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    Re: Table 8, Bowditch
    From: Ark Shvetsky
    Date: 2009 Jan 13, 09:58 -0800

    Thank you, Lu.� It makes it clear
    ----- Original Message ----
    From: Lu Abel 
    To: NavList@fer3.com
    Sent: Monday, January 12, 2009 11:52:59 PM
    Subject: [NavList 7034] Re: Table 8, Bowditch
    For reasons difficult to explain unless you're a graduate electrical 
    engineer or physicist, the lower the frequency of electromagnetic waves, 
    the more they "cling" to the surface of the earth.� Despite their very, 
    very high frequencies, radar microwaves are about 1/10,000 the frequency 
    of light waves.� And that makes them cling to the surface of the earth 
    (especially conductive surfaces such as seawater) a bit more tightly 
    than light.� Or to put it in terms more familiar to us as celestial 
    navigators, they refract more highly.� Thus the difference between the 
    multiplicative factors of 1.22 vs 1.15 for distance-to-horizon for radar 
    vs light.
    An extreme example of this "clinging" occurs at very low radio 
    frequencies, where the radio waves can follow the surface of the earth 
    for hundreds or thousands of miles.� Loran capitalizes on this 
    phenomenon -- it runs at just 100KHz (1/100,000 of the frequency of 
    radar) and so-called ground waves are what make the system work.� They 
    follow the curvature of the earth for hundreds of miles.� Loran compares 
    the time it receives signals vs the time several transmitters transmit 
    their signals and, based on the speed of light, computes the distance of 
    the receiver from the various transmitters.� Skywaves -- ones that 
    bounce off the ionosphere -- are useless for Loran distance finding 
    because of the variability of the height of the ionosphere and hence the 
    time it takes for the signal to reach the receiver.
    Lu Abel
    inuik@yahoo.com wrote:
    > Greeting,
    > I'm trying to understand why for calculating the approximate radar range at 
    which any given target will return an echo-it is suggested that Bowditch 
    Table 8 can be used with the distances increased by 4-5%% to allow for the 
    slight curvature of radar waves over the geometric horizon but that provision 
    is apparently not used for the visible horizon calc?
    > Or the answer is the formulas: for radar 1.22 Sq.Rt of H
    > for visible horizon 1.15 Sq.Rt of H
    > I think the light refraction is affecting the distance to the visible 
    horizon, so why 4-5% is added to radio waves?
    > Ark
    > >
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