1,720,965 research outputs found
Minimum switching thruster control for spacecraft precision pointing
No full textMaintaining the attitude of a spacecraft precisely aligned to a given orientation is crucial for commercial and scientific space missions. The problem becomes challenging when on/off thrusters are employed instead of momentum exchange devices due to, e.g., wheel failures or power limitations. In this case, the attitude control system must enforce an oscillating motion about the setpoint, so as to minimize the switching frequency of the actuators, while guaranteeing a pre-defined pointing accuracy and rejecting the external disturbances. This paper develops a three-axis attitude control scheme for this problem, accounting for the limitations imposed by the thruster technology. The proposed technique is able to track both the period and the phase of periodic oscillations along the rotational axes, which is instrumental to minimize the switching frequency in the presence of input coupling. Two simulation case studies of a geostationary mission and a low Earth orbit mission are reported, showing that the proposed controller can effectively deal with both constant and time-varying disturbance torques
Propulsion options for station-keeping of small LEO satellites
No full textThe need to increase the competitiveness of commercial space missions has raised the
interest in operating small spacecraft at very low altitudes, rather than larger spacecraft at higher altitudes. This paper presents a parametric study tailored to the identification of propulsion options suitable for station-keeping of satellites in the 10−100 kg class, orbiting in the altitude range 250 − 500 km
An adaptive groundtrack maintenance scheme for spacecraft with electric propulsion
No full textIn this paper, the repeat-groundtrack orbit maintenance problem is addressed for spacecraft driven by electric propulsion. An adaptive solution is proposed, which combines a hysteresis controller and a recursive least squares filter. The controller provides a pulse-width modulated command to the thruster, in compliance with the peculiarities of the electric propulsion technology. The filter takes care of estimating a set of environmental disturbance parameters, from inertial position and velocity measurements. The resulting control scheme is able to compensate for the groundtrack drift due to atmospheric drag, in a fully autonomous manner. A numerical study of a low Earth orbit mission confirms the effectiveness of the proposed method
All-Electric spacecraft precision pointing using model predictive control
No full textA new approach to ACS (attitude control system) design is presented for three-axis precision pointing of all-electric spacecraft. An model predictive Control (MPC) law is proposed, whose objective is to keep the spacecraft attitude and angular velocity within given bounds. The proposed methodology allows the designer to explicitly take into account both the fuel consumption and the number of firings of the thrusters, providing a suitable way to tradeoff these objectives by means of a scalar parameter. The approach is general enough to be applied to the three-axis attitude control problem, even in the presence of coupled dynamics. Simulation results show that the achievable accuracy is suitable for both communication and Earth observation GEO missions. Moreover, the fuel consumption and number of firing cycles make the proposed ACS a viable alternative to systems based on momentum wheels
Propulsion options for very low Earth orbit microsatellites
Full text linkThe growing competitiveness in the commercial space market has raised the interest in operating small spacecraft at very low altitudes. To make this feasible, the space industry has started developing propulsion options tailored specifically to these platforms. This paper presents a review of emerging micropropulsion technologies and evaluates their applicability to microsatellite missions in the altitude range 250–500 km. The results of the proposed analysis are demonstrated on two different remote sensing applications
Optimal Low-Thrust Orbit Transfers Made Easy: A Direct Approach
Full text linkThe optimization of low-thrust, multi-revolution orbit transfer trajectories is often regarded as a difficult problem in modern astrodynamics. In this paper, a flexible and computationally efficient approach is presented for the optimization of low-thrust orbit transfers under eclipse constraints. The proposed approach leverages a new dynamic model of the orbital motion and a Lyapunov-based initial guess generation scheme that is very easy to tune. A multi-objective, single-phase formulation of the optimal control problem is devised, which provides a convenient way to trade off fuel consumption and time of flight. A distinctive feature of such a formulation is that it requires no prior information about the structure of the optimal solution. Simulation results for two benchmark orbit transfer scenarios indicate that minimum-time, minimum-fuel and mixed time/fuel-optimal instances of the control problem can be readily solved via direct collocation, while incurring a significantly lower computational demand with respect to existing techniques
SSCAM: Micro-satellite platform for Earth observation
No full textThanks to recent technological advances in space component miniaturization, small spacecraft can assure a fast and affordable access to space, while providing a level of performance similar to larger ones. Nevertheless, only a few Earth observation micro-satellite have been demonstrated in orbit so far. This paper presents a small commercial platform concept tailored to the bservation of the Tuscany Region (Italy) area. A preliminary mission analysis is performed and some potential applications are
discussed. In particular, it is shown that several state-of-the-art imaging devices can be considered as the payload of the proposed platform
Laser-induced fluorescence spectroscopy on neutrals for plasma studies in Hall thrusters
Full text linkHall thrusters offer an excellent balance between specific impulse and thrust-to-area ratios. However, adapting them to new power requirements is challenging because of the complexity of plasma dynamics and material interactions. This study focuses on using a minimally intrusive diagnostic tool, laser-induced fluorescence (LIF) spectroscopy, to investigate the atomic population within an electric thruster. We developed a comprehensive LIF spectra simulator incorporating factors such as the natural gas mixture and the Doppler Effect. By leveraging the anomalous Zeeman Effect, we achieved accurate non-intrusive measurements of the local magnetic field vector, a critical parameter for the thruster design process. We propose a new data elaboration strategy that combines measurements taken with two orthogonal laser polarizations inside the vacuum chambers. This strategy was successful in decoupling the neutrals’ temperature and magnetic field values inferred from LIF spectra. An experimental campaign within a laboratory model Hall thruster operating with Krypton validated this approach and demonstrated the technique’s potential. Three different LIF excitation schemes were used, with small discrepancies in the inferred values of the model variables. The transitions starting from the level 5s′1/21 led to the best precision in the evaluation of the neutrals’ temperature and the local magnetic field. The neutrals’ drift velocity experiences an apparent acceleration until the exit of the thruster channel, which could be due to charge-exchange collisions or slow neutrals’ population depletion caused by ionization
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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