We have a received a request under the Freedom of Information Act 2000 for the following:
“I believe your department provides consultancy and technical guidance to ACPO on the use of speed detection devices. I am an electronics engineer who has spent several years doing targeting and tracking on military weapons projects and have become interested in the devices being used for civilian purposes, both from a personal interest and technical standpoint.
I have a copy of the ACPO guidelines and have a couple of technical issues which are not clear from reading the guidelines and wonder whether you could clarify them both to me and wonder whether it would be wise to suggest amending the guidelines in a manner that someone less technical than me, such as a policeman would be able to readily understand. Questions and points are numbered below.
1) Section 14.8- The sentence reads “The use of any repeater radio in the vicinity of a laser speed meter must be avoided.” I believe the term “vicinity” is a little vague, but I would
interpret it as a distance where the power from it is equivalent to the 0.5m suggested minimum distance for a GSM phone. Do you believe that it is reasonable to use a normal inverse square law for this calculation, or should a more conservative calculation be used because a mobile handset is pretty close to onmi-directional, whereas a repeater mast with it’s almost t dipole has an almost flat pattern. If you do believe a more conservative (or different) calculation should be used, could you suggest what it might be.
2) Section 14.2 The sentence reads “by aiming the device at the target vehicle in the area around the registration plate”. Could you please be a little more explicit about the term
“around”. Is the purpose of this sentence to indicate a flat perpendicular area such as the number plate. Many modem cars are designed such that there is an air intake just below the number plate, which would be included in the term “around”. I understand the purpose of each pulse is to accurately measure distance, which would be difficult if each pulse is not hitting the same point on the vehicle. I was wondering whether that was the purpose of the reference to “slip factor”. If! might suggest the addition of the word “equi-distant perpendicular before “area around the registration plate”
3) I understand it is now standard practice for these devices to be used just after the bottom of a hill (hi likely vehicle speed), with a fixed vertical mount. This results in the vehicle
driving through the beam( vertically) rather the beam being “held steady” on the target area. Do you have any comment on this practice and can you tell what testing has been
done with these devices “fixed” like this.
4) Calculation of speed. It has been speculated that the LTI20:20, doesn’t simply do a distance/time calculation, using two distance measurements and the time interval between
them, but may use a more complicated formula, such as averaging (maybe even ARMA [ Regressive Moving Average] or ARIMA), over possibly several hundred pulses. I had assumed it was simply the distance/time with the target of’ the second pulse being indicted by the crosshairs on the photo (second since it has the speed and it would need both distance-measurements to compute this.). If the device has used measurements taken perhaps hundreds of measurements before and a complicated formula what testing on the correctness of this formula has been done, If it is as I had originally assumed simply a distance change between two samples/time calculation (neglecting any tolerances and
checks) could you please confirm so.
5) Do you happen to know where I could buy a copy of an LTI20:20 user manual. I did try contacting the manufacturers but did not get any reply. I understand there are two main models and would be interested in either/both.
6) I have also read speculation as to tho “self-checks” and have been disappointed by the vast amount of obvious gibberish written. Such gibberish has included it does 20 distance measurements then switches to do 40 speed ones in less than a third of a second” Aside from the impossibility of measuring anything but distance; the 60 required measurements would not fit in a third of second with the 8 ms sample period. Are you able to state what consistency checks the device does, in the absence of the technical manual which might tell me?
7) I understand that the beam width of the device is 3 milli radians, but suspect that this is not the -3db points as it would make the required distance measurements impossible to the accuracy required. I suspect that this is instead the detectable beam width, and is used by “lidar detectors” My understanding of normal IR range finders is that the “beam width” actually decreases with distance as a certain intensity is required at the target to get a reflection back to the detector The area of signal where the signal is greater than this threshold gets smaller as distance increases. This makes the devices work better at longer distances than would otherwise be expected by the 3 millirad beam width. Do you have any formulae for this effect?
8) I was a little disappointed to discover that it is not mandatory for police officers using these equipments to either have a copy of or read the ACPO guidelines If I might I would suggest that such officers are required to have a copy and should read the guidelines at least annually and each be issued their own copy Sadly it appears that officers using the LTI20:20 are not even aware of the basic principals of the device such as that it is InfraRed, I would have expected this type of basic knowledge.”