Innovative Solutions in Nano and Pico-satellite Communications

This research activity develops in the rapidly and constantly growing field of
avionics for small satellites. The relatively widespread availability of low-cost piggyback
launch opportunities recently provided to an heterogeneous set of entities
the access to orbit. Universities, industries, local governments and even amateurs,
all became interested in space and the rather unique opportunities offered by its
environment.
This led to the development of a large number of nano and pico-satellite missions,
respectively with spacecrafts of mass lower than 10 kg and 1 kg orbiting in Low Earth
Orbits (LEO) under 700 km of altitude. Such tiny satellites are usually built with
commercially available electronic components not qualified for the space environment,
allowing for savings along the whole development cycle in recurring and non recurring
costs. Design re-use extends the cost reduction to the system level, with an aggressive
exploitation of existing technologies and, possibly, of space-flown architectures.
Communication subsystems, a small but critical set of elements common to every
mission, are not exempt from such a philosophy. On-board networks, on-board
transceivers, antennas, ground stations, and the protocols in between them are all
critical elements for a spacecraft mission and, at the same time, some of the most
specialized and complex ones. Design re-use is then sought at every level, to the point
of favoring “old and trusted” technologies in spite of lower performances and reduced
flexibility.
While this was acceptable for pioneer pico-satellite missions, with the growth
of the scientific goals the traditional trade-offs are not appropriate anymore. Even
further, the stream of innovations coming from the ground mobile market is not being
adequately exploited and today outdated communication architectures set — rather
than match — mission capabilities and achievable goals.
This research aims at finding new solutions to common problems becoming prevalent
in this field. Better trade-offs are needed in the ground and flight communication
segments and in the elements linking them. Better performances are achievable with
an increase in system complexity, always taking into account energy, mass and cost
constraints.
In chapter 1 we will start from the communication systems of the ground segment,
v
the moving antennas that track and provide communication with LEO satellites as
they fly by over them. A typical pass of a LEO satellite over a ground station, from
rise to set, lasts about 10-15 minutes on average. For a single ground station and an
high-inclination orbit, the number of passes/day can be as low as 2. This limits the
contact time with the spacecraft with understandable restrictions on the spacecraft
control and data retrieval capabilities of the mission. At the same time, the ground
station itself is heavily underused and a networking effort with other universities and
individuals, the GENSO network, will be able to provide relevant improvements.
During the design and developement of the GENSO network other limitations
of the current, typical, pico-satellite architecture became apparent. The concern for
security of transmitted data and, thus, of the mission itself, demonstrated that the
lack of confidence in the network may hinder the adoption rate of leading missions.
While all the communications within the network are kept secure by industry-standard
public-key encryption protocols, the signals at a certain remote station will
leave the network to be transmitted to space. The current paradigm of pico-satellite
missions relies on security through obscurity and, at the very least, this can be easily
undermined by statistical analysis of the outgoing data. Chapter 2 investigates the
security problems and proposes a strategy based on peer reviewed standards to solve
them. The choices are shown to be compatible with the requirements of nano and
pico-satellite missions.
Nonetheless, for a proper and permanent solution based on asymmetric, public-key
protocols, the need for on-board programmable logic becomes apparent. The current
approach, relying on simpler and lower power microcontrollers, is quickly becoming
dictated more by convenience than by energy savings. This point, along with many
others, will be addressed later in chapters 4 and 5.
Chapter 3 focuses instead on attitude determination systems and a gap in the
capabilities of the available sensors. While this topic may sound unrelated to communication
systems, it will be shown how an all-analog phase-tracking receiver is
potentially able to provide an accurate attitude reading in a less structurally-invasive
manner compared to existing solution.
Even though this solution may better fit micro than nano-satellites, during its
implementation it quickly became apparent that the reliability and power savings
offered by analog components come with a relevant board area penalty. At a given
complexity level, the fast evolution of digital architectures is becoming a much more
flexible approach.
As will be envisioned in chapter 4 and shown later in chapter 5, the commercially
available digital solutions are becoming compatible with the tiny resources available
on-board pico-satellites. But, more importantly, they are being demanded by the
growing needs of missions.
More work is needed to make an all-digital pico-satellite transceiver a reality, but
this will be discussed with some closing remarks in chapter 6.