Spacewire time distribution protocol implementation and results

Aeroflex Gaisler has developed, under European Space Agency (ESA) contract 4000104519, a draft ECSS protocol for the transmission and synchronization of CCSDS Unsegmented Code (CUC) time in SpaceWire networks. The working name of the protocol is “Time Distribution Protocol”. Apart from transmission and synchronization of time across the SpaceWire network, the protocol also provides guidelines to achieve highly accurate time synchronization by mitigating jitter and latency affecting SpaceWire Time-Code transmission in a SpaceWire network. The protocol also provides guidelines for correcting clock drift appearing in local SpaceWire nodes. A prototype implementation of the protocol was performed and analyzed for intended functionality. The implementation of jitter and drift mitigation is based on a simple time interval measurement of incoming SpaceWire Time-Codes using local clock. Statistical information is gathered, which is then used to calculate an average correction value that is applied to modify a frequency synthesizer which provides inputs for the time generation. By controlling the frequency synthesizer the time is maintained stable without drift arising from oscillator or crystal used to generate the local clock. Distributed interrupts are used for latency measurement between two nodes in a SpaceWire network. Time-stamping of reception and transmission of distributed interrupts provides the values needed to calculate latency. The calculated latency value is used for correcting the time maintained in the system. By performing CCSDS Unsegmented Code (CUC) transmission and latency, jitter, drift mitigation a stable time keeping in a system is achieved.