Originally Posted by
jimbonzzz
Most police laser is 904/905nm infrared laser. The laser diode used in the guns is commonly around 50 Watts peak power, though could be more or less.
"LED" jammers usually use 8 infrared LED's per transponder. These LEDs are commonly at 870nm, and can jam the 904/905nm guns because they're close enough that they will still be received by the laser guns, and the LEDs also emit closer to 904/905 when they heat up. A transponder using several LEDs will still have much less power than a transponder using laser diodes.
"Laser Diode Jammers" usually use one laser diode per transponder, though recently some jammer makers have made designs with two laser diodes per transponder. These laser diodes are 904/905nm with 10-100 Watts peak power, depending on the jammer. It is important to note that the jammers do not usually use optics to the extent that the laser guns do in order to collimate the beam, thus the jammer beams spreads out more to ensure wide coverage.
As far as how the guns are jammed, it works like so:
The laser guns are "gated", which means that they'll only accept a pulse and use it for speed calculations when they're expecting to receive a pulse. The guns are only expecting to receive a pulse betwen the time it sends out it's pulse, and before that pulse is reflected back. Any jamming pulses must be received in this time window too, in order to have any effect. Once the gun's own pulse is reflected back, all bets are off: the gun uses it's own reflected pulse for calculations, and doesn't accept any pulses until it sends out another pulse.
For example, a Kustom ProLaser III sends out 200 pulses per second, or one pulse every 5000 microseconds. At 1000 feet, the "time of flight" of the gun's pulse is 2 microseconds (about 1 foot per nanosecond for total travel of 2000 feet). So, any jamming pulses must be received by the gun in that 2 microsecond window. If a jamming pulse reaches the gun during the next 4998 microseconds, it has no effect at all and is essentially ignored by the gun. Then, the cycle repeats for the next pulse.
How do the jammers get a pulse into this narrow time window? The jammers receive a few of the laser gun's pulses, and time between them. Then, they are able to "predict" when the next pulse from the gun will be received at the jammer. A number of nanoseconds before the predicted pulse is expected to be received, they send out one or more jamming pulse(s). This ensures that a jamming pulse is received at the gun before the gun's own reflected pulse is, and is during the time window where the gun will accept a pulse and use it for speed calculation. Since the jamming pulse makes it back to the gun first, the gun's own reflected pulse is ignored.
Jammers recognise individual guns based on the pulse rate, this is called using a "look up table". With a look up table, jamming can be customized for the specific laser guns. This is done so that pulse timing can be custom-tailored for each laser gun, for optimal jamming and avoiding jam codes. In addition to a "look up table", some jammers use a default algorithm to attempt to jam unrecognised guns. Not all jammers use a default algorithm.
The laser guns generally need 30-60 consistent pulses in order for a speed reading to be displayed. In general, the pulse timing in the time window can varied slightly for each pulse. This varies enough so that they do not correspond to any speed, so no speed is displayed. But other techniques might also be used, such as discussed below.
As for avoiding jam codes: the guns use different schemes to detect jamming. In order to attempt to avoid jam codes, a few different techniques are used, such as precisely timing jamming pulses in the time window to fake conditions which might be present during regular targeting. These conditions might cause other operational error codes to appear on the gun, but codes which do not indicate jamming. Another method might be to limit pulse amplitude in certain situations.
There was an older jamming technique, used in the Lidatek LE-10 and Target LE-850. Instead of precisely timing jamming pulses with incoming pulse, these jammers first detect the laser, and then transmit jamming pulses at a very high frequency, generally 4 MHz. At this frequency, there is a pulse transmitted every 250 nanoseconds. This ensures that there will always be at least one jamming pulse which makes it into the time window for the gun, as long as the vehicle is about 125 feet away from the gun or further (light travels at about 1 foot per nanosecond, if the vehicle is 125 feet away the time of flight of the gun's pulse is 250 nanoseconds). This scheme should successfully jam all guns. But, there are some drawbacks. This scheme will definitely cause jam codes on the guns. And, the laser diodes are not rated to operate at 4 MHz, but jammer makers found that they could operate at this frequency for a very short time without burning up. So, jamming is only possible for a short period of time, generally a number of seconds. After that, the laser diode must cool before it can jam again.
Well, I hope this covered it :wink:
Jim
LIDAR facts
Reply With Quote
Bookmarks