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

For atmospheric missions of tethered satellite systems, the orbital mo tion must be controlled in addition to the librational motion. This pa per studies the simultaneous control of orbital motion, librations, an d tether length variations by integrating them into a single dynamical system. The equations of motion are derived using the Lagrangian form ulation. 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 ac ting on the system, assuming the atmospheric density to vary exponenti ally with the altitude. Orbital perturbations caused by the aerodynami c forces are estimated. A method to control the orbit is designed, usi ng thrusters installed on the main satellite. The on-off control thrus t is applied along the local horizontal direction and the semi-major a xis is taken as the control parameter for switching the thrusters on a nd off. The librations are controlled by a linear (for deployment and station-keeping) or nonlinear (for retrieval) modulation of tension in the tether.