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2009, Spaceflight Mechanics
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19 pages
1 file
This paper analyses the problem of the autonomous control of a satellite in Low Earth Orbit (LEO) using an optimum controller. The type of controller considered is a standard Linear Quadratic Regulator (LQR). As the problem can also be treated as a two-satellites-formation control in which there is no cancellation of the common perturbative forces, its formulation is similar to that used in formation flying control problems. The driving orbit control requirement is to keep the satellite orbit within a maximum absolute distance of 250 m (r.m.s.) from a sun-synchronous, phased and frozen reference orbit. The control action is realized by means of in-plane and out of-plane thrusts whose cost is minimized by the optimal solution of the control problem. The PRISMA dual satellite mission flight software development and test environment is used as a first test-bed to validate the control algorithms. The PRISMA flight software and test environment allows a very realistic validation of the proposed control techniques. The TerraSAR-X mission scenario is use as a second test-bed for the validation of the control algorithm as it is a very representative example of LEO satellite for Earth observation with high demanding orbit control accuracy requirements.
AIAA/AAS Astrodynamics Specialist Conference and Exhibit, 2008
In this thesis, several control techniques are applied to an occulter satellite for a given formation flying mission. This research is in collaboration with the Flight Dynamics Analysis branch at the NASA Goddard Space Flight Center in Greenbelt, Maryland. The spacecraft is part of a leader-follower configuration which orbits about the Earth/Moon-Sun L2 libration point in a lissajous orbit. A controller is required to maintain a distance of 50,000 km between the occulter and the leader satellite in the radial direction with respect to the orbit. The occulter is allowed a tolerance range of 10 m within the "shadow" of the leader. In addition, the controller must also minimize the fuel usage (Deltav) needed to maintain the occulter's trajectory. The control techniques analyzed in this paper consist of several linear (PID, Linear Quadratic Regulator, and Hinfinity) and one nonlinear controller (Sliding Mode Control). All control techniques are compared based on the overall minimization of trajectory error and fuel usage, ease of implementation, and robustness against disturbances and perturbations such as solar radiation pressure and thruster misalignments. The results of this research show that of the control techniques analyzed, the Linear Quadratic Regulator (LQR) and Sliding Mode Control (SMC) satisfy the mission requirements. While LQR uses less fuel to satisfy given mission requirements, the SMC would also be a suitable choice of control if a mission weighted accuracy over fuel usage.
Journal of Aerospace Engineering, 2007
In this paper, a controller is designed for a satellite formation flying system around the Earth based on an uncertainty model derived from a nonlinear relative position equation. In this model, nonzero eccentricity and varying semimajor axis are included as parametric uncertainties. J 2 perturbation, atmospheric drag, and actuation and sensor noise are bounded by functional uncertainties. The controller design based on the nominal mission ͑an 800 km altitude circular reference orbit͒ is capable of achieving desired performance, is robust to uncertainties, and satisfies fuel consumption requirements even in a challenge nonnominal mission ͑a 0.1 eccentricity and 7,978 km semimajor axis elliptic reference orbit͒ with the same control gain. In this nonnominal mission, the designed controller is able to keep formation with almost the same level of the ⌬V budget ͑43.86 m/s/year͒ as used in the nominal mission ͑39.65 m/s/year͒. For comparison, linear quadratic regulator ͑LQR͒ and sliding mode controllers ͑SMC͒ are developed and extensively tuned to get the same ⌬V consumption as that of the designed controller for the nominal mission. However, as shown in the simulation, the designed linear robust controller ͑LQR͒ and nonlinear robust controller ͑SMC͒ have a serious ⌬V consumption penalty ͑1.72 km/ s / year for SMC͒ or are unstable ͑for LQR͒ in the nonnominal mission.
TerraSAR-X is an advanced synthetic aperture radar satellite system operated in a 505 km altitude sun-synchronous repeat orbit. A tight orbit control requirement, driven by interferometric applications, is formulated as a 250 m radius “tube” defined about an Earth-fixed reference orbit. In this paper we review the orbit control requirements and constraints and discuss the implemented guidance and control concept. Since the launch in 2007 more than six years of in-flight experience have been gained, including almost 500 orbit control maneuvers. The presented flight results proof that both the implemented reference orbit and the orbit control concept work remarkably well, and that the tight control requirement is fully met.
2009
A DYNAMICS AND CONTROL ALGORITHM FOR LOW EARTH ORBIT PRECISION FORMATION FLYING SATELLITES Jesse Koovik Eyer Doctor of Philosophy Graduate Department of Aerospace Science and Engineering University of Toronto 2009 An innovative dynamics and control algorithm is developed for a dual-nanosatellite formation flying mission. The principal function of this algorithm is to use regular GPS state measurements to determine the controlled satellite’s tracking error from a set of reference trajectories in the localvertical/local-horizontal reference frame. A linear state-feedback control law—designed using a linear quadratic regulator method—calculates the optimal thrusts necessary to correct this error and communicates the thrust directions to the attitude control system and the thrust durations to the propulsion system. The control system is developed to minimize the conflicting metrics of tracking error and ∆V requirements. To reconfigure the formation, an optimization algorithm is designed...
Journal of Aerospace Engineering, 2007
In this paper, a controller is designed for a satellite formation flying system around the Earth based on an uncertainty model derived from a nonlinear relative position equation. In this model, nonzero eccentricity and varying semimajor axis are included as parametric uncertainties. J 2 perturbation, atmospheric drag, and actuation and sensor noise are bounded by functional uncertainties. The controller design based on the nominal mission ͑an 800 km altitude circular reference orbit͒ is capable of achieving desired performance, is robust to uncertainties, and satisfies fuel consumption requirements even in a challenge nonnominal mission ͑a 0.1 eccentricity and 7,978 km semimajor axis elliptic reference orbit͒ with the same control gain. In this nonnominal mission, the designed controller is able to keep formation with almost the same level of the ⌬V budget ͑43.86 m/s/year͒ as used in the nominal mission ͑39.65 m/s/year͒. For comparison, linear quadratic regulator ͑LQR͒ and sliding mode controllers ͑SMC͒ are developed and extensively tuned to get the same ⌬V consumption as that of the designed controller for the nominal mission. However, as shown in the simulation, the designed linear robust controller ͑LQR͒ and nonlinear robust controller ͑SMC͒ have a serious ⌬V consumption penalty ͑1.72 km/ s / year for SMC͒ or are unstable ͑for LQR͒ in the nonnominal mission.
AIAA Guidance, Navigation, and Control Conference and Exhibit, 2006
Journal of Guidance, Control, and Dynamics, 2009
Autonomous formation flight is a technical challenge of great interest for many scientific missions. Among other applications, the design of synthetic apertures is a promising benefit of using distributed spacecraft. Even if numerous studies exist in the literature, the formation flying concepts and applications were until now rather theoretical. The TanDEM-X Autonomous Formation Flying system presented in this paper will be implemented in the upcoming TanDEM-X mission and contributes as such to increase the readiness level of this technology. The paper focuses on the design of a guidance, navigation, and control system enabling the autonomous relative control of two spacecraft flying on near-circular orbits. Emphasis is given to the practical implementation within an onboard embedded computer, which requires a simple, resource-sparing, and robust design of the system. Therefore, the algorithms are tailored to minimize the usage of onboard resources and to allow the harmonious integration of the relative control system within the space segment. The validation of TanDEM-X Autonomous Formation Flying performed using a hardware-in-the-loop testbed shows that control performance at the meter level is expected.
Space 2000 Conference and Exposition, 2000
Microcosm, under Air Force Research Laboratory (Space Vehicles Directorate) and internal funding, developed and recently flew the first fully autonomous, on-board orbit determination and intrack and cross-track control system. Results show the technology maintaining in-track position to ± 1 km indefinitely while using less propellant than traditional orbit maintenance. This technology provides a capability never previously available: specifying a satellite's position months, if not years, in advance with great ease and accuracy with simple geometric calculations rather than complex orbital mechanics and propagation. This will allow all system components (ground based and on-orbit) to know factors such as the current location of all satellites in the system, location and direction to the nearest satellite, parameters of current or future ground passes, when satellite transitions occur, and when a given satellite will next be over any location for all future times. For constellations, the technology eliminates the need for re-phasing as the in-track position is maintained as well as the altitude. Implementing autonomous orbit control significantly reduces operations costs, eliminates many of the traditional payload planning cycles, and creates added system robustness. This paper provides results of the flight demonstration and discusses additional applications of this technology.
2001
Fully autonomous, on-board orbit control was flight demonstrated on UoSAT-12. This technology was developed to reduce operations cost. It does this using less propellant than traditional orbit maintenance, but also opens a new array of potential mission applications. Because the position of the spacecraft is controlled and, therefore, known at all future times, users can plan applications without updating the spacecraft ephemeris; simple ground equipment can know the position of all system satellites at any time; and the ground track of the spacecraft can be made to follow a pre-defined path. Other applications are also described.
The novel concept of multiple spacecraft formation flying as a substitute for a single large vehicle will enhance future space mission performance. The benefits of a spacecraft formation include more cost effective synthetic aperture radar for observations, "graceful degradation" of the formation, flexibility of the satellites altering their roles, reduction of cost owing to the reduction of mass launched into orbit etc. A significant challenge in the domain of control design is to contrive a formation maintenance controller that will enable the member spacecrafts to maintain a desired relative orbit with minimal propellant expenditure. This thesis examines linear and nonlinear LEO formation control methodologies, one of each type, with the aim of evaluating them from a propellant budget, thrust level and error dynamics standpoint. A Linear Quadratic Regulator has been applied on J 2 -perturbed Clohessy-Wiltshire dynamics. In order to remove the restriction of the applicability of Cartesian local vertical local horizontal frame based control laws to only circular leader orbits, a sliding mode controller acting on a full nonlinear dynamical model has been implemented. This work also studies the effects of leader orbit eccentricity, inclination and formation radius on formation keeping fuel demand and tracking error. Finally, conclusions are drawn regarding the suitability of the control laws considered and various recommendations made.
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