نشریه علوم و فناوری فضایی
سال ششم شماره 1 (پیاپی 14، بهار 1392)

This paper presents the control design for large angle and high rotation rates maneuvers using reaction cold gas thrusters. Navigation system provides suborbital attitude changes in terms of quaternion. Cold gas thrusters with pulse-width pulsefrequency modulation provide nearly proportional control torques. The use of quaternion as attitude errors for large angle feedback control in a suborbital capsule is investigated. Numerical simulations demonstrate the practical feasibility of a three-axis large angle maneuver.

Providing of sufficient input pressure of pumps is Pressurizing system tank mission. In structure of objective propulsion system, pressurizing system of fuel tank is hot gas kind that is feeding with engine which caused of operation relational. In this paper, interaction of pressurizing system of fuel tank and liquid rocket engine has been surveyed. Objective system consists of four main subsystems: liquid rocket engine, fuel tank, gas mixer and pipes. The procedure is based on a nonlinear mathematical model that dynamically describes operation of favorite system by regard to interaction of subsystems. Then all of the equations have been simulated in Matlab Simulation environment. Finally, results of propulsion system hot test are compared with model that shows acceptable accuracy of simulator code.

Astrounats, as envoys of mankind in outer space, are always faced with innumerable perils. Thus, their rescue and return has always been in the forefront of Humankind’s endevours to regulate outer Space activities. outer Space Treaty as lex generalis and The rescue and return Agreement as lex especialis are both relevant in this regard. Although there has never been an opportunity to put their provisions on this matter into test, no one can deny the magnitude of the duty to rescue and return in developing mankind’s activities in ouer space and its harmonizing effect on interntional co-operation. However scientific and technological advancement of the last decade coupled with the ever increasing possibility of space tourism endevours in outer space has caused new debates concerning the possibility of applying the provisions of the rescue and return agreement to space tourists. This article is an attempt to find an answer to this question.

This paper presents robust fault detection based on adaptive thresholds for a three axis satellite. For this purpose, first we described the attitude control system (ACS) as a quasi linear parameter model. Next, an interval observer has been designed that based on, effect of the satellite parameter uncertainties has been propagated into the alarm limits and so the adaptive thresholds are generated. In this paper, it is shown that the developed method minimizes the missing alarm rates; also this approach detects small or incipient faults more effectively than the classical fault detection algorithms with constantthresholds. In the next part of paper, we propose an isolation algorithm using the fault tree approach. Also, an accommodation system has been designed based on reconfiguration of available actuators. Accordingly, after isolation of faulty reaction wheels, the accommodation system turns them off and replaces the suitable magnetic tourqers instead of the faulty reaction wheels and so the attitude control error is maintained limited.

SAR radar imaging system is a supplement for electro optic remote sensing system and has been used over the past two decades. Understanding the behavior of these systems is difficult, because the complexity of these systems, especially in the mapping of the signal space to image space. This problem, development and use of this technology from the performance perspective and opposing with it from the performance passive defense, severely limited. In this paper a model for the performance of SAR imaging systems is provide. This model is like the impulse response function for linear systems independent of time. In other words, the proposed model is the impulse response of twodimensional SAR systems and by using it the SAR imaging process can be simulated. Using this model, many phenomena such as speckle noise shaping, can be described and the factors and parameters that are effective in the imaging system can be analyzed. Validity of the two-dimensional impulse response or functional model that presented in this paper, by comparing the results obtained in this paper and the expected results reported in reliable sources in this area has been proved.

In this paper, a precise propagation model which takes into account the effects of the atmospheric drag and gravitational harmonies is developed and presented using available Two Line Element (TLE) data. Moreover, the prediction of the trajectory of space objects (e.g. the operational and non-operational satellites and space debris) and their orbital data is performed. Then, the 2009 prominent collision between the Cosmos2251 and Iridium33 satellite is simulated and the maximum probability of their collision is computed by implementing the propagation algorithm and probabilities Theory, and finally the results are discussed. Therefore, the precise position and velocity of each space object at any time, as well as their collision probability will be determined, and if necessary, the time available to enact collision avoidance maneuver will be obtained. The success and accuracy of an avoidance maneuver is affected by the precision of the propagation model, the exact computation of collision probability, and finally the maneuver mechanism which are utilized.

In this paper, in order to control the relative motion for spacecraft formation flying, an optimal sliding mode controller is presented. This controller is designed based on the linearized equations of relative motion in circular orbit and applied to nonlinear system that is subjected to external disturbance. Firstly optimal controller is designed based on linear quadratic (LQ) method, and then integral sliding mode control technique is used to robustify the controller. It is assumed that spacecrafts move in low-earth orbits and J2 perturbation is considered as external disturbance. Using Lyapunov second method, the stability of the closed-loop system is guaranteed. The performance of the proposed controller in tracking the desired trajectory is compared to sliding mode controller and simulation results show the effective performance of the proposed controller.

Precision spacecrafts require high levels of pointing stability. Small levels of vibration can cause a significant reduction in image quality. There are many possible disturbance sources on spacecraft (mechanical systems or sensors), but the reaction wheel assembly (RWA) is anticipated to be the largest. Therefore, accurate models of reaction wheel disturbances are necessary to predict their effect on the spacecraft performance and develop methods to control the undesired vibration. In this paper, two types of reaction wheel disturbance models is presented. The first is a steady-state empirical model that was originally created based on a prototype RWA hard-mounted test data. The model assumes that the disturbances consist of discrete harmonics of the wheel speed with amplitudes proportional to the wheel speed squared. Experimental data obtained from RWA designed and manufactured by Aghalari and et al. are used to illustrate the empirical modeling process and provide model validation. The model captures the harmonic disturbances of the wheel quite well, but does not include interactions between the harmonics and the structural modes of the wheel which result in large disturbance amplifications at some wheel speeds. Therefore the second model, a nonlinear analytical model, is created using energy methods to capture the internal flexibilities and fundamental harmonic of an unbalanced wheel. Then the analytical model has been extended to capture all the wheel harmonics as well as the disturbance amplifications that occur due to excitation of the structural wheel modes by the harmonics. Finally experimental data obtained from hard-mounted test of RWA is used to determine the model parameters for both types of models and a comparison between the models and data is presented.