FW: [at-l] altimeter

[jbkramer@afn.org (Bryan Kramer)]

The random noise (Selective Availability) that was added to degrade the
signal has been turned off for more than a year now, May of 2000
actually. You should have noticed that your location is now shown with a
10 or 15 ft uncertainty as opposed to 30 to 50 feet before they turned
off SA.

The geometry of altitude measurement makes it inherently less accurate,
the manufacturers claim that the error for altitude measurements is 2X
or more times the horizontal error. In the flatlands near MSL you can
often find the gps claiming it is at -100 feet or more. Another problem
with altitude measurements is that the gps is using an idealized
geodesic model of the earth, the more the real shape of the earth varies
from that model the worse the measurements are. Also the altitude
measurements tend to jump around a lot, this is part of the difficulty
in measuring it. The GPS manufacturers sell models with built in
barometric altimeters and that should tell you how accurate they think
those measurements are.

Bryan



> -----Original Message-----
> 
> it's not the mathematics.   I've written software for those 
> sats.   if you
> get 4 birds, it's unbelievably accurate.   What you are 
> seeing is purposeful
> inaccuracies put in for national defense.   It would really suck if a
> foreign country could use our satellites to home in on us.
> 
> t.
> 
> ----- Original Message -----
> From: "Bryan Kramer" <jbkramer@afn.org>
> To: "'t.'" <tjfort@netdoor.com>
> Sent: Friday, February 01, 2002 5:13 PM
> Subject: RE: [at-l] altimeter
> 
> 
> I think the mathematics of GPS makes the altitude readings
> highly suspect. We often find ourselves several hundred feet 
> below sea level here in Florida, according to our GPS anyway.
> 
> Bryan
> 
> >
> > Have you considered GPS?
> >
> >
> > t.
> 
>