Re: Off Road storing of untaxed vehicles

Derek Moody wrote: 
In article <do22rv$ee6$1@xxxxxxxxxxxxxxxxxx>, Steve Firth
<URL:mailto:%steve%@xxxxxxxxxxxx> wrote:

Derek Moody wrote:

Have a care: The gps will calculate your speed from your displacement
against time and display it as an 'average' over an unspecified time. Any
bend/wiggle in the road within that time span will reduce the displacement -
resulting in a 'speed' figure that is lower than the true value.

Umm no, wrong.

You may care to look up "pseudo range rate" before continuing. No current GPS set determines velocity in the way that you describe. 

Umm no, wrong.    ;-)

I admit to oversimplifying and illustrating it with an extreme example
in order to stress the point.

The pseudo range rate is an averaging system, smoothed over time.  Even a
static gps receiver without smoothing and clock correction would appear to
jitter around - especially in a built up or sky obstructed area where
sallites might additionally go in and out of sight or into reflections (the
sats move even if you don't) - hence the smoothing and clock correction.

What I didn't say was that the displacement origin is a moving, multiple one
- along the smoothed previous position line - and that the smoothing
function is weighted to favour recent data.  Even so iIrc the last few
percent of the smoothing may be from more than a minute back.

AIui surveying gps and portable dgps transmitters only approach minimum
error after several minutes - up to a quarter of an hour of averaging being
needed at times but these devices also take account of a lot of extra
factors to get positions within a few centimetres.

No, you're still confusing position fix with velocity determination, as your comment on dGPS and surveying shows. Although what you say is true, it has no relevance to the issue of determining velocity, you're simply describing the difficulty in obtaining an accurate position fix.

The velocity determination from pseudo range rate is "good enough" as in Good enough to given an RMS error of 0.1kt. That means that whereas PRR derived velocity may well be subject to an error (what isn't) in real world use that error will not exceed 0.5-1mph which is more than good enough for determining the velocity of a vehicle.

You may care to quibble about how many angels dance on the head of a pin but it won't detract from the fact that your initial assumption that GPS velocity is derived from serial position fixes was wrong and that you subsequent explanation abovbe tended towards the "balls" end of the spectrum.

HTH.
.

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