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Refer to figure.
At which position would an aircraft at an altitude of 5000 ft AMSL receive the strongest signal from VNC NDB (27°04.3’N, 082°26.4’W)?
  • A
    C (26°52.7’N, 082°23.5’W)
  • B
    A (27°11.0’N, 082°03.5’W)
  • C
    D (27°13.6’N, 082°18.8’W)
  • D
    B (26°51.2’N, 082°55.4’W)

Refer to figure.

The learning objectives state: State that the range of an NDB over sea is better than over land due to better ground wave propagation over seawater than over land.

  • Furthermore, in general, the closer aircraft will receive the stronger signal. At position C, the aircraft would receive the signal unaffected by ground clutter and terrain, resulting in the strongest signal.

Coastal Refraction

Coastal refraction is caused by radio waves travelling faster over sea than over land. This results in a change in the propagation direction when a wave crosses a boundary between sea and land.

The refraction at the boundary of land and water is a function of the propagation speeds over land and water plus the incidence angle relative to the boundary. Radio waves speed up over water, causing the wave front to bend away from its normal path and pull it towards the coast.

This phenomenon is negligible when the aircraft is perpendicular (90°) to the coast as the wave continues to propagate straight through the boundary. The magnitude of the refraction increases as the incidence increases.

The effect of coastal refraction can be minimized by flying higher.

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