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PostPosted: Thu May 27, 2010 17:30 
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Gatso speed cameras inaccurate in cold weather?

Motorist challenges speeding fine after discovering expert evidence on camera reading reliability
Posted: 27 May 2010
by Visordown News

Gatso: Not suitable for sub-zero temperatures?

A MOTORIST is challenging a speeding fine after discovering Gatso speed camera units may provide inaccurate readings in cold weather.

Andrew Fowler, 48, is to take his case to court after the dad-of-three was snapped allegedly travelling at 41mph in a 30mph limit, on a road he uses every day on his way to work.

The electrical engineer claims to have expert evidence showing Gatsos can give false readings in sub-zero temperatures.

Mr Fowler believes that vital timing circuitry inside the unit is affected by cold weather, making speed readings inaccurate.

If successful, Mr Fowler's case could throw 1000s of speeding fines already handed out into question.

"It's me versus the system, there's no two ways about it, and it is pretty daunting to be honest," said Mr Fowler.

"I'm nowhere near being a rich man but I don't mind how much this costs me because it is for a good cause, this is for all motorists.

"If I'm successful everyone will be at their crown court with speeding cases. Everyone will be able to appeal."

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PostPosted: Thu May 27, 2010 20:27 
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Presumably his defence stretches to suggesting that the white marks in the road move closer together the colder it gets...


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PostPosted: Thu May 27, 2010 20:50 
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Here's the shot:

Image

I'm sure somebody could work out his speed.


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PostPosted: Thu May 27, 2010 20:54 
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Photos here: http://www.dailymail.co.uk/news/article ... -cold.html

Tech stuff quoted from here: http://www.telegraph.co.uk/motoring/new ... meras.html

article wrote:
and at lower temperatures vital timing circuitry is slowed inside the unit.

Speaking as a seasoned electronic design engineer: I find it inconceivable that anyone could design a frequency reference that significantly slows at -5C (significant means more than 1%) into such a critical piece of equipment (evidence capture). For example, crystal oscillators usually have a drift of up to 100ppm over the operating temperature range, which is 0.01% (or 1% of 1%). That's certainly nowhere near the claimed 37% error.

Nothing has been said about any error from the radar sensor.
This does beg the question of whether the counter for the radar pulses is also driving/being driven from the timer for the photo delays; I suspect not. Again I would find it inconceivable that anyone would do such a silly short cut.

article wrote:
He added: "If it's so cold the capacitor on the circuit can't hold its charge or is damaged the flash will not go off in time.

So why would it go off later? Does he know how a flash trigger works? Would the displayed time delay not be wrong too?
What about the radar sensor; nothing would have been triggered unless that was in error too.

I don't buy it, and he's obviously clutching at straws! If anything, I think it much more likely his speedo was affected by the cold weather.

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PostPosted: Thu May 27, 2010 20:57 
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weepej wrote:
I'm sure somebody could work out his speed.

I got 42mph from the photos (4.75 markers, 2m apart, over the assumed 500ms).

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PostPosted: Thu May 27, 2010 21:10 
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Steve wrote:
I don't buy it


I agree - I don't buy the timing bit either.
However, i could accept that the camera might have been falsely triggered by the oncoming traffic on the other side - I've seen that happen many times.

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PostPosted: Thu May 27, 2010 21:17 
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Pete317 wrote:
However, i could accept that the camera might have been falsely triggered by the oncoming traffic on the other side - I've seen that happen many times.

How do you know it was falsely triggered? Could they have been some sort of self-tests?

Even if due to the 'shutter effect': then to have the speed matching the secondary one to within 10%?

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PostPosted: Thu May 27, 2010 21:20 
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Google aerial view link

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PostPosted: Thu May 27, 2010 21:27 
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Steve wrote:
How do you know it was falsely triggered?


I didn't say that it was falsely triggered, just that I do accept that it could happen - not that it did happen in this case.

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PostPosted: Thu May 27, 2010 21:30 
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Steve wrote:
Speaking as a seasoned electronic design engineer: I find it inconceivable that anyone could design a frequency reference that significantly slows at -5C (significant means more than 1%) into such a critical piece of equipment (evidence capture). For example, crystal oscillators usually have a drift of up to 100ppm over the operating temperature range, which is 0.01% (or 1% of 1%). That's certainly nowhere near the claimed 37% error.



Unlike you ,Steve ,I'm a humble HND + HNC electronics & comms tech( mostly maintenance rather than design) , but from experience any circuitry such as this involving an accurate clock circuit /accurate oscillator ,should be enclosed in a temperature controlled oven at the generation stage ,and all components and the complete circuitry after the controlled environment would be expected to be subjected to a soak cycle/cycles over at least the range of temperatures to be expected with minimal variation. In fact the only place that temperature variations used to be a factor were in Claire's part of the world - due to the nature of the terrain - very little top soil to cover cables and protect them from temperature variations ,so that comms circuits passing through that neck of the woods reflected the state of the weather to other parts of the country -more so as the frequency increased .But back to the drift - I concur with your figures .

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PostPosted: Thu May 27, 2010 21:33 
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Pete317 wrote:
Steve wrote:
How do you know it was falsely triggered?


I didn't say that it was falsely triggered, just that I do accept that it could happen - not that it did happen in this case.

Someone misunderstood someone :)

I was talking about the false triggering that you saw (not of this reported offence):
Pete317 wrote:
...I've seen that happen many times.

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PostPosted: Thu May 27, 2010 21:36 
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Steve wrote:


Which does sort of beg a couple of questions:

1) Is a 30mph limit really necessary along that stretch of road?
2) Why is the camera after the pedestrian crossing?

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PostPosted: Thu May 27, 2010 21:45 
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Steve wrote:
Speaking as a seasoned electronic design engineer: I find it inconceivable that anyone could design a frequency reference that significantly slows at -5C (significant means more than 1%) into such a critical piece of equipment (evidence capture). For example, crystal oscillators usually have a drift of up to 100ppm over the operating temperature range, which is 0.01% (or 1% of 1%). That's certainly nowhere near the claimed 37% error.


As an equally seasoned electronic designer I can only concur with that, Steve. I have just specified a crystal oscillator module which is accurate to 20ppm from -20 to 100 degC. And that isn't anything special - costs less than $10.

article wrote:
He added: "If it's so cold the capacitor on the circuit can't hold its charge or is damaged the flash will not go off in time.

Capacitors fail at high temperature,s not low.

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PostPosted: Thu May 27, 2010 21:45 
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Steve wrote:
I was talking about the false triggering that you saw (not of this reported offence):


Sorry, I also misunderstood you :oops:

I've seen many cameras flashing with nothing passing them, save for traffic on the other side of the road. One camera I know of in Cirencester does this all the time.

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PostPosted: Thu May 27, 2010 22:00 
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dcbwhaley wrote:
Capacitors fail at high temperature,s not low.


I agree - the capacitor story sounds a bit fishy (apologies to Steve :wink: )
A timing anomaly might have been more plausible. I don't know what's inside the Gatso, but I do know that several microcontrollers have PLLs which are set by pin configuration, and that a bad solder joint on one of the pins (which could be affected by temperature) could conceivably change the clock frequency by 3/4, leading to both the doppler decoder reading 4/3 high and the camera timer being 4/3 slow. But I'm just speculating.

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PostPosted: Thu May 27, 2010 22:02 
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Pete317 wrote:
A timing anomaly might have been more plausible. I don't know what's inside the Gatso, but I do know that several microcontrollers have PLLs which are set by pin configuration, and that a bad solder joint on one of the pins (which could be affected by temperature) could conceivably change the clock frequency by 3/4, leading to both the doppler decoder reading 4/3 high and the camera timer being 4/3 slow. But I'm just speculating.



I prefer my speculation that the white lines on the road move closer together.


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PostPosted: Thu May 27, 2010 22:12 
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weepej wrote:
I prefer my speculation that the white lines on the road move closer together.


It might have been vaguely funny the first time you said it - repeating it just makes you look foolish.

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PostPosted: Thu May 27, 2010 22:25 
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dcbwhaley wrote:
As an equally seasoned electronic designer I can only concur with that, Steve. I have just specified a crystal oscillator module which is accurate to 20ppm from -20 to 100 degC. And that isn't anything special - costs less than $10.

Did you mean cents? I've specced crystals of 34ppm (+85, -40C) for about 4c (for mass production)
Anyway, I think that point is agreed!

dcbwhaley wrote:
Capacitors fail at high temperature,s not low.

Aluminium electrolytics (the capacitor of choice for flash reservoirs) go high ESR at low temps, but not enough to not give a flash at -5C. But yes, they certainly don't lose their charge at -5C.

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PostPosted: Thu May 27, 2010 22:27 
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weepej wrote:
I prefer my speculation that the white lines on the road move closer together.

:doh:

Let's not let the quoted technical arguments get in the way of a good trolling, eh?

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PostPosted: Thu May 27, 2010 22:31 
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Pete317 wrote:
...and that a bad solder joint on one of the pins (which could be affected by temperature) could conceivably change the clock frequency by 3/4, leading to both the doppler decoder reading 4/3 high and the camera timer being 4/3 slow.

That makes sense when arguing a common clock, but if that was really the case then the primary and secondary measurements couldn't ever be classed as "independent".

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