Aerospace Science and Technology
Shahrokh Zohrabzadeh Bozorgi; Abolghasem Naghash
Abstract
In this paper, a few hybrid satellite constellations including combinations of LEO and GEO satellites for providing satellite navigation and positioning services for users in Iran have been designed and proposed. The performance of the constellations has been analyzed based on DOP values variations. ...
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In this paper, a few hybrid satellite constellations including combinations of LEO and GEO satellites for providing satellite navigation and positioning services for users in Iran have been designed and proposed. The performance of the constellations has been analyzed based on DOP values variations. It is shown that theoretically, it is possible to provide satellite positioning and navigation service with acceptable DOP values based on the introduced hybrid pattern including three GEO satellites and a constellation of about 30 to 60 LEO satellites in 3 or 4 orbit planes. The design has been performed based on studying the skyplot of the Iranian territory considering the GEO satellites as fixed points, and then determining the effect of the instantaneous position of the LEO satellites on the DOP values. A few LEO constellations have been designed to provide best DOP values based on the skyplot analysis results. Then, scenarios including similar GEO satellites and different patterns for LEO satellites have been simulated for half a sidereal day. The performance of the hybrid constellations provides satisfactory results with the average PDOP values of less than 4 which is acceptable. Optimizing the resulted pattern can lead to more desirable performance. In addition to navigation mission, hybrid constellations can perform other missions. Therefore, the proposed constellations can be operated as multi-mission space platforms.
Aerospace Science and Technology
Ali Motamedi; Abolghasem Naghash
Abstract
The purpose of this paper is to present a Multi-Input Multi-Output (MIMO) linear controller based on the eigenstructure assignment method for a fixed-wing Unmanned Aerial Vehicle (UAV) in longitudinal and lateral-directional channels. To this end, a six degree-of-freedom model of the aerial vehicle is ...
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The purpose of this paper is to present a Multi-Input Multi-Output (MIMO) linear controller based on the eigenstructure assignment method for a fixed-wing Unmanned Aerial Vehicle (UAV) in longitudinal and lateral-directional channels. To this end, a six degree-of-freedom model of the aerial vehicle is considered, where dynamic modes of the system in each channel are analyzed, and the effect of each dynamic mode on state and output variables of the system is investigated. Then, the eigenvalue and eigenvector parameters of the designed controller are appropriately assigned for the system dynamic modes in each channel. In addition, the system requirements of each dynamic mode are satisfied with the proposed controller, and the adverse interaction between the system state variables is minimized. The capability and effectiveness of the designed controller in a desired maneuver are demonstrated with a nonlinear model simulation of a fixed-wing UAV. In this regard, the results in longitudinal and lateral-directional channels are presented.
Reza Soltani Nezhad; Abolqasem Naghash
Abstract
This article examines the fault tolerance control (FTC) system for the Boeing 747. to reach this goal, firstly 6 degrees of freedom equations are simulated and linearized by using dynamic inversion method. Then, the system is controlled by a linear proportional-integral-derivative controller. To control ...
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This article examines the fault tolerance control (FTC) system for the Boeing 747. to reach this goal, firstly 6 degrees of freedom equations are simulated and linearized by using dynamic inversion method. Then, the system is controlled by a linear proportional-integral-derivative controller. To control this system, the angular velocity loop (r, q, p) must be closed and use cascade control to achieve this goal. Faults such as actuator and sensor failure are injected into the system, and by adding an integrated gain to the controller, the system resists these faults. Also, a redundancy system has been used in sensors to prevent sensor faults. Moreover, a linear Kalman filter has been used to eliminate noise in the system. If an actuator is locked in the Boeing 747, the faulty actuator is removed from the control system. Then, a healthy actuator or other remaining actuators will eliminate the effects of this fault.
Alireza Mohamadifard; Abolqasem Naghash
Volume 8, Issue 2 , September 2011, , Pages 87-95
Abstract
A near-optimal midcourse trajectory shaping guidance algorithm is proposed for both air and ballistic target engagement mission attributes for generic long range interceptor missile. This guidance methodology is based on the maximum final velocity as the objective function and maximum permissible flight ...
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A near-optimal midcourse trajectory shaping guidance algorithm is proposed for both air and ballistic target engagement mission attributes for generic long range interceptor missile. This guidance methodology is based on the maximum final velocity as the objective function and maximum permissible flight altitude as the in-flight state constraint as well as the head-on orientation as the terminal state constraint for anti-ballistic trajectory. Guidance algorithm utilizes the combination of the Generalized Vector Explicit Guidance or GENEX guidance with the Bezier curve-generating functions. Nominal Bezier curves are fitted by choosing control points intuitively. Waypoints are then selected on the curve to divide it to suitable portions according to the curve’s length and curvature. These waypoints are then fed into the GENEX guidance law. To avoid acceleration command jumps, an algorithm is designed to switch to the next waypoint at a distance from the current currently-approaching waypoint. To provide near-optimality and meet the in-flight and terminal constraints, all the guidance algorithm parameters including Bezier control points, waypoints, switching distances and the GENEX law gain are optimized using Genetic Algorithm by setting the mentioned cost function and constraints. Simulation results show better performance compared to nominal trajectories while ensuring the flight altitude constraint for air target and head-on orientation for ballistic target.
maryam malekzadeh; Abolqasem Naghash; H.Ali Talebi
Volume 7, Issue 2 , September 2010, , Pages 81-91
Abstract
Two robust nonlinear controllers along with a nonlinear observer have been developed in this study to control a 1D nonlinear flexible spacecraft. The first controller is based on dynamic inversion, while the second one is composed of dynamic inversion and µ-synthesis controllers. The extension of dynamic ...
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Two robust nonlinear controllers along with a nonlinear observer have been developed in this study to control a 1D nonlinear flexible spacecraft. The first controller is based on dynamic inversion, while the second one is composed of dynamic inversion and µ-synthesis controllers. The extension of dynamic inversion approach to flexible spacecraft is impeded by the non-minimum phase characteristics when the panel tip position is taken as the output of the system. To overcome this problem, the controllers are designed by utilizing the modified output re-definition approach. It is assumed that only one torque on the hub is used. In particular, the assumption that all sate variables are measurable is not realistic; hence sliding mode observers is used to estimate states. Actuator saturation is considered in the design of controllers. The performances of the proposed controllers are compared in terms of nominal performance, robustness to uncertainties, vibration suppression of panel, sensitivity to measurement noise, environment disturbance and nonlinearity in large maneuvers. To evaluate the performance of the proposed controllers, an extensive number of simulations on a nonlinear model of the spacecraft are performed. Simulation results show the ability of the proposed controller in tracking the attitude trajectory and damping panel vibration. It is also verified that the perturbations, environment disturbance and measurement errors have only slight effects on the tracking and damping responses.
Nematollah Ghahramani; Abolqasem Naghash; Farzad Towhidkhah
Volume 5, Issue 3 , July 2008, , Pages 99-105
Abstract
In this paper, a new incremental predictive guidance method based on implicit form of velocity to be gained algorithm is proposed. In this approach, the generalized incremental predictive control (GIPC) approach is applied to the linearized model for compensating the guidance error. Instead of using ...
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In this paper, a new incremental predictive guidance method based on implicit form of velocity to be gained algorithm is proposed. In this approach, the generalized incremental predictive control (GIPC) approach is applied to the linearized model for compensating the guidance error. Instead of using the present state in popular model based predictive controller (MPC), in the new method both previous and present states are utilized. GIPC approach introduces a feedback action including the weighted difference of the process states and the summation of the control action increments. To evaluate the robustness and performance of the proposed approach, the parameter uncertainties of the guidance and control are considered and a comparison with standard GPC is performed by extensive computer simulations. The results show a significant improvement in the robustness as well as tracking performance for the perturbed initial value of velocity to be gained or the reference signal.
Abolqasem Naghash
Volume 2, Issue 3 , September 2005, , Pages 9-18
Abstract
This work is an example of application of nonlinear programming to a problem of three-dimensional trajectory optimization for multistage launch vehicles for geostationary orbit missions. The main objective is to minimize fuel consumption or equivalently to maximize the payload. The launch vehicle considered ...
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This work is an example of application of nonlinear programming to a problem of three-dimensional trajectory optimization for multistage launch vehicles for geostationary orbit missions. The main objective is to minimize fuel consumption or equivalently to maximize the payload. The launch vehicle considered here, Europa-II, consists of 5 thrust phases and 2 coast phases. Major parameters of the coast arcs such as inclination, eccentricity and true anomaly of attachment points are not prespecified and should be found in optimization problem. The fairing should be jettisoned whenever aerothermal flux falls below a certain value. An aerodynamic heating constraint for atmospheric part of the flight is also considered. The problem is solved by direct collocation method and results are compared with those in Ref. [1] where an indirect multiple shooting method with an inner loop for parameter optimization is used. Advantages of present work with respect to methods used in that reference are then discussed.