ADCS HWAR project

1. Jan Peeters Salazar
06035621
AA279C Project: HAWK
Jan Peeters Salazar
Table of Contents
Section 1.....................................................................................................................................................................2
Satelllite Choice....................................................................................................................................................2
Satellite Orbit Parameters...................................................................................................................................2
Mission Requirements.........................................................................................................................................3
Attitude Determination and Control System: Sensors and Actuators ........................................................3
Systems Layout and Mass Distribution............................................................................................................4
Inertia Properties and Principal Axis................................................................................................................5
Barycenter Coordinates and Orbit Propagation Scheme ..............................................................................7

2. Jan Peeters Salazar
06035621
Section 1
Satelllite Choice
Inspired by Cyrrus Foster’s lecture during AA279, the satellite chosen for this project was inspired by
the Flock 1 constellation launched by Planet Labs in April 2013. One of these satellites is presented in
the image below:
Keeping close resemblance to the original Dove, this project satellite, the HAWK will exhibit the
following mission characteristics:
• Orbit: LEO (400 km, ISS altitude)
• Target Attitude: Earth pointing (for imaging)
• Parameterization: quaternions (industry standard)
• Sensors: magnetometers, gyros and star-pointing camera (photodiodes also included, but not
covered)
• Actuators: magnetorquers and reaction wheels
Satellite Orbit Parameters
The constellation comprises 28 HAWK 3U cubesats with a 3-5 m resolution camera as the primary
payload. For this project, it will be assumed that HAWK operates on the same orbit plane as the
International Space Station (ISS) with orbit parameters 1:
• Semi-major axis: 𝑎 = 6775 𝑘𝑚
• Eccentricity: 𝑒 = 0.0006899
• Inclination: 𝑖 = 51,6426°
• Right ascension of ascending node: Ω = 0.8840°
• Argument of perigee: 𝜔 = 49.4428°
1 hhttp://accms04.physik.rwth-aachen.de/~schwerin/meet_160702/thorsten1_160702.pdf

3. Jan Peeters Salazar
06035621
Mission Requirements
Among the vast array of requirements that such a mission may have, the set presented below is both
applicable to the payload (the Earth imaging camera) and the course (Attitude Determination and
Control). PlanetLabs has not made public the mission requirements of the Dove satellites; in lieu,
similar Earth-imaging missions such as the BIRD mission of (Cervone, 2014) and the FireSat analysis
of (Wertz & Larson, 2010) were used as reference. Considering that requirements should be verifiable,
achievable, logic, integral and definite, 7 key ADCS requirements for the HAWK satellite are:
• ADCS_Req_1: The ADCS shall have a pointing accuracy of at least 1 deg, 3σ on all three axis
on Earth pointing mode.
• ADCS_Req_2: The ADCS shall have a reliability of 95% on the nominal projected life cycle
• ADCS_Req_3: The ADCS shall be single-redundant on 3 axis at all times
• ADCS_Req_4: The satellite shall have a maximum maneuvering rate of 3 deg/s during high-
drag to low-drag attitude maneuvers
• ADCS_Req_5: The ADCS shall have an accuracy of 0.25 deg, 3σ including determination and
control errors
• ADCS_Req_6: The ADCS shall have a stability of 0.1 deg for the duration of one shot of the
Dove’s Bayer Masked CCD camera
• ADCS_Req_7: The ADCS shall avoid any vibrations corresponding to the natural frequency
of the telescope payload and the Bayer Masked CCD camera or any of its constituent
components
Attitude Determination and Control System: Sensors and Actuators
The HAWK satellite will be designed to operate in 5 different modes:
• A nominal mode where the satellite will be performing Earth-tracking activities
• A power acquisition mode, where the satellite will be pointing at the Sun for battery recharges
• A high-drag mode, to increase/decrease the relative spacing with other satellites of the
constellation
• A safe mode, in case of any subsystem malfunction
• An end-of life mode, after which the satellite should return to the atmosphere
Following the previous requirements and the modes defined above, the HAWK satellite will be
designed to use a combination of 3 sensors in its attitude determination subsystem:
• Two independent three-axis magnetometers with post-calibration capabilities for nominal
conditions (0.1 deg, 3σ) with redundant connections
• One Sun sensor for use during power acquisition mode (3 deg, 3σ). Redundant connection.
• One Star camera (0.5 deg, 3σ) for fine pointing. Redundant connection.
The combination of these devices should be enough to comply with the ADS requirements.
The actuator selection for the attitude control subsystem is driven primarily by the pointing accuracy but
also by the high-drag/low-drag maneuvering rate. Following the requirements, the HAWK satellite will
use a combination of 3 actuators in its attitude control subsystem:
• One set of 4 reaction wheels in 4-point pyramid configuration (ensuring redundancy) for nominal
conditions

4. Jan Peeters Salazar
06035621
• Two magnetic “coil type’ torquers for moment dumping. Single redundant connections.
Systems Layout and Mass Distribution.
Following the Cubesat size convention and the (limited) data provided by PlanetLabs, a very simplified
layout model of the HAWK satellite is presented in this sub-section.
From PlanetLabs’ specifications, the Flock-1 Dove satellites weigh about 5 𝑘𝑔. Since the HAWK
satellite is going to significantly outperform the competition, it is assumed that it has a weight of
4.0 𝑘𝑔. For simplification, it is assumed that HAWK only has 4 components: bus, payload (telescope
and Bayer Masked CCD camera), solar panels, and systems (ADCS, EPS, C&DH and
Communication.). Furthermore, a mass fraction of each component was assumed, yielding the
following masses:
• Bus – 0.6 kg (15%)
• Payload – 1.4 kg (35%)
• Systems – 1.2 kg (30%)
• Solar Panels – 0.8 kg (20%)
The shape and layout of each one of the 4 satellite components is defined as follows:
• Bus: the satellite “shell” was modeled as a hollow rectangle 10 x 10 x 30 cm, in compliance
with the 3U Cubesat convention. 0.5 cm thickness
• Payload: the telescope and Bayer Masked CCD camera were jointly modelled as a 25 cm long,
9 cm diameter solid cylinder located on the front of the satellite (positive 𝑥̂ direction)
• Systems: all the other systems were represented as a 9 x 9 x 5 cm cuboid located at the back of
the satellite
• Solar panels: following similar dimensions as the Flock-1 Dove satellites, the solar panels were
modelled as a 70 x 10 cm sheet with 0.5 cm thickness located on top of the satellite (negative
𝑧̂ direction)
The component distribution and dimensions are presented in the figure below:

5. Jan Peeters Salazar
06035621
Inertia Properties and Principal Axis
Now that the satellite is fully defined, a Solidworks model was created true to the geometries and
masses discussed above. The exploded view of the Solidworks model is presented below:

6. Jan Peeters Salazar
06035621
Defining the 𝑥̂ 𝐵 (body) unit vector as the axis of symmetry of the cylinder and the 𝑧̂ 𝐵 unit vector as
the nadir direction, Solidworks yields the following mass moment of inertia matrix (taken at the center
of mass):
𝐼 𝐵 = [
0.04 0 0.0 …
0 0.04 0
0.0 … 0 0.06
]
The HAWK satellite is symmetric about the 𝑥 𝐵 and 𝑧 𝐵 axes, resulting in a zero cross-moments of
inertia 𝐼 𝑥𝑦 and 𝐼 𝑦𝑧. However, since the systems cuboid is placed asymmetrically, the cross-moment of
inertia 𝐼 𝑥𝑧 is very small but non-zero, nonetheless. This means that there is another set of axes, the
principal axes, that only contains diagonal terms. Performing the eigenvalue-eigenvector operation
yields the following transformation matrix between the body-axes and the principal-axes:
𝑇𝑃←𝐵 = [
1 0 0.04
0 1 0
−0.04 0 1
]
Transforming the moment of inertia matrix from the body frame to the principal frame yields the
updated principal moment inertia matrix:
𝐼 𝑃 = [
0.04 0 0
0 0.04 0
0 0 0.06
]
Superimposing both coordinate frames on a simple render of the HAWK satellite yields the following
image:

7. Jan Peeters Salazar
06035621
Barycenter Coordinates and Orbit Propagation Scheme
Given the simplification of the HAWK satellite, all surfaces are at right angles of each other. This
means that all surface-normal unit vectors are either in the ±𝑥̂, 𝑦̂, 𝑧̂ direction. Furthermore, the Adam-
Bashforth integration scheme developed in AA279A will be used to propagate the orbit.