Aerospace and Electronic Systems Magazine June 2017 - 40

Design of The 3U

Figure 9.

Magnetorquer assembly.

radiation tolerant ferromagnetic random access memory (FRAM)
up to 180 KB/s write speed are available for high speed data write
and read operations and storage of flight critical ADCS data.
Four fine digital sun sensors are placed on each of the satellite side panels. This ensures that even if satellite spins around the
longitudinal body axes, accurate sun vector measurements would
still be available most of the time during sunlight. Sun sensors are
not required for proper operation of attitude control but together
with magnetometer readings enable full 3-axis attitude estimation in order to verify spacecraft's pointing accuracy in orbit. The
fine sun sensor measures the incident sun spot onto the surface of
position sensitive photo-diode detector (PSD) and then calculates
sun vector direction based on the geometrical dimensions of the
instrument. The sensor cap with 0.5 mm pinhole is used to limit
the incident light to a smaller beam to increase precision (refer to
Figure 10). A differential measurement circuit ensures that temperature transients in the device do not affect measurement accuracy. The accuracy of the sensor is ±0.5 deg and field of view is
60 deg conic.

ADCS Detumbling Controller
The aim of the detumble controller, as stated in ADCS requirements earlier, is to reduce the initial spin of the satellite from 50
deg/s to below 0.3 deg/s1 in the three satellite body reference axes
within three days. Furthermore, the detumble controller must function as a recovery mechanism in situations where e.g. the thruster
has caused the satellite to spin up due to thrust vector misalignment
during propulsion maneuver.
A commonly used control algorithm for detumbling of satellites in LEOs, is the B ˙ or B-dot controller [15]. It relies on magnetic coils or torque rods as control actuators. LituanicaSAT-2 has
one magnetic coil on its Z axis and two magnetic torque rods on
X and Y axes.
Generally two types of B-dot controllers are used: B-dot proportional controller and a B-dot bang-bang controller [15]. The
control law is based on the measurement of the rate of change of
body fixed magnetometer signals. For the B-dot proportional controller the control law is
1

Note that the natural spin of the velocity pointing body is equal
to 1 revolution per orbit period or 0.06 deg/s.

Figure 10.

Fine sun sensor assembly.

M ctrl = −kB

(1)

where Mctrl is the commanded magnetic dipole moment, k is a gain
constant, and B is the derivative of the earth's magnetic field vector
in spacecraft's body fixed frame.
For a bang-bang system, instead of using proportional control
to calculate the dipole command, the maximum torquer strength is
always used

( )

M ctrl = − M max sgn B

(2)

here Mmax is the maximum torquer dipole.
In both cases a control torque is defined as
Tctrl = M ctrl × Be

(3)

The magnetic moment is controlled by adjusting the output
of pulse-width modulation (PWM) duty cycle to magnetic torque
coils.

VALIDATION OF ADCS PERFORMANCE
SIMULATION OF ORBITAL ENVIRONMENT
The performance of a certain ADCS design is a function of satellite attitude dynamics under environmental torques, which depend
on the expected orbit, altitude, the satellite geometry, and mass
properties. In order to design and quantify the performance of a
certain satellite, a high fidelity simulation is often implemented
of the satellite parameters and the disturbance torques affecting it
[10]. To verify the performance of specific ADCS design solutions
for LituanicaSAT-2, a 6 degree of freedom (DOF) simulation of the
satellite's motion in orbit had to be performed. Matlab Simulink
has been used to implement orbit and attitude propagators. Orbital
motion of the satellite is simulated by implementing SGP4 simplified perturbations model [16]. The attitude propagator models the

IEEE A&E SYSTEMS MAGAZINE

JUNE 2017

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine June 2017