The HESSI magnetic attitude control system

Abstract

The High Energy Solar Spectroscopic Imager (HESSI) mission represents one of NASA’s low-cost Small Explorer spacecraft missions developed to study the physics of solar flares to much greater detail than ever achieved in the past. The spacecraft is spinstabilized about its solar imaging axis and maintains a Sun pointing orientation to within 0.2 degrees using active magnetic precession and nutation control. The precession control logic is based on a simple proportional control loop operating around the two-axis outputs Ii-om a fme Sun sensor and the measured local magnetic field horn a three-axis magnetometer. Although the spacecraft is spin-stabilized about its major principal axis, the continuous precession torques generate unstable nutational energy without sufficient passive or active nutation control. Hence, to ensure ample nutation control while minimizing cost and weight, an active nutation controller is implemented based on the transverse rates estimated from the fme Sun sensor outputs and the spin rate estimated from the magnetometer. The resulting combined active precession/nutation controller is stable as long as the nutation time constant is smaller than the precession time constant. The magnetic control system is also capable of acquiring Sun-pointing orientation from any arbitrary attitude upon release from the launch vehicle. The simplified control design requires no magnetic field model or ephemeris propagator. I. The HESS1 Spacecraft and Mission The High Energy Solar Spectroscopic Imager (HESSI) mission, representing one of NASA’s low-cost Small Explorer (SMEX) missions, is scheduled for launch aboard a Pegasus XL in July of 2000. The primary mission goal is to explore the basic physics of particle acceleration and explosive energy release in Copyright