Linux on a Small Satellite

With less than a year to design and build a satellite, this team used existing sensor hardware, industry-standard parts, shell scripts and our favorite OS to make the project come together.
Networking Architecture of COTS Processors

The core Copperfield-2 payload processor provides two key functions for the mission. First, it is a sensor system that receives sensed data, processes the data and interacts with onboard communications equipment to transmit the results to other sensors and ground stations. Secondly, it serves as a general-purpose computer system that provides the infrastructure for storage and data handling. In fact, multiple general-purpose processors are part of the Copperfield-2 payload, each communicating by way of an Ethernet network. A COTS Ethernet switch serves as the center of the star Ethernet architecture.

Table 1. TacSat-1 Copperfield-2 Ethernet-Connected Embedded Systems

High-Speed Interface (HSI)Bright Star Engineering (custom adapter board)Linux 2.4 custom distributionPowerPC MPC823
IDM UHF ModemInnovative ConceptsProprietaryPowerPC 860
Copperfield-2 MR.DIG CardAeronix/NRLLinux 2.4 custom distribution (DENX ELDK-based) PowerPC PowerQuicc II 8260
RF Front End ControllerBright Star Engineering (custom adapter board)Linux 2.4 custom distributionStrongARM SA1110
Gateway to the Bus Legacy Equipment

To capitalize on the Ethernet, TCP/IP, standards-based architecture of the UAV payload while remaining compatible with the satellite bus' legacy OX.25 interfaces—which provide a means for downlinking science data and state-of-health telemetry—a different embedded computer module was designed specifically to serve as the bridge. This module is called the high-speed interface (HSI) and provides a 2MB synchronous serial bus connected to the spacecraft communication controller. The HSI hardware is implemented as a combination of FPGA hardware and a BSE ipEngine general-purpose PowerPC 823 embedded processor.

In the HSI, the FPGA provides the hardware necessary to meet timing requirements for the data link, decoupling the processor from the synchronous data link. The PowerPC runs a Linux 2.4-based kernel, and the HSI FPGA interface is implemented as a standard Linux device driver. No special real-time extensions are used, and a Linux-based application provides the interface between the TCP/IP networking stack, using standard protocols and the device driver implementation. The HSI system allows multiple processes and Ethernet-connected computers to access the data stream sent to the spacecraft. The PowerPC communications controller on the Copperfield-2 processor easily could have handled the HSI tasks on TacSat-1. However, due to the extremely limited availability of hardware and the desire to increase parallel development opportunities, this interface was developed independently.

Rapid Payload Software Development with Existing Tools

The most “custom” part in any satellite program often is the payload control software. Because many of the Copperfield-2 payload components with processors run Linux, interesting software options are available. Much of the payload software was implemented as bash (Bourne again shell) scripts. During the rapid development of the payload software, the philosophy was to attempt to divide the software development into two parts, custom and reused software modules. This philosophy called for minimizing custom code to limited functions and programs with specific purposes. Occasionally, we did find that existing utilities did not quite fit the requirements, and these were modified or replacements were written.

These specific custom programs and drivers allowed for control of payload elements through small command-line utilities that could be tested completely and easily in their limited functionality. These programs were developed with the UNIX command-line functionality in mind, along with data input through standard in (STDIN) and data output through standard out (STDOUT). Developing software utilities with interfaces such as these in mind has been the standard for many legacy operating system concepts from the earliest UNIX developments. We intended to continue that strategy and build upon it, as it provides an amazingly flexible way of constructing thorough capabilities with simple although powerful utilities.