Aerospace and Electronic Systems Magazine October 2017 - 8

Current Use of Linux in Spacecraft Flight Software

IPEX
IPEX was a 1 kg CubeSat developed by Cal Poly San Luis Obispo
and Jet Propulsion Laboratory (JPL) that aimed to validate technologies for on-board instrument processing and autonomous operations. The main computer had a 400 MHz Atmel ARM9 processor with 128 MB RAM, a few megabytes of radiation resistant
PRAM, 512 MB NAND Flash, and a 16 GB microSD card. Linux
2.6.30 was used. The satellite additionally carried a Gumstix Earth
Storm single-board computer with an 800 MHz ARM processor,
512 MB RAM, 512 MB NAND Flash, and also running Linux. The
satellite had several cameras as payload, and had high processing
power requirements. The two computers were linked with a serial
link. An on-board software, CASPER, managed the spacecraft resources autonomously.
The flight software of IPEX was based on extending and adapting Linux facilities. The System V init process was used to start
and restart main components of the flight software. Operations
were based on uploading observation and ground contact schedules. Uplink and downlink bandwidth was limited, which made
operations harder. The developers noted that more software could
have been preloaded on the computer, since nonvolatile storage
was abundant, while it was very hard to upload large pieces of
software during the mission. IPEX computer used reboots to clear
problems possibly caused by radiation, which grew more frequent
as the mission progressed. The satellite stopped communicating in
early 2015, but its mission of demonstrating on-board autonomy
was successfully completed [17], [18].

STRAND-1 AND PHONESAT SATELLITES
The Android OS is based on Linux, and at least two projects have
used Android smartphones to build satellite on-board computers.
Smartphones contain many technologies useful for small satellites,
such as low-power yet capable processors, gyroscopes, accelerometers, cameras, and communication interfaces.
STRaND-1 was a CubeSat built by Surrey Space Centre and
Surrey Satellite Technology Ltd that aimed to explore new ways
of including smartphone technologies to CubeSat platforms. While
the satellite main computer ran FreeRTOS, STRaND-1 used in its
payload μClinux in a Digi-Wi9C single board computer and Android on a Google Nexus One smartphone. The satellite main computer communicated with the Digi-Wi9C via I2C, while Digi-Wi9C
communicated with the smartphone via USB and Wi-Fi using for
example the telnet protocol. One of the aims of the mission was
to test transferring control of the satellite to the smartphone and
its Android applications. The satellite was launched in early 2013,
and experienced initial communication problems but was recovered and continued its mission in summer 2013 [19], [20].
The NASA PhoneSat technology demonstration project aimed
to demonstrate building very low-cost nanosatellites using smartphones and other consumer technology. The project has launched
several smartphone-based nanosatellites that used Google Nexus
One and Nexus-S smartphones as on-board computers. Several
PhoneSats have been launched since 2013, and the smartphone
cameras have successfully taken pictures of the Earth [23].
8

Figure 4.

Two Dove satellites running Linux being deployed from the International Space Station. Photo: NASA.

LIGHTSAIL-1
LightSail-1 was a CubeSat developed by the Planetary Society and
launched in 2015. Its purpose was to test the deployment of a solar
sail in space. The LightSail-1 avionics consisted of two processor
boards with different tasks. The main board handled the spacecraft
command, control, data collection, and telemetry, and the payload
interface board focused on attitude control and managing the solar
sail payload deployment. The main board was a Tyvak Intrepid v6
single-board computer and used Linux, and the payload interface
board used a 16-bit PIC33 processor. Flight software was written
in C for both the Linux-running Intrepid and the PIC processor.
While the Linux system could run several processes and support
many libraries and interfaces, the PIC processor ran a simple 5 Hz
control loop for attitude control and solar sail deployment.
During the mission, a problem in the Linux system was discovered that caused a temporary loss of control. A comma-separated
values file grew out of bounds due to a bug, causing the system to
halt; fortunately, a radiation-induced single-event effect caused the
main computer to reboot, temporarily fixing the problem, and allowing a bug fix to be applied in orbit. After the bug fix, the solar
sail deployment was successfully performed just in time before the
low-flying satellite deorbited [21], [22].

DOVE SATELLITES
Planet, formerly Planet Labs, aims to provide high-resolution imagery of the whole Earth at high refresh rates by using constellations of tens to hundreds of CubeSats. The Dove satellite family
used by Planet consists of three-unit CubeSats that are in effect
telescopes with cameras, support electronics, and attitude control.
Two Dove satellites are shown in Figure 4. Planet has extensively
used components from the smartphone industry to compress the
electronics to a very small size, thus making as much room as possible for the camera optics. Like smartphones and other consumer
electronics, Planet does not use separately boxed electronics, but
all the spacecraft electronics are integrated to a single package.
Each satellite is very low cost, and expendable; there is thus a satellite-level redundancy built into the constellation.

IEEE A&E SYSTEMS MAGAZINE

OCTOBER 2017

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