Radar and AIS for Position Confirmation

Marine radar transmits microwave pulses and measures the time for echoes to return, giving range and bearing to targets. Radar range is generally more accurate than radar bearing — range is determined by precise timing, whereas bearing depends on the width of the radar beam (typically 2°–6° on a yacht scanner). A radar fix from two or more ranges (using Variable Range Markers) is therefore more reliable than one using bearings alone.

To obtain a radar fix, take ranges and/or bearings of identifiable charted features — headlands, isolated rocks, or distinctive coastline shapes. Plot these on the chart just as you would visual bearings. Two radar ranges from well-separated features give a good fix. Three ranges or a combination of ranges and bearings improve confidence and highlight errors.

Modern chart plotters offer radar overlay, superimposing the radar image on the electronic chart. This allows the navigator to visually compare what the radar sees with charted features. If the radar picture does not align with the chart, it may indicate a GPS offset, incorrect chart datum, or radar misalignment — each of which must be investigated.

AIS (Automatic Identification System) receives broadcasts from other vessels and shore stations containing their identity, position, course, and speed. AIS is invaluable for collision avoidance and identifying nearby traffic, but it is not a primary navigation tool. AIS depends on other vessels transmitting — not all vessels carry AIS (small craft, fishing boats, and military vessels may not). AIS positions are also GPS-derived, so they share the same GPS vulnerabilities.

AIS-equipped aids to navigation (AtoNs) — both real and virtual — provide charted reference points that can help confirm position. However, AIS should supplement, not replace, visual and radar observations. AIS data can be delayed, and virtual AtoNs exist only as electronic signals with no physical structure to verify visually.