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    Re: Role of CN at sea, was Re: Averaging sights ...
    From: Derrick Young
    Date: 2004 Oct 13, 17:07 -0400

    Regardless of what method is used to determine a vessel's position, it is
    subject to failure.  It can be GPS (a lot more frequent than most folks
    realize), LORAN-C, SATNAV (boy that is showing my age), RDF (even older!),
    even CN.
    
    It does not matter what instrument is used to determine your position.  You
    should be prepared with multiple instruments and methods.
    
    The reason for this is simple to understand.  Think in terms of plotting
    LOPs and COPs.  Do you shoot one, two, three or more?  if you shoot more
    than two, why?  Part of the answer relies on the "quality of the fix".  The
    more LOP/COPs that you take, the easier it is to determine that your have
    bad data.  Additionally, when you take the LOPs and COPs, you should be
    noting what instrument is being used (steering compass, hand held bearing
    compass, fixed bearing compass, sextant, etc.)  What do you do when your
    position from your LOP/COPs don't intersect at close to the same location -
    you evaluate the LOP data based on the instrument and visual inspection of
    the data.  This allows you to make a value judgment to determine that you
    transposed some numbers (or in my case, flat missed a digit or two).  If you
    compare this with the position shown by your GPS, you can evaluate both your
    navigation and plotting skills as well as determining if your GPS is working
    like it should.  If the crew of the Royal Majesty had used multiple methods
    to determine position, would they have run aground?
    
    In going through the boats at my marina and talking with the various rec
    skippers, there is the over reliance on GPS.  Most have no other method of
    determining their position.  They do not seem to understand the information
    their GPS is providing.  But they are able to read the display and it says
    course (actually immediate past heading), speed (over ground) and position
    (including elevation).  They don't understand why they should be concerned
    with the quality of the GPS fix.  All that they know is that they must be
    where the device indicates - and that always causes issues when they run
    aground in a mud flat that is clearly marked on the chart but not where
    their GPS says they are.
    
    Having multiple GPS units does not improve the accuracy of the fix
    determined by using them any more than having multiple LORAN-C or sextants.
    I am not saying that they should or should not have multiple
    GPS/LORAN-C/SATNAV/RDF or even sextants.  What I am saying is that we need
    to understand the various types and quality of failure modes for each given
    class of instrument.
    
    Once you understand each of the different failure modes or dependencies,
    then based on where you are boating (and don't forget the costs), you make
    your choices.
    
    On my boat, I have 2 GPS units (different battery sources), LORAN-C, a RDF
    receiver, a sextant and a couple of hand bearing compasses.  I have three
    computers onboard with various nav programs and e-charts, but I have also
    constructed a chart table, with a piece of lexan over the surface.  Since
    the majority of my boating involves more piloting (going to/from
    known/charted objects), I use the sextant, LORAN-C and GPS.  I use the RDF
    only rarely - not enough to be well practiced with it.  I have gotten to
    where I can do COPs based on horizontal and vertical angles quickly.  These
    get transferred to my charts and log book.  Then I back them up with GPS
    and/or LORAN-C TDs - depending on where I am at the time - all are notated
    as to
    
    Does that make good sense?  To me it does - your mileage will vary.
    
    With this, I think that I am going to sit back and read the rest of the
    thread.
    
    derrick
    
    
    

       
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