Attitude Determination & Control System
The attitude determination and control system (ADCS) is required to point the SNOE satellite spin axis within 5° of the orbit normal, and to maintain the spin rate between 4 and 6 RPM. Of the 15 orbits per day, one or two will be used for attitude control. Attitude determination measurements will be obtained throughout all orbits.
For control actuation, the satellite uses two magnetic torque rods (10 A-m2 each), one directed along the spin axis and the other directed normal to the spin axis. Control commands are generated on the ground, in the form of a sequence of timed commands. These control timelines are uploaded to the satellite, and executed open loop. Spin axis direction is controlled using the spin axis torque rod; spin rate is controlled by commutating the transverse torque rod. Some coupling of these control modes is unavoidable. Passive nutation damping is provided by a alcohol-filled ring damper, mounted perpendicular to the central plate of the satellite. Active nutation can be implemented if needed, using a B-dot law on the spin axis torque rod only. The ADCS system is controlled by an independent electronics module, which provides the command and data interface to the flight computer.
Attitude determination calculations are performed on the ground, as part of the attitude control operations. The attitude measurements come from two pyroelectric horizon crossing indicators (HCIs) and a three axis fluxgate magnetometer. The HCI’s are mounted in a V-configuration with respect to the spin axis, each with a 45 degree half-cone angle.
