Towards a distributed real-time system for future satellite applications

[摘要] ENGLISH ABSTRACT:The Linux operating system and shared Ethernet are alternative technologies with the potential toreduce both the development time and costs of satellites as well as the supporting infrastructure.Modular satellites, ground stations and rapid proto typing testbeds also have a commonrequirement for distributed real-time computation. The identified technologies were investigatedto determine whether this requirement could also be met.Various real-time extensions and modifications are currently available for the Linux operatingsystem. A suitable open source real-time extension called Real-Time Application Interface(RTAI) was selected for the implementation of an experimental distributed real-time system.Experimental results showed that the RTAI operating system could deliver deterministic realtimeperformance, but only in the absence of non-real-time load.Shared Ethernet is currently the most popular and widely used commercial networkingtechnology. However, Ethernet wasn't developed to provide real-time performance. Severalmethods have been proposed in literature to modify Ethernet for real-time communications. Atoken passing protocol was found to be an effective and least intrusive solution. The Real-TimeToken (RTToken) protocol was designed to guarantee predictable network access tocommunicating real-time tasks. The protocol passes a token between nodes in a predeterminedorder and nodes are assigned fixed token holding times. Experimental results proved that theprotocol offered predictable network access with bounded jitter.An experimental distributed real-time system was implemented, which included the extension ofthe RTAI operating system with the RTToken protocol, as a loadable kernel module. Real-timetasks communicated using connectionless Internet protocols. The Real-Time networking (RTnet)subsystem of RTAI supported these protocols. Under collision-free conditions consistenttransmission delays with bounded jitter was measured. The integrated RTToken protocolprovided guaranteed and bounded network access to communicating real-time tasks, with limitoverheads. Tests exhibited errors in some of the RTAI functionality. Overall the investigatedtechnologies showed promise in being able to meet the distributed real-time requirements ofvarious applications, including those found in the satellite environment.

[发布机构] Stellenbosch University