How_it_Works





			RADAR in general Firstly you need to understand the basis of RADAR, which is still the most common method of detecting moving objects. It stands for Radio Detection And Ranging. In the UK, only one method of radar detection is relevant to the detection of the speed of a moving vehicle. This is Doppler radar, named after the 19th Century Austrian physicist who discovered the phenomenon. (that could be worth something if you ever get on Who Wants To Be A Millionaire - so remember that). Time for some basic physics then. Radio travels in waves, and waves have both amplitude and frequency. When radio waves bounce off an object, they inheret some of the properties of that object. So if the object is stationary, (as in, it's standing still, not that it's paper and office supplies you fool) then the radio waves which are reflected will return with the same amplitude and frequency that they were emitted. However, if the object is moving towards the radio source, then the reflected waves get all bunched up and compressed, and their frequency goes up. Conversely, if the object is moving away from the source, then the reflected waves get all spaced out, and their frequency goes down. Still with me? Take a look at this drawing to figure it out: Speed-measuring devices can detect this change in frequency between the emitted waves and the reflected waves. The difference between the two is directly proportional to the speed of the object being measured. The device calculates this speed and displays it either in miles per hour or kilometres per hour. Everyone has heard Doppler effect, even if they don't know what it is. It's why a train horn or ambulance siren appears to change sound as it approaches you. The sound waves emitted from the siren or horn get squashed up because the vehicle is coming towards you, so their frequency goes up. It's exactly the same principle for radar, except that it uses radio waves instead of sound waves. In the UK, the police forces use devices which in turn use interrupted-cw Doppler radar. Typically this is transmitted in something like a 60ms burst once a second. You ought to be aware that speed-measuring devices will always display the speed of the strongest returned signal. So when two similar sized cars are in view together, the closest one will return the strongest signal. Similarly, a small car being followed by a big truck will cause the truck to return the strongest signal. This is why radar-based devices are neither good nor recommended at picking out a single vehicle in a line of traffic. RADAR in detail Getting adventurous eh? Fancy reading a little more on radar do you? Actually, it is still very relevant that you read this section, but probably more so if you've been caught and want to know if the device was used taking all factors into account. Radar coverage Radar coverage depends on several factors. the radar unit power the sensitivity of the receiver unit the characteristics of the aerial in the unit the size of the target the target's distance from the observer the position of the target in the aerial beam This shows the typical coverage factors for a handheld radar unit such as a Muniquip K-GP: The car-sized vehicle would not be correctly detected beyond the 200 metre range. Similarly, the van or truck-sized vehicle would not be detected beyond the 300 metre range, and the HGV would be out-of-range by 400 metres. Note that if this exact scenario existed in the real world, the unit would detect the strongest return - in this case it would most likely be the HGV. Cosine Factor You remember cosines don't you? Sines, cosines and tangents - all that mathematical guff at school that made you nod off into a nice, deep slumber at the back of the class. Well pay attention Double-0-7! Radar only works to detect the true speed of a vehicle if the radar is in the vehicle's path If the radar is positioned at an angle to the path of the vehicle, as in a GATSO camera, the apparent speed of the vehicle is reduced. The reduction in speed is proportional to the cosine of that angle. For example, at an angle of 15 degrees, a vehicle travelling at 40mph would only be measured at 38.6mph. Seriously. Look - cos(15) = 0.966. So 40mph x 0.966 = 38.6mph. GATSO cameras and other roadside devices have circuitry designed to compensate for cosine factor. The units must be aligned correctly according to the manufacturers instructions in order for the circuitry to do it's job correctly. In short: Cosine factor is always in favour of the driver. Reflections and site selection So we know that doppler radar does rather depend on reflection from a moving object to work correctly. What about stationary objects then? Well, in some cases, such as street signs or lamp posts, because those objects don't typically tend to move (unless you've had a pint too many) they don't have any effect on Doppler radar. However the same objects if large enough can act like mirrors, reflecting radar signals from objects moving outside the typical area of coverage. It is entirely possible for a street sign to reflect radar both ways from a vehicle around a bend from the radar operator, thus giving them a reading even when no vehicle is in sight. Now before you go getting ideas about using this as a get-out for a speeding offence, it doesn't take a rocket scientist to figure out that double-reflected doppler radar is going to be considerably weaker than a direct line-of-sight reflection. In layman's terms - it's bloody unlikely that the "no parking" sign behind you reflected the radar onto that dual carriageway. However, the ACPO manual does indicate that "sensible precautions should be taken to select a site free from reflecting objects and with a clear, unobstructed view of the road." It goes on to state that it is an "important requirement to check a site before starting measurements. The operator should always select a site with a clear view of oncoming traffic which is free of any large objects such as bus shelters, large road signs and metal fences or crash barriers." Another interesting point is that officers are advised that in the event that they decide to measure speeds on a dual carriageway or motorways, they must not do so from the central reservation because of the problems of stray reflections from the other carriageway. Unmanned cameras though are not subject to the same recommendations. ACPO guidelines prohibit the use of handheld radar speed measuring devices from within a vehicle. No if's or but's. Doesn't matter if the window is up or down, the ACPO do not allow for it. Scientific advice backs this up, so much so that they even reckon the device should use a power supply independant of any vehicle. Quite why, I'm not sure. Dorset Mayor proves Gatso wrong - January 2004 Kris Haskins, the deputy mayor of Portland in Dorset, was fined £60 and had three penalty points imposed when his van was apparently photographed at 51mph in a 30mph zone. He asked to see the pictures and worked out that his vehicle had been travelling at 13.42mph. The Dorset Safety Camera Partnership said the speed camera had been triggered by reflections of vehicles waiting in traffic. They admitted other drivers could have been trapped by the same mistake. Radar bounces off other vehicles and surfaces and it can lead to the Gatso calculating the wrong speed. The police dont often check the photos to confirm the reading - so if you have any doubt about the claimed speed, persist in asking to see the photo's. Calibration and testing Officers using hand-held radar devices must perform calibration verification tests at the start and end of each tour of duty of the device. A record of these checks must be made - usually in the officer's pocket books. These start- and end-of-duty checks should be carried out by driving a police vehicle with an approved, certified calibrated speedometer through the beam at a speed compatible with the site chosen. The error margin allowed is + or - 2mph. Recommended operating technique Officers are only allowed to use hand-held radar devices when they are on foot. They are only to be used to corroborate an officer's prior suspicions about a vehicle's speed. ACPO guidelines state that "the only enforcement situation should be when one vehicle is isolated in the field of view of the radar device and operator." In other words, if you're in a line of traffic, the police cannot be sure that they have actually measured your speed and not that of another vehicle in the same line. They must track their target (you) for at least three seconds once the device has 'locked on' to you and started reading your speed. Steady speed readings need to be taken over these three seconds, and the police are allowed a margin of error. So for example, if plod are looking in their display and it shows 38-37-38-39-37 then they can safely assume that you were doing between 37 and 39mph. However, if it goes ape and shows 38-37-46-39-37 then they're obliged to abandon the speed measurement. Let's explode another myth here too: there is no legal requirement for the officer to show you the speed they measured on the device when you're pulled over but they are 'obliged' to do so. Evidence in court Officers who get called into court for cases where your speed was measured with a radar-based device must be able to provide evidence that they recorded all the relevant information at the time including vehicle types, directions, speeds (duh!), and interestingly, the presence of any other vehicle that might have affected the measurement. Bear in mind though that laser-based devices are very accurate, and any half-competent officer won't have any trouble sighting their target (ie. you) correctly. If they don't have photographic evidence of your alleged infraction, the equipment is regarded as having corroborated the officer's prior opinion. Operators of these devices should normally do so from positions where they will be clearly visible to the public. ie. hiding in the bushes and standing behind bridges is not catered for. Radio Interference Most modern devices that the police use can detect the presence of third-party radio interference and will show the officer on the display that something is not right. It's not really feasible for the police to come up with 100% bombproof guidelines on this particular subject, suffice to say that the officer operating the device ought to do something about it if he receives this warning indication. Radiation hazards Not to you, but to the police. They reckon that if you hold the gun closer than 25cm to their body, aerial-end first and squeeze the trigger, they'll irradiate their nuts. The Americans have even gone so far as to spoof this fact in an advert for the Yahoo search engine. -------------------------------------------------------------------------------- Roadside RADAR. Roadside radar-based devices are things like GATSO and Truvelo cameras. They can measure the speed of traffic both approaching the device and receding from the device, usually by setting a switch inside somewhere. Site location criteria Local councils would have us believe that if they followed their own criteria for site selection for unmanned cameras, then they'd all be there to prevent accidents. How many times have you found a camera nestling behind a corner on a blind bend, contravening guideline 1? Site is at least 400 metres in length. Number of injury accidents in the preceding 3 years is 8 or more. Number of fatal/serious accidents is 5 or more. Number of loss of control accidents is 3 or more. Number of excessive speed in respect of conditions or limit accidents is 3 or more. 85 percentile speed greater than the ACPO guidelines. Site conditions are suitable. No other engineering measures appropriate, for example speed humps. On a more technical note, the devices should generally be placed on the side of the road where they are to measure the speed. So if you've been flashed by a camera on the other side of the road when you were driving towards it, chances are that the range control had been badly adjusted and it detected you whilst photographic the carriageway on it's side of the road. The normal practice is to monitor a road with a maximum of 3 lanes whilst placing the device within 4 feet of the edge of the carriageway. Calibration Like so many other devices, these units must be calibrated by driving a police vehicle with an approved, certified calibrated speedometer through the target zone. -------------------------------------------------------------------------------- LIDAR in general Radar is starting to show it's age. The new kid on the block is, and has been for some time, LIDAR. This stands for Light Detection and Ranging. These are usually infra-red laser-based devices and are far more accurate and reliable than radar-based devices, not to mention a lot lighter. This makes them ideal to be built in to handheld devices such as the LTI20.20. Laser-based LIDAR devices can be used either manned or unmanned, and with or without cameras attached to them. Pretty flexible then. Their basic method of operation is different from the doppler-method described above. LIDAR-base devices work on a time-of-flight mode. ie. they measure the time it takes light to travel from the gun's transmitter to the object, and back again. LIDAR devices typically pulse at between 45Hz and 72Hz - i.e. 45 to 72 times a second. The speed of light is 300million metres/second. Let's take two pulses from a LIDAR device as an example. Pulse 1 emitted from the device and detected by the receiver 0.000001333 seconds later. Well, speed equals distance over time. So distance = speed x time. The speed of light is 300,000,000m/s and the time is 0.00000133s so the distance is 300,000,000 x 0.000001333 = 399metres. That's the distance the light travelled from the gun to the target and back again, so the distance to the object is half that, or in this case 199.5metres Pulse 2 is emitted 1/45th of a second later, and detected 0.000001325 seconds after that. The same calculation again gives the distance of 300,000,000 x 0.000001325 = 397.5metres round-trip, or a device-to-target distance of 198.75metres. Now it's another calculation using the above figures. The difference in distance between the two readings is 0.75metres. The time between pulses was 1/45th of a second so the object travelled 0.75 metres in that time, or 0.75 x 45 = 33.75metres/second. It's now even simpler maths to scale that up from metres/second to km/h - 33.75m/s x 3600 = 121500metres in an hour which in turn is 121.5km/h. Now bear in mind that all that those calculations happen 45 times a second (or more). The device needs two or three consecutive readings all with similar results to give a finite speed which is why LIDAR devices take 0.3 seconds to lock-on and read your speed. Bear this in mind before you buy a laser-detector then - if you can react in 0.3 seconds and reduce your speed, then you're lying. This is important so read it carefully. The ACPO guidelines state that "operators should bear in mind that the device confirms and corroborates prior personal observations." What this means is that officers who stand at the side of the road indiscriminately targetting every vehicle that comes into their line of sight are in breach of their own guidelines. Remember this if you get stopped because it is important! ACPO guidelines also prohibit the use of handheld laser or optical speed measuring devices from within a vehicle unless the window is down. ie. the beam will not be projected through a window or off a mirror. -------------------------------------------------------------------------------- LIDAR in detail Good god you're a real sucker for technical information if you're still reading at this point. Still, always keen to satisfy a reader's desire for more knowledge, here's the more detailed information on LIDAR. Range Would you believe handheld devices like the LTI20.20 are capable of measuring your speed as far away as 600 metres (2000ft)? The newest devices can go further than that. You could be targetted before you can even make out the officer who is targetting you. This of course has it's own ramifications. If you can't see him, how can he see you? The sights on most devices are not telescopic, and the police are advised against making measurements at the extreme limits of the device's range. Now - have you ever held a camera with a big zoom lens and tried to keep the image steady in the viewfinder? Difficult isn't it. Steadiness of the devices therefore does affect operating range, but it doesn't affect accuracy, simply because of the speed of light. (ie. bloody quick). Heavy rain, spray, mist or fog do affect a device's range, but don't affect it's accuracy. Again, as with radar-based devices, they police aren't under any legal obligation to show you the reading on the device once you're pulled over, even though they're encouraged to do so. Cosine Effect Not a million miles removed from the cosine error that affects radar-based devices. I won't go into it again - you can read about it above - it's exactly the same for laser-based devices. However, one important note is that when an officer is going to do an over-bridge measurement - ie. when he's parked on a bridge over a motorway for example, he's encouraged to be as close as possible to the centreline of the lane being targetted to reduce cosine effect. More to the point, he must carry out a height check from the level of operation (ie. the bridge) to the road surface below and multiply this figure by 10. This then becomes the minimum distance the ACPO allow for speed measurement. So for example, if he's on a bridge 10 metres above the motorway, the closest he's allowed to measure the speed of a vehicle is 100 metres. Calibration and testing This is the subject of much debate with people who have been caught for speeding. Many people believe that laser-based devices go out of calibration at the slightest knock. Not true. All these devices are now continually self-calibrating. What the police do is not calibration, but calibration verification. Actually, the device itself performs verification at power on, but the police must also check the device at the start and end of each tour of duty of the device. A record of these checks must be made - usually in the officer's pocket books. If a calibration defect is found, the police must return the device to it's manufacturer. However, once a year, the devices must be calibrated by the manufacturer or a certified agent, and a certificate of calibration should be issued to, and held by the police. A visible sticker showing the date of calibration must be fixed to the device. The start- and end-of-duty checks should be carried out depending on how the device is to be used: Hand-held device Checked by driving a police vehicle with an approved, certified calibrated speedometer through the beam at a speed compatible with the site chosen. The error margin allowed is + or - 2mph. Roadside or tripod Again, checked by driving a police vehicle with an approved, certified calibrated speedometer through the beam, this time at a predetermined speed, at each location the device is to be used. Evidence in court Officers who get called into court for cases where your speed was measured with a laser-based device must be able to provide evidence that they recorded all the relevant information at the time including vehicle types, directions, speeds (duh!), and interestingly, the presence of any other vehicle that might have affected the measurement. Bear in mind though that laser-based devices are very accurate, and any half-competent officer won't have any trouble sighting their target (ie. you) correctly. If they don't have photographic evidence of your alleged infraction, the equipment is regarded as having corroborated the officer's prior opinion. Operators of these devices should normally do so from positions where they will be clearly visible to the public. ie. hiding in the bushes and standing behind bridges is not catered for. -------------------------------------------------------------------------------- Time and distance devices This covers a multitude of nasty pieces of kit, including some LIDAR based devices in time-distance mode. The most commonly-used time-distance device though is the all-conquering piece of über-equipment that all officers love - the VASCAR! The first thing you absolutely must be aware of is that time-distance devices are exempt from Home Office Type Approval. This means that the 1999 case where police evidence was thrown out on the grounds that the PolicePilot wasn't type-approved was won on an extreme technicality. It hasn't set a precedent, and don't expect any favours from the police or courts if you try to use this as an excuse. So now we've cleared that up, any device which measures time and distance is covered by these guidelines. Elementary maths will tell you that speed can be calculated by dividing distance by time. Think about it - mph = miles per hour = distance (miles) in (per) time (hour). By the police's own admission, the level of accuracy required to operate something like a VASCAR unit is high, and does require a considerable amount of practice and exposure to the device. For the police to measure your speed like this, your vehicle must be checked between two reference points. The ACPO guidelines define these as "identifiable points on or near the carriageway either permanent or temporary." This can be those white squares you've driven over so many times but never asked what they were for, the shadow from a bridge or pole, a piece of street furniture (signs etc) or any number of other things, right down to the point where a road surface changes colour. However, the police vehicle must pass between the same points as you. To do this, they can either be following you, or in the case of wanting to sit at the side of the road or on a bridge, they have to drive through these two markers first and set them in the device so it 'knows' the distance you are being measured over. As long as the two points don't move, the police can make a note of the location and if they come back to it at a later date, they can simply dial-in the distance measured last time around. So, a time-distance device can be used to check your speed in one of 5 modes: Follow checks They're following you. You pass point A, they start the timer. They pass point A, they set the start point. You pass point B, they stop the timer. They pass point B, they set the end point. Being followed checks Nearly identical to the above, but the police car is in front of you watching you in their mirrors. Crossing check This is a variation on the two methods above. The police sit and watch from the side of the road. As you whip across a known point, they start the timer. They then move off and as they cross the same point, set the start point. The rest is as above. Pre-fed distance checks They pass points A and B setting the start and end points. The device now has a distance set in it. They now park up at the side of the road and as you pass points A and B, the timer is started and stopped. Dial-in distance This is a variation on pre-fed distance checks where they can come back to a site that they already know the distance between the markers from an earlier session. The minimum distance the police are allowed to use a time-distance device over, is 0.125 miles - one-eighth of a mile. Under exceptional circumstances, they're allowed to go down to 0.07 miles but only in pre-fed or dial-in distance modes where the start and end points are not shadows, and the area being checked cannot physically allow a speed above 40mph. Basically, if they really want to measure speed in this sort of confined space, they're better off using a different type of device. Calibration Time-distance devices must be calibrated when they're installed in a vehicle, if the tyres are changed on a vehicle, or if they're taken out and re-installed. On top of that, they must be checked at least once per week. (most VASCAR units deactivate automatically after 7 days which means the operators have no choice). As with the other devices, they must be checked at the start and end of each tour of duty for the device. -------------------------------------------------------------------------------- Sensor operated devices Truvelo, Speedmaster, Speedman - all operated by tubes across the road and a box of tricks at the side of the road. All classify as sensor-operated devices. The tubes are either piezo-electric coax tubes (deformation causes a current to flow) or hollow rubber tubes (deformation causes a change in air pressure). Technically, these devices must conform to the same rules as the time-distance devices outlined above, because that's essentially what they are. The tubes are placed a known distance apart on the road, and the time is calculated between each tube registering a vehicle. Unlike time-distance devices though, sensor-operated devices must be Home Office Type Approved. Calibration Once again, sensor-operated devices must be calibrated by driving a police vehicle with an approved, certified calibrated speedometer across both tubes. A tolerance of + or - 2mph is allowed. -------------------------------------------------------------------------------- Certificates of competence Whenever an officer is trained to use any speed-measuring device, he or she is awarded a certificate of competence at completion of training, but only of course if he or she actually did manage to prove that they were adept at using the devices. These certificates are generally signed by the chief constable and the instructor. In court, the officers can be called upon to produce their certificates of competence along with any other paperwork directly pertaining to the case being tried (proof of calibration verification for example). Copyright © Paul Gander 1995 - 2006 This site and all contents unless otherwise noted are copyrighted.The author will respond expeditiously to any intellectual property infringement.Reproduction in whole or in part in any form or medium without express written permission of Paul Gander is prohibited. Click here for full legal and copyright details pertaining to the Speedtrap Bible. All rights reserved. [http://www.speed-trap.net] [http://www.speed-trap.co.uk]