Vol.11 no.3 2019

Commentary : Contributing to the SpaceWire international standard (H. HIHARA et al.)−149−Synthesiology - English edition Vol.11 No.3 (2018) 3.2 Concurrent scheduling of time slotThe framework for maintaining sufcient real-time property on the SpaceWire network was greatly simplified based on the Japanese proposal mentioned earlier, and a design guideline was published as SpaceWire-D.[18] In the initial SpaceWire-D standard proposal, only one communication transaction per one time slot was allowed. This was based on the European claim that if multiple transactions were allowed in one time slot, the real-time property could not be veried, and the proposal was made as a standard of simple scheduling.However, we have developed and operated a data handling system that enabled multiple transactions of communication within one time slot for many years, and had sufcient orbital experience. Although the practicality of this specification was empirical, JAXA knew the European culture that placed importance on formal and logical verification. Therefore, a government-academia-industry joint research plan was promoted, and through joint R&D by JAXA, Nagoya University, and the industries participating in the project, we created a guideline that could handle the European way of thinking in which importance was placed on the logical (formal) verification.[19] Based on this experience, we made proposals to implement multiple transactions of communication within one time slot, and this was reected in the above specication as the concurrent scheduling of time slots. Here, the European claim that multiple communication transactions within one time slot could not be verified was reviewed, and specs were created as design guidelines to realize verifiable real-time performance. This was a result of the fusion of empirical knowledge that reflected the experience of the development and operation of the Japanese satellites, and the explicit knowledge of Europe that placed importance on logical integrity and veriability.3.3 Plug-and-playThe SpaceWire RMAP standard has several similarities to the specification of the peripheral interface module (PIM)[1] that had been conventionally used in Japanese scientific satellites as the communication standard. Based on the orbital operation experience in Japan, in ASTRO-H, the RMAP functions were utilized to define the address range that could be commonly referenced with the addressing mode called the standard RMAP address space that encompassed the common address space for the whole network. In this address range, when a certain address was accessed, the address and the communication service were linked so the data exchange (communication service) could be done with a communication protocol corresponding to that address. This feature was referenced at the SpW WG, and in the SpaceWire plug-and-play standard (current Network Discovery Protocol),[20] the specification was set so the standard RMAP address space set in ASTRO-H could be applied. As a result, the concept of “plug-and-play that links the satellite onboard equipment as if we plugged into an outlet” was realized.Plug-and-play is a concept that is generally applied to consumer products, and in Japan, it was thought that the application to spacecraft onboard equipment was not very realistic. On the other hand, PIM that was Japan’s proprietary standard was similar to the plug-and-play concept dened by Europe, and this led to the actual specication proposal.3.4 Results of Japan-Europe joint developmentASTRO-H was developed with the goal of connecting each equipment “like plugging them into an outlet” so they could be immediately tested or operated. Therefore, a test and validation environment was prepared considering unit tests, procurement plans, and subsystem tests, not only the development of equipment and subsystems. Moreover, expecting that the development would be conducted under wide-ranging international cooperation, the joint R&D for the RMAP conformance tester was conducted jointly with the University of Dundee that was overseeing the specications for SpaceWire subcontracted by ESA.[16] This allowed the development of test specications and pass-fail determination including responses under off-nominal conditions that are not clearly written in the specications to be conducted in Japan. The devices that were developed in various countries were brought to Japan, and this allowed thorough development of a full redundancy network for large 2.7-ton satellites.In the RMAP conformance tester, there were about 80 % of off-nominal test cases that were mutually understood and extracted in the process of the Japan-UK joint R&D. The off-nominal test conditions are not clearly written in the standard specification. However, careful investigation of off-nominal test conditions not only enables exact test and validation, but also detects insufficiencies and defects in the setting of nominal test conditions. As a result of such steady R&D, the RMAP standard which matched the understanding and requests of the personnel of Japan and UK (and Europe) was established. The RMAP conformance tester is used around the world as a de facto standard, and this includes both cases of nominal and off-nominal test. As a result, both the nominal and off-nominal conditions were guaranteed to have conformity and understanding, and if Japan purchased overseas equipment that complied with the SpaceWire international standard, they could be installed onboard the Japanese satellite system. For the tester for SpaceWire, joint development is being conducted with other European companies, and continuous cooperation is pursued to maintain conformity and international understanding.4 Comparison of behavior patterns of each countryIn the previous chapter, we summarized the technological elements in the Japanese proposals that were reflected in the SpaceWire international standard and reviewed the standard proposal activities. To extract the success factors

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