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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
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Table 1. Different battery technology capacities
We can clearly see that using the Li-Ion batteries a program
can save quite a lot on the mass and volume properties of a
satellite. In the early days of the Li-Ion development, these
batteries were considered a risk and had not been tested in
flight conditions. Also since their recharge and power
consumption requirements were quite strict, designer tended to
shy away from the use of Li-Ion cells for development. But
with the extended use of the Li-Ion type batteries and the
development of monitor and charge electronics, assembling a
battery usage circuit has become significantly easier. Today a
cell phone battery, which uses prismatic cell structures, can
power a store-and-dump type of satellite almost through its
whole life cycle.
There have also been some breakthroughs in solar cell
efficiency. Through the introduction of multiple junction cells
like triple junction solar cells, efficiencies of up to 27.5% have
been achieved.
Yet besides these innovations and breakthroughs in solar cell
and battery technology, the power subsystem is a very tricky
area, still filled with areas of “black magic.” Designs with
improvements in these areas come rather from the industry
than educational or non-profit institutions and therefore the
price is set by these companies. The price of this subsystem
can not be brought down just through an initiative by
universities by building such hardware in-house and free flow
of information. The end users and research institutions have to
be innovative when getting the required solar cells and
batteries. In many cases the aerospace-rated batteries have a
terrestrial application counterpart; by thoroughly testing these
in vacuum and thermal chambers some of these terrestrial
batteries can be used on space missions. For the acquisition of
solar cells, institutions can contact solar cell manufacturers for
"reject" high-grade cells that have not met very strict
constraints but still are efficient enough for small programs.
Of course, the best design strategy in building a small power
subsystem is to bring down the power consumption of other
subsystems as well as to keep the number of voltage levels
required by the whole satellite to a minimum. This can be
achieved by having a technology comparison meeting at the
start of the project and deciding on components for each
subsystem that require as little power as possible but also
share a common supply voltage. This way the amount of bulky
DC-DC converters on board can be kept to a minimum, which
also keeps the thermal household more stable.
3.5 Solutions in Mission Operations
Mission operations, although neglected in many small satellite
development projects, is one of the bigger contributors to the
overall mission cost of a project. Mostly, ground stations do
not come cheap and the amount of coverage one can get out of
only one ground station is so limited that the satellite has to
incorporate means to store and transfer larger amounts of data,
which transfers to the project as added communication and
345
command and data handling costs. However, mission
operations is also a field where cooperation can be easily
achieved and major cost savings can be accomplished.
However, the first initiative before achieving cooperation
between institutions and countries is to establish
communications hardware standards. The reason for
establishing such standards is more to establish frequency and
networking standards rather than to provide a shorter list of
communications equipment. The goal is to make every ground-
station capable of establishing a link with any satellite and
through the use of the internet, route the data to the end user
without having to reconfigure the ground station’s hardware or
software.
To accomplish the use of multiple ground stations by multiple
users, a network of ground stations can be connected to a
resource manager website, where the operators of the satellites
have individual accounts. Through this website the operators
can enter in their orbital parameters and desired contact times.
Then the resource manager can allocate ground station time
based on these parameters, as well as other parameters such as
the occurrence of a critical event in orbit, satellite status
(Nominal, Safe mode, Deployment) and also fairness. In this
manner, a true world-wide ground station network can be
established and become greater than the sum of its parts.
Through the use of TCP/IP for communication, not only
between the ground stations but also between the spacecraft
and the ground station, the hardware and maintenance costs of
ground stations can be minimized since there are widely
available TCP/IP hardware and software solutions.
3.6 Solutions in Software
Software is one of the first areas where projects today can and
are saving money since the development of software requires
less expensive hardware than the other subsystems and is only
intensive in labor which many research institutes and schools
can provide easily in form of research assistants and students.
This does not mean, however, that software comes completely
free and one can not accomplish cost savings in that field.
Today many satellites run on either commercial real-time
operating systems or custom written firmware. The wide-
spread use of embedded linux applications has brought a new
possibility for the satellite designer. This new operating
possibility is not only completely free, but also comes with a
big community of software developers and libraries that can be
employed when putting together the software for the
spacecraft. These systems similarity with desktop linux
systems also means that software developers will be in known
territory, programming with a system familiar to them which in
turn reduces development times.
The use of TCP/IP in the communication scheme between
ground station and spacecraft, as suggested earlier, also has the
added benefit of minimizing the required software
development. Once the driver has been written to put TCP/IP
on the physical communication link between the satellite and
the ground-station, the rest of the software is a trivial piece
that has already been solved and the communication between
the two nodes is not much different than bringing up a website
on your home computer. In case of our suggested
