How accurate is the SV speedo?

[mikerj]

Originally Posted by squirrel_hunter

And you know thats accurate because?

Now I know that position for civilian applications is not as accurate as it could be (based on first hand system developer accounts), so why would the speed be?

Because speed is calculated differently to position, a combination of position vs time and doppler shift is used to derive speed.

A GPS system will be more accurate than all but the most carefully calibrated speedometers under the majority of conditions. Even consumer grade devices have a typical accuracy of 0.1mph, though under some conditions that can degrade a little, maybe +-0.5mph.

[timwilky]

Originally Posted by davepreston

depends om tyre size does it mmmmmmmmmmmmmm
my thou runs off the gears

Yeah, so what!

That just means the speedo is proportional to the final drive etc. So even worse accuracy than an 650 if you change your sprocket ratios. Tyre wear will still also be a minor factor.

[kaivalagi]

Originally Posted by squirrel_hunter

I'm just wondering you see if by your reasoning more than one has the same results, how do you know which one is accurate, the GPS or the speedo? And based on what?

Thought you might be interested in this from here:
"GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.
As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed."

And this for kicks: http://www.heraldsun.com.au/news/vic...-1111113864822

Originally Posted by mikerj

Because speed is calculated differently to position, a combination of position vs time and doppler shift is used to derive speed.

A GPS system will be more accurate than all but the most carefully calibrated speedometers under the majority of conditions. Even consumer grade devices have a typical accuracy of 0.1mph, though under some conditions that can degrade a little, maybe +-0.5mph.

Thanks, sometimes I know but forget why I do, not enough room up there sometimes for the details

[squirrel_hunter]

Originally Posted by mikerj

Because speed is calculated differently to position, a combination of position vs time and doppler shift is used to derive speed.

If position is inaccurate, but still required and used in the speed calculation, why then is speed accurate? Further Doppler Shift as I understand it is the same theory that is used in radar guns as issued to the police to calculate speed, however these devices have been proved to be inaccurate, due in part to target slippage, something that a moving device could suffer from...

Originally Posted by kaivalagi

Thought you might be interested in this from here:
"GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time.

As I said above the calculation is still subject to position which is a known inaccurate. Though the points on wheel size and drive ratios are good if the parts have been changed from the original.

Originally Posted by kaivalagi

And this for kicks: http://www.heraldsun.com.au/news/vic...-1111113864822

Yes but they say that the ticket should not have been revoked.

[mikerj]

Originally Posted by squirrel_hunter

If position is inaccurate, but still required and used in the speed calculation, why then is speed accurate?

Doppler is the primary mechanism for speed calculation, I honestly can't remember why or when a contribution of trackpoints is added. A doppler based system can have exceptional accuracy

Originally Posted by squirrel_hunter

Further Doppler Shift as I understand it is the same theory that is used in radar guns as issued to the police to calculate speed, however these devices have been proved to be inaccurate, due in part to target slippage, something that a moving device could suffer from...

Are you maybe thinking about laser speed guns? They can suffer quite badly from slip error due to the relatively narrow divergence of the beam and the range over which they can be effective. Radar guns have less of a problem as the beam diverges much more, but it can still happen. GPS simply isn't affected by this since a reflected signal isn't used at all, the GPS receiver directly measures the relative shifts in the satellite carrier frequencies.

[kaivalagi]

Originally Posted by squirrel_hunter

If position is inaccurate, but still required and used in the speed calculation, why then is speed accurate?

I'm happy with this principle of the speed being much more accurate regardless of positional error, but I went looking for proof. Here's more on it: http://eprints.ucl.ac.uk/146583/

From the text:
"The speed determined by the GPS receiver was within 0.2 ms(-1) of the true speed measured for 45% of the values with a further 19% lying within 0.4 ms(-1) (n = 5060). The accuracy of speed determination was preserved even when the positional data were degraded due to poor satellite number or geometry. GPS data loggers are therefore accurate for the determination of speed over-ground in biomechanical and energetic studies performed on relatively straight courses. Errors increase on circular paths, especially those with small radii of curvature, due to a tendency to underestimate speed."

0.5 metres per second being an absolute worst case scenario is a smidgen over 1 mph, so worst case when speed is measured on an oval track like in the test (not the best accuracy to begin with as mentioned) we are looking at 0.5 mph either way...

If you still don't buy into the concept I am not sure what else I can say to convince you....all I can say is read about some of the science behind it

edit: what he ^^^ says also

A few quotes from websites saying they are accurate, a university study on it suggesting the same, quite a few people whose experience suggests their various GPS devices all agree, both in this thread and in others on the subject.

SH, if you think they are inaccurate at measuring a constant speed in a straight line which is what we are talking about here (ie: 70mph = 77), to more than 1 mph, then instead of asking why we think the sat nav IS accurate, then questioning the replies, why not produce some evidence as to why you think they are NOT?

Not very according to my Sat Nav.

[squirrel_hunter]

Originally Posted by mikerj

Doppler is the primary mechanism for speed calculation, I honestly can't remember why or when a contribution of trackpoints is added. A doppler based system can have exceptional accuracy

Thats a fantastic paper, thanks.

Originally Posted by mikerj

Are you maybe thinking about laser speed guns? They can suffer quite badly from slip error due to the relatively narrow divergence of the beam and the range over which they can be effective. Radar guns have less of a problem as the beam diverges much more, but it can still happen. GPS simply isn't affected by this since a reflected signal isn't used at all, the GPS receiver directly measures the relative shifts in the satellite carrier frequencies.

Yep was thinking more about laser than radar. But its interesting to know that GPS isn't effected in the same way.

Originally Posted by kaivalagi

Here's more on it: http://eprints.ucl.ac.uk/146583/

Cheers for that artical as well.

[squirrel_hunter]

Originally Posted by -Ralph-

A few quotes from websites saying they are accurate, a university study on it suggesting the same, quite a few people whose experience suggests their various GPS devices all agree, both in this thread and in others on the subject.

SH, if you think they are inaccurate at measuring a constant speed in a straight line which is what we are talking about here (ie: 70mph = 77), to more than 1 mph, then instead of asking why we think the sat nav IS accurate, then questioning the replies, why not produce some evidence as to why you think they are NOT?

I've never said that GPS is inaccurate, nor do I think its inaccurate. I have simply been wondering how it is so accurate compared to a speedo. I have previously read that GPS is more accurate than a speedo, but no one has provided the answer as to why. So when it popped up again I asked and kaivalagi and mikerj have been kind enough to provide me with an answer, thanks guys.