From: ronkean@juno.com
Date: Tue Dec 21 1999 - 00:24:04 MST
On Mon, 20 Dec 1999 19:56:54 -0800 "Michael M. Butler"
<butler@comp-lib.org> writes:
>
> > In an automobile, even with the
> >receiver and antenna sitting on the dashboard, it is unusable.
>
> "My mileage varies." I have screwed-in a dash mount and gotten
> readings all over the place without an external antenna.
>
An important determinant of GPS performance is satellite geometry. In
addition to the importance of there being a sufficient number of
satellites with sufficiently strong signals reaching the receiver,
accuracy is a strong function of the diversity of the directions in which
the satellites lie, relative to the receiver. If all the satellites
being received are low in the sky (near the horizon), with none overhead,
readings of altitude will be of relatively poor accuracy. If the
receiver is not getting signals from any satellites below, say, 70
degress elevation, then altitude may be accurate, but the horizontal
position (latitude/longitude) will be relatively uncertain.
Some receivers have a 'DOP' or 'GDOP' readout, which means 'dilution of
precision' or 'geometric dilution of precision'. It's a number ranging
from 1.0 for best diversity, up to 10 or more if the geometry is lousy.
The receiver multiplies the DOP number times the expected error based on
signal strengths to obtain the 'expected error' readout.
If a GPS receiver is sitting on a car's dashboard it may receive signals
from several satellites thru the windshield, having a radio view of about
half the sky. But the other half of the sky would be blocked by the
car's steel roof, which would likely result in somewhat impaired accuracy
due to poor geometrics.
Ron Kean
.
.
.
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