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The Solar Dynamics Observatory (SDO) was designed and built at the Goddard Space Flight Center, launched from Cape Canaveral on February 11, 2010, and reached its final geosynchronous science orbit on March 16, 2010. The purpose of SDO is to observe the Sun and continuously relay data to a dedicated ground station. SDO remains Sun-pointing throughout most of its mission for the instruments to take measurements of the Sun. The SDO attitude control system (ACS) is a single-fault tolerant design. Its fully redundant attitude sensor complement includes sixteen coarse Sun sensors (CSSs), a digital Sun sensor (DSS), three two-axis inertial reference units (IRUs), and two star trackers (STs). The ACS also makes use of the four guide telescopes included as a part of one of the science instruments. Attitude actuation is performed using four reaction wheels assemblies (RWAs) and eight thrusters, with a single main engine used to provide velocity-change thrust for orbit raising. The attitude control software has five nominal control modes, three wheel-based modes and two thruster-based modes. A wheel-based Safehold running in the attitude control electronics box improves the robustness of the system as a whole. All six modes are designed on the same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integral gains to zero. This paper details the final overall design of the SDO guidance, navigation, and control (GN&C) system and how it was used in practice during SDO launch, commissioning, and nominal operations. This overview will include the ACS control modes, attitude determination and sensor calibration, the high gain antenna (HGA) calibration, and jitter mitigation operation.The Solar Dynamics Observatory mission is part of the NASA Living With a Star program, which seeks to understand the changing Sun and its effects on the Solar System, life, and society. To this end, the SDO spacecraft carries three Sun-observing instruments: Helioseismic and Magnetic Imager (HMI), led by Stanford University; Atmospheric Imaging Assembly (AIA), led by Lockheed Martin Space and Astrophysics Laboratory; and Extreme Ultraviolet Variability Experiment (EVE), led by the University of Colorado. The basic mission is to observe the Sun for a very high percentage of the 5-year mission (10-year goal) with long stretches of uninterrupted observations and with constant, high-data-rate transmission to a dedicated ground station to be located in White Sands, New Mexico. These goals guided the design of the spacecraft bus that will carry and service the three-instrument payload. Overarching design goals for the bus are geosynchronous orbit, near-constant Sun observations with the ability to fly through eclipses, and constant HGA contact with the dedicated ground station. A three-axis stabilized ACS is needed both to point at the Sun accurately and to keep the roll about the Sun vector correctly positioned with respect to the solar north pole. This roll control is especially important for t...

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T HE Solar Dynamics Observatory (SDO) is a NASA spacecraft designed to study the Sun. It was launched on February 11, 2010 into a geosynchronous orbit, and uses a suite of attitude sensors and actuators to finely point the spacecraft at the Sun. SDO has three science instruments: the Atmospheric Imaging Assembly (AIA), the Helioseismic and Magnetic Imager (HMI), and the Extreme Ultraviolet Variability Experiment (EVE). SDO uses two High Gain Antennas (HGAs) to send science data to a dedicated ground station in White Sands, New Mexico. In order to meet the science data capture budget, the HGAs must be able to transmit data to the ground for a very large percentage of the time.Each HGA is a dual-axis antenna driven by stepper motors. Both antennas transmit data at all times, but only a single antenna is required in order to meet the transmission rate requirement. For portions of the year, one antenna or the other has an unobstructed view of the White Sands ground station. During other periods, however, the view from both antennas to the Earth is blocked for different portions of the day. During these times of blockage, the two HGAs take turns pointing to White Sands, with the other antenna pointing out to space. The HGAs handover White Sands transmission responsibilities to the unblocked antenna. There are two handover seasons per year, each lasting about 72 days, where the antennas hand off control every twelve hours. The non-tracking antenna clews back to the ground station by following a ground commanded trajectory and arrives approximately 5 minutes before the formerly tracking antenna slews away to point out into space.The SDO Attitude Control System (ACS) runs at 5 Hz, and the HGA Gimbal Control Electronics (GCE) run at 200 Hz. There are 40 opportunities for the gimbals to step each ACS cycle, with a hardware limitation of no more than one step every three GCE cycles. The ACS calculates the desired gimbal motion for tracking the ground station or for clewing, and sends the command to the GCE at 5 Hz. This command contains the number of gimbals steps for that ACS cycle, the direction of motion, the spacing of the steps, and the delay before taking the first step.The AIA and HMI instruments are sensitive to spacecraft jitter. Pre-flight analysis showed that jitter from the motion of the HGAs was a cause of concern. Three jitter mitigation techniques were developed to overcome the effects of jitter from different sources. The first method is the random step delay, which avoids gimbal steps hitting a cadence on a jitter-critical mode by pseudo-randomly delaying the first gimbal step in an ACS cycle. The second method of jitter mitigation is stagger stepping, which forbids the two antennas from taking steps during the same ACS cycle in order to avoid constructively adding jitter from two antennas. The third method is the inclusion of an instrument No Step Request (NSR), which allows the instruments to request a stoppage in gimbal stepping during the times when they are taking images.During the commissioni...

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...