From: dudley@birch12.cray.com (Dudley Knappe)
Subject: Re: GPS fix quality (was: Celestial vs. GPS )
Reply-To: dudley@birch.cray.com
In-reply-to: roy@mchip00.med.nyu.edu's message of 30 Dec 1992 21:54:01 GMT
Organization: Cray Research, Inc.
Date: 30 Dec 92 17:48:41 CST


	[stuff about Magellan 5000 signal analysis mode deleted]

|> roy@mchip00.med.nyu.edu (Roy Smith) writes:
|> 
|> 	While this is certainly good, it doesn't solve the problem I
|> originally complained about, which is that any dummy can (and many do) buy a
|> Loran, read enough of the manual to figure out how to plug it in and turn it
|> on, look at all those digits on the display, and think that he really does
|> know where he is to within a 1/10th of a mile, automatically, all the time,
|> without having to think.  

	I resemble that remark.  I originally trusted the GPS implicity until
it told us we were making @147 kts and were 100 miles away from where dead 
reconning told us we were.  It is a good thing that we were in the middle of
the gulf at the time, we should have been driving the boat down Bourbon St. for
all the GPS knew.

|> If you have to go into some special diagnostic
|> mode to get the accuracy information, most people are never going to bother,
|> or even realize that it's there, or if they do, know what to do with the
|> information 
	
	Just for the record, the main lcd display has two icons that light up
(flash?, I don't remember now) when the unit thinks there may be a problem
with the signals or the fix.  Then you could go to the satellite display and
look at the quality of all the signals.  The gripe I have is that the icons
would turn on and off based on signal quality but seemed to oscillate if the
quality was marginal.  If you glanced at the GPS during an off oscillation,
you might think you had a good fix, when in fact you did not.  You really had
to look at it over a period of a minute or so to see if the signal icon would
come on.

|>        ...              Maybe by having the box automatically reduce the
|> number of digits displayed    ...

	Stu's LORAN would display position digits that it was unsure of as
flashing numbers.  You could always tell how confident the unit was of it's
fix.  (That is not to say that even if the LORAN was confident, that it
couldn't be wrong.)

|> What would be really cool (but I don't know if LCD displays are quite up to
|> this yet) would be to show your position graphically superimposed on a
|> chart, not as a dot, but as an ellipse of the proper size and shape, or even
|> better, a series of ellipses for 50%, 75%, 90%, 99%, etc confidence levels.
|> Or maybe, on a grey-level display, darker in the center, ramping down in
|> intensity as you get further away.
|> 

	Great idea! where do I buy one? ;-)

	Your point is made.  In some cases the instrument displays the
position so confidently that you are taken in.  Or you become very dependent
on the technology because it hasn't failed you yet.  Or you are tired and
aren't as sharp.  Or ...  IMHO, electronic nav aids should be taken as
advisors, you still need to use your brain to determine if they might be lying
to you or not.

--
Dudley Knappe, Software Development Division            Cray Research, Inc.
Phone: (612) 683-5529                                   655F Lone Oak Drive
E-mail: dudley@cray.com or uunet!cray!dudley            Eagan, MN 55121
From: Dan.Hannah@f764.n153.z1.ship.wimsey.bc.ca (Dan Hannah)
Sender: UUCP@ship.wimsey.bc.ca