Formation-flying Spacecraft Research Articles

Pulsed Plasma Thruster plume analysis

Micro-Pulsed Plasma Thrusters (μPPTs) are a promising method for precision attitude control for small spacecraft in formation flying. They create an ionized plasma plume, which may interfere with other spacecraft in the formation. To characterize the ions in the plume, a diagnostic has been built that couples a drift tube with an energy analyzer. The drift tube provides time of flight measurements to determine the exhaust velocity, and the energy analyzer discriminates the ion energies. The energy analyzer measures the current on a collector plate downstream of four grids that repel electrons and ions below a specified energy. The first grid lowers the density of the plasma, therefore increasing Debye length. The second and fourth grids have a negative potential applied to them so they repel the electrons, while the third grid's voltage can be varied to repel lower energy ions. The ion energies can be computed by differentiating the data. Combining the information of the ion energies and their velocities identifies the ion masses in the PPT plume. The PPT used for this diagnostic is the micro-PPT developed for the Dawgstar satellite. This PPT uses 5.2 Joules per pulse and has a 2.3 cm 2 propellant area, a 1.3 cm electrode length, and an estimated thrust of 85μN [C. Rayburn et al., AIAA-2000-3256]. This paper will describe the development and design of the time of flight/gridded energy analyzer diagnostic and present recent experimental results.

Sensing Technologies for Formation-Flying Spacecraft in LEO using CDGPS and an Interspacecraft Communications System

: This paper describes research on sensing technologies for formation-flying spacecraft in low earth orbit. The approach is based on a combination of carrier-phase differential GPS (CDGPS) sensors and GPS-like transceivers that are combined with the interspacecraft communications system. By providing additional inter-spacecraft range and Doppler measurements to the navigation solution, these on-board transmitters could facilitate the use of CDGPS for orbits in which adequate visibility/geometry to the NAVSTAR constellation is not available. In addition, the carrier-phase cycle ambiguity can be rapidly initialized using relative vehicle motions. The paper discusses this augmentation concept in detail and presents a new algorithm that significantly improves the bias initialization process. The paper also addresses some key measurement errors associated with formation-flying spacecraft separated by long baselines (10–100 km). Experimental results on a new terrestrial testbed are presented to show the vehicle formation undergoing short path initialization and formation maneuvers.

The need for a systems perspective in control theory and practice

Control systems have had a profound impact on society as theories, techniques, and algorithms have migrated from the laboratory to thousands of products. As the control community continues to improve its solutions, ideas are being generated for a new era of applications that are fundamentally different from their predecessors. Where applications once employed control systems for greater performance, new applications require control systems for their very existence. Examples of these types of systems are ultra-agile military aircraft, large-space-structure observatories, and formation-flying spacecraft constellations. This control-enabled class of systems (also called high-performance systems) is changing the role of control science in engineering. We examine the new role of control science in high-performance systems and its implication for control theory. We describe the traditionally serial role of control science in systems design and the evolving iterative system-control design process used for high-performance systems. Next, we discuss several high-performance systems that use this iterative design process. Then, we describe past work that is related to systems-control analysis-synthesis (S-C-A-S), followed by an examination of fundamental control theory concepts that may be extended as part of an S-C-A-S theory. Finally, we present conclusions.