“From SpaceWire to SpaceFibre”
SpaceWire is a high speed data interface, that has been specifically developed for the use on board of spacecraft. The ECSS standard ECSS-E-50-12A, which defines SpaceWire links, nodes, routers and network, is a good example for the global cooperation among space agencies, industry and universities. A big number of developments of components and equipments is available and under development around the globe, all compatible and based on this standard.
The current SpaceWire implementations sponsored by ESA target a maximum data rate of 200 Mb/s, and a maximum cable length of 10 meters. These limitations are factors imposed by the hardware which introduces jitter and skew onto the transmitted Data and Strobe signal.
In a number of sensor systems like High Resolution Synthetic Aperture Radar, Hyper Spectral Imagers or High Speed High Resolution Cameras the produced data rate is in the order of several Gb/s. To achieve such aggregate data rates over SpaceWire would require the bundling of several SpaceWire cables in parallel making such a link costly both in terms of hardware and mass.
Recognizing the need for higher data rates and the limitations imposed by the current SpaceWire specifications, ESA has started the development of “SpaceFibre”, an optical extension to the SpaceWire standard. The SpaceFibre research, specification and a first bread boarding has been carried out in the frame of the ESA technology development program.
SpaceFibre is based on fibre optic link technology and shall provide a bi-directional, point to point link connection at data rates up to 10 Gbit/sec in the final version. The breadboard developed can already demonstrate a data rates up to2.5 Gbit/sec. Besides the higher data rate SpaceFibre provides galvanic isolation for simpler integration, a harness mass advantage and a maximum link length of more than 100m. The last property is especially beneficial when used on ground in EGSE set-ups. The bread board further demonstrated the operation of SpaceFibre over copper to bridge shorter distances
The paper mainly discusses the required modifications in the evolution from SpaceWire to SpaceFibre and the trade-offs made in order to be able to integrate SpaceFibre links into a SpaceWire network but not drive the SpaceFibre development by the SpaceWire legacy. The modifications introduced compared to SpaceWire do not only affect the physical and the signalling level but also the character level and exchange level.
The higher data rate and longer link length required the evolution from Data and Strobe to embedded clock transmission using 8B/10B encoding.
Besides the stand alone usage of SpaceFibre, it is possible to integrate SpaceFibre links into a SpaceWire network via special SpaceWire–SpaceFibre routers, which have been bread boarded as well. A number of SpaceWire links can be multiplexed on one SpaceFibre link in the first router, run via the SpaceFibre to a second router and there split again into several physical SpaceWire links. Alternatively the data could be routed further through other SpaceFibre links. This concept of having several virtual channels running over one physical link appears to be very powerful and should be further pursued in SpaceFibre.
The described interoperability between SpaceWire and SpaceFibre is regarded to be very important in order to secure the return of investments made in the SpaceWire technology and at the same time being able to serve the full range of high and very high data rate applications.
The experiences made with the breadboard development need to be critically reviewed. Ultimately the results of this review should serve as input to a new standardisation effort under involvement of the SpaceWire community.
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