Module 12 - Visual Aids to Navigation
Fog Signals
Some lighthouses and navigation marks have fog signals — sound signals activated in poor visibility. These include diaphones (two-tone), sirens, horns, and bells. The character of the fog signal (e.g., 'Horn(2) 30s' = 2 blasts every 30 seconds) is shown on the chart.
Sound travels unpredictably over water — it can be reflected, refracted, and absorbed, making it unreliable for estimating distance or direction. Fog signals confirm you are near a mark but should not be relied upon for precise positioning.
- Fog signals: diaphones, sirens, horns, bells
- Character shown on chart (e.g., Horn(2) 30s)
- Sound over water is unreliable for distance/direction
- Confirms proximity but not precise position
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