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What is Laser/LiDAR? How is it used?  How does it measure your speed?

Laser, or LiDAR, is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light.  A LiDAR speed gun is a device used by the police for speed limit enforcement which uses LiDAR to detect the speed of a vehicle. Unlike Radar speed guns, which rely on Doppler shifts to measure the speed of a vehicle, these devices allow a police officer to measure the speed of an individual vehicle within a stream of traffic.  It relies on the principle of time-of-flight of two (or more) short 905nm wavelength (near infrared – NIR) LASER pulses.  

 

The police officer takes aim (usually at the licence plate or secondary reflector such as headlight, chrome grill-work, etc.). Some LiDAR units have a tone to tell the operator if they are getting a good return signal and the tone varies from target to target so the operator can sample a number of vehicles and select the fastest moving vehicle if they choose. The 4 milliradian cone presents an area of illumination of only 1 square meter at 300 meters distance. Therefore it is able to easily get a separate reading from any vehicle even when there are many vehicles in a group. Vehicles in the “shadow” of other vehicles cannot be speed measured. The police officer presses the trigger of the LiDAR gun and a short 30 nanosecond laser pulse [Pulse A] “flies” towards the target and hits the target vehicle. At the same time the pulse is released into flight [Release Time A] is captured by a high resolution high-speed timer built into the LiDAR gun. Additional laser pulses are also sent at a rate of 1000 pulses per second (1kHz). The advent of high-speed timers able to measure time in was a critical technology making “time in flight” measurement of light possible. So the LiDAR measures time-in-flight of each pulse and taking 2 readings over a period of time of 3/1000 second (theoretically) to determine velocity of the vehicle. Stalker LIDAR LR hand-held units advertise a target speed acquisitions tome of 0.4s (minimum time). Another reading (or more) is used to plot velocity of the vehicle and the system looks at variance to indicate errors so can reject a reading and restart the process again until a clean reading is received. It is expected the police LiDAR requires 1/250th second to get a single accurate and usable (error checked) vehicle speed reading.  The laser pulse is reflected from the target vehicle (license plate preferable). Testing protocol for LiDAR units actually use a piece of retro-reflective material the size of a standard automobile license plate on a black background for testing. LiDAR guns use 50uW (microwatt) laser diodes providing 30 nanosecond pulses at a rate of 1kHz (1000 pulses per second) producing 1000 50uW pulses. Reflection is enhanced by “scatter” caused by strong reflective surfaces on modern automobiles (e.g., chrome bumpers, chrome grill work, headlight reflectors).

 

“Scatter” is what modern LiDAR detectors detect in order to give users advanced notice that LiDAR is being used ahead. This is merely an internal calculation based on the speed-of-light and the laser pulse’s time-in-flight divided by 2 (halved — to account for a one way trip time). Since a number of pulses must be measured and to determine a velocity and then variances considered in error checking it is highly likely the Police LiDAR gun is capable of getting a reading of target vehicle speed in as short a time as 1/250 second (theoretically).  Laser shifting/jamming comes in two varieties.  The first is called active, in which units create slurry of 905nm pulses to try to confuse the Police LiDAR.  The second is passive, in which the light is defracted using absorption.

 

Why do you need Laser Jamming? Is it legal?  

Laser/LiDAR is an instant on detection method, giving you no time to react appropriately. Laser/LiDAR uses light, and light frequencies are regulated by the FDA.  The FDA does not have any provision preventing you from legally jamming light frequencies, so jamming laser/LiDAR is legal in Massachusetts and most states (please check local laws to verify, as AVI,llc cannot be held responsible in any misinformation of the laws).  Radar frequencies are regulated by the FCC, which does not allow any manipulation of radar frequencies, so it is not legal to jam.  The good news- radar is typically always on, can be detected from a distance due to its scatter, giving you adequate time to react as needed.  Laser is an instant-on type of detection.  By the time you are hit, it is already too late to react, as the officer already has your speed.  This is where laser jamming is most critical- it gives you a buffer to react and not have your speed detected.

 

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