Aerospace and Electronic Systems Magazine October 2017 - 5

The term Linux strictly refers to the OS kernel originally developed by Linus Torvalds since 1991. Linux is used in embedded
systems because of quality and availability of code, wide hardware
support, implementations of communication protocols and application programming interfaces, available development tools, favorable licensing conditions, vendor independence, and cost. The
Linux kernel can be run in various Linux systems from very small
embedded computers to supercomputer clusters. Each system has a
different purpose and different software packages and applications.
Linux has been ported to many architectures: as of this writing, the Linux kernel readme file lists at least 22 supported architectures. According to Yaghmour [6], architectures mostly used

in embedded Linux applications include ARM, AVR32, Intel x86,
M32R, MIPS, Motorola 68000, PowerPC, and SuperH. Linux is
also available for the LEON processors commonly used in the
European space industry. Thus Linux is available for most of the
architectures commonly used in space applications mentioned in
the previous section.
A generic on-board software architecture using Linux is depicted in Figure 2. Linux provides OS facilities such as virtual
memory, processes, communication sockets, and files. The kernel
is responsible for driving the devices, managing input-output access, scheduling processes, enforcing memory sharing, and handling signals. The kernel is typically started by a bootloader, and
is never stopped while the embedded device is running. The Linux
kernel has support for many data buses and communication protocols useful for spacecraft developers, including at least CAN bus,
Modbus, SPI, I2C, and serial port [6]. CAN is one of the on-board
data buses already used in the space industry, and is directly supported by Linux [1].
Many commercial embedded Linux distributions exist for various platforms, but embedded developers may also build their custom distributions using the kernel source code. Buildroot is one
commonly used tool for this purpose. uClibc, a lightweight C library, is often used to replace the standard GNU C library used

Figure 1.

Figure 2.

result has tended to be very tightly coupled, and not well suited for
later use in other projects [5].
When OS have been used in spacecraft, common selections
have included VxWorks and RTEMS. Both VxWorks and RTEMS
are RTOSs designed for embedded systems and commonly used in
safety-critical hard real-time applications [1]. University small satellite missions have also used other general purpose RTOSs, such
as FreeRTOS [4].

EMBEDDED LINUX

Generic on-board software architecture.

OCTOBER 2017

Generic on-board software architecture with Linux kernel. Adapted from [6].

IEEE A&E SYSTEMS MAGAZINE

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