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Speed Up Your Data: How to Design High-Speed Digital Signals for an Onboard Network in Spacecraft
Stéphane Hermant, R&D Project Manager, and Kévin Provost, R&D Project Engineer - Axon’ Cable
The quantity of data transmitted between devices on board spacecraft can reach up to several tens of Gb/s, and that volume is constantly increasing. Data transmission is much more than transmitting 0s and 1s. The type of information, the speed, and the signal integrity are key issues which also must be considered. The high-speed links have to be adapted to the environment. When designing the links for spacecraft, engineers must take into account parameters for outgassing, resistance to radiation, and weight-saving.
More and more data
The increasing number of devices on spacecraft, as well as the quantity of data to be transmitted, make the use of more adapted networks necessary. In some cases, the bus network topology to the MIL-STD-1553-B with a data rate close to 1 Mb/s needs to be replaced by point-to-point links with faster data transmission including high-speed data links up to several tens of Gb/s. These networks may be made with several physical links to allow redundancy or to share the bandwidth. As an example, SpaceWire according to ECSS-E-ST-50-12 (ESA standard) is a data-handling network for use on spacecraft. It connects instruments, mass memory, processors, downlink telemetry, and other onboard sub-systems. SpaceWire provides high-speed (up to 400 Mb/s), bi-directional, full-duplex data-links, which connect SpaceWire enabled devices. Data-handling networks can be built to suit particular applications using point- to-point data-links and routing switches.
Data rates must be compatible with current and future needs
Today, onboard digital signals require more and more dedicated media with specific features. Low-speed data bus use multi-purpose cables and connectors including D-sub and Micro-D types but as data rates rise, it is necessary to have more technical media. The SpaceWire network designed to connect high data rate sensors, processing units, memory devices, and telemetry/telecommand sub-systems on spacecraft provides speeds from 2 to 200 Mb/s. Now, however, data rates are starting to reach 400 Mbit/s when a matched impedance connector is used. Multi-Gb/s networks like SpaceFibre accommodate faster communication and improve the data rate by a factor of more than 10 with up to 10,000 Mb/s.
To be compatible with these standards, engineers have to take into account several parameters: impedance connection, signal losses, skew, and crosstalk.

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