In a DP Class 2 or 3 vessel with four Position Reference Systems, why should the DPO consider the two DGNSS as just one PRS?

The rationale for considering the two DGNSS as just one Position Reference System (PRS) in a DP Class 2 or 3 vessel with four systems stems from the inherent vulnerabilities associated with common-mode failures. When both DGNSS systems are utilized, they may operate in a manner that renders them susceptible to similar errors or signal interruptions. If a common-mode failure occurs—such as interference from a signal distortion or a satellite outage—both DGNSS systems could fail simultaneously, resulting in a significant loss of positioning data.

Ignoring this connection could lead to a false sense of security, where operators believe they have redundancy in their position reference systems. By recognizing the two DGNSS as one PRS, the dynamic positioning operator (DPO) maintains a clearer understanding of the risks and ensures they are adequately prepared for potential signal failures. This recognition helps in maintaining operational safety and integrity by fostering a more cautious approach to reliance on multiple systems that may be subject to identical failures.