GNSS Limitations and Cross-Checks

GPS (and the broader term GNSS, which includes Galileo, GLONASS, and BeiDou) provides position accuracy typically within 5–15 metres under good conditions. However, this accuracy depends on satellite geometry, atmospheric conditions, and the local environment. Accuracy degrades when available satellites are clustered in one part of the sky rather than spread evenly — this is quantified by Dilution of Precision (DOP) values.

HDOP (Horizontal Dilution of Precision) relates to the spread of satellites in the horizontal plane and directly affects the accuracy of your lat/long fix. GDOP (Geometric Dilution of Precision) considers all dimensions including altitude and time. An HDOP of 1.0 is ideal; values above 3–4 indicate degraded accuracy, and above 6 the fix should be treated with caution.

Multipath error occurs when satellite signals bounce off nearby surfaces — cliffs, large vessels, bridge structures, or your own rigging — before reaching the antenna. The reflected signal travels a longer path, causing the receiver to miscalculate range and therefore position. Multipath is worst in confined waters near steep terrain or alongside large ships, precisely where accurate positioning matters most.

GPS can also be affected by solar activity (ionospheric disturbance), deliberate jamming, or spoofing. RAIM (Receiver Autonomous Integrity Monitoring) is a feature in many marine GPS units that cross-checks range measurements from redundant satellites to detect faulty signals and warn the navigator. RAIM requires a minimum of five satellites (six to exclude the faulty one).

A prudent navigator never relies on a single source of position information. GPS positions should be cross-checked against independent sources: visual bearings of charted objects, radar ranges and bearings, depth soundings compared to the chart, and transit lines. If the GPS position places you in one location but the depth sounder reads differently from what the chart shows at that position, investigate immediately.