This module provides the attitude guidance output for a sun pointing mode. This is used
for safe mode, or a power generation mode. The input is the sun direction vector which does not
have to be normalized, as well as the body rate information. The output is the standard attitude
reference state message. The sun direction measurement is cross with the desired body axis that
is to point at the sun to create a principle rotation vector. The dot product between these two
vectors is used to extract the principal rotation angle. With these a tracking error MRP state
is computer. The body rate tracking errors relative to the reference frame are set equal to the
measured body rates to bring the vehicle to rest when pointing at the sun. Thus, the reference
angular rate and acceleration vectors relative to the inertial frame are nominally set to zero.
If the sun vector is not available, then the reference rate is set to a body-fixed value while
the attitude tracking error is set to zero.