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A LiDAR speed gun is a device that police use to measure vehicle speed, to see if the target vehicle is exceeding the speed limit. It uses LiDAR to detect the speed of a vehicle. Unlike Radar speed guns, which rely on doppler shifts to measure speed, these devices let a police officer measure the speed of an individual vehicle within a stream of traffic. ==How police LiDAR guns work== Note: This article uses some Imperial units—for reference: 1 mile = 1.6 kilometers; 1 yard = 0.91 meters; 1 foot = 30.5 cm; 1 Mile Per Hour (MPH) = 1.6 Kilometers Per Hour. LiDAR relies on the principle of time-of-flight of two or more short 905 nm wavelength (near infrared - NIR) LASER pulses. The police officer aims the LiDAR through a telescopic monocular (2X - 8X, depending on model) built into the LiDAR gun. The scope helps the police officer see the target vehicle before the driver sees the police officer—generally at a distance of 1000 ft and up to 4000 ft. The police officer aims the pulsed 4 milliradian laser at the license plate. License plates are coated with a retro-reflective coating that reflects the laser pulses back to the LIDAR gun receiver aperture. Range varies by LiDAR gun manufacturer, target vehicle aim-point reflectivity, and weather conditions (temperature, humidity, precipitation). The LiDAR can record vehicle speed anywhere from ~5 feet to ~4,000 feet away. Most police Lidar units use a magnification of 2X. An 8X magnification scope makes acquiring and tracking a quickly moving vehicle more difficult. LiDAR gun manufactures have begun to concentrate on extending the speed-detection range of the devices (2014). Some LiDAR units produce a tone to indicate they are receiving a good return signal. The tone may vary from target to target, so the operator can sample multiple vehicles and select a particular one. The 3-4 milliradian cone presents an area of illumination of about 1 square meter at 300 meters distance. Therefore, the police officer can select a single vehicle out of a group. A vehicle in the "shadow" of another vehicle cannot be measured. To operate the device, the police officer presses the LiDAR gun trigger. The gun emits short LiDAR laser pulses, with a pulse width (duration of pulse) of 30 nanoseconds or less. Depending on the LiDAR gun, the number of pulses per second (pps) ranges from 100 to 380 in the USA or up to 600 pps for countries outside the USA. The LiDAR gun's internal software uses an algorithm that rejects inaccuracies. All manufacturers use proprietary error rejection methods. LiDAR speed measurement takes place in these steps: # On a trigger pull, the gun sends a series of short (typically 30 nanosecond) laser pulses (100-600 per second) and starts a timer. # The gun stores the time that each pulse's reflection reached the gun's detector. # The gun uses elapsed “time-of-flight” to determine the distance each pulse traveled, and uses the difference between pulse distances to calculate speed.〔White Paper: "An Overview of avalanche photodiodes and pulsed lasers as they are used in 3D laser radar type applications" by Bruno Dion, CMC Electronics, Inc.〕 Distance_traveled = Distance_2 - Distance_1 Speed = Distance_traveled / Elapsed_time Current LIDAR guns typically acquire and validate target vehicle speed in under half a second (250 to 400 ms). LiDAR guns must meet a law enforcement accuracy requirement of +1 MPH or -2 MPH. All attain +/-1 MPH (same as RADAR). Since the LiDAR gun may be off, and is only triggered when the police officer targets a vehicle, there are no "stray" LiDAR signals to detect, so driver-operated RADAR/LiDAR detectors are not very effective. A police officer targeting many vehicles at long range may create some stray laser reflections that a LiDAR detector may detect, but this isn't likely. If a RADAR/LiDAR detector in a targeted vehicle sounds an alarm, it does no good because the gun has completed the speed measurement long before the driver can react. In addition to fast target acquisition, some LiDAR guns produce pulse patterns that are not evenly spaced in time. Many LiDAR detectors use photo-diodes to detect 904-905 nm laser through a narrow band-width filter (to reject irrelevant wavelengths) and detect only even pulsing. These "stealth" LiDAR signals are invisible to most LiDAR detectors. Other than distance, other factors that degrade vehicle speed measurement include: * Refraction by differences in air density due to "heat waves" off a hot road surface, etc. * Weather conditions, such as rain, heavy fog, snow—which have a negligible impact on the laser pulse but may impair the police officer' ability to target. * Windshield glass tends to scatter the IR pulses. This is usually negligible, but if the windshield is wet with rain, fogged, or splattered with snow, that can reduce LiDAR range. * Front or rear vehicle targeting makes a difference because, generally, automobile rears present a stronger reflection. In states that don't require front plates, or in the case of motorcycles, the rear is surely the strongest reflection point However, police set up to detect vehicles from the front around 90% of the time so they can wave offending vehicles over without having to chase them. * Darkness can impede the ability of police officer to identify and target and a vehicle. LiDAR units do not yet include light amplification or "night vision." * Occlusion by the sun, where the sun is behind the target, can prevent the LiDAR unit from reading. The Sun may "wash out" incoming LiDAR pulses. The sun also impedes targeting. Aiming the LiDAR into the sun may cause LiDAR gun faults (rejected readings) even though a narrow bandpass filter (+/-5 nm; 899 nm-909 nm) at the LiDAR gun receiver aperture rejects light outside the Laser's operating range (sun, HID Headlights, etc.)〔(Article: They Have Lasers! | Road & Track Magazine, Nov. 1991, Page 106)〕 # Cosine error is an issue with LiDAR, just as it is with RADAR speed measurement. The closer the LiDAR gun is to directly in-front or behind the target vehicle, the more accurate the reading. Police often use off-center angles of as much as 15 degrees, but they know this cosine angle works against them. The gun calculates a lower-than-actual speed because the calculated distance between pulses is less because the car is moving at an angle to the gun, instead of directly towards or away. This is why LiDAR units often operate close to the edge of the road. # Police misuse (sweeping) refers to the act of sweeping the LiDAR gun while pressing the trigger, so that—particularly at long range where angular separation between targets is slight—pulses from more than one target create a false reading. # Police in motion, in a moving police vehicle, must stop the vehicle to get an accurate speed measurement. For example: A police officer is trying to read a group of vehicles over a mile away (near the range limit). The officer is operating the unit "hand held" (without a tripod). "Camera shake" makes the LiDAR pulse beam sweep across two or more targets during a single read. If the initial pulses reflect off a vehicle that is further away and final pulses bounce off a vehicle that is much closer, this may make the LiDAR "think" it read a single vehicle. The pulse time-in-flight between the initial and final vehicle being much shorter, the LiDAR unit provides a reading much higher than the target vehicle speed. A specific "sweep error" scenario: A car is traveling at the 45 MPH posted speed limit alongside another vehicle that's further forward, also traveling at the posted speed. If the officer's unsteady hand makes the LiDAR beam sweep from the first vehicle to the second, forward vehicle in a single read, the LiDAR gun might produce a reading as high as 80 MPH, depending on the distance between the vehicles. The officer may think this is the target vehicle's actual speed and ticket the driver. The probability of this is high because "pulling" the trigger instead of "squeezing" it can make the gun sweep. Police officers are trained in handgun marksmanship and learn to "squeeze" the trigger correctly, but at ranges of a mile this effect can be amplified and go unnoticed by the police officer who, with good intentions, may still believe a violation occurred. 7. Calculating distance traveled by target: Now having the two distance of the vehicle when pulse A hit it and pulse B hit it the LiDAR can calculate the distance traveled by the target vehicle between pulses as follows: (to Target B ) - (to Target A ) = (Traveled by Target ). Some LiDAR guns have logic that tries to detect when a vehicle is operating some form of LiDAR "Jamming" signal and may report suspected jamming. There are many instances of false positives, however, so police are never sure if a vehicle is operating a jamming device. Also, some LiDAR guns are more susceptible to LiDAR jamming than others. LiDAR jammers assume the Police Officer is within 30 degrees of center front (in most cases) or 30 degrees of center rear (in lesser cases). The Police Officer must operate within these angles to limit cosine error, which always favors the speeder. In fact, many police officers try to get as close to 0 degrees as possible to produce the most accurate reading. They may stand near the edge of the road, or even leaning out into it. Jammers generate a large number of short 905 nm laser pulses in a 30 degrees wide (or less) beam. This makes the LiDAR gun detect so many returning pulses that it becomes "confused." The error-correction algorithm ends up rejecting the readings and prevents the LiDAR gun from determining vehicle speed . LiDAR units are becoming more sophisticated, and some can detect jamming attempts. LiDAR gun manufactures buy every new LiDAR detector and jammer and analyse them to determine how to improve LiDAR gun software. Some may offer police-user upgradable firmware. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「LIDAR speed gun」の詳細全文を読む スポンサード リンク
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