How Accurate Are GPS Speed Readings?

[BBB61]

I have a Streetpilot III and I have found it interesting to note the speed readings given by the GPS and compared them to a variety of vehicles I have driven. The most recent vehicle to undergo a comparison was my 2004 Triumph Sprint ST. According to the GPS readings, the speedo on my Triumph is over reading in the region of 15-20% at speeds between 30 and 70 mph. The odometer is only over reading by 1%.

I have taken this accuracy (or lack of it) up with Triumph. The readings I am getting are outside what is allowed by the current EU regulations (maximum of 10% over read + 2.5mph, no under read). Triumphs' latest response was to say "we can only assume that you are making comparisons against GPS, if this is the case then we would advise that this method should not be used due to speed inaccuracies".

I have been careful to note the speed readings on level roads without gradients as I believe a gradient can affect the GPS reading. Would anyone be in a position to comment on the accuracy of the Garmin speed readings?

Bob

 

[MikeP]

Bob,

I was concerned enough with the variation between mine and various vehicles I've used it in that I used a calibrated stopwatch (still some around) and checked it on a certified measured mile.

Several runs and the satnav (BM Nav II) is under 1% variation.

All the vehicles I've used it in/on, the speedo's over-read by roughly the same, around 10 to 11% (handy to know with the number of camera's about these days!).

 

[martinc.d]

Hi Bob,
Posted a thread before on this subject and on the 1150 GS speedo i am roughly 5 mph across the hole range different ie, gps 20mph---bike 25mph.
were as my car is 2-3mph, HGV however is around 1mph out or can be spot on bearing in mind they are calibrated.

MD

 

[boundless]

GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.

From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

 

[GuidoT]

Hi Bob,
Posted a thread before on this subject and on the 1150 GS speedo i am roughly 5 mph across the hole range different ie, gps 20mph---bike 25mph.
were as my car is 2-3mph, HGV however is around 1mph out or can be spot on bearing in mind they are calibrated.

MD

I agree about the GS speedo. On my Guzzi, the speedo, cycle computer and GPS all agree (above 20) when the speedo needle settles down. The GS reads 5 mph faster from 20mph to 80mph.

 

[BBB61]

Bob,

I was concerned enough with the variation between mine and various vehicles I've used it in that I used a calibrated stopwatch (still some around) and checked it on a certified measured mile.

Mike,

Are the marker posts on motorways accurate enough to measure speedo accuracy against? And you mentioned measure miles. Do you know if there is a list of the locations of these published anywhere?

Bob

 

[Mouse]

Found this at http://www.cheshire.gov.uk/roads/FAQs.htm

Look for the little circular signs on certain stretches of motorway. The first one is yellow, the second is a quarter red, the third is half-red, the fourth is three-quarters red and the last one is totally red. They mark out a measured mile.

 

[BBB61]

Cheers for that Mouse

Bob