Implementation of a reliable date bus for the Delfi nanosatellite programme
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Abstract
The Delfi-n3Xt nano-satellite is the second Dutch universitysatellite currently being developed at the Delft University of Technology (TUD) as successor of the Delfi-C3 that has been successfully launched in April 2008. Compared to Delfi-C3, the Delfi-n3Xt platform provides significant advancements to the platform: a high-speed downlink, three-axis attitude control and a single-point of failure free battery. In total five payloads will be flown that generate a considerable larger amount of data compared to Delfi-C3that implies, as well, a robust and adequate design for the data handling system that interlinks the various embedded systems on board. This paper examines the design and implementation of a fault tolerant data bus architecture as part of the satellite Command and DataHandling Subsystem (CDHS). Delfi-C3 carries an I2C protocol based implementation that currently experiencesproblems with data corruption and timeouts and is therefore subject of scrutiny andanalysis in this paper. In particular, the relationship between error rates, master-slave speeds and processing overheads is evaluated in detail. After a tradeoff study betweenseveral bus standards for Delfi-n3Xt, the choice is once again an I2Cimplementation, but with significant hardware and software improvements over the previous design. In terms of hardware, shielding and bus protection considerationsare included in the very early stages of design. With respect to software, special care is taken in dealing with the varying clock speeds between slaves and masters, correct data handling and the feasibility of error detection and correction codes, as the amount of data generated by thepayloads of the Delfi-n3Xt is significantly higher. The final result of this research is the selection of the most adequate reliability techniques and their implementation. This I2Cbus targeted middleware is intended for usage in the complete Delfi nanosatelliteprogramme at TUD and for several other space applications in general.