Aerospace and Electronic Systems Magazine October 2017 - 6
Current Use of Linux in Spacecraft Flight Software
in desktop applications. BusyBox is often used to include many
UNIX-like tools in embedded applications where only limited resources are available [6]-[8].
μCLinux is a version of Linux developed for processors that
lack a memory management unit, but provides the same basic
functionalities as Linux [6].
Linux has been designed as a general purpose OS and thus does
not directly support hard real-time applications [6]. However, solutions have been designed to address this limitation and we discuss
these in the next section about advantages and drawbacks.
LINUX IN ORBIT
This section reviews Linux use in various spacecraft. The analysis
is limited to a selection of missions that have academically published their Linux use. The selection includes educational, governmental, and commercial missions. More than a hundred spacecraft,
many by Planet and SpaceX, have flown Linux. The discussed
missions are summarized in Table 1.
Possibly the first study of using Linux in spacecraft dates to the
NASA FlightLinux project that ran from 1999 to 2002. The project aimed to provide an on-orbit demonstration of the Linux OS;
the demonstration target was the UoSat-12 satellite operated by
Surrey Satellite Technology Ltd. The rationale for this demonstration was to try extending the COTS philosophy used in hardware
to the software domain by using an off-the-shelf OS with many
available software packages, that is, Linux. The kernel and a decompression program were fitted within a space of approximately
400 kB, and the system only had 4 MB of RAM available. Linux
provided networking and filesystem facilities and supported highlevel programming languages, such as Java. The kernel setup routine was modified to support the UoSat-12 hardware. The planned
UoSat-12 test was very primitive, such as printing "Hello World!"
to a serial port [30].
Many CubeSat missions, including the Aalto-1 mission operated by Aalto University [10], have used Linux in their flight
computers. Aalto-1 uses an AT91RM9200-based computer with a
customized 3.4 kernel prepared using Buildroot. Aalto-1, shown
in Figure 3, was launched on June 23, 2017. As the mission began
recently and is still ongoing, it is not analysed in this article.
A microsatellite example is TacSat-1, which used Linux
in its Copperfield-2 payload in PowerPC MPC823, PowerPC
PowerQuicc II 8260, and StrongArm SA1110 computers. Linux
was considered possible since there were no hard real-time
requirements for the payload software, and thus no real-time
adaptations of Linux were used. The payload computers, communicating via TCP/IP, processed sensor data and provided
payload data storage and handling infrastructure. Linux also
provided scripting engines as an additional benefit; Perl and Python were evaluated, but shell scripting was decided to be used.
Many payload functionalities were implemented as scripts, and
the data flow between software was handled via standard input and output. This allowed quick interfacing with existing,
off-the-shelf Linux software. Most of the custom software developed for TacSat-1 handled conversion between TCP/IP and
OX.25 protocols. Development was possible on x86 PCs, while
6
the target itself had PowerPC architecture. However, TacSat-1
was never launched [9].
The following subsections discuss some of the missions in
Table 1.
QUAKESAT
One of the earliest CubeSats to fly Linux was QuakeSat, which
was a 3U CubeSat built at Stanford University for QuakeFinder
LLC. It aimed to study extremely low-frequency magnetic signals
in order to possibly predict earthquake activity. QuakeSat also
aimed to demonstrate the usefulness of COTS electronics and the
CubeSat standard for the development of low-cost space missions.
QuakeSat's command and data handling system operated with
timed commands to perform payload data gathering when historically active earthquake areas were within the range of the instrument. QuakeSat had no digital on-board mission data processing.
All electronics were embedded on a single circuit board. Diamond
Systems Prometheus PC-104 processor board was used as the
command and data handling system hardware, and a version of
Linux provided by Diamond Systems was used as the OS. Linux
was chosen since many of the required device drivers were already
available. The processor was clocked down to 66 MHz, and the
system had 32 MB of RAM and a 192 MB Flash disk for storage.
In addition to Linux, the flight software consisted of some 10,000
lines of code, some of which included already existing AX.25 and
modem drivers. Flight software features included communication
with an on-board UHF modem, AX.25 libraries, other radio utilities, payload data collection and compression, data downlink, and
command execution. Bzip2 compression was used for the payload
data.
The satellite had only passive magnetic attitude control. The
QuakeSat command and data handling system performed timebased operations, with operation times selected on the predicted
position of the satellite. The command and data handling system
was able to store commands and data for one day. Schedule files
were uploaded to the satellite, and payload data files were compressed and downlinked from the satellite. The satellite operator
was also able to query real-time telemetry from the satellite. The
operator also needed to synchronize the satellite clock every three
days. The satellite performed well in orbit, and collected useful
scientific data [11], [12].
UWE-1 AND UWE-2
UWE-1 and UWE-2 were 1 kg CubeSats built by the University of
Würzburg, and they were launched in 2005 and 2009, respectively.
The UWE satellites aimed to test various small satellite technologies, including the use of internet protocols in space. Both used a
similar on-board computer, which included a 16-bit Hitachi H8S
2764 microprocessor. The computer had 8 MB of RAM and 4 MB
of nonvolatile flash memory. The 16-bit processor did not have a
memory management unit, requiring the use of μClinux. The computer consumed only 300 mW during nominal operations; the low
power consumption was one of the reasons to select the microcontroller, as it allowed more power for other experiments. The
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