Speed camera inaccuracies?

http://news.bbc.co.uk/1/hi/england/wear/7033353.stm

(put in Idle rather than Bike, as can speed in any vehicle and case illustrated is in a car)

Curious story, inventor has allegedly more accurate GPS sat tracking device which states he was doing 29.18 in a 30 zone rather than 42 as claimed by the prosecution.

CPS took an interesting line in abandoning the case saying that the police officer involved had retired and therefore couldn't give evidence to the required standard to maintain a prosecution. Interested as to what evidence the CPS expected the police officer to give? Surely he was not expected to argue the relative scientific merits of the technologies involved? Can only assume therefore that the prosecution was based on either a "I pressed a button as target went under bridge A and pressed another button as he went under bridge B", or "I followed at a consistent distance and my calibrated speedo read 42mph".

Assuming that inherently unreliable measuring methods were used by the police in this case (ie. time/distance based on human interaction, whether using visual markers or maintained distance), then we're left with a staggering margin of error - potentially the police being 12mph out of whack on a 30mph target. Even if we go with 50/50, the police being out by 6mph in one direction and the defendant being out by 6mph in the other direction - we're still left with a 6mph difference on a 30mph target - that's 20%. That's staggering.

Not wanting to police-bash, but could this in any way set a precedent for convictions based upon human interaction coupled with a time/distance calculation?

[Bluepete]

So, do we believe this man? What evidence does he have that his recording of speed is accurate?

Maybe he is telling porkies 'cos he has something to loose. A laser recording a speed shows the speed at an exact point in time (less than 1/20th of a second)

CPS usually withdraw cases involving retired Cops in minor cases, it's just too much faff!

What BluePete said is correct.
This man effectively won the case as it was dropped by the CPS. It's important that we don't read too much into a media report like this. For us to make a judgement about this specific case we'd need a lot more detailed information about the circumstances.
It is interesting though.

[Bluepete]

Just to add, I thought the US Gov't controlled the accuracy of the GPS system? How has this man managed to get such accuracy when no-one else can?

Make a big enough smokescreen and no-one will bother to look through it.

[the_lone_wolf]

Originally Posted by BluePete

Just to add, I thought the US Gov't controlled the accuracy of the GPS system? How has this man managed to get such accuracy when no-one else can?

Make a big enough smokescreen and no-one will bother to look through it.

regardless of the case outcome or merits, i use the GPS network to get accuracies down to less than a tenth of a millimetre on stationary equipment, under 10mm for a moving receiver that fits in a small rucksack, nothing to do with the US govt...

Yeah they do happen, a few years back I was sent a speeding NIP from herts police stating that I was going 40mph in a 30 zone.

I knew it wasn't and wrote back explained its was white van speeding around that got flashed not me and in fact was doing 30 mph. Got a letter back saying that they have made a mistake and was no further action on me, that were going after white van man!

Result so it does happen!

[TSM]

If you use a device with multiple receivers with good clock sync and multiple satilite views then you can increase the acuracy of position.

US goverment opened up GPS signals to better resolution a few years ago, before only special GPS decryptors could get acurate positions.

If he is recording the NEMA output of the receiver it could be cross refrenced and checked for acuracy of the data stream.

Mobile handheld speed traps have been found again and again to be inacurate under certian circumstances. Stationary cars/vans/bikes showing as moving 5-10mph just because the operator was moving the device just as he pressed the trigger, bikers with black leathers are hard to track. The front of a bike has no flat areas like a car numberplate, the screen and headlights are usaly angled, this can cause false reading. Tracking over long distances can increase the error margin, especialy when used in hand held config.

There has been reports of the LT 20/20 which is sold under a diffrent name in the US but are the same and made by the same company, shown to have the errors.

Originally Posted by BluePete

So, do we believe this man? What evidence does he have that his recording of speed is accurate?

Maybe he is telling porkies 'cos he has something to loose. A laser recording a speed shows the speed at an exact point in time (less than 1/20th of a second)

CPS usually withdraw cases involving retired Cops in minor cases, it's just too much faff!

Oh yep, I totally agree, I listed two possibilities and you're right, there's actually three, the third being that the defendants equipment (or testimony) was 100% wrong and the case simply got dropped as it was too much trouble.

I'm admittedly only going on the posted link as to answer the "what evidence does he have". The article suggests he has at least some that is at least scientifically testable, the margin of error then being up to the court to weigh up I guess. This may or may not be the case of course, but it would appear that he has a fair bit to lose in blatant false testimony, he's staked his invention, his company and his government funded research grant on that. Of course it could also be argued that he took a gamble, knowing the chances of the CPS dropping it, and knowing that if they did it would (in the eyes of the media) validate his research and probably increase the stock value of his company several fold.

The laser thing, how does that work then? (if it's not secret squirrel)? Does it bounce a beam from Hawaii 5-0 car to target to ascertain distance, do the same again a 20th of a second later to work out distance variation, then look up the calibrated speedo and work out speed? If so, then the methodology seems pretty valid (discounting targetting interference). If we take the story on face value, ie. one reasonable methodology produces vastly different results than another, then I think that needs testing at least. Perhaps unfortunately, it would seem that today at least it won't be tested in a court.

[Bluepete]

http://www.pepipoo.com/LTi2020_screen.htm

First hit on google

Originally Posted by BluePete

Just to add, I thought the US Gov't controlled the accuracy of the GPS system? How has this man managed to get such accuracy when no-one else can?

Make a big enough smokescreen and no-one will bother to look through it.

Originally Posted by the_lone_wolf

regardless of the case outcome or merits, i use the GPS network to get accuracies down to less than a tenth of a millimetre on stationary equipment, under 10mm for a moving receiver that fits in a small rucksack, nothing to do with the US govt...

Yes they do indeed own all the satilites and the rights to it. They have only in the last few year allowed civilian applications to share the same degree of acuracy.

There may well be other companies wit hsats up there but all the car sat navs like tom tom are using the satalites from th americal military.

have no worries they only done it because they have something a bit better now which they sare with their closest allies