From: hellsop_at_ninehells.com
Date: Sun May 22 2005 - 10:43:32 PDT
On Sun, May 22, 2005 at 07:24:45PM +0200, Newton wrote:
> I don't know what WAAS/EGNOS means but there was a very popular setup to
> enhance GPS reception from 10m to 1m just before, I think, the war in
> Yugoslavia. Just at the beginning of this war the US DOD disabled the
> artifical GPS inaccuracy of about +/-10m and automagically all GPS of the
> world had 1m accuracy :-)). This was to not give away the GPS decoding
> algorithm to all the allied forces but provide higher accuracy, of course to
> both - alliances AND enemy. Which makes still sense because the enemy
> normally knows its own terrain very well :-)
>
> During that time we used a GPS reference receiver close to the capitol of
> Switzerland in Berne, which broadcasted the correction data within the RDS
> information by all swiss governmental ultra short wave radio stations. The
> so called DGPS (differential GPS) had a small radio receiver with automatic
> tuning and RDS detection and we had to buy an annual license for about
> 500USD (!!!). During the war and since then the whole DGPS was not needed
> anymore but as I heared from pilots a similar functionality is provided by
> all airports too, but locally only - of course.
WAAS is very similar to the system outlined above: There are two
groundstations in North America at known positions that receive the
satellite signals, calculate their "positions" from that data to figure
an offset that accounts for atmospheric distortion and upload that data
to be added to the periodic ephemeris data that comes back down.
WAAS-enabled recievers use this to apply their own corrections to their
readings. Most of the time, this results in about a 2/3 decrease in the
diameter of precision. The 3m figure comes from applying that
decrease to the usually-quoted 10m DOP, but yes, often one sees much
better than that.
> Long mail - short content: Why does one need 3m accuracy? Even 10m should be
> fairly enough and 1m should be normal until the next war...
Depends on the application. For MOST things that the average citizen is
going to be using GPS for (personal navigation in cities, or to marked
waterways, or for geocaching), 10m is plenty accurate.
--
"Friendship is born at that moment when one person says to another, 'What!
You too? I thought I was the only one!'"
--C.S. Lewis
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