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Engineering note

Redundancy that never switches is just spare parts

Two antennas and three links do not make a resilient system. What makes it resilient is a documented handover policy, tested while somebody is aboard to watch it fail.

Why two antennas, and what they actually buy

A second antenna buys two things: continuity when the first is being serviced, and a second look at the sky when the vessel is in a marginal footprint or obstructed by its own superstructure. The first is a maintenance argument, the second a performance one — and they lead to different arrangements.

For continuity you can keep one antenna per vessel and accept the outage, or carry a spare. For performance you need both live, which means either a combining arrangement or two receivers and a switch.

Switching has to be automatic and observable

A manual changeover panel is a procedure that works until the person who knows it is asleep. What is needed is an automatic switch on signal quality, plus an indication on the bridge so somebody can see which path is live — and a log, so the reason for the last switch can be reconstructed.

The diagnostic value matters more than the switch itself: a system that switches twenty times a day is telling you the primary path is marginal, and that is a sizing answer, not a switching answer.

The link handover policy

With cellular, microwave and satellite available, the policy decides which one carries what. The documented configuration runs cellular alongside VSAT with automatic priority switching; a microwave link slots in as the primary where line of sight exists.

A workable policy is ordered by cost, not by speed: microwave first where the path exists, cellular second inside coverage, satellite last — with the exception list written down (voice always on satellite if the cellular link is congested, CCTV backhaul never on the metered link).

  • Order by recurring cost: microwave → cellular → satellite
  • Name the exceptions explicitly (voice, CCTV backhaul, remote support)
  • Set a hysteresis so the system does not flap between links
  • Log every switch with the reason and the measured quality
  • Test the failover with somebody aboard, on a schedule

What to write in the handover file

The handover file should state which link is primary, what triggers a switch, what the crew should do when the bridge indication changes, and who to call. Without that, redundancy becomes a conversation at 03:00 between an officer and a phone.

The same file should record the test: date, what was switched, what was observed. A failover that has never been tested is an assumption.

Checklist for the drawing and the yard

  1. 01The purpose of the second antenna is stated: continuity or performance
  2. 02Switching is automatic, with a bridge indication and a log
  3. 03Link priority ordered by recurring cost, exceptions written down
  4. 04Hysteresis set so the system does not flap
  5. 05Failover tested with somebody aboard, result recorded in the handover file

Questions on this note

Failover design

Is a second antenna worth it on a small vessel?+

For continuity, rarely — the cost of a spare radome exceeds the cost of the outage on a coastal boat. For performance in an obstructed position or a marginal footprint, sometimes yes, and the cheaper answer is usually a better mounting position.

Can cellular and satellite share one router?+

Yes, and that is the normal arrangement: one router with two WAN interfaces and a policy for which traffic uses which. What matters is that the policy is written down and the switch is logged.