GPS versus Court of Law

[Gon Fishun]

Been reading in some of the forums that a lot of people rely on their GPS for the actual speed they are travelling at and not their vehicle speedo. If you are booked for speeding and you argue it to the court room, is the Judge going to believe the police radar/ camera? or your GPS. ;D

I have a Navman and it registers between 6-4 kph less than the vehicle speedo, but when the vehicle is travelling at 100kph by its speedo I still get past like I,m sitting still, and yes I have travelled behind wifeys car and we get the same speedo readings within 2-3 kph . :P
IMO Gps signals have a ways to travell and a lot of pollution to get through and cannot be accurate. :)

[Matt76]

Gps is a lot more accurate than your speedo.

[Matt76]

I have a Navman and it registers between 6-4 kph less than the vehicle speedo, but when the vehicle is travelling at 100kph by its speedo I still get past like I,m sitting still,

That's because your speedo is out lol, most cars are.

[oldboot]

The problem with GPS is that is measuers speed from point to point between samples....on a curved road it can read well under your road speed..especially in situations where it is not getting a continuous or regular fix.

If ya realy want to get into this whole speeding legality thing...google "aussie speeding fines"

If ya going to chalenge a police measuing instrument, ya realy betta know what you are doing.

cheers

[Gon Fishun]

Been doing a bit of googling and everybody reckons the GPS is the way to go for accurate speed.
Was going to try it in the boat but, it might not go as fast as I was thinking it might.:-?

Oldboot -
Nothing to challenge, trying to keep nose clean.;D

[Matt76]

When I was racing motorbikes (very poorly lol) I used an electronic lap timer that reads off a transducer mounted on the pit wall to get my practice times, a mate of mine modified a mobile phone and used its GPS system to read his laps and give lap times and even section split times, it was quite amazing, I was a huge sceptic but it was amazingly accurate. Any lap times quicker than mine I quickly dismissed as the GPS being inaccurate though haha.

[NAGG]

That's because your speedo is out lol, most cars are.

A simple thing like tyre wear can change your speedo accuracy - most cars would be out
+ or - 2-5%
GPS accuracy can vary minute by minute ..... but is reasonably accurate depending on signal and how quickly the unit processes the information

[Apollo]

That's because your speedo is out lol, most cars are.

I tested a few vehicles I drive against my handheld GPS. Set the vehicle at 100kph on the speedometer on flat road and read the GPS.

Wife's EL falcon - 97kph
My Courier - 100kph
My work Navara - 92kph
My ex work VX Commodore - 103kph

Fair difference in accuracy

Steve

[SatNav]

1. Keep in mind "GPS speed" is simply not based on differences from point to point as the fundamental three (3) outputs from a GPS is Position, Velocity & Time (PVT) which are computed as part of the adjustment solution.

2. GPS accuracy (position) has little direct affect on velocity (speed) as position is not the primary data source that determines speed.

3. Typically GPS speed will have an error of less than 0.1km/h ignoring the absurd possibilities that could be raised.

[Scott nthQld]

The problem with GPS is that is measuers speed from point to point between samples....on a curved road it can read well under your road speed..especially in situations where it is not getting a continuous or regular fix.

If ya realy want to get into this whole speeding legality thing...google "aussie speeding fines"

If ya going to chalenge a police measuing instrument, ya realy betta know what you are doing.

cheers

That's why you don't challenge to coppers instruments, sure ask for a calibration certificate, but if you want to pursue it further, attack your car for incorrect speedo. Regardless of how fantastic a bit of techlnology is, it will never measure anything 100% correctly. When it comes to measuring anything, be it lenght,, weight, distance, speed etc, it will always be out by upto 1/2 the value of the next significant figure, ie, I take a ruler and measure someing and it measures 1 cm, by the nature of measurement, there will be an inaccuracy of upto 1/2cm either way, that is, that thing I just measured might only be 0.5cm, or upto 1.5cm. if i was to measure the same thing in millimetres, it might be 12mm, but again, it could be 11.5mm or upto 12.5mm.

So, going from that, most cars speedo's will measure speed (on the analogue dials) in 5km increments, 5, 10, 15 20km etc, so by the very nature of the scale, the true speed could be upto 2.5km different either way to what is displayed. And that iss assuming that the car's speedo is 100% accurate. That's just pretty basic maths and physics right there, from year 8 or something I think.

But then you have variances from the vehicle itself, whether tyre pressures are up or down compared to when the vehicle was calibrated, to even the overall diameter of your tyres and tread depth. You can even go as far back the the manufacture of the tyres and rims, because they are measured, again, the measurement will be out by upto 0.5 of the last signifacant figure (tolerances is engineering speak).

So you leave the police instuments alone if they can provide calibration certificates and attack your vehicle and basic principles of measurement for their is no way you can tell 100% whether or not you are speeding, given the equipment supplied on the vehicle, so how can you stop yourself speeding when you don't know if you are or not to begin with? Dunno how that would wash in court, but it makes sense to me, very logical, its just the law isn't all about logic and reasoning

[winston]

Just to add insult to injury, my BT 50 at indicated 100 Kph was travelling at 90 Kph when referenced to my GPS. This means that my 10000 km service was done 1000 Km prematurely and at 100000 Km indicated it would have actually travelled 10000 Km less i.e. one service. Cheers Winston.

[Matt76]

I tested a few vehicles I drive against my handheld GPS. Set the vehicle at 100kph on the speedometer on flat road and read the GPS.

Wife's EL falcon - 97kph
My Courier - 100kph
My work Navara - 92kph
My ex work VX Commodore - 103kph

Fair difference in accuracy

Steve

My Navara speedo is way out too. Not sure how it is acceptable to be that far out.

[testlab]

If you want to even think about challenging police speed readings then start here:
http://www.measurement.gov.au/Services/Training/Pages/Introductiontoestimatingmeasurementuncertainty.asp x
and then work up to getting your GPS calibrated and accepted as a source for legal metrology. Then give up.

I did the full NMI uncertainty of measurement course and five of the NSW Police instrument calibration lab guys were there doing it too.

In my opinion GPS is much more accurate than a car speedo if the stabilty of the GPS has settled. It can in some circumstances equal the accuracy of modern laser and radar but not all the time. A radar/laser operates over a short distance with minimal sources of error and it be easily calibrated and its uncertainty of measurement quantified. It is not subject to "outside influences" affecting its internal reference unlike GPS.

I have a $50000 GPS time reference in my lab. It takes up to 72 hours to achieve its stated accuracy of 5x10e-12 (5 millions of a millionth of a sec) and I cannot use it as a legal source of measurement !!!!!! These things are used throughout the telecomms, aviation and media industries to produce incredibly accurate sychronisation signals.

Its not legal to use it for measurement because the uncertainty of measurement (not the accuracy) cannot be quantified, in turn because the GNSS/GPS system uncertainty cannot be quantified.

So good luck using a consumer grade GPS to challenge a system that is a source of established legal metrology. Previously in NSW some successful challenges to police speed measurement were based on adminstrative errors (using uncalibrated equipment, then later without known measurement uncertainty). They learnt from their mistakes.

[marty666]

Also under ADR standards speedos cant read over 100% accurate so the manufactures put in a fudge factor normally of 3% so that you will never be doing more than what is on the speedo, which can simply be affected by your mobile phone in your pocket picking up wifi signals as you drive seeing as all modern cars use digital signals and normally over a can bus as well.

[FishHunter]

Just to add insult to injury, my BT 50 at indicated 100 Kph was travelling at 90 Kph when referenced to my GPS. This means that my 10000 km service was done 1000 Km prematurely and at 100000 Km indicated it would have actually travelled 10000 Km less i.e. one service. Cheers Winston.

The rangers (same car different badge) are just as inaccurate. A customer tried to have words with me about this as his odo was over reading and the effect on his resale value.

I pointed him in the direction of the ADR's as it was within specs and suggested he change tire size if it was such an issue in his life.

[TheRealAndy]

My navara is also out. On the highway, i go on GPS speed. Been trhough plenty of radar traps and speed cams and never recieved a ticket.

I also had the fortune of testing it out on those radar signs (you see them in roadwork zones in NSW a fair bit) a few times, and the GPS matches up with the signs