Time Synchronisation Without GPS Belowdecks
A red-state event has to be timestamped to within milliseconds across all sensor cells. Belowdecks, GPS is not available. PTP over the EtherCAT ring is.
A coherence check is only valid if all the cells being compared are reporting on the same clock. Belowdecks on a steel-hulled vessel, GPS is unavailable or unreliable, and we needed a synchronisation source that did not depend on it. PTP-over-EtherCAT solves the problem inside the ring.
Where the clock comes from
- Segment master holds the deck reference clock, disciplined from vessel time at the bridge.
- PTP messages propagate around the ring with per-node compensation for the link delay.
- Each cell holds local time to within ~1 µs of the master across normal operation.
- Cells log a sequence number alongside the timestamp so out-of-order arrivals are detectable.
Questions, answered
Why can't GPS provide timestamps for sensor cells belowdecks?+
On a steel-hulled vessel, GPS is unavailable or unreliable belowdecks. But a coherence check is only valid if all compared cells report on the same clock, so the system needs a synchronisation source that doesn't depend on GPS. PTP-over-EtherCAT solves it inside the ring.
How is time kept in sync across all the cells?+
The segment master holds the deck reference clock, disciplined from vessel time at the bridge. PTP messages propagate around the ring with per-node compensation for link delay, keeping each cell within about 1 µs of the master. Cells also log a sequence number alongside the timestamp so out-of-order arrivals are detectable.
Is PTP-over-EtherCAT a custom solution?+
No — it is the same Precision Time Protocol standard the rest of the industrial-automation world uses. GPS belowdecks is a polite fiction; PTP over the EtherCAT ring is the actual, well-proven answer.
Continue the thread
Why EtherCAT Ring Topology for Marine Sensors
Star-topology fieldbuses fail open when one run is damaged. A redundant ring keeps the segment master talking to every node, even with a cut cable.
The Data Pipeline From Deck to Bridge
A walk-through of every hop the data takes — sensor cell, segment master, vessel server, bridge console — and the latency budget at each.