Crash course in detection physics

[Messiah]

This was all pretty good until i saw the line "whilst i must agree that radar works on bouncing a high freq pulse of sound".

The only detection too that uses sound is sonar, and this is due to the high refraction level of light through water which makes light useless for detecting vessals. RADAR on the other hand (and LIDAR) both use pulses taken from the electromagnetic spectrum ie light but at different wavelengths. Sound wouldnt be too useful for car speed detection as the higher the sound frequency the lower distance it can be used at and the lower the frequency the less 'acurate' the measurement. This coupled with the fact that sound can be easily distorted by enviromental factors make its a pretty dodgy tool to use.

Now on to the Number plate issue. Now if we set up a few assumptions ie...

1. The Radar/Lidar device has a very narrow beam

2. The cop holding it has a very steady hand

3. The same cop has the worlds greatest aim

Then yes tilting your number plate down by 10Degs will cause the beam to reflect into the ground before it reaches the emitter.

Unfortunately the last 2 assumptions are incalcuable and the first is very unlikely (which makes the last 2 redundant anyway). In reality you'll have the following problems.

1. The motion of the car coupled with the small corrections in the coppers aim will mean that the beam will not stay focused on that one area

2. The edges in the number plate lettering would cause enough refraction to atleast reflect a fraction of the beam back to the emmiter.

3. Since we know that the beam is not restricted to the number plate we have to assume that the entire fron end of the car will act as a potential radar/lidar cross section.

Now dont get me wrong, i'm not honking on about people trying to be sneaky and get out of stuff they ****ed up. Nor am i saying that all this is utter crap. Instead i'm trying to put it all in perspective.

It is possible to make changes to your car that will reduce your cars radar cross section, however the changes neccesery to reduce it to a level which would make it undetectable (or at the very least untargetable) would require so much boddy modding your nice shiny skyline front end would look more at home in the aussie solar challenge.

The real trick is to use a combination of what alot of you have suggested, and turn your car into an "unclean" target. here are a few additions.

1. Use a high gloss transparent spray on yr number plate (i have heard hair spray works but i could be wrong).

2. Mount Ir flood lights near yr number plates. I have a few of these lying around. They produce no visual light but anything wishing to detect an IR reflection from your car will instead receive IR "noise", and anything trying to use IR as an illumination device for say photography will instead find the pics washed out and over exposed.

3. avoid fixed speed cameras and coppers

4. dont speed.

Yeah ok the last 2 were cheating but eh! what ya gonna do. Now nothings fool proof, and i dont endorse any of these sneaky measures, so if u get nicked using any dont point them to me. On the other hand, if anyone has a spare of NVG's available (passive that is) and cares to test its effectiveness i'd love to hear the results.

Just my 2p.

Messiah

[awill]

IR spot lights hey.

hmm

Where might we be able to purchase these and would they look neat enough on the front of a car??

oh also how much would they be does anyone know?

[GTRman1992]

IR spot lights is an interesting idea, and would probably work well. The high reflective number plate won't work on IR photography and also draws attention to yourself.

What about the old fashon radar, which is not light. These would still nail you with all ideas above.

I think the idea of tilting number plates etc is good for both methods of getting done as the more you reduce your radar footprint, the closer the device will have to be to get you, so you may have a chance of spotting it and punching the centre pedal.

[Warpspeed]

Forget about the high intensity LEDs around the numberplate, I have tried it. If you use a night vision viewer you can see how things look at infrared easily. Your numberplate looks like it has Christmas tree lights around it. Quite pretty actually, but you can easily still read the numberplate.

I do not understand how tilting the numberplate is going to help. I can easily see a numberplate that is angled 45 degrees away from my line of vision. It certainly does not disappear or become invisible. If I can see it, so can a RADAR or LIDAR.

[craz0]

The last time I got done for speeding I was Driving my Mazda SP20 and they told me once I pulled over that they couldn't get a signal of my car until I was right in front of them. I was driving down the Highway with the sun in my eyes (so I didn’t see them coming) and they where coming the opposite way they had the gun mounted on the side of there car, one of the police officers was very interested about the front end of the car asking me if it was standard and what it was made out of and that, he was telling me that the car was like absorbing the radar and they only got a signal once they where a couple of meters away from me and he reckons it was of my bonnet or roof. I can’t remember if the car was clean or not probably covered in bug guts and road grime, most off the time I have a car bra on it when I’m on the Highway.

Thought it might ad a bit to the conversation.

[OZ GTR35]

Guys,

I’m x-military and have been trained and worked on radar's for about 5 years. Its all crap, the only way to jam radar is to transmit the same signal back to the radar to confuse the receiver and give a false reading. The cops radars are that sensitive that your head alone would send back enough signal for them to measure a phase shift in your momentum.

So in 100 words or less how does a cops radar work?

The cop transmits a pulsed signal (can be from sound waves to giga wavelengths, in the NSW cops case IR) at an object, the object reflects a small amount of that signal back and the radar measures the phase shift between the sent signal and the received signal. If you are moving towards the radar then you will compress the signal and if you move away from the radar then you de-compress the pulsed signal.

The classic example is the train bell at a rail way crossing. The trains bell rings at the same frequency but as the train moves towards you it sounds like a higher frequency as the waves are compressed, once the train is passed, it sound lower in frequency as the wave is decompressed.

The ever slight difference in pulsed waves can be measured and hence determine your speed.

As the cops use the IR frequency there is virtually no way in blocking this without a disco ball which radiates IR in all directions, painting your car in IR absorbing paint means you would need to paint yourself and the windscreens tyres etc.

The cops are good aims as they can pick up bikes if they are on a tripod.

All in all you cant do much that’s why they changed from the old K-band radars which you use to be able to block with a 1 watt broad frequency TX.

[Ustasa]

Oz,

What about passive jamming as in adding a constructive/destructive signal onto the original pulse? Ie. front grille converted into a antenna?

How hard would it be to set up an active jammer which is powered and can send out noise on the appropriate radar frequency?

[OZ GTR35]

Works well for the old radar frequencies, but you will need something else to transmit IR to combat the new hand help radars. A disco ball of IR transmitters at the right IR frequency(s) would help to disrupt the signal.