SpaceCAN - A low-cost, reliable and robust control and monitoring bus for small satellites

摘要

The control and monitoring bus is the backbone for routing commands and telemetry among spacecraft subsystems. Typically, a central processing unit is commanding other intelligent nodes (such as the power system, communication system, and payloads) and collects status information from them. The data to be exchanged on this bus is of moderate volume but must be transmitted in a reliable way. Most commonly for small satellites, and in particular CubeSats, the I2C bus is used as main system bus. This is a problem because the I2C bus is not fault tolerant and has been reported as the source of mishaps to a number of satellite missions. We propose to replace I2C bus with CAN bus, which is heavily employed in automotive and industrial applications since decades and has been qualified for use in space. For this, we have developed the SpaceCAN protocol, which utilizes CAN as transport technology and combines elements of ECSS CAN Bus Extension Protocol for reliability and robustness, and the ISO-TP protocol for large message transfer. With the help of a low-cost prototype we validate the protocol features and contrast its performance against the I2C bus. The results demonstrate that SpaceCAN is superior to I2C in terms of reliability and robustness to failures. The main obstacle to an immediate replacement of I2C is the increased power consumption of the CAN bus, for which we outline strategies for improvement. The entire prototype source code is available under a permissive open source license.