||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.