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6 Answers

What does line of sight really means ?

Asked by: 3679 views ,
Aircraft Systems, Student Pilot

I'm working on my private pilot and found a few videos that mentioned the fact that radio or VOR are not "line of sight" system and other system, such as Fly Watch are. I can't find anything about this in the literature and have no idea what this really means. Can I have some help here. Thanks.



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6 Answers

  1. Kris Kortokrax on Dec 25, 2013

    If you Google “line of sight VHF” you will find out all you need to know, including pictures and formulas for determining line of sight distance based upon antenna height.

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  2. Mark Kolber on Dec 26, 2013

    And, perhaps surprisingly, it’s pretty much plain English: with high-powered binoculars on a day with unlimited visibility, would you be able to see the antenna from where you are? Or would it be unseen because of the curvature of the earth or terrain? (That’s not 100% accurate since some atmospheric signal refraction can take place to extend the range, but close enough for the level of science typical in flight instruction).

    As Kris says, try Google for more detail explanations, calculators, and graphical examples such as this one (I don’t know if we can embed photos, so this might or might not work; I’ll try to embed but also give a link):

    Link: http://www.free-online-private-pilot-ground-school.com/images/line-of-sight.gif

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  3. Bidochon on Dec 26, 2013

    Thanks guys. I should have been more precise with my question as I am mostly puzzled about why some of them (VOR, DME, ILS) are “line of sight” and others (radio communication for example) do not seem to be.

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  4. Kris Kortokrax on Dec 26, 2013

    VHF radio communication is line of sight. HF radio is not. You are not likely to use HF, unless you are conducting extended overwater flights.

    Radio wave propagation is fairly involved, however, a rudimentary disussion can be found at:


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  5. Best Answer

    Wes Beard on Dec 26, 2013


    Line of sight is a property of radio waves. It is proportional to the frequency of the radio wave. For example, an HF radio or NDB is a low frequency down in the kilohertz (kHz) range. They have a very long amplitude and tend to bounce off of the ionosphere. AM radio is in also in this range. They do not have a very strong line of sight property.

    In the U.S., some AM radio stations must turn off their signal at night due to intereference from other radio stations hundreds of miles away. Which brings up an interesting point, the ionosphere decreases in altitude during the night and the result is low radio waves travel further.

    VOR and VHF radio waves are in the megahertz (mHz) range and do not bounce off the ionosphere as easily as low radio waves. This results in the radio wave having a stronger line of sight property though these radio waves also have the ability to bend, to a certain extent, around mountains and bounce off the ionosphere.

    DME is in an even higher frequency range than a VOR or VHF radio. Thus the line of sight property is the strongest of the three mentioned. I have not been able to pick up a DME signal even when I am a few hundred feet below a mountain range. Thus I can infer that a DME signal does not bend very well at all.

    The Instrument Flying Handbook has a good discussion on radio wave propagation.

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  6. Bidochon on Dec 26, 2013

    Thanks a lot to Wes, Kris and Mark. I have now a better idea of the concept.

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