DYNAMICS AND CONTROL OF COUPLED ORBITAL AND LIBRATIONAL MOTION OF TETHERED SATELLITE SYSTEMS

For atmospheric missions of tethered satellite systems, the orbital motion must be controlled in addition to the librational motion. This paper studies the simultaneous control of orbital motion, librations, and tether length variations by integrating them into a single dynamical system. The equations of motion are derived using the Lagrangian formulation. Dynamics and control during all the three phases-deployment, station-keeping, and retrieval-are considered. Analytical expressions are obtained for the generalized forces due to the aerodynamic drag acting on the system, assuming the atmospheric density to vary exponentially with the altitude. Orbital perturbations caused by the aerodynamic forces are estimated. A method to control the orbit is designed, using thrusters installed on the main satellite. The on-off control thrust is applied along the local horizontal direction and the semi-major axis is taken as the control parameter for switching the thrusters on and off. The librations are controlled by a linear (for deployment and station-keeping) or nonlinear (for retrieval) modulation of tension in the tether.