Hybrid Leader Follower and Sensor Scheduling for Large Spacecraft Networks

A decentralized estimation architecture for large formations of spacecraft is introduced that, when coupled with a local controller, parameterizes the degree to which a node is a leader or a follower, eliminating the rigid classification of nodes as strictly leaders and followers. Measurements are provided by a single range/bearing sensor similar to the Autonomous Formation Flying Sensor, currently being developed at JPL, and a scheduling algorithm that maximizes the information collected. Assuming the presence of free body dynamics, the controller employed is a thrust-limited, optimal bang-bang controller that is activated when a spacecraft reaches an ellipsoidal boundary defined about a reference state. The resulting architecture is compared via simulation to a system where each spacecraft has full knowledge of the fleet. NASA is currently studying several large scale spacecraft formation missions, many to be flown at the Earth-Sun libration points. The location aords scientists a much clearer view of the universe, while also providing a convenient gravitational pull to be Earth following. Large formations of spacecraft present interesting challenges to the scientific community including: low fuel usage control for given precision requirements, fleet estimation given limited sensor and communication resources, and accurate relative state tracking to reduce fuel in the presence of the other challenges. Developing scalable algorithm tools is critical to the success of these missions. Consider as an example, the case of precision control to minimize fuel. Typical single spacecraft systems are allowed to drift with respect to an external frame, as it is not crucial (and would be fuel expensive) to reposition the satellite. Multiple satellite missions can similarly relax the positioning of the formation in an external reference frame over small time horizons, but each individual spacecraft must control its relative position with respect to the fleet (and minimize fuel) for mission performance and safety. 1‐3 Moreover, sensitivity of these factors grows with the number of spacecraft in the system. Further, while formations of 2-3 spacecraft can be designed with full relative sensing and communication to the fleet, aording an accurate estimate of all spacecraft in the fleet, formations of 16 or more spacecraft will certainly be constrained in the number of relative spacecraft measurements, as well as the number of communication links and available bandwidth. Future NASA missions with large scale formations of spacecraft currently under study include the MicroArcsecond X-ray Imaging Mission (MAXIM), 1 Terrestrial Planet Finder (TPF), 2 and Stellar Imager (SI). 3 MAXIM is an X-ray interferometer composed of 33 spacecraft and will be able to image the event horizon of a black hole. TPF shall enable scientists to find and study extra-solar planets similar to our own. While several designs are currently under review, a TPF design by Lockheed Martin is composed of 4, possibly 6, free flying spacecraft which will function as an infrared interferometer. The motivating example for this work is the Stellar Imager (SI), 3 shown in Figure 1(left). The SI mission postulates that stellar activity is key to understanding life in the universe. SI is a large, space based UV optical sparse aperture telescope/Fizeau Interferometer designed to study the sun. It is designed to be flown at the Earth-Sun libration point, as