Abstract—This paper addresses the problem of coordinating multiple spacecraft to fly in tightly controlled formations. The main contribution of the paper is to introduce a coordination architecture that subsumes leader-following, behavioral, and virtual-structure approaches to the multiagent coordination problem. The architecture is illustrated through a detailed application of the ideas to the problem of synthesizing a multiple spacecraft interferometer in deep space. Index Terms—Control architecture, coordinated control, interferometry, spacecraft formation flying. I.

Formation Flying is quickly revolutionizing the way the space community conducts autonomous science missions around the Earth and in space. This technological revolution will provide new, innovative ways for this community to gather scientific information, share this information between space vehicles and the ground, and expedite the Human exploration of space. Once fully matured, this technology will result in swarms of space vehicles flying as a virtual platform and gathering significantly more and better science data than is possible today. Formation flying will be enabled through the development and deployment of spaceborne differential Global Positioning System (GPS) technology and through innovative spacecraft autonomy techniques. This paper provides an overview of the current status of NASA/DoD/Industry/University partnership to bring Formation Flying technology to the forefront as quickly as possible, the hurdles that need to be overcome to achieve the formation flying vision, and the team's approach to transfer this technology to space. It will also describe some of the formation flying testbeds, such as Orion, that are being developed to demonstrate and validate these innovative GPS sensing and formation control technologies.

Carrier-phase Di#erential GPS #CDGPS# for orbit and attitude determination is emerging as a very promising low cost alternative to more conventional methods, such as sun sensors, magnetometers, and star trackers. Relative spacecraft position and attitude determination are important for missions involving formation #ying, suchas those proposed for stellar interferometry under NASA's New Millennium Program, and LEO missions for coordinated Earth observing. This paper describes research on the development of a GPS based relative position and attitude sensing system in a laboratory environment for future application on spacecraft formations. Previous papers have discussed the demonstration of spacecraft rendezvous and capture in 2D, in a fully functional indoor GPS environment using simulated spacecraft on an airbearing table. The work presented in this paper extends Presented at Institute of Navigation GPS-97, Kansas City, Missouri, September 1997 the previous GPS based sensing to a formation of three prototype spacecraft, and experimentally demonstrates very accurate real-time solutions ## 2 cm error# for the three relative vehicle positions. A trade study of estimation algorithms and architectures is presented. Search based techniques for the carrier phase integer ambiguity resolution problem for vehicle formations are also demonstrated on simulation data.

This paper considers the problem of reorienting a constellation of spacecraft such that the fuel distributed across the constellation is both conserved and expended uniformly. Results are derived for constellations with an arbitrary number of spacecraft, assuming that Assistant Professor, Department of Electrical and Computer Engineering. Member AIAA. y Assistant Professor, Department of Mechanical Engineering. Member AIAA. z Senior Research Scientist and Technical Supervisor, Automation and Control Section. Associate Fellow of AIAA. 1 the constellation is in free space, that the spacecraft mass is timeinvariant, and that the thrusters can produce thrust in any direction. An open-loop control algorithm is derived by minimizing a cost function that trades off total fuel minimization and fuel equalization. The associated optimization problem is shown to be amenable to standard algorithms. Simulation results using a four-spacecraft constellation are given. Introduction Multiple s...

This paper considers the problem of rotating a formation of satellites from one orientation to another, using an optimal amount of fuel. The formation is constrained to have the same shape at the beginning and end of a maneuver. However, the shape is unconstrained throughout the maneuver. This implies that to optimize fuel, each spacecraft travels in a straight line, from its beginning to end position. A cost function, motivated by the entropy function from information theory, is used to trade off fuel minimization verses equalizing the fuel across the constellation to avoid fuel starvation. 1 Introduction Multiple spacecraft formation flying is emerging as an enabling technology for a number of planned NASA missions. An example is the proposed separated spacecraft interferometry missions. Since the life expectancy of a satellite is limited by its fuel, fuel optimization is critically important to formation control algorithms. For various application of spacecraft formation flying, i...

There are an increasing number of applications requiring precise relative position and clock offset information. The Global Positioning System has demonstrated precise and drift free position and timing information using CodeDivision -Multiple-Access (CDMA) spread spectrum technology. This technology is widely used and relatively inexpensive, making it attractive in applications beyond the scope of typical satellite based GPS. In situations with limited or no visibility of the GPS satellites, ground transmitters that emulate the signal structure of the GPS satellites (pseudolites) can be used as additional or replacement signal sources. Transceivers (which transmit and receive GPS signals) can be used to improve standard pseudolite positioning systems. If their locations are known, transceivers can be used to remove the need for the reference antenna typically necessary in standard differential systems. By using either the GPS satellite signals or other transceiver signals, a self-surv...

Formation flying of multiple vehicles requires accurate information on the relative position and attitude between the vehicles. Carrier Phase Differential GPS (CDGPS) can provide the necessary accuracy in position and attitude sensing to successfully carry out such missions. In missions with little to no visibility to the NAVSTAR constellation, CDGPS can still be used by augmenting the available signals with onboard transmitters broadcasting the L_1 carrier signal modulated by a unique C/A code. The feasibility of this augmented GPS system is demonstrated with a two-vehicle formation in an indoor environment.

This paper considers the problem of rotating a constellation of spacecraft using on/off thrusters such that the relative distance between the spacecraft is maintained to within a specified tolerance. The basic idea is to resolve all of the constraints into a sphere around the desired trajectory of each spacecraft. As the constellation rotates, the sphere sweeps out a tube that constrains the motion of the spacecraft. The control is activated when the spacecraft encounters the constraint sphere, the direction and magnitude of the thrust are calculated to maximize the travel of the spacecraft before the next required thrust, thus minimizing thrust. 1 Introduction Multiple spacecraft formation flying is emerging as an enabling technology for a number of planned NASA missions. Of particular interest to this paper is separated spacecraft interferometry. Some interferometry missions impose the constraint that the constellation be rotated, or retargeted, while maintaining the relative distan...

Abstract. Distributed space systems are often cited as a means of enabling vast performance increases ranging from enhanced mission capabilities to radical reductions in operations cost. To explore this

of a dissertation submitted by Jonathan R. T. Lawton This dissertation has been read by each member of the following graduate committee and by majority vote has been found to be satisfactory.