EDITORIAL
2
INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL Int. J. Robust Nonlinear Control 2002; 12:95–96 (DOI: 10.1002/rnc.677) EDITORIAL A recent topic of great interest is the use of small satellites flying in close formation. A possible use for satellite formations is distributed sensing for high-resolution, synthetic-aperture radar and imaging. Formations of satellites could operate cooperatively to act as a sparse aperture with an effective dimension larger than can be achieved by a single, larger satellite. Essentially, the impracticality of a large, filled aperture is eliminated through the use of a sparse aperture of separate satellites, the relative positions of whose individual apertures form a set of relative position baselines. Satellites orbiting in close formation will have relative geometry based on a carefully designed set of orbits to achieve desirable relative positions. Small differences in orbital elements between the satellites’ orbits will lead to a formation with useful relative geometry that rotates as a whole as the formation orbits the Earth. In the ideal case, these satellites maintain their relative geometry indefinitely as they orbit. Unfortunately, perturbations such as gravity harmonics from a non-spherical Earth, drag, and solar radiation pressure tend to perturb the orbits from their ideal trajectories. Since each satellite is in a slightly different orbit, the perturbations tend to affect the satellites differently, causing the cluster to disperse. Hence, close formations of satellites require active control for formation-keeping. The goal of research in the dynamics and control of satellite formations is to understand the relative motion behaviour in a realistic setting and develop control algorithms for formation- keeping, formation management and reconfiguration. There are a number of challenges to the design of formation-keeping control laws for formations of satellites. One challenge is to develop useful desired relative orbits that require a minimum of manoeuvring to maintain. Another challenge is to find a feedback technique that optimally uses propellant to overcome the orbital perturbations that tend to disperse the formation. Formation design attempts to minimize the amount of formation-keeping fuel required, while feedback control attempts to minimize the amount of fuel actually expended. Finally, it will be necessary to find control strategies to reconfigure the geometry of the formations to accommodate changes in the mission or in the number of satellites within a formation. The papers in this special issue will provide new ideas and insights into various aspects of the dynamics and control of satellite formation flight. The first three papers in this issue address the formation control problem. The first paper deals with the determination of initial conditions and the design of fuel-balancing orbit control laws for a formation of satellites. A method for determining the initial conditions that result in quasi-periodic relative orbits over the short-term, in the presence of the earth oblateness perturbation, and an intelligent control concept that exploits the physics of the relative motion dynamics are proposed. The second paper presents an adaptive, output feedback control design methodology for the spacecraft formation flying problem. A control law is designed to provide a filtered velocity measurement and a desired adaptive compensation with semi-global, asymptotic relative position tracking. The third paper discusses a decentralized architecture for autonomous establishment and maintenance of satellite formations. Such an architecture does not require a central supervisor, as the spacecraft can cooperatively track planned manoeuvres and trajectories in the face of disturbances and uncertainties, while processing only local Received Copyright # 2002 John Wiley & Sons, Ltd. Revised
-
Upload
andrew-sparks -
Category
Documents
-
view
212 -
download
0
Transcript of EDITORIAL
INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROLInt. J. Robust Nonlinear Control 2002; 12:95–96 (DOI: 10.1002/rnc.677)
EDITORIAL
A recent topic of great interest is the use of small satellites flying in close formation. A possibleuse for satellite formations is distributed sensing for high-resolution, synthetic-aperture radarand imaging. Formations of satellites could operate cooperatively to act as a sparse aperturewith an effective dimension larger than can be achieved by a single, larger satellite. Essentially,the impracticality of a large, filled aperture is eliminated through the use of a sparse aperture ofseparate satellites, the relative positions of whose individual apertures form a set of relativeposition baselines.
Satellites orbiting in close formation will have relative geometry based on a carefully designedset of orbits to achieve desirable relative positions. Small differences in orbital elements betweenthe satellites’ orbits will lead to a formation with useful relative geometry that rotates as a wholeas the formation orbits the Earth. In the ideal case, these satellites maintain their relativegeometry indefinitely as they orbit. Unfortunately, perturbations such as gravity harmonicsfrom a non-spherical Earth, drag, and solar radiation pressure tend to perturb the orbits fromtheir ideal trajectories. Since each satellite is in a slightly different orbit, the perturbations tendto affect the satellites differently, causing the cluster to disperse. Hence, close formations ofsatellites require active control for formation-keeping.
The goal of research in the dynamics and control of satellite formations is to understand therelative motion behaviour in a realistic setting and develop control algorithms for formation-keeping, formation management and reconfiguration. There are a number of challenges to thedesign of formation-keeping control laws for formations of satellites. One challenge is todevelop useful desired relative orbits that require a minimum of manoeuvring to maintain.Another challenge is to find a feedback technique that optimally uses propellant to overcome theorbital perturbations that tend to disperse the formation. Formation design attempts tominimize the amount of formation-keeping fuel required, while feedback control attempts tominimize the amount of fuel actually expended. Finally, it will be necessary to find controlstrategies to reconfigure the geometry of the formations to accommodate changes in the missionor in the number of satellites within a formation. The papers in this special issue will providenew ideas and insights into various aspects of the dynamics and control of satellite formationflight.
The first three papers in this issue address the formation control problem. The first paperdeals with the determination of initial conditions and the design of fuel-balancing orbit controllaws for a formation of satellites. A method for determining the initial conditions that result inquasi-periodic relative orbits over the short-term, in the presence of the earth oblatenessperturbation, and an intelligent control concept that exploits the physics of the relative motiondynamics are proposed. The second paper presents an adaptive, output feedback control designmethodology for the spacecraft formation flying problem. A control law is designed to provide afiltered velocity measurement and a desired adaptive compensation with semi-global, asymptoticrelative position tracking. The third paper discusses a decentralized architecture forautonomous establishment and maintenance of satellite formations. Such an architecture doesnot require a central supervisor, as the spacecraft can cooperatively track planned manoeuvresand trajectories in the face of disturbances and uncertainties, while processing only local
ReceivedCopyright # 2002 John Wiley & Sons, Ltd. Revised
measurement information. Use of such a controller simplifies and improves the robustness offormation operations, since it distributes an autonomous capability for orbit determination andrelative navigation, formation maintenance, and manoeuvre trim among all of the spacecraft. Inan example formation flying scenario, the decentralized approach successfully maintains theformation in the face of uncertainties and nonlinear perturbations, and produces identicalresults to those of a corresponding centralized controller.
The fourth paper addresses a novel approach to the formation control problem, in which anon-propulsive means is proposed for countering the differential orbit plane precession, which isthe major perturbation caused by earth oblateness. The approach taken is to make use of thesolar radiation pressure acting on a relatively small surface, termed a solar wing. The resultingtorque causes the orbit to precess so that, if the wing is sized correctly, the motion from the solarradiation perturbation can be made to cancel, on average, the motion from the earth oblateness.Hence, the formation can be maintained without the use of propellant. The long-term orbitaleffects of the solar wing are shown to be highly nonlinear, with strong coupling between theorbital inclination and the longitude of the ascending node.
The fifth paper discusses co-ordinated attitude control for spacecraft formations. The paperaddresses optimal control for attitude stabilization and attitude tracking, co-ordinationstrategies of multiple satellites, and co-ordinated controller design to implement the co-ordination strategies. Optimal control laws are derived for attitude stabilization and tracking.Then, an innovative design in a perceptive frame is developed for the problem of co-ordinatedattitude control. The controller is able to accommodate a variety of co-ordination strategies thatare important to satellite missions. Several useful co-ordination strategies are introduced. In thepresence of disturbances, the relative attitude error of the distributed satellites is significantlyreduced using the controllers developed in this paper.
The last two papers in this issue discuss the fuel-optimal formation reconfiguration problem.The first of these papers presents fuel- and time-optimal algorithms for a co-ordination andcontrol architecture developed for distributed spacecraft systems. This architecture includesboth the low-level station-keeping control and a high-level co-ordinator that designs trajectoriesto reconfigure the formation. The trajectory and station-keeping optimization algorithmsare based on the solutions of linear and integer programming problems. The overall controlapproach is demonstrated using a realistic nonlinear simulation environment. Finally, in the lastpaper, generation of fuel-optimal manoeuvres for spacecraft formation reconfiguration ismodelled and analysed as a multi-agent optimal control problem, including the consideration oftask assignment among agents for terminal targets in the optimization process. An efficienthybrid optimization algorithm is developed to address this problem. The proposedmulti-agent optimal control methodology uses calculus of variations, task assignment, andgenetic algorithm at different stages of the optimization process, and employs a distributedcomputational architecture. The task assignment is constructed using the principle of optimalityfrom dynamic programming and guarantees global optimal assignment.
Andrew SparksAir Force Research Laboratory
AFRL/VACA, 2210 Eighth StreetWright Patterson Air Force Base
OH 45433-7521, USA
Copyright # 2002 John Wiley & Sons, Ltd. Int. J. Robust Nonlinear Control. 2002; 12:95–96
EDITORIAL96
