Ahmet Sofyalı

TURKSAT-3USAT: A 3U communication CubeSat with passive magnetic stabilization

TURKSAT-3USAT is a three unit CubeSat developed for voice communication at low earth orbit. The payload is a VHF/UHF transponder operating at amateur bands. The transponder and all other subsystems, except the stabilization, are doubled for redundancy. Where possible, both COTS systems and in-house development is employed. The energy is provided using lithium polymer batteries together with super capacitors. Satellite stabilization is accomplished using passive magnetic attitude control system with hysteresis rods. Based on the recent United Nations regulations and CubeSat standard, the satellite is to include a de-orbiting system which will make it reenter the atmosphere following completion of its life time. The 3USAT is foreseen to be ready for launch, early 2012.

Time-varying sliding mode in rigid body motion controlled by magnetic torque

In this work, the problem of rigid body attitude control by magnetic torqueing is considered. The aim of the work is to derive an asymptotically stable solution to this problem, which is known to have two challenging properties: instantaneous underactuation due to the structure of the magnetic torque production law; time-variance due to the dependence of that law on the time-varying local geomagnetic field vector. To ensure an asymptotically stable motion towards the reference state in inertial space, a time-varying sliding manifold is proposed in this paper. The manifold has two parts. The first part is a linear function of states and is well-known in literature to be specific to the problem of rigid body attitude control by momentum exchange- or reaction-based torqueing. The second part consists of two integral terms with respect to time, whose integrands are respectively the angular orientation of the body in inertial space and the component of the control vector along the local geomagnetic field. These designed time-integral terms enable the application of the equivalent control method to the considered problem and make the state vector converge to the reference state in sliding mode. With their inclusion in the sliding vector, there exists a time-varying sliding mode in nonlinear rigid body motion controlled by magnetic torque, which is proven by the satisfaction of the reaching condition for the general reaching law. The presented exemplary results of simulation studies, which are carried out under both ideality assumption and non-ideal conditions that are modelled with high-fidelity, verify the mathematical results.

Single Station Antenna-Based Spacecraft Orbit Determination via Robust EKF against the Effect of Measurement Matrix Singularity

AbstractIn this study, a robust extended Kalman filter (EKF) for estimation of a spacecraft’s position and velocity by single station antenna tracking data against the effect of measurement matrix singularity is proposed. The designed filtering algorithm estimates the position and velocity of the spacecraft on direct nonlinear range-azimuth-elevation measurements. The proposed robust EKF is examined through the geostationary satellite orbital motion simulations and the recommendations on its application for various spacecraft missions are given.

Integral Sliding Mode Control of Small Satellite Attitude Motion by Purely Magnetic Actuation

Abstract This paper deals with the purely magnetic attitude control problem. Nonlinear equations of attitude motion under environmental disturbances are presented. It is observed that the environmental disturbances affecting the control system appear as unmatched uncertainties. The controlled dynamic system can be represented in regular state-space form, which allows appropriate controller design. Attenuation of the disturbance effects on the steady-state behavior of the attitude angles is still an important problem in small satellite missions. Therefore, the integral sliding mode control method is used to solve the purely magnetic attitude control problem. The control torque vector at the output of the controller acts on the spacecraft after successive manipulations in magnetic actuation and interaction steps. The magnetic attitude control system is designed by using Lyapunovs direct method in the framework of sliding mode control theory. The performance of the resulting control system is evaluated through realistic simulations, and it is seen that the integral sliding mode controller has a superior steady-state performance with respect to the nominal controller.

Three-axis attitude control of a small satellite by magnetic PD-like controller integrated with passive pitch bias momentum method

The attitude acquisition problem of a magnetic small satellite under effect of environmental disturbances is solved by two methods. The first method stabilizes the nonlinear attitude in three-axis by purely electromagnetic actuation. The second method exploits the inertial stiffness provided by a pitch momentum wheel to gain insensitivity to disturbances. The simulation results obtained for the same initial conditions show the superiority of the non-aided method in terms of transient duration and energy consumption, on the other hand the disturbance rejection capability of the passively momentum-biased method. The steady state error can be reduced if the pitch bias momentum is employed passively whereas it is determined by the total magnitude of the disturbance torques in the case of purely magnetic control. The momentum wheel has a regularizing effect on the dynamics; it leads the system from irregularity to nonlinear stability then the magnetorquers become able to carry the states to equilibrium, also zero.

Computational phase portrait analysis of two nonlinear small satellite models

Two small satellite models are selected that are subject to gravity gradient in circular, low-altitude orbits. The second one is passively controlled by a pitch momentum wheel. The aim of this work is to represent how the passive pitch bias momentum method affects the nonlinear attitude dynamics. The analysis is based on phase portraits, Poincaré sections, and time responses. The phase portraits show that there are multiple equilibrium points lying on the Euler angle axes. Global behaviors remind motion about a saddle point located at the origin. The dynamics of the second model seems to have reduced nonlinear characteristics according to corresponding, less attracted motion pattern compared to the pattern for the first model observed in phase portraits and according to scattered points that do not build island-like structures, which is the case for the first model, in Poincaré sections. The time responses obtained using low initial attitude angles indicate that the nonlinear responses of the second model bring stable nonlinear motion to mind whereas the responses of the first model are divergent, so unstable. The pitch momentum wheel induces nutation that leads to high-frequency oscillation besides the low-frequency oscillation.

Search for the general relativistic effects on the motion of a spacecraft

Effects of general relativity on the motion of a spacecraft moving on a particular trajectory around Earth is investigated numerically in the context of circular restricted three body problem. It is seen that the general relativistic effects are worth to be included in the calculations, since they are observable to the Earth based observation devices terrestrial or spaceborne after a reasonable period. Even the effects of frame-dragging of rotating Earth can be detected by the current devices.)