What the Examiner Is Really Testing
The examiner wants to know whether you trust your kit intelligently or blindly. A pass-standard candidate can name the principal electronic position-fixing systems, articulate each system's specific failure modes and error sources, explain how errors are detected, and describe the corrective action — including when to revert to another method. Quoting "GPS can be wrong" is not enough; you must say why, how you would know, and what you do.
GNSS (GPS and Other Constellations)
Limitations and error sources:
- Satellite geometry (HDOP/PDOP): poor spread of satellites increases positional error. Check DOP values on your receiver.
- Ionospheric and tropospheric delay: signal refraction causes timing errors; worse at low elevation angles.
- Multipath: signal reflects off superstructure before reaching the antenna — common on yachts with complex rigs.
- Selective availability: now switched off for civilian users, but legacy consideration.
- Signal spoofing and jamming: increasingly significant in conflict zones and some coastal areas. Spoofing can present a plausible but entirely false position; jamming causes total loss of fix.
- Receiver autonomous integrity monitoring (RAIM): the receiver self-checks using redundant satellites. Below the minimum satellite threshold, RAIM fails and the receiver may not alert you — it can hold a last-known position.
Detection and correction:
- Cross-check GPS position against an independent source — radar fix, visual bearing, depth contour, or a second independently-sourced GNSS (e.g., a chartplotter and a separate handheld on a different constellation).
- Monitor for sudden, unexplained position jumps or heading/speed anomalies inconsistent with log and compass.
- In spoofing-suspect areas, maintain a running DR and compare continuously.
Radar-Based Position Fixing
Limitations:
- Range and bearing accuracy depend on target conspicuity, sea clutter, rain clutter, and radar horizon (affected by height of scanner).
- Parallel indexing assumes accurate chart data and correct EBL/VRM calibration.
- False echoes, side-lobe returns, and blind sectors (caused by mast, superstructure).
Correction: Use multiple position lines; favour range over bearing when possible as range is more accurate. Regularly verify EBL and VRM calibration against known distances.
Depth (Echo Sounder)
Useful as a position line when crossing a charted depth contour. Errors arise from transducer draught offset, speed of sound variation with salinity/temperature, and bottom type. Always apply the instrument's draught correction.
Structured Spoken Answer
In the oral, lead with the system, then its specific failure mode, then how you detect it, then your corrective action. End by reinforcing the principle: no single electronic system is sufficient alone; the OOW maintains a running DR and cross-checks continuously. SOLAS requires the OOW to make use of all available means — that phrase underpins why multi-system cross-checking is a legal obligation, not just good practice, and the oral examination syllabus underpinned by MSN 1858 assesses whether candidates can demonstrate this in practice.