Solar Dynamics Observatory Guidance, Navigation, and Control System Overview

Angela M. Russo, Scott R. Starin, and Melissa F. VessNASA Goddard Space Flight Center Code 591, Greenbelt, Maryland 20771AbstractThe Solar Dynamics Observatory (SDO) was designed and built at the Goddard Space Flight Center, launchedfrom 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 remainsSun-pointing throughout most of its mission for the instruments to take measurements of the Sun. The SDO attitudecontrol system (ACS) is a single-fault tolerant design. Its fully redundant attitude sensor complement includessixteen coarse Sun sensors (CSSs), a digital Sun sensor (DSS), three two-axis inertial reference units (IRUs), andtwo star trackers (STs). The ACS also makes use of the four guide telescopes included as a part of one of the scienceinstruments. Attitude actuation is performed using four reaction wheels assemblies (RWAs) and eight thrusters, witha single main engine used to provide velocity-change thrust for orbit raising. The attitude control software has fivenominal control modes, three wheel-based modes and two thruster-based modes. A wheel-based Safehold running inthe attitude control electronics box improves the robustness of the system as a whole. All six modes are designed onthe same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integralgains to zero. This paper details the final overall design of the SDO guidance, navigation, and control (GNCAtmospheric Imaging Assembly (AIA), led by Lockheed Martin Space and Astrophysics Laboratory; and ExtremeUltraviolet Variability Experiment (EVE), led by the University of Colorado. The basic mission is to observe theSun for a very high percentage of the 5-year mission (10-year goal) with long stretches of uninterrupted observationsand with constant, high-data-rate transmission to a dedicated ground station to be located in White Sands, NewMexico. 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 flythrough eclipses, and constant HGA contact with the dedicated ground station. A three-axis stabilized ACS isneeded both to point at the Sun accurately and to keep the roll about the Sun vector correctly positioned with respectto the solar north pole. This roll control is especially important for the magnetic field imaging of HM I.The mission requirements have several general impacts on the ACS design. Both the AIA and HMI instrumentsare very sensitive to the blurring caused by jitter. Each has an image stabilization system (ISS) with some ability tofilter out high frequency motion, but below the bandwidth of the ISS the control system must compensate fordisturbances within the ACS bandwidth or avoid exciting jitter at higher frequencies.Within the ACS bandwidth, the control requirement imposed by AIA is to place the center of the solar disk nomore than 2 arc