Date: Fri, 12 May 2000 13:45:05 -0400
Reply-To: Bill Knight <Knight.Bill@ESC.STATE.NC.US>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Bill Knight <Knight.Bill@ESC.STATE.NC.US>
Subject: Re: GPS
In-Reply-To: <B541AEB1.1399%happycampers@picusnet.com>
Content-Type: text/plain; charset="iso-8859-1"
I believe SA has been discontinued as of May 1st. We
should be
able to get an accurate fix now. You can check the
www.garmin.com
site for more information about the withdrawl of SA.
Bill
-----Original Message-----
From: Vanagon Mailing List
[mailto:vanagon@gerry.vanagon.com]On Behalf
Of Dave C
Sent: Friday, May 12, 2000 12:52 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: GPS
on 5/11/2000 10:28 PM, David Beierl at dbeierl@IBM.NET
wrote:
> At 18:31 5/11/2000, Andrew Grebneff wrote:
>> I'd find one useful both in the van (speedo)
> Andrew, I don't think you'd find it a satisfactory
speedometer for a car --
> it doesn't measure speed directly at all, just takes
successive positions
> and integrates them over a period of time to produce
a moving-average
> speed.
GPS is "acceptably accurate" above 11 mph. Below 11mph
its not reliable due
to SA. The moving average David refers to is usually
an option that can
usually be toggled on or off. The displayed speed is a
calculation between
the last automatic position fix and current position
measured every few
milliseconds. Call the manufacturer and speak with
someone in tech support
to get a firm grasp on what "acceptable" is to that
manufacturer.
I wouldn't take out my speedo though, just supplement
it.
> Every time you turn your vector changes, and the
displayed speed
> drops because you're now averaging vectors in
multiple directions and so
> forth.
I think David's referring to the (VMG) Vector Made
Good function which is
different than the displayed speed over ground (SOG).
It calculates speed at
which you approach a destination regardless of whether
one is on course. If
one is one direct course then VMG and SOG will be the
same.
> Worst case, I could easily imagine it displaying a
speed of zero
> while you went in tight circles at 30 mph. All
perfectly legitimate from a
> navigation standpoint, but...
Some one indicated that there could be 100's of meters
of error. Not so, In
the US, Selective Ability (SA) employed by the owners
fo the system (DoD)
for civilian receivers is between 15m and 100 meters
(which is hundreds of
feet) horizontally and between 100m-156m vertical.
100m supposedly happens no more than 1:20 position
fixes and then there is a
much larger error of 300m no more than 1:1000
position fixes.
BUT,newer GPS units are adept at reducing this further
through an averaging
algorithim. The GPS knows how much error is induced
and over time averages
these to reduce the amount of error by about half. If
you want even more
accuracy then opt for DGPS Differential GPS which
needs radio signal to
correct fro the error of SA to around 15 meters.
In practice, civilian GPS users should enjoy accuracy
of 50m or less for
more than 50% of the time.
Dave C.
Williamsburg, VA
zzzzzzzzz__
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