ago. This paper shows that neuro-fuzzy systems can be used effectively to design the solar arrays of electrical power subsystem of a remote sensing satellite in conceptual design phase. In the design of neuro-fuzzy system, Takagi-Sugeno inference system, hybrid training algorithm and Gaussian membership functions are used. The simulation results obtained in this modeling have an accurate accuracy compared to the experimental data and classical calculations of remote sensing satellites

Monopropellant thruster of atitude control system is a requirement for the development and functionalization of satellites in space, which have expensive and high-tech technology. Hydrazine thrusters are currently the most widely used thrusters for guidance and control systems of re-entry and manned spacecraft. In this paper, design and computation of an injector with hollow-cone spray with two tangential inlets as a fuel injector of a 10N monopropellant hydrazine thruster is presented. This injector has designed based on Bazarov method so that can generate a spray with a common (not so big) spray angle and very thin liquid sheet. Therefore it will be suitable from aspect of limitation of the catalyst bed length and also gives finer atomization. The phenomenon of creating and developing an air core in the internal flow of these injectors and its simulation is complex due to the existence of two turbulent swirl flows in two different phases, liquid and gas, which have an interface. For this injector, simulation of the internal flow has been performed to predict the output flow characteristics and ensure the formation of the gas core inside it. These characteristics include the spray cone angle, liquid sheet thickness, the output velocity distribution of the injector nozzle, and etc. For this purpose, volume of fluid (VOF) method has been used and flow turbulence has been simulated using the k-e model. The results of this study is presented in detail in the paper.

In this paper, the aim is to simulate night-sky images for use in star-sensor designing software. For this purpose, a comprehensive and precise algorithm was developed to simulate night sky images based on the ideal pinhole method and the use of Gaussian distribution functions. Then, in order to create more realism, sources of random and systematic errors, the elongated images due to the high dynamics of the platform, as well as the asymmetric back-lighting of the moon, the sun, and the planets of the solar system have been simulated. Finally, considering the importance of realism in the problem-solving simulation approach, the use of precision ray tracking method as an alternative to the ideal pinhole method is suggested.

Demand for positioning services with proper coverage and high accuracy has been increased rapidly in recent years. Global positioning systems solves many users’ requirements and presents positioning services with global coverage and proper accuracy. However, satellite-based positioning systems are made and control by Developed countries and their precise location services are rarely available to civilians and other countries. Also, the signals of the satellite systems are fallible and low-power, and are not suitable for closed locations. The use of a local positioning system along with global positioning is one of the programs that governments are urging in the future to enhance the reliability of the positioning, increase coverage of users and reduce the system error. In this research, after introducing local positioning system and its most important applications, its indicators will be compared with international positioning standards and its market in Iran will be analyzed.

Prediction of satellite orbital position is a critical requirement for all satellite ground stations. In this paper, a new viewpoint for predicting orbital position of satellites is presented. In contrast to traditional methods which are based on Kepler's law, the proposed method, is solely based on past observation of a given satellite. In contrast to traditional algorithms which have neglected some of the orbital perturbations, the most important feature of this method is considering all orbital perturbations by using real data. TLEs (Two Line Element sets) are the most available real data and are used in this research as the main data source. Using the capability of neural networks for time series prediction over available data, results in a fast and accurate orbital position predictor. The comparison between the output of our proposed method, SPG4 (Simplified General Perturbation version 4) propagator and real orbital position of a given satellite, shows the effectiveness of this algorithm.

In this paper, the combination of reaction wheels and thrusters is applied to attitude control of a satellite. First, governing equations of satellite attitude dynamics are given using quaternion and PID controller is designed based on the satellite quaternion to determine the applied control torque. By applying the reaction wheels physical constraints such as its maximum torque, its maximum momentum and maximum power on the desired torque, reaction wheels angular momentums and torques are found. The obtained results show that the unsaturated reaction wheels capabilities in attitude control. Results also show that the wheels saturation leads to error in control and increases the Euler angles and quaternions. Satellite thrusters are utilized to reaction wheels de-saturation and attitude control simultaneously. Three different strategies are proposed in this paper for wheels de-saturation using thrusters. Two well known methods, pulse width modulator (PWM) and pulse width pulse frequency (PWPF) modulator are used to attitude control using thrusters. All methods are compared together and the optimal method is proposed for the satellite attitude control. This paper results can be useful in design and control of different class of satellites.