On the control of orbital drift of geostationary tethered satellites

This paper shows that the state of a dumbbell-type tethered geosynchronous satellite, in the neighborhood of a stable spoke equilibrium, is completely controllable by tether length in a central inverse squared gravity field in the subspace defined by a constant moment of momentum. The effectiveness of simple open-loop tether length changes on the adjustment of the system state is investigated, as is the usefulness of optimal control in orbit stabilization and orbit adjustment for both the linearized and full nonlinear models. HE dynamics of tethered satellite systems has been stud- ied extensively during the last twenty years.1"9 Due to recent missions, there has been a renewed interest both in the dynamics and in the control of tethered satellites. The first such experiment flown occurred in 1992 and consisted of a Shuttle-tethered subsatellite system in which tether lengths of approximately 20 km were planned. Shuttle-based subsatellites are also planned for the near future with a tether length of approximately 100 km. The difficulties associated with tether deployment and retrieval are well known in the literature. In the case of the manned Shuttle, the tether dynamics during retrieval are of major concern with respect to safety. These safety problems would be far less serious in the case of un- manned tethered satellite systems of the dumbbell type, which have been suggested for many years, and more recently again by Netzer and Kane.10 The question of using attitude-orbit coupling for the control of such a simple tethered system is therefore finding renewed interest. The present paper investigates attitude-orbit coupling, with particular attention being paid to the use of tether length con- trol for satellite system repositioning. The use of tether deploy- ment for satellite system relocation has been presented previ- ously and is discussed in Ref. 11. The present paper goes beyond this, building on the work in Ref. 12, which showed that a dumbbell-type tethered satellite with tethers of negligi- ble mass is completely controllable in the neighborhood of the spoke equilibrium in a subspace of the state space defined by the constant moment of momentum. This result is reconsidered in the present paper. Under the same simplifying assumptions as in Ref. 12, simple control maneuvers in the neighborhood of the spoke equilibrium will be studied. In addition, a control law for orbit-attitude stabi- lization in the neighborhood of the spoke equilibrium will be tested, the control law being obtained via linear quadratic optimal control theory. If the orbit-attitude coupling is suffi- ciently strong it could possibly be used not only to stabilize the system's attitude but also to adjust the orbit. In particular, by means of a suitable tether control, a satellite in a geostationary orbit may be moved from one point of the orbit to another. Orbit-attitude coupling of tethered satellites therefore in prin- ciple permits the compensation of the in-plane drift of geosta- tionary satellites without propellant expenditure. Since some of the present generation of geostationary satellites have their useful life limited by the total propellant available, tether con-