From: Newton (newton_at_spymac.com)
Date: Sun May 22 2005 - 10:24:45 PDT
> Still trying to decide what GPS receiver to buy, was reading=20
> about WAAS/EGNOS precision enhancements and it seems to me=20
> that the ideal receiver should support this standard (which=20
> will give ~3m accuracy!)=20
3m - This is - IMHO - ridiculous. The realistic resolution which can be
reached with the current state-of-the-art receivers is 1m for LON/LAT =
and
about 3m for height (this is the state of about 5 years ago but it =
shouldn't
have changed much). This limitation is given due to the wavelength of =
the
signal and the type of processing. The next better resolution =
improvement
can be reached by RTK-GPS which are at least ten times more expensive. =
These
Real Time Kinematic GPS receivers measure not only by counting multiples =
of
a wavelength, but are able to determine the time difference between two
different waves, respectively between the waves of all visible =
satellites.=20
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 :-)=20
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.=20
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...
Marco
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