LT120.20 Speedmeter
FOI 4881 We have a received a request under the Freedom of Information Act 2000 for the following: “Thank you for your reply dated 6th September…
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FOI 4881
We have a received a request under the Freedom of Information Act 2000 for the following:
“Thank you for your reply dated 6th September, and your answers which were very helpful. I would however like to clarify a few. I’ll use the same numbering system as both our letters.
1) I appreciate the EMC testing done, however was just tying to establish what I believed was normal practice, that interference is based on power, and if frequencies are equivalent then equal power at different distances would have the same effect. Thanks to your answer I now appreciate you may not be in a position to respond to the dipole/omnipole question. Having read your EMC testing document, it doesn’t make any reference to the two GSM bands (900 and 1800). Could you please confirm both were tested, and for the record confirm the equivalent power at equivalent frequencies assertion?
2) 1 now understand the guidance for “number plate”, but am surprised that a distance measuring has no requirement to be held on a equidistant point on the vehicle. My experience of electronic instrumentation is that relying on error generation to avoid incorrect readings is bad practice (and not robust), and instruments should be used correctly to avoid the errors in the first place. The fewer errors that are generated the fewer that will statistically get through detection.
3) Very helpful answer, however it appears pretty common practice to use it in this manner (I know of at least two regular sites)
4) 1 have spent some considerable time and effort doing research on how the device attempts to makes it’s calculation. One of the big questions is whether the patented formula for speed calculation that the company holds is actually used in the device. This is not a simple distance over time calculation and I’m not convinced I could analytically prove it. I have come across many descriptions of the 30/40 pulse generally claimed to be used in the “measurement phase” Perhaps I should elaborate on what I managed to uncover arid perhaps you would be kind enough to fill in the gaps that I have not been able fill. The best description (and I believe the most reliable as I suspect it’s taken from a manual) ca from a New Zealand police training website. It states
The LTI20:20 emits a continuous series of pulse whilst the trigger is held and measurements are taken during this period. Distances are measured and speed computed. When enough matching speed calculations have been seen (it said 30, not your 40, though maybe different countries have different firmware) It described matching to within 1mph, and states the same measurements are used for speed and distance display. I have also seen claims that the calibration certificate states the “period” of the measurement, and this is required for legal reasons to fix the time and exact location of the reading. The only period I can see on calibration certificates is the time between pulses, which imply the last two pulse are used.
Your answer doesn’t actually contradict this, but it sort of implies the device takes a distance at pulse zero and a. distance forty pulses later and divides by thirty nine times the sample Interval to get the speed, totally negating any advantage from the higher sample rate that could be gained by reducing potential errors in vehicle movement. I don’t think algorithm as particularly likely, nor is the “average” of 39 numbers, which is mathematically equivalent. There are much better algorithms for finding the most accurate result from “noisy” data than an average – I speak as an “adaptive control” engineer. Am I to take your answer to mean the previous 40 or so measurements are used to verify the target lock and match to the displayed result.
5) It was worth a try, I have so far been unable to obtain a copy, can you imagine why such a user manual might be deemed commercially sensitive as I have had suggested to me. This idea sounds a little bizarre as I understand one is supplied with every instrument.
6) Thank you, these so like standard instrumentation self checks to me and don’t involve ‘real calibration”
7) 1 appreciate you do not have any formulaes, however my description is merely the application of physics, I am sure you would agree that the receiving sensor has a minimum power which it can detect. I an sure you must agree that the further it has to reflect from it’s target the weaker that the signal will be. This implies that the signal power/strength hitting the vehicle has to be stronger the further the vehicle is. So even from this it is obvious that the reflecting area that is power enough to generate a received signal will decrease with increased range. On the transmit side the mirror condition will exist such that the greater the distance power per unit area will go down (inverse square), hence a smaller effective angle Will be achieved. I believe this is the explanation of why the instrument’s accuracy is not effected by range. If this were not the case the area at 800m of a 3 milliradian beam (whether detected or 3dB point) would be wider than a car width hence the reflecting area would be irregular and your suggestion would be generate an error code. I appreciate there may there may be other effects that allow accuracy to not be effected by range, but I have yet to hear any plausible ones. Can you suggest any?
8) I appreciate that though I understood part of your remit was to suggest practice that was beneficial to the reliable operation of the device, I was just a little shocked to discover there are operators who are happy to state they have no idea how it operates, don’t know the guidelines haven’t actually seen a copy for a number of years, and seem to think it’s infallible if pointed in the general direction of a car. I believe part of the battle to convince people such devices are reliable is confidence they are operated by competent and knowledgeable individuals. The dumb “it just works” argument has never been an effective persuasion to me – I’ve seen to many “infallible” things fail, - Microsoft windows being one, their error traps catch most faults but occasionally you get one that locks a PC solid and the mains switch is the only solution. This question was more a helpful suggestion than a real question.”
We released the following information on 8 November 2006.