OPTIMAL SPACECRAFT ATTITUDE CONTROL USING COLLOCATION AND NONLINEAR PROGRAMMING

Introduction I N recent years, NASA has been able to retrieve disabled satellites from orbit and has even repaired one satellite, the Solar Maximum satellite, while in orbit. However, there are many satellites in orbits too high for the shuttle to reach. If such a satellite fails to deploy properly or malfunctions after some use is made of it, there is currently no way of retrieving it. A remotely operated satellite [or orbital maneuvering vehicle (OMV)] is needed for retrieval of such satellites. A prerequisite for retrieval is the detumbling of the satellite. Optimal open-loop controls for detumbling of a satellite were developed by Con way and Widhalm. A nonlinear feedback control for the same problem was developed by Widhalm and Conway. Webber improved the nonlinear feedback controller of Ref. 2 and was able to make the cost comparable to that of the open-loop control formulation. Here, a new method, direct collocation with nonlinear programming (DCNLP), is employed to find the optimal openloop control histories for detumbling a disabled satellite. The satellite system is assumed to have docked in such a way that the tumbling motion of the disabled satellite is not disturbed. The controls are torques and forces applied to the docking arm and joint and torques applied about the body axes of the OMV. The results of Ref. 1 have been reproduced, verifying the solution, but the method used for this work is simpler to implement and more robust. Solutions have been obtained for cases in which various constraints are placed on the controls and in which the number of controls is reduced or increased from that considered in Ref. 1.