To those of you with GPS speedos

adouglasmhor

The_ICT_Engineer wrote: »

Actual I own Dutton Commander, Which I can just drive into the thames.

Nae luck, how did you get lumbered with that pig?

http://www.dailymail.co.uk/news/article-562564/Businessmans-amphibious-car-banded-floating-coffin-judge.html

Lum

adouglasmhor wrote: »

Does it really? I thought it measures 3d vector speed by taking bearings from 3 or more satellites simultaneously, also some fitted ones use dead reckoning from engine pulses or other fitted input from the vehicle, but I am willing to be proved wrong do you have any up to date links etc.?
http://www.satnavforensics.com/how-sat-nav-works.php

Dead reckoning based on engine data is subject to the same problems that normal speedos have. Also most fitted ones don't display the GPS speed because that would show a discrepancy between the GPS and the speedo which manufacturers are reluctant to do.

Hills are a problem because many cheaper devices simply do not bother to read or store the elevation data. Their primary function is navigation so they don't need this info. Of course it would be nice if they took it into account, and if the map data had it, because in some cases it may be a shorter distance to go around the mountain than up it and down the other side.

Curves are a problem because they tend to record your position at a set sample rate, such as 1 per second, and assume that you have travelled in a straight line to your new position. If you're going around a corner then you will have actually travelled a greater distance than what the satnav thinks you have travelled and thus it will underread.

The more expensive GPSes with higher sample rates and that take elevation into account do not have these issues, or at least they are reduced to the point that they are negligible, but most people just go into Halfords and buy the crap one that's on offer for 50 quid.

TrickyWicky

adouglasmhor wrote: »

Nae luck, how did you get lumbered with that pig?

http://www.dailymail.co.uk/news/article-562564/Businessmans-amphibious-car-banded-floating-coffin-judge.html

Ah, so we have one who is trying to show off and using a duff craft to do it. Not much to brag about really is it.

Out on our drive we got a car thats SORN and doesn't start. Woo, look at me!!!

Lum

Sounds like you'd have gotten a better vehicle if you subcontracted out to Jeremy Clarkson instead.

benjus

Lum wrote: »

Dead reckoning based on engine data is subject to the same problems that normal speedos have. Also most fitted ones don't display the GPS speed because that would show a discrepancy between the GPS and the speedo which manufacturers are reluctant to do.

Hills are a problem because many cheaper devices simply do not bother to read or store the elevation data. Their primary function is navigation so they don't need this info. Of course it would be nice if they took it into account, and if the map data had it, because in some cases it may be a shorter distance to go around the mountain than up it and down the other side.

Curves are a problem because they tend to record your position at a set sample rate, such as 1 per second, and assume that you have travelled in a straight line to your new position. If you're going around a corner then you will have actually travelled a greater distance than what the satnav thinks you have travelled and thus it will underread.

The more expensive GPSes with higher sample rates and that take elevation into account do not have these issues, or at least they are reduced to the point that they are negligible, but most people just go into Halfords and buy the crap one that's on offer for 50 quid.

Do you have technical knowledge of how these things work? Because if they do use doppler shift, as the link I posted earlier suggests, the points you mentioned are irrelevant.

Of course, it could well be the case that some makes/models of GPS use change in position/time while others use doppler shift.

rodenal

My speedo reads 5% over

The car itself (using readings from the obdII as previous posters) logs speed to within 1% of a GPS nav

Wig

The_ICT_Engineer wrote: »

Actual I own Dutton Commander, Which I can just drive into the thames.

Not at 70mph you can't, well you could, but there would be a big splash.

But I for one enjoyed your replies.