Robust digital autopilot design for spacecraft equipped with pulse-operated thrusters

Abstract

The analysis and design of attitude control systems for spacecraft employing pulse-operated (on-off) thrusters is usually accomplished through a combination of modeling approximations and empirical techniques. A new thruster pulse-modulation theory for pointing and tracking applications is developed from nonlinear control theory. This theory provides the framework for an autopilot suitable for use in digital computers whose performance and robustness properties are characterized analytically, in the design process. Given bounds on the anticipated dynamical modeling errors and sensor errors, it is shown that design specifications can be established and acceptable performance ensured in the presence of these error sources. Spacecraft with time-varying inertia properties can be accommodated, as well as clustered thruster configurations that provide multiple discrete torque levels about one or more spacecraft axes. A realistic application of the theory is illustrated via detailed computer simulation of a digital autopilot designed for midcourse guidance of a hypothetical interplanetary spacecraft.