Re: Questions about EGNOS.

From: David Lee (davidlee_malvern_at_dont.use.this.bit.hotmail.com))
Date: 12/03/04 

Date: 3 Dec 2004 18:18:05 GMT

Nick Hopton wrote...
> David Lee wrote.
> >I can't remember the
> >WAAS/EGNOS time-out period on a Garmin but I'm sure that it's way over
> >conservative and no more than a couple of minutes at most, so even when
> >it's fully operational EGNOS/Garmin for UK terrestrial navigation will
> >really just be a waste of money unless you can stick to south facing
> >slopes! :-(
 
> I think that might be pitching it a bit strong, David. I don't know if
> this will work with your Garmin, but after I do a factory reset on my
> Geko 301 and then turn on WAAS it often shows very strong signals from
> #33, #35 and #39. It even tries to use the data from these satellites
> for correction purposes and the 'D's start to appear. But as you say,
> this goes away after a few minutes. But I think it's more than possible
> that one day we will get usable signals from EGNOS, even if we're not on
> south-facing slopes. My location incidentally is Reading, in the south
> of England.
>
> Just now it's not the accuracy of the GPS which is bothering me, it's
> the accuracy of the OS-maps. I've just plotted the track of a walk that
> I did earlier today and it says that I walked about ten miles down the
> middle of the Kennet and Avon canal <g>. This isn't the first time,
> either.

Different firmware in the Gecko - I'm pretty certain that the etrex range
is set to ignore WAAS/EGNOS signals broadcast with the test bits set.

Regarding your wet feet (or rather lack of wetness) - it's not the accuracy
of the map that's at fault. Try measuring the width of the canal as
represented on your map - it will be just a bit more than it is in reality!
 Problem is that roads, canals etc are represented by "conventional signs"
and the width is fixed by convention not ground-truth. I don't know how
wide the canal is where you are but it must be less than 40 feet, which
scales to less than 1/100 of an inch on a 1:50,000 OS map. Hence the
tow-path will run right down the middle of the canal as represented on the
map, which shows that your Gecko is recording a pretty accurate track.

David

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