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[JamesV6CDX]

Do they over read? If so, how much by?

[Marks DTM Calib]

They appear to over read by about 3-4mph across the full speed range

[Entwood]

All non-calibrated speedos over-read, as they are not allowed, by law, to under read ... so set a tad high to cover inaccuracies.

IMHO my 3.2 speedo over reads by about 4-6 mph compared to sat-nav speeds under test conditions.

Totally subjective opinion .. but I believe it to be pretty reasonable info

[Marks DTM Calib]

All non-calibrated speedos over-read, as they are not allowed, by law, to under read ... so set a tad high to cover inaccuracies.

IMHO my 3.2 speedo over reads by about 4-6 mph compared to sat-nav speeds under test conditions.

Totally subjective opinion .. but I believe it to be pretty reasonable info

Be aware that GPS speeds often under read due to the way they calculate position based on time signals

[Bigron]

I always understood that they were deliberately set to be about 10% fast, for reasons of flattery and to avoid the greed cameras!

Ron.

[Bigron]

Marks, GPS will always under-read on any gradient, the steeper the gradient the greater the error due to the fact that they can only calculate "linear" speeds, i.e. they cannot take into account the distance travelled vertically, either up or down so it seems that you have not travelled as far in a given amount of time, hence seemingly slower.

Ron.

[Marks DTM Calib]

Marks, GPS will always under-read on any gradient, the steeper the gradient the greater the error due to the fact that they can only calculate "linear" speeds, i.e. they cannot take into account the distance travelled vertically, either up or down so it seems that you have not travelled as far in a given amount of time, hence seemingly slower.

Ron.

Its more than that in reality, the vertical and horizontal position information has the same level of error, this can only be improved with differential setups but there mega bucks and require annual lcensing to receive the correction data.

Maybe one day the US will release the extra clock byte to improve the accuracy to that the military enjoy....but until then....

[Bigron]

Ok, thanks for the additional information, Marks; I had been unaware of the horizontal inaccuracies, always thought they were kosher!
Just think, with the full-spec GPS they could have you to within 4 inches, yes?

Ron.

[Marks DTM Calib]

Ok, thanks for the additional information, Marks; I had been unaware of the horizontal inaccuracies, always thought they were kosher!
Just think, with the full-spec GPS they could have you to within 4 inches, yes?

Ron.

100mm or better yes.

Standard GPS can do to about 2-4m (it can be better if static for long periods or is able to track other static objects).

The stuff we use is DGPS so gets correction signals from satellites or shore based stations (at a cost) which supports accuracy to 1m.

If we are designing for ships which require accurate positioning e.g. semi-subs or PSV's and the like, we have to use Syscan equipment and targets because the GPS just wont cut it as we need accuracy to about 100mm

[Bigron]

Wow!
Thanks.

Ron.