ADF Coastal Effect
When the NDB beam crosses the coast, the beam is refracted (bent) as it travels slightly faster over water than over land.
This refraction causes the aircraft to seem closer to the coast than it actually is and this causes a bearing error.
How to minimise the error?
There are a number of things we can do to minimise the bearing error, firstly we can choose a beacon where the bearing angle will cut the coast at 90o – this is much easier to say than do! The middle aircraft in the diagram shows a 90o cut.
Another way you can minimise the error is to use an NDB that is close to the coast. In the diagram the top aircraft is using the NDB that is near the coast, although the refraction error will be dependent on the angle the beam crosses the coast, the resultant bearing error will be less if the NDB is near the coastline.
On the bottom part of the diagram, the aircraft where the NDB is further inland, the refraction angle is the same but the bearing angle is greater, thus there is more error.
Another way to minimise the error is to fly higher this is because the waves are refracted more when closer to the waters surface. Thus fly higher for less refraction.
Here are some Questions on this subject:
Q What causes coastal refraction?
A The difference in speed of the EM-wave over land and over sea.
Q Which radio navigation aid is mainly affected by the shoreline/coastal effect?
A NDB
Q The shoreline/coastal effect is caused by refraction of the radio waves. What is one factor influencing the error of the bearing?
A 90o cut, NDB close to the coast and/or Flight altitude
Q Errors caused by the effect of coastal refraction on bearings at lower altitudes are maximum when the NDB is.
A Inland is the worst position for an NDB and when the bearing crosses the coast at an acute angle.
Q There are two NDBs, one 20 NM inland, and the other 50 NM inland from the coast. Assuming that the error caused by coastal refraction is the same for both propagations, the extent of the error in a position line plotted by an aircraft that is over water will be:
A Greater from the beacon that is 50 NM inland.