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    Re: Loran
    From: Lu Abel
    Date: 2006 Dec 12, 08:18 -0800

    I am an electrical engineer, but not an expert on radio propagation. 
    Here's my understanding:
    Loran uses a very low frequency radio signal (100 kHz, by way of 
    reference the bottom of the US AM broadcast band is at 500 kHz).  As 
    such it has a very long wavelength (3 km).  These sort of waves would 
    tend more to bend than bounce on terrestrial-sized features like 
    buildings.  But for large features like cliffs, there certainly could be 
    a bending that would appear to a distant observer as a change in arrival 
    Still, as I understand it, the largest effects the time of arrival of a 
    Loran signal (and remember, Loran works by measuring difference between 
    the receipt of signals from the various transmitters in the chain) are 
    (a) speed of propagation in the atmosphere vs free space, (b) refraction 
    effects (indeed, the ability to receive Loran signals at long distances 
    totally depends on refraction causing the signal to cling to the surface 
    of the earth); (c) effects of the electrical conductivity of the earth 
    and ocean on the speed of propagation (think of an electrically 
    conductive earth/sea as exerting a drag on Loran's radio wave).
    Loran engineers speak of Primary Factor, Secondary Factor, and 
    Additional Secondary Factor as corrections to the expected time of 
    arrival of a signal,  PF and SF were calculated, ASF had to be measured 
    in the field.
    If you're in the US and have ever taken a coastal navigation course, 
    you're likely to have used the 1210TR chart, a training chart that's a 
    reprint of a mid-80s version of a 1:80,000 coastal chart embracing the 
    southern coast of Rhode Island.  There are Loran-C TD lines printed on 
    the chart, with the marginal notes stating "not corrected for ASF."   In 
    fact, I think electronics advanced more quickly than NOAA, to my 
    knowledge no chart ever had it's TD lines corrected for ASF whilst Loran 
    receivers soon had ASF corrections built into their L/Lo displays.
    Bill wrote:
    > Lu wrote:
    >>The atmosphere, refraction, and even the
    >>conductivity of the earth (influenced by things such as moisture
    >>content) affects the speed of radio waves.  Loran designers could
    >>compensate for some of this with calculations, but effects caused by
    >>things such as ground moisture required experimental measurement.
    >>Without knowing these systematic errors, Loran had great repeatability
    >>(coming back to an exact set of Loran coordinates would typically bring
    >>one to within � 50 feet of the position where they were originally
    >>measured) but initially poor absolute accuracy (L/Lo as calculated by
    >>the box and/or from TD LOPs plotted on charts) (typically 1/4 mile or
    > If I recall older texts, Loran radio waves could also be subject a change in
    > direction/speed by to bouncing off bluffs, cliffs, and other land obstacles.
    > Is that fact?
    > Bill
    > > 
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