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Shoreline Effect: Why NDB Signals Bend As They Cross The Coast

NDB Shoreline Effect Header

Remember non-directional beacons (NDBs)? While you probably navigate with VORs and GPS, NDBs are still out there. They're prone to errors and interference, and they can be confusing to track.

Every once in a while, we come across an NDB question. Earlier this week, a pilot asked why shoreline effect bends an NDB course.

Good question. There's not a lot of great information out there to answer it - so we researched this post.

What Is Shoreline Effect

Have you ever heard of shoreline effect? It's been called "land effect" and "coastal effect," but it's really "coastal refraction."

NDBs send out low or medium frequency radio waves. They actually generate two waves - a ground wave that follows the Earth, and a sky wave that bounces off of the ionosphere. In this case, were only talking about the ground wave.

When the ground wave crosses a shoreline and moves out to sea, it bends towards the coast. But why?

Coastal Refraction: Changes In Conductivity From Land To Sea

What's more conductive - land or sea? The sea, right? If lightning strikes the ground, it quickly dissipates. But, if it strikes a pool - Zap!

These electrical differences affect radio waves, as well. As low frequency waves cross the shoreline, they move abruptly from an area of low conductivity (land) to an area of high conductivity (water). That rapid change in conductivity changes the radio wave's "phase velocity."

What's that? Imagine the crest of a wave moving across the ocean - the speed that the crest moves is the wave's phase velocity. Radio waves are no different.

As the phase velocity changes, the wave's frequency (what you dialed into the radio receiver) stays the same. The laws of conservation of energy and momentum come into play, and the wave bends. This is called refraction. For a more in-depth look at the physics, check out Snell's law.

Visualizing Shoreline Effect: Video

Have you ever seen a mirage? Or, have you ever noticed how objects seem to bend when they're placed into water?

Take a look at the video below, where we insert a straw into a glass of water. Imagine the straw is the radio wave, and the air above the glass is the land.

Notice how the straw doesn't appear to bend at all when it's perpendicular to the water's surface, and that it looks the most bent when it's nearly parallel to the water's surface.

What you saw in the video above is nearly the same as shoreline effect. The waves bend the most when they're nearly parallel to the coastline, and don't bend when they're perpendicular to the coast.

How Your Ground Course Changes

If you're tracking an NDB bearing and flying over the sea, what will happen to your bearing pointer as you cross the shoreline?

As you cross the shoreline, your bearing pointer suddenly shifts - pointing away from the shore. You'll need to change your ground track to stay on course.

When you're over the sea, shoreline effect bends the radio waves towards the coast - so you fly a track closer to the shore. Once you cross over the shoreline on to land, the effect is gone and the radio waves point directly toward the radio station.

Take a look at the animated GIF below.

NDB Shoreline Effect

As you cross the shore, your bearing pointer swings. You turn to correct, putting you back on track towards the NDB.

What Do You Think? Do You Still Use NDBs? Let Us Know In The Comments.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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