Satellite Configuration Design - Finite Element Analysis for Satellite Structures - page 21

All objects must stay out of the payload's field of view. The only practical way
to orient the payload to its target is to rotate the satellite. This is usually the
simplest approach, because fixed mounting of the payload is more easier than
using a gimbaled mechanism. A high precision MBEI has requirements for
accurate pointing. This means the mounting structure must be stiff and provides
direct load path into the satellite's primary structure. Structural distortions between
the payload and the ADCS sensors must be minimized. Distortions can result from
on-orbit structural vibration, on-orbit thermal effects, and any yielding or joint
shifting during launch or ground operations. Making the mounting structure stiff
avoids problems from on-orbit vibration and lunch effect. Thermal deformation
can be controlled by selecting the right materials and by controlling temperatures.
2.6.2 Attitude Determination and Control
Subsystem
The selected method of control drives the satellite's shape. The satellite config-
uration, in turn, can derive the types and sizes of actuators. Small Sat is preferred
to be symmetric, this will reduce aerodynamic drag and solar radiation pressure,
hence a net torque. To minimize this torque, the Small Sat's center of mass should
be as close as possible to its center of pressure, which is the centroid of the
satellite's projected area. This is provided also by creating a symmetrical front
area, so four solar arrays, symmetrical about the satellite's center of mass, will be
used. Symmetry also reduces gravity-gradient torques, as does a compact shape.
The configuration of tree-axis control satellite, like Small Sat, is the most severe
constraint for ADCS and structural design. Making appendages of Small Sat as
short as possible makes it easier to keep natural frequencies above the control
system's bandwidth. This will avoid resonance phenomena which lead to structural
fracture.
The star sensor of Small Sat requires a narrow field of view, so it must be
protected from any obstacles. Bright sunlight can damage the star sensor or causes
it to shut down. Therefore, the star sensor mounting will be turned by a certain
angle to protect it from sunlight. The Magnetometer must be installed at enough
distances from high magnetic field components like ADCS actuators, reaction
wheels, and magnetorquer. Alignment is very important for ADCS sensors, so they
are grouped on one platform, which is stiff and thermally stable to reduce errors
from distortions.
For reaction wheels, a common approach is to align them with the satellite axes
and add a wheel at the critical axis to provide redundancy. If any one wheel fails,
the redundant wheel can compensate. The Y-axis shown in Fig. 2.5 is the most
critical one for the stability of Small Sat, so the redundant wheel is installed on the
Y-axis. The same approach is followed for the angular velocity meters ''gyros'',
but the redundant one is added at a skewed axis. The configuration of Small Sat