Question 1

What is the key practical difference between GNSS jamming, GNSS spoofing, and ionospheric interference, from the DPO's point of view?

Accepted Answer

Jamming typically degrades or blocks the signal, causing a loss of lock or large position errors that DGNSS quality indicators (HDOP, number of satellites) usually flag as poor; spoofing feeds a false but plausible signal, which can show as a smooth, confident-looking position that is simply wrong, sometimes without obvious quality degradation; ionospheric interference causes signal delay errors that vary with geomagnetic activity and can affect differential correction accuracy across a wider area This distinction matters because spoofing is the most dangerous of the three — a jammed receiver usually flags itself as degraded, but a well-executed spoof can present a confident, "good-looking" but false position with no obvious quality warning, which is why cross-checking against an independent, non-GNSS reference is the real defence.

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.