From: kverruyt@nl.oracle.com (Kees Verruyt)
Newsgroups: rec.boats
Subject: Re: Celestial vs. GPS (was Re: Bermuda Trip - 7/'93)
Date: 30 Dec 92 13:51:36 GMT
Organization: Oracle Europe

In article <29093@oasys.dt.navy.mil>, hanners@oasys.dt.navy.mil (Robert Hanners) writes:
|> In rec.boats, roy@mchip00.med.nyu.edu (Roy Smith) writes:
|> >bartley@spss.com (Dennis Bartley) writes:
|> >> You really don't have to be a rocket scientist to use Loran or GPS,
|> >> and with just a little thought you can tell the position it's giving
|> >> is in error.
|> >
|> >        I think part of the problem is that most people don't understand the
|> >difference between precision and accuracy, and assume that just because a
|> >Loran shows you your position to 1/10th (or even 1/100th) of a minute it
|> >must be correct.
|> 
|> I'm not too familiar with the features on Loran and GPS but I do have
|> an appreciation of the potential error propagation that these devices,
|> especially the former, possess.  Intrinsic to the position calculation
|> algoritm of the unit is the ability for the unit to calculate range from
|> the transmitters and, depending on the memory/cost, some degree of the
|> propagation perturbation potential (percent range over land, for example).

Of course, that's one of the reasons why I like GPS better - since there are
more satellites than the three needed for a 2d fix, some of the errors are
averaged away (hopefully).
 
|> Also there's the direct ability to calculate potential error due to
|> nearly parallel intercepts and of course the unit can get a direct read
|> on signal strength.
|> 
|> My question is this: Why isn't it reasonable for the unit to calculate and
|> display a measure of this potential error, as individual values (for
|> the discerning user :-) ) or as a total?  Or is this in fact a feature
|> of some makes?  The error itself could be a statistical measure such
|> as 50%, standard deviation, 2 x sd, or whatever.  Preferrably such
|> weighting would be user selectable.

You're quite right -- and so think some of the GPS manufacturers since I
remember that at least one brand has the ability to display such an error-value.

Don't remember which one though. I probably saw it at the Paris (France) Boat
Show, where I must have seen at least 20 different brands of GPS equipment. 

|> 
|> Now I do appreciate some of the difficulties in affixing real values to
|> some types of error and that some kinds of error cannot be reasonably
|> quantifiable at all.  (This would be an advantage of displaying individual
|> errors rather than simply the total.)  But it seems that manufacturers
|> might see that the advantage of such analyses for the knowledgable sailor
|> would outweigh the perception to the less knowledgable user that their
|> instrument is "error prone".
|> 
|> What do you think?

Well I won't buy a GPS if it doesn't show me at least some indication of how
large the error is.

|> 
|> -- Bob
|> 
|> ------------------------------------------------------------------------------
|> Robert J. Hanners         "My comments; only mine."          *DIASTOLE* 676673
|> CD, NSWC, Annapolis, MD                              hanners@oasys.dt.navy.mil

-- 
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Kees Verruijt                           Rijnzathe 6
Software Engineer Gateway development   3454 PV  DE MEERN
Oracle Europe                           The Netherlands
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   or: kverruyt@oracle.nl               Fax: +31 3406 65603