Linux on a Small Satellite

The extensive use of TCP/IP-based systems and the common Linux operating system provided unique opportunities for a distributed development environment. Early in TacSat-1, our custom PowerPC 8260 development hardware had limited availability. The design cycle for much of the payload software began on Intel x86-based computer systems, migrated to generic PowerPC embedded processors and eventually made its way to the final target. The software design team was distributed spatially and tied together through a virtual private network (VPN) architecture. Remote power control devices allowed developers who were operating off-site to cycle power on hardware components. A Web-based collaboration tool allowed the posting and dissemination of critical communications and interconnection control documents (ICDs). Some developers also used instant messaging technology to stay in contact with one another. Recent additions to the collaborative working environment include the use of E-Log to maintain an on-line database of lessons learned. We also are working to integrate Bugzilla capability into the system to replace our relatively crude Message Forum-based problem report (PR) tracking.

The TCP/IP nature of the payload data network allowed developers to test communications between payload elements at each step in the design process, from developing on a standard PC to final communications before inserting the custom hardware required to communicate with the bus. Even after complete integration of the payload into the bus, an Ethernet test port allowed network access to the satellite, which was invaluable for collaborative debugging of the system. Test ports also allow access to serial consoles for most of the payload components and, in some cases, JTAG or other hardware debugging ports.

The payload software design team consisted of experienced satellite and ground station software experts, as well as team members accustomed to the TCP/IP data transport and Web/CGI application development, plus embedded systems experts. Although quite different from the typical satellite software design team, this combination provided nearly the perfect balance of skills and innovative methods to maximize the use of existing software designed for aircraft applications. The extensive remote collaboration, interface testing and networking capability provided a smooth bus-payload integration.

The core of the payload control software, including many of the command and control scripts, were developed in a span of less than four months, from start to finish. Additional scripts were inserted into the core payload control software infrastructure to bring on-line additional sensor capabilities as those sensors became available. New capabilities and patches may be uploaded to the satellite as requirements dictate.

Few satellite programs have the sponsor-supplied latitude or the ability to take risks that the TacSat-1 initiative provides. In this context, the TacSat-1 program allows innovative leveraging of both GOTS and COTS hardware components, as well as novel approaches to creating payload software that provides maximum flexibility and standards-based operation. The modular nature of the Copperfield-2 allowed rapid hardware integration, proving the concept of a modular payload that scales from UAV applications to a spacecraft application, all using Linux and GNU software as a foundation. At the time of this writing, TacSat-1 was scheduled to launch in February 2005.

The author acknowledges the essential contributions to the TacSat-1 effort by Stuart Nicholson, consultant, and Eric Karlin, Mike Steininger and Brian Davis of SGSS, Inc., for core Payload Control Software. Brian Micek of Titan Corp., Chris Gembaroski, Don Kremer, Tim Richmeyer and the Copperfield-2 team at Aeronix, Inc., Jeff Angielski of the PTR Group for Linux porting, device drivers and sensor support software. Also, thanks to Wolfgang Denx and the Linux PowerPC community for their contributions toward making PowerPC Linux stable and robust.

Resources for this article: /article/8066.