Solar Dynamics Observatory (SDO) Ascent Planning and Momentum Management

Abstract

The Solar Dynamics Observatory (SDO) has a mission to study the sun in unprecedented detail and its effects on Earth. SDO will reach its final mission orbit through a sequence of ascent maneuvers including an engineering burn, six major perigee raising burns, and three trim burns. Once it is in its mission orbit, there will be two station-keeping maneuvers a year to keep SDO at 102 degrees W longitude +/-0.5 degrees. To achieve its goals it must maintain almost continuous ground contact. To maximize continuity of ground contact, momentum unloading maneuvers must be reduced to a minimum. Members of the Flight Dynamics and Attitude Control System teams collaborated to design systems and operational plans to properly manage the momentum during operations.The main objective of the SDO Momentum Management Planning (MMP) Utility is to provide a time and a target momentum for the next wheel momentum unload (usually referred to as 'delta-H maneuver'). This is achieved by dynamical propagation of the total angular momentum coupled with a simplified version of the onboard wheel control algorithm to predict time evolution of each of four wheel speeds based on the anticipated history of the system momentum. A two-step procedure was designed to predict times when one of the wheel speeds would approach its upper limit if no delta-H maneuver is scheduled to prevent it. Based on this prediction, the MMP Utility provides the recommended time and a target momentum for the next delta-H maneuver. Reliability of MMP recommendations is restricted by the accuracy of a predicted external torque used for dynamical propagation. theoretical predictions for three torques: Solar-Radiation (SR), Gravity-Gradient (GG), and Magnetic-Disturbance (MD), which play a dominant role in the SDO momentum buildup. While both GG and MD torques are described by simple conventional formulas, computation of the SR torque requires a rather sophisticated model developed by SDO G&C engineers.A serious complication comes from the fact that the average torque responsible for the momentum buildup is by order of magnitude smaller than the amplitude of torque oscillations, so a small effect must be predicted in the presence of large oscillations. The MMP provides a backup procedure utilizing polynomial fits to the inertial angular momentum computed from wheel speeds telemetered over a period of at least several days. The obtained polynomial fits are then used to extrapolate the average torque into the future. Simulations show the extrapolated average torque is usually reliable for no more than three weeks. This prediction is sufficient for scheduling the time of the next delta-H maneuver but not of subsequent maneuvers. NomenclatureA = attitude matrix b = null bias 0 e = 4D normalized vector used to define the null component of the reaction wheel momentum H k = momentum of the k th reaction wheel H = 4D vector formed by reaction wheel momenta H  = 4D projection of H orthogonal to 0 e o H = 4D vector formed by 'signed' (2 and 4) reaction wheel momenta o H  = 4D pr...

High gain antenna calibration on three spacecraft

“…The Solar Dynamics Observatory [5] was launched on Feb. 11, 2010 and achieved its mission orbit on March 16. Its orbit is geosynchronous at about 102 deg west longitude and inclined at about 28.5 degrees.…”