TECHNICAL REPORT
SelfieSat
Authors:
Antonio Caiazzo
Davide Candela
Stefania Ferrentino
Giovanni Nardone
Salvatore Sarno
April 2015
Abstract
Abstract
The project is aimed at the design of a nanosatellite that, detaching from a
mothership, is able to take pictures of the latter, both for artistic and
diagnostic purposes. Main objectives were to minimize the fuel or power
consumption, to make the nanosatellite autonomous and reusable, and to
offer a simple re-charging configuration. Basing on these key-points a
Cubesat-like system, named SELFIESAT, has been designed and its internal
components have been selected choosing among those available into the
market-place. Particular attention has been paid to the SELFIESAT storage
mechanism placed on the mothership, and to an automous camera pointing
device.
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Table of Contents
Table of Contents
SELFIESAT ............................................................................................................................ 3
INTRODUCTION ..................................................................................................................... 3
CAMERA & CAMERA POINTING ............................................................................................. 4
ATTITUDE CONTROL ............................................................................................................. 5
PROPULSION ......................................................................................................................... 6
ELECTRICAL POWER ............................................................................................................. 6
COMMUNICATION TOOLS ...................................................................................................... 7
ROTATING PLATFORM........................................................................................................... 7
ORBITAL TRANSFER .............................................................................................................. 8
MATERIALS AND COMPONENTS ........................................................................................... 10
FEASIBILITY ANALYSIS .................................................................................................. 11
List of Figures
Fig. 1 – SelfieSat anatomy......................................................................................................... 3
Fig. 2 – NanoCam C1U ............................................................................................................ 4
Fig. 3 – Star Sensor ST-200 ...................................................................................................... 5
Fig. 4 – MAI-101 Miniature 3-axes Reaction Wheels................................................................. 5
Fig. 5 – 100 N Electrospray Thruster System .......................................................................... 6
Fig. 6 – Nanopower BP4 ........................................................................................................... 6
Fig. 7 – UHF Transceiver ......................................................................................................... 7
Fig. 8 – Rotating Platform ........................................................................................................ 7
Fig. 9 – Ejection toward and external orbit ............................................................................... 9
Fig. 10 – Rendez-vous manoeuvre ............................................................................................. 9
Fig. 11 – Truss structure ......................................................................................................... 10
List of Tables
Table 1 – SelfieSat Components .............................................................................................. 10
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SelfieSat
SelfieSat
Introduction
Belonging to the category of the nanosatellites, SelfieSat has been conceived as a cubic
satellite whose dimensions are 250x250x250 mm. However this has been thought about
only as a first design, since the chosen dimensions are the maximum acceptable for a
nanosatellite. Therefore the final configuration may be smaller than the first attempt
one, depending on the systems that the satellite has to host.
Fig. 1 – SelfieSat anatomy
At first, the design phase afforded by the team required the definition of some key
points intended to make the best choices possible. Consequently the following basic
ideas have been taken as a guide for the whole work:

minimum fuel and power consumption, in order to maximize the lifetime;

flexibility, thought as the ability to reach any desired orbit, either higher or
lower with respect to the mothership one;

reusability, since the nanosatellite has to be safely re-joint with the mothership
when its mission is over;

autonomous mothership pointing, so that the camera is always aligned with its
target;
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SelfieSat

simple recharging configuration, in order to make the docking station as simple
as possible and to guarantee to SELFIESAT a continuous and always sufficient
power supply.
Camera & Camera Pointing
The milestones described in the previous subsection have been fixed in the team
members’ minds when discussing about the inner subsystems needed for a full and safe
accomplishment of the mission.
In this scenario, given that the main goal of the satellite is to take pictures of its
mothership, it appears clear that a Nanocam has to be placed inside the cube. Therefore
one of its sides displays a lens, necessary to let the camera looking outside. In particular,
the selected camera, a NanoCam
C1U, is characterized by a field of
view of nearly 10°. This implies that,
supposing that the mothership has an
end to end length of 20 m and
considering the worst case (solar
panels orthogonal to the camera
axis),
the
minimum
distance
necessary to focus the whole satellite
is 75 m.
Fig. 2 – NanoCam C1U
In addition, since the nanosatellite has to continuously point toward the mothership, a
system, inspired to the Automatic Direction Finder and to the Non Direction Beacon
avionic systems, has been employed for camera pointing. In particular SelfieSat hosts
two loop antennas, each orthogonal to the other, which receives the signal transmitted
by an omni-directional antenna, mounted on the mothership satellite. The transmitter
broadcasts some pulse signals whenever the onboard computer gives a start to the
mission. At this stage the loop antennas, rotating around their own axes, sense both the
azimuth and elevation angles with respect to the mothership and hence the camera can
be pointed towards its target.
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SelfieSat
Attitude Control
During SelfieSat mission there are two kind of sensors aimed at measuring the attitude
of the nanosatellite with respect to ether the stars and the mothership. The first one is a
Star Sensor which is placed aside the camera, whereas
the second one is composed by the two loop antennas
discussed in the Camera & Camera Pointing
subsection. In particular, the Star Sensor is conceived
to be operative when SelfieSat, despite being on its
route towards its target orbit, has its camera turned off.
On the other hand, whenever the mothership transmits
a signal announcing the eminent start of the mission,
the loop antennas become operative.
Fig. 3 – Star Sensor ST-200
Other possible choices are:

the mission starting time is computed by the CPU on-board SelfieSat on the
basis of the Star Tracker: in this case, SelfieSat has to communicate this status
to the mothership;

both the mothership and SelfieSat evaluate the mission starting time: in this
case no radio communication is required.
Once the correct attitude has been detected, the desired positioning is achieved by
means of three reaction wheels.
Fig. 4 – MAI-101 Miniature 3-axes Reaction Wheels
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SelfieSat
Propulsion
The presence of a 3-axes Reaction Wheels allows SelfieSat to use a single thruster. In
particular the nanosatellite is equipped with an Electrospray Propulsion System. This
solution:

maximizes the useful volume or equivalently it minimizes the overall size if the
volume is kept constant: this is due to the absence of a tank;

maximizes the lifetime since an electrical engine does not require fuel resupply;

simplifies the SelfieSat-mothership joint since no fluid has to be transferred
between the two systems.
Fig. 5 – 100 N Electrospray Thruster System
Electrical Power
SelfieSat hosts a four lithium-ion cells Battery-Pack. This component can be recharged
both on-board and during the mission.
In particular, when SelfieSat is docked,
in order to minimize the number of
connections with the mothership, the
re-charging process is performed by a
wireless inductive system, similar to
those
used
on
Earth
for
the
smartphones. On the other hand, in
Fig. 6 – Nanopower BP4
order to make long missions possible,
the nanosatellite is enveloped in a solar
panels skin. Despite the low surface exposed to the solar radiations, this solution is
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SelfieSat
expected to be effective due to the fact that SelfieSat missions are conceived to be
performed in the orbit of Mercury, where the sun is nearly three times brighter than on
the Earth surface.
Communication Tools
SelfieSat configuration comprises an antenna and a UHF Transceiver aimed at
receiving/sending data in realtime from/to the mothership.
Fig. 7 – UHF Transceiver
Rotating Platform
A rotating platform arranged on the mothership houses SelfieSat. It is conceived to use
the cube-sat camera on-board and can be thought as a swivel chair:

it
is provided with the
inductive
recharging system, described in the
Electrical Power subsection;

moreover it accommodates both the
physical
interface
for
the docking
between SelfieSat and the mothership,
and a port for data download.

an electromagnetic device is imagined
inside; it makes the storage/docking
Fig. 8 – Rotating Platform
mechanism a release mechanism too.
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SelfieSat
As for the latter system, it gives the right magnetic thrust to eject the cube-sat and
eventually drives it to the desired transfer orbit, either external or internal. The voltage
is indeed selected according to the ΔV required by the maneuver.
A different solution, concerning the expulsion mechanism, may consider a system made
of retractable springs. This mechanical device requires both a larger volume and a
complex design. In fact a motor unit is necessary for spring compression. Moreover the
electromagnetic system allows to reduce significantly the current consumption with
respect to the springs retractable concept: ideally, with the first solution we have just a
current impulse whereas with the second one a current step is needed.
The only disadvantage regarding the electromagnetic system is an eventual
electromagnetic interference with the onboard instruments. Therefore, comparing all the
aspects concerning the two solutions and taking into account the electromagnetic system
greater reliability, the electromagnet has been preferred.
Orbital Transfer
The storage mechanism rotation allows SelfieSat to be placed on the desired orbit. A
proper modulation of the electro-magnetic device gives the ΔV required to perform the
first orbit transfer. Hohmann trajectories are considered, being them the most efficient
in terms of consumptions. The cubesat electric thruster later supplies the second ΔV
necessary to insert SELFIESAT into the desired orbit. However in the perspective of the
fuel consumption-time required to perform the maneuver, the orientation of the rotating
platform allows to perform a generic orbital transfer, so that the maximum flexibility is
achieved.
When SelfieSat mission comes to an end, the nanosatellite is recovered through a
rendezvous maneuver. Finally the docking phase is assisted by the electromagnetic
device described in Rotating Platform subsection.
Examples of SELFIESAT ejection toward an external orbit and of Rendez-vous are shown
in Fig. 9 and Fig. 10.
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SelfieSat
Fig. 9 – Ejection toward and external orbit
Fig. 10 – Rendez-vous manoeuvre
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SelfieSat
Materials and components
Aluminum alloy has been chosen for both the external and internal structure thanks to
its weight and resistance properties.
To further reduce the weight of the nanosatellite a
truss structure has been considered for the internal
structure. Moreover since the environmental
conditions are expected to be harsh, the external
structure is enveloped by sunshade made of heatresistant
ceramic
cloth,
inspired
to
the
MESSENGER system.
Fig. 11 – Truss structure
It is important to point out that SelfieSat is not a
Sci-fi project, since its design is totally based upon existing technologies and
commercial subsystems typically employed to build CubeSats. In particular all the
components lying under the shell of the nanosatellite are listed below.
Table 1 – SelfieSat Components
System
Model
Camera
NANOCAM C1U
Thruster
100 N ELECTROSPRAY THRUSTER SYSTEM
Star Tracker
ST-200
Solar Panels
CLYDE SPACE SMALL SATELLITE SOLAR PANELS
CPU
COMPUTER NANOMIND A712D
Battery
NANOPOWER BP4
Transponder
UTRX HALF DUPLEX UHF TRANSCEIVER
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Feasibility Analysis
Feasibility Analysis
Costs
It is possible to introduce a preliminary analysis of the costs of the main components of
the nanosatellite. In the following list, the prices are based on the information available
on the CubeSat shop site:
System
Cost
CAMERA - NANOCAM C1U
11.500 €
BATTERY PACK – NANO POWER BP4
1.500 €
REACTION WHEELS – MAI 101 MINIATURE 3-AXIS
13.900 €
ON BOARD CPU – NANOMIND A712 D
4750 €
TRANSPONDER - UTRX HALF DUPLEX UHF TRANSCEIVER
8600 €
SOLAR PANELS – NANOPOWER SOLAR P110U-A/B
2750 €
The total estimated cost of the listed components is equal to 43.000 €.
Obviously this is only a preliminary result; a further analysis is necessary in order to
consider the costs about the Star Sensor ST-200, the 100 µN Electrospray Thruster
System and the realization of a rotating platform required for our propose.
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