Question 1

The client notes that the DGNSS units are now showing stable, mutually-agreeing positions again and asks the DPO to re-enable them to restore full redundancy, since the job is behind schedule. How should the DPO respond?

Accepted Answer

Be cautious — DGNSS units agreeing with each other again does not prove the interference has ended, since a spoofing source could cause two units to agree on the same false position; the safer verification is to confirm DGNSS agrees with Fanbeam (the unaffected, different-technology reference) before trusting it, not just that the two GNSS units agree with each other This is the core lesson of the whole cascade: two GNSS units agreeing with each other proves nothing about whether a shared external cause (spoofing in particular) is still affecting both. The only meaningful check is cross-technology agreement — DGNSS against Fanbeam — not GNSS-against-GNSS.

Question 2

A DP2 vessel is holding position. Both DGNSS units (normally independent) suddenly show an identical, sudden position jump of 40 metres in the same direction at the same instant, while the Fanbeam shows no corresponding change. What should the DPO immediately suspect?

Accepted Answer

A GNSS-wide problem affecting both satellite receivers simultaneously (interference, spoofing, or a satellite constellation issue) rather than two independent sensor faults — the fact that both units moved identically and together is the key clue, since genuinely independent reference systems failing identically at the same instant is far less likely than a shared external cause The signature here is diagnostic: independent systems agreeing with each other but disagreeing with a genuinely different technology (Fanbeam) is a classic common-cause signature pointing at the GNSS signal environment itself, not the receivers. This is exactly the pattern seen in real GNSS jamming and spoofing incidents.

Question 3

The DPO checks the Fanbeam reading against the suspect DGNSS units and confirms the vessel has not actually moved 40 metres — the Fanbeam position is consistent with the pre-event position. What should the DPO do immediately?

Accepted Answer

Manually exclude both DGNSS units from the DP position solution immediately, relying on the Fanbeam (and any other unaffected reference) — do not wait for the DP system's automatic voting to resolve this, since two affected units may outvote the one correct reference if left to automatic logic This is precisely why blind trust in majority voting is dangerous during a common-cause GNSS event: two affected systems can outnumber the one unaffected system, and naive voting logic would pick the wrong answer. The DPO's own judgement, informed by cross-checking technology types, must override simple vote-counting here.