Aerospace and Electronic Systems Magazine June 2017 - 35

behavior of atomic and molecular oxygen as a key parameter of
the lower thermosphere. Atomic oxygen is the dominant species
in these regions and therefore its measurement is crucial in the
validation of atmosphere models. Moreover, erosion of spacecraft
surfaces due to interaction with atomic oxygen is a serious concern
and merits in-situ study in its own right [3].

Technology Demonstration Payload
LituanicaSAT-2 will carry a technology demonstration payload
called "Chemical Propulsion System for Small Satellites" (EPSS).
The primary objective of this experiment is to demonstrate the orbital maneuvering and drag compensation capabilities of a CubeSat using an integral green monopropellant microthruster. The proposed system from start-up company NanoAvionics makes use of
a "green", non-toxic fuel, replacing the hydrazine-based propellant
used by large-scale satellite technologies and corresponds to the
European Space Agency's (ESA) and the National Aeronautics and
Space Administration (NASA) Clean Space Initiative.

SYSTEM ARCHITECTURE
The choice of the spacecraft bus architecture is a very important design step and drives the system complexity and development cost.
The main design parameters that are addressed during system architecture development are fault tolerance, choice of interfaces, and
distribution of functions between satellite subsystems. There are two
main system design architecture types usually employed by satellite
designers: distributed or centralized (monolithic). In a distributed
architecture, the subsystems are highly autonomous with strictly
defined interfaces and tasks whereas a monolithic architecture is
based on a centralized on-board computer that performs most of the
tasks and has the highest level of autonomy inside the spacecraft. A
decentralized philosophy is clearly attractive due to enhanced fault
tolerance and the possibility of parallel development and testing of
the subsystems [4]. The downside is that each subsystem must have
enough processing power and data storage to carry out its tasks [4]
and this creates additional burden on software development. Furthermore completely decentralized architecture might not be possible due
to conflicting interfaces between commercial off the shelf (COTS)
subsystems or specific requirements for a particular subsystem.
The LituanicaSAT-2 system architecture can be seen in Figure 2.
The spacecraft consists of 6 subsystems: OBC, ADCS, EPS, COM,
JUNE 2017

science unit payload, and propulsion (experimental unit). The system
architecture is a mixture of distributed and centralized philosophies.
A fully distributed philosophy could not be implemented due to specific hardware requirements of the payload: both the science and the
propulsion payloads had to be controlled by time tagged scripts sent
directly by on board computer via UART (universal asynchronous
receiver/transmitter) data interface. Furthermore the COTS EPS
subsystem only supported I2C data interface and could not be connected to the main system bus which was implemented using more
robust CAN (controller area network) interface. To overcome this
problem it was decided that OBC will act as a network router by
making it's peripherals - EPS and payload - accessible directly to any
other subsystem via a higher level CubeSat Space Protocol (CSP).
COM, ADCS, and OBC subsystems act as separate nodes in the CSP
network and can operate independently of each other.

DESIGN OF THE FUNCTIONAL UNIT
The functional unit is the core-operating unit of the satellite that
is usually called simply a "satellite bus". It is composed of OBC,
EPS, ADCS, and COM subsystems. Inside the satellite structure,
the avionics of the functional unit form a stack of 3 printed circuit
board (PCB) assemblies:
1. motherboard assembly;
2. power conditioning and distribution unit (PCDU) "GomSpace
P31u" from GomSpace;
3. OBC, ADCS, and COM controller board assembly "SatBus
3C1" developed by Vilnius University and start-up company
Nanoavionics.
The motherboard assembly holds x and y axes magnetorquer rods
and a deployment mechanism for deploying satellite solar arrays
and antennas. It also serves as an interface hub to the science unit.
The PCDU board is the only COTS hardware, which as the name
implies is responsible for storing and delivering power to other satellite subsystems. The 3rd board is the most complex and contains
the on board computer, ADCS computer with inertial sensors, and
the ultra high frequency (UHF) radio.
Apart from avionics bus, the functional unit also contains a Z
axis magnetorquer coil, RF splitter board assembly, quad monopole antenna array, external ADCS sun sensors and solar arrays for

IEEE A&E SYSTEMS MAGAZINE

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