Precision tethered satellite attitude control

Tethered spacecraft possess unique dynamic characteristics which make them advantageous for certain classes of experiments. One use for which tethers are particularly well suited is to provide an isolated platform for space-borne observatories. The advantages of tethering a pointing platform 1 or 2 km from a space shuttle or space station are that, compared to placing the observatory on the parent spacecraft, vibrational disturbances are attenuated and contamination is eliminated.;In practice, all satellites have some requirement on the attitude control of the spacecraft, and tethered satellites are no exception. It has previously been shown that conventional means of performing attitude control for tethered satellites are insufficient for any mission with pointing requirements more stringent than about 1 deg. This is due mainly to the relatively large force applied by the tether to the spacecraft. A particularly effective method of implementing attitude control for tethered satellites is to use this tether tension force to generate control torques by moving the tether attach point relative to the sub-satellite center of mass. A demonstration of this attitude control technique on an astrophysical pointing platform has been proposed for a Space Shuttle flight test project and is referred to as the Kinetic Isolation Tether Experiment (KITE).;The current work is concerned with the theoretical development of both a large angle slew and long term, precision pointing control algorithm for tethered satellites, and the simulation of the KITE mission in an Earth laboratory environment. To that end, a scaled, one-dimensional, air-bearing supported laboratory simulator of the KITE satellite configuration has been constructed and is described in detail. The system equations are derived and a suitable control law is described. The precision control algorithm consists of a Linear Quadratic Gaussian, full-state feedback control law in conjunction with a multi-variable Kalman filter. The control algorithm has been shown to regulate the vehicle orientation to within 0.60 arcsec RMS. This level of precision was achieved only after including a mass center estimator and accurately modeling the effects that the nonlinear actuator added to the system model.;In addition, a tether dynamics simulator has been constructed in order to implement the natural dynamic behavior of a 2 km long tether in the earth laboratory environment. The tether simulator is used to test the ability of the control algorithm to regulate the air-bearing vehicle orientation in the presence of variations in the tether tension magnitude and direction. Results of experiments show that, for the level of variation in tension magnitude and direction expected on orbit, neither longitudinal nor in-plane lateral tether dynamics will prevent the control algorithm from achieving long term, precision attitude control on the order of 1 arcsec.