Date:         Fri, 12 May 2000 12:51:39 -0400
Reply-To:     Dave C <happycampers@PICUSNET.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Dave C <happycampers@PICUSNET.COM>
Subject:      Re: GPS
In-Reply-To:  <4.3.1.2.20000511221952.03a8a100@127.0.0.1>
Content-type: text/plain; charset="US-ASCII"

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
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