New pulse-modulation technique for guidance and control of automated spacecraft

A comprehensive approach to problems such as automated on-orbit rendezvous and soft landing on a planetary surface is presented for spacecraft employing pulse-operated (on-off) propulsion systems. Using a technique derived from robust control theory, a new class of guidance algorithms that modulate the duration and frequency of thruster firings is developed. These algorithms allow analytical characterization of transient errors, limit cycle deadband, and the set of possible terminal conditions in the design process, without the use of dynamical approximations such as linearization. With this approach, the desired performance is ensured in the presence of dynamical modeling errors with known bounds; the effects of navigational errors can be minimized to the extent that they can be bounded. A realistic application is illustrated via computer simulation of a hypothetical mission scenario, in which a robotic spacecraft equipped with an aided-inertial guidance system soft lands on the planet Mars. N missions carried out with robotic spacecraft, self-contained guidance and control are often desirable and in some cases a necessity. Although a wealth of theory exists for applications such as automated orbital transfer, rendezvous, station keeping, and soft landing, among others, much of this knowledge base presumes the use of continuous throttleable or fixed-thrust propulsion, whereas in practice many spacecraft are equipped with on-off gas jets or thrusters because of their relative simplicity and low cost. This paper describes a new technique for pulse-modulated operation of these devices intended for use in the aforementioned problems. A representative , but not exhaustive, review of previous work emphasizing mission applications is as follows. Much of the ren- dezvous literature is devoted to the determination of optimal trajec- tories and their associated thrust profiles. Both theoretical and op- erational aspects of rendezvous are discussed in the survey papers by Jezewski et al.1 and Leonard and Bergmann.2 Parten and Mayer3 and Young and Alexander4 describe the rendezvous and docking procedures developed for the Gemini and Apollo piloted missions, respectively; these procedures have also been used in Space Shut- tle missions. Automated rendezvous schemes have been proposed for the Space Shuttle5'6 but not implemented. Previous missions in- volving soft landing have all been accomplished with throttleable propulsion systems. The early work leading to the guidance algo- rithms used in the Apollo missions was performed by Cherry7 and Battin.8 The operational Apollo lunar module guidance system is described by Klumpp.9 The automated guidance schemes used by the Surveyor and Viking spacecraft were similar: the terminal de- scent system developed for the Surveyor lunar lander is described by Cheng et al.,10 and Ingoldby11 discusses the corresponding system for the Viking Mars landers. Another prevalent feature of the literature is the use of linear approximations of the spacecraft's dynamics to obtain an analyt- ically tractable problem. Given the nonlinear behavior of on-off thrusters, the closed-loop dynamics of systems employing these