ESA GNC Conference Papers Repository
Flight Performance of the IRIS Attitude Control System
The Interface Region Imaging Spectrograph (IRIS) small explorer (SMEX) spacecraft was launched into a near-perfect 600 km sun-synchronous dawn dusk orbit by a Pegasus launch vehicle dropped from a Lockheed L1011 aircraft near Vandenberg, California on June 27, 2013. The primary science objective of IRIS is to gain understanding of how the solar atmosphere is energized through the interface region of the sun by combining advanced numeric modeling with images and spectra provided by the UV imaging spectrograph of IRIS. Following separation from the launch vehicle and deployment of the solar arrays (SA), the attitude control system (ACS) first detumbled the spacecraft (S/C) efficiently using three magnetic torque rods (MTR) and a science-grade DTU magnetometer (MGM). Next, the four reaction wheels were used to rotate the S/C to bring the sun inside the 128x128° FOV of the digital sun sensor (DSS) using information from five coarse sun sensors (CSS), after which a transition to the coarse control mode followed where the ACS uses just the DSS and MGM attitude sensors. Steady state coarse control ensures that the SAs are pointed to the sun to a better than 1° accuracy. Following checkout of the dual head ?ASC DTU autonomous star tracker (AST), sampled at 2 Hz, a transition was commanded from the coarse control mode, which is part of the suite of safe hold modes, to the inertial sun pointing (ISP) mode, where, using the AST and ephemerides, the Z axis is pointed to the sun and the X axis is kept in the plane defined by the polar axis of the sun and the apparent direction of the sun. The boresights of the two AST heads, which are at 93° angles from the Z axis, are pointed in opposite directions to ensure that one head is always clear from the Earth . The AST accuracy is sufficient to bring the sun to within the +- 80 arcsec linear range per axis of the solar Guide Telescope (GT), thus allowing a transition to the fine sun pointing (FSP) mode where the GT, which is co-boresighted with the science telescope, is used for control about two axes while the AST is used for control about the Z axis. The instrument also employs the GT for image stabilization, achieved by 2-axis control of the secondary mirror of the telescope. In addition to describing the ACS design, the paper mainly addresses the ACS flight performance, which turned out to be excellent with a pointing stability of better than 0.5 arcsec (3?) about the X and Y axes. Feed-forward compensation designed to minimize attitude disturbances due to angular momentum unloading was also found to be highly effective, limiting excursions totypically less than 1/3rd of the 10 requirement. As a result, science observations are not affected by unloading operations. We will discuss problems discovered during IOC for which solutions had to be found to ensure a highly successful mission.