Terminal timing synchronisation in DVB-RCS systems using on-board NCR generation

In DVB-RCS systems, Return Channel Satellite Terminals (RCST) receive a stream of Network Clock Reference (NCR) packets from a central Hub station to regenerate their internal clocks and aid network synchronisation. Where the NCR is generated at a ground Hub station the delays between Hub and satellite (and the reverse path) must be measured; an allowance for the measurement error is made in the guard time. The bandwidth efficiency gains of locating the NCR clock on-board the satellite are evaluated: for high transmission rates the gain may be worthwhile whereas for lower rates a negligible benefit exists. In some applications, on-board processing may be used to switch packets on the forward link (for routing to different beams). With an on-ground NCR source, the satellite switching most likely results in NCR delay variance (jumps) beyond acceptable limits for correct RCST frequency regeneration. An on-board NCR clock aids a system architecture that encompasses forward link switching and can be justified in these circumstances.