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    Re: Lat/Long Calculations in Loran
    From: Lu Abel
    Date: 2001 Jul 22, 4:14 PM

    Herbert:
    
    Thanks for your reply.  In my original post I should have noted that I had
    already researched the Loran-C handbook, the USCG NavCen web site, and done
    a Google search.  All came up empty with any specifics on how L/Lo
    calculations are actually done in Loran receivers.
    
    As you point out, a number of sources including the Loran-C Handbook
    provide the equations for the hyperbolic LOPs.  But the ones in the Loran-C
    Handbook at least are flawed:  (1) they assume the Master and Secondary
    stations are on the same parallel of Latitude, and (2) the work all their
    mathematics in miles of distance rather than L/Lo (which where we want to
    end up).  In other words, they are the standard equations found in a
    standard high school algebra text, not equations suitable for real-world
    use.  And there is absolutely no information on how one might intersect a
    pair to get a fix.
    
    And, again, using the hyperbolic LOP equations might or might not be the
    way it was done in real Loran units.
    
    Lu Abel
    
    
    At 09:02 PM 7/22/2001 +0000, you wrote:
    >It's a combination of both. You have to start with TDs to get an approximate
    >position. Because the TDs give you hyperbolic surfaces, and you know your
    >altitude, you get a system of  3 quadratic equations in 3 dimensions.
    >
    >Then you have to correct for Primary Phase Factor, Secondary Phase Factor and
    >Additional Secondary Phase Factor. The latter is for varying propagation speed
    >of the signal over different types of surfaces. Since the route of the signal
    >and hence the decomposition into the various segments of the signal path are
    >only known after the approximate position of the receiver is known,
    >iteration is
    >unavoidable.
    >
    >I cannot speak for Micrologic or Furuno, but anything less then the above
    >would
    >not have been state of the art.
    >
    >A technical description of Loran-C can be found in "Loran-C User Handbook",
    >1992, COMDTPUB P16562.6 available from the US GPO.
    >
    >Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
    >
    >
    >Lu Abel wrote:
    >
    > > Does anyone have specific knowledge of how Loran receivers calculate L/Lo?
    > >
    > > It seems to me there are two possibilities:
    > >
    > > 1.  Pseudo-ranging as is done in GPS receivers.  This involves guessing at
    > > a position (L, Lo, and time) and seeing if it's consistent with the
    > > received signals.  If not, the position is adjusted and rechecked.  This is
    > > repeated until the AP is consistent with all received signals and therefore
    > > can be called a fix.
    > >
    > > 2.  Starting with the TDs as received and grinding them through some set of
    > > equations which produce L/Lo directly from the TDs.
    > >
    > > I'd appreciate knowing how folks like Micrologic and Furuno actually
    > did it.
    > >
    > > Thanks.
    > >
    > > Lu
    

       
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