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From: Young, Derrick (no email)
Date: Wed Oct 13 2004 - 17:07:11 EDT
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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