Re: GPS speed indications

On Fri, 04 Sep 2009 18:17:26 -0500, Larry Sheldon
<lfsheldon@xxxxxxxxx> wrote:

This is a pure question without preconceived notions as to what the
answers might be.

What I think I know is that the unit-in-hand locates satellites and does
timings to determine where it is in three dimensions.

What I don't know is how often they do that.

Seems like if the interval is very long it would not know about altitude
changes, ziggy-zaggity twisty roads and while it would know position
accurately it might not know exact distance traveled between fixes and
thence would not be able to calculate speed accurately.

The consumer road-targeted GPSes that I've used all seem to be on the
order of a second or so between updates (and it's not just predicting
from the current direction/map; it notices turns at intersections
pretty quickly). I doubt you'd be able to zig-zag a car fast enough
to add enough extra distance to fool it.

My older Nuvi's speed reporting is very accurate (it matched the speed
on my wife's radar ticket last year :-)

Elevation is the least accurate item these GPSes report (because of
the angle of the satellites), and even that's always been within
50-100 feet of the roadside "Elev 5000ft" type signs and a digital
barometric altimeter I had along. The only inaccuracies I normally
see are map related -- it thinks I'm on the highway instead of a
frontage road, etc.