Question 1

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 2

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.

Question 3

With both DGNSS excluded, the vessel is now running on Fanbeam alone, with no independent cross-check. What constraint does this place on the operation?

Accepted Answer

Single-reference operation with degraded redundancy — the same ASOG escalation logic applies as for any single-reference situation; additionally, Fanbeam's line-of-sight dependency means any loss of the target (fog, obstruction, range) would now mean total reference loss with no GNSS fallback available while the interference persists Proving Fanbeam correct in this instance does not change its known limitation — line-of-sight dependency. With GNSS unusable due to the interference and Fanbeam now the sole reference, the vessel has lost its normal fallback path if Fanbeam itself loses lock, which is a meaningfully different (and more constrained) risk picture than normal multi-reference operation.

DP — GPS Spoofing (Cascade) Practice Questions