For DP Class 2 and 3 operations, at least three Position Ref...

For DP Class 2 and 3 operations, at least three Position Reference Systems should be deployed and selected into the DP system. Why should they not all be based on the same principle?

A DPO is only trained on one type of Position Reference System B So that if one PRS fails the vessel remains fully operational C Because of the improved accuracy available from this configuration D To protect against Commen Mode Failure