GNSS Limitations and Cross-Checks

MCA guidance reports that GNSS commonly produces horizontal accuracy around 5–25 m for 95% of observations, while stressing that equipment, augmentation, antenna view and environment change the result. Navigate to inherent accuracy and safety margin, not the number of displayed decimals.

HDOP describes how satellite geometry scales horizontal ranging uncertainty. Lower is generally favourable geometry, but there is no portable pass/fail value that proves a trustworthy position across receivers and situations. Actual error also depends on ranging error, atmosphere, multipath, receiver processing and antenna environment. HDOP does not validate a chart, waypoint, datum or signal integrity.

Multipath is a reflected-signal path that can bias range in harbours, near cliffs, structures or large vessels. Jamming can deny or degrade the signal; spoofing can create a plausible wrong position. A GNSS-fed chart, radar overlay, AIS display and second screen may all inherit one wrong position, so display count is not source independence.

RAIM or other receiver integrity monitoring compares redundant measurements against a receiver-specific threshold and may raise an integrity warning. Availability and behaviour depend on equipment, satellite geometry and configuration. A clear indication reduces one class of concern but cannot prove the waypoint, chart, antenna offset, datum or absence of sophisticated interference.