DP Mini Guide: PRS — Bad or Unreliable Reference

Crew Connect DP Academy · Mini Guide 03

PRS — Bad or Unreliable Reference

Topic: Position Reference Systems · Sub-topic: PRS Quality  ·  Level: DPO  ·  Read time: 8 min

1. What / Why / How

Accuracy, availability, reliability and integrity are four distinct properties of a PRS. A reference can be available (turned on, reporting a number) while being neither accurate nor reliable. Drift, noise, offset, position jumps and a falsely plausible value are all different failure signatures worth being able to tell apart.

2. Recognition / Risk / But

Compare reference residuals, trends, the DP model's behaviour and independent sources against each other. The risk: the DP controller may act on incorrect information that never triggered an alarm.

But: rejecting a genuinely good reference solely because it disagrees with two references that share a common mode can also be the wrong call — disagreement doesn't automatically mean the odd one out is the faulty one.

3. Now / What next / Decision / Why

Identify the discrepancy precisely, cross-check independent-principle sources and the operational context, preserve the evidence (don't just clear the alarm), and apply your vessel's approved reference-management procedure rather than an ad-hoc judgement call.

4. Consequence / Reassess / Escalate

A position jump is followed by apparent stabilisation. The key question: is "stable again" sufficient evidence of recovery? It is not — revise the operational status based on verified cause, not on the number simply looking normal again.

5. Recover / Verify / Human factor

Verify root cause or credible restoration through independent checks before trusting the reference fully again. Trap: confirmation bias, and normalisation of a small recurring offset ("it always does that") that quietly erodes the standard you'd apply to a first-time fault.

Sources: IMCA M252 (position reference systems and sensors), IMO MSC.1/Circ.1580, IMCA DPE 04/18 and DPE 02/22 (reference selection/set-up learning cases). This guide is educational content, not a substitute for accredited DP training, vessel-specific FMEA/ASOG, or full-mission simulator assessment. Crew Connect never infers compliance with a specific vessel's applicable requirements — always verify against your own approved documents.