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    ELoran and electronic compasses
    From: Lu Abel
    Date: 2008 Apr 27, 10:27 -0700

    A bit off topic, but ...
    
    A friend sent me an article (perhaps more accurately described as a
    publicity release) about eLoran that claims its signal can be used in
    electronic compasses.   The precise sentence in the article is:
    "Moreover, eLORAN can do things GNSS cannot, such as acting as a static
    compass."
    
    First of all, the statement about GPS is inaccurate, since one can buy
    electronic compasses that work even when they are static by comparing
    the phase differences between the receipt of GPS signals at two or three
    antennas separated by a foot or less.
    
    What makes me very curious, though, is how an eLoran-based electronic
    compass would work.
    
    "GPS compasses" work by observing the phase differences between signals
    received at two or three different receivers, these phase differences
    give the direction to the satellite and by knowing its location in space
    it's simple math from there to calculate the direction of true north.
    But the phase difference method works only because GPS signals have such
    a short wavelength that there are considerable phase differences between
    signals received at antennas even a short distance apart.
    
    I have not been able to find any information on how eLoran-based
    electronic compasses would work.   Loran signals are very
    long-wavelength signals (25,000 times the wavelength of GPS signals), so
    measuring their phase differences in any reasonably small electronic
    compass setup would seem impractical.
    
    Maybe measure the bearing to each of the transmitters?   But I have a
    hard time believing that could be done accurately enough to create an
    electronic compass with sub-one-degree accuracy.
    
    Can anyone point me to an explanation, or is the sentence I quoted above
    simply hyperbole from an eLoran supporter?
    
    Thanks
    
    Lu Abel
    
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