QUESTION 4.5.2

DP position-reference systems

OilBridge

What is being checked

The inspector is checking whether independent position-reference systems are correctly weighted, monitored and protected from common-mode error, with sensor offsets available to DPOs, and whether plans, records and interviews demonstrate that the procedure is applied during real voyages rather than completed only as paperwork.

Ready onboard

0/6 complete

PROCEDUREThe procedure defines selection, deselection, weighting, pooling, offset entry, plausibility limits and response to disagreement for each fitted PRS.EVIDENCEDemonstrate current PRS status, offsets, reference origins, alarm history, maintenance and a recent controlled change or rejection of a faulty reference.RESPONSIBILITYThe Master and responsible officers can identify who prepares, verifies, approves and monitors each stage.OFFSETSLongitudinal and transverse antenna or target offsets are documented and match software configuration.INDEPENDENCEReferences do not rely unknowingly on the same GNSS corrections, power, network or external target.DIVERGENCEDPOs recognise slow drift, frozen data and plausible common-mode error rather than waiting for a hard alarm.

Practice questions

Answer aloud, then open the preparation point.

1What is the operational purpose of dp position-reference systems?

Independent position-reference systems are correctly weighted, monitored and protected from common-mode error, with sensor offsets available to dpos.

2Show how the procedure was applied on the last voyage.

Demonstrate current PRS status, offsets, reference origins, alarm history, maintenance and a recent controlled change or rejection of a faulty reference.

3Which decision requires the Master's judgement?

The DPO and Master decide whether available references remain sufficiently independent and reliable for the activity and environmental condition.

4What happens if the planned control becomes invalid?

Reject the suspect reference, verify position by independent means, move through ASOG status, suspend or abort before redundancy and excursion margins are lost.

5How are changes communicated and recorded?

Brief the affected bridge and machinery personnel, amend the controlled plan or record, obtain required approval and retain the reason and revised limits.

Where an observation may arise

PAPER COMPLIANCE

Records exist but the bridge team cannot relate them to the actual voyage or operation.

CHANGE UNCONTROLLED

A material change is not reassessed, approved, briefed or recorded.

OFFSET ERROR

Incorrect sign, origin or value shifts the calculated vessel position.

COMMON MODE

Multiple references are counted as independent despite shared failure.

BAD REFERENCE RETAINED

A divergent or frozen reference remains weighted into the solution.

Master's practical note

Ask which two references can fail together. Three displayed positions do not provide threefold assurance if they share satellite corrections, network or power.

Equipment boundary and references

Equipment boundary: Applicable only to oil vessels with DP or shuttle-tanker operations. Required number and diversity depend on equipment class, activity, ASOG and field requirements.

References

OCIMF SIRE 2.0 Question Library Part 1, Version 1.0, Question 4.5.2
IMO MSC.1/Circ.1580
IMCA M 252
Applicable class DP rules

Last reviewed: 2026-09-11