radar reflector- is it still needed if you have radar fitted.

You get caught because

1. Metal reflects a radar signal better than GRP, your car is one big radar reflector.

2.A police radar gun is pointed directly at you usually at a short range, i.e. you come around the bend and there is Mr. Policeman pointing his radar straight at you. It doesn't send the beam out in a 360 degree pattern, hitting you car once every revolution like a marine radar, the beam is narrow and pointed at you until he puts the gun down and stops you to give you a nice b*ll*cking.

3. His gun only needs to time the return from your car in order to work out your speed, so a small low power reflection is enough, he is not trying to generate a picture on a CRT or LCD screen so high power not required.
 
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"I have never owned a car with a radar reflector fitted"
Oh, but you have. Some years ago I did extensive research with police radar guns (in a partially successful attempt to devise a 'stealth' motorcycle). The headlights proved to be very capable radar reflectors.

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Agreed. Very difficult to make a car that doesn't reflect. But I did know someone who made a modulated reflector, that sent back a signal showing a speed 30 mph higher than he was actually doing. "Me constable? No. What does your meter say now that I'm stopped? I think it needs servicing." His job? With a manufacturer of such devices, servicing the ones that were returned.
 
I would be interested to hear how you're friend fooled the gun in this way. As I understand it there is no information carried in the radar signal about a car's speed. Radar guns work, as I understand them, by measuring either the doppler effect of the returned signal or by carrying out a number of measurements of the round triptime of the signal. It seems to me that you would have to defeat some primary law of physics to mess with either method.

Could be wrong though, often am /forums/images/graemlins/smile.gif
 
There are those who argue that the best radar reflectors on a yacht are the diesel engine and the "hole" in the water made by the yacht's hull.
What does puzzle me is that there appear to be no definitive studies published that would settle the arguments once and for all.
 
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I would be interested to hear how you're friend fooled the gun in this way. As I understand it there is no information carried in the radar signal about a car's speed. Radar guns work, as I understand them, by measuring either the doppler effect of the returned signal or by carrying out a number of measurements of the round triptime of the signal. It seems to me that you would have to defeat some primary law of physics to mess with either method.


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The doppler manifests itself as a shift in frequency. Just have a system which receives the RADAR signal and re-transmits x Hz above.
 
I believe most police radars use doppler effect to get the speed - so it would be theoretically possible to fit an active transponder that responds on a slightly lower frequency than the incident wave hence fooling the radar that you are going slower.

Could be very expensive testing it though /forums/images/graemlins/cool.gif
 
Possibly an oscillator slightly shifted from the police frequency so as to mix the appropriate beat frequency at the gun would submerge the reflected return from the car.
 
One of the magazines did a study some years ago.

The results were so mixed as to be of little value.

There most certainly was definitive rsults that showed anthing like:

reflected signal without reflector = xdb
reflected signal with reflector = x+30db

Which is what I would like to see.

I suspect most of the problems come with the user of the radar - as someone pointed out there is a skill to tuning, using and interpreting the screen.

Perhaps above all there is the need for someone to be looking at the thing.

Also there are so many eccentric views about how these things work and what they are and are not capable of that one needs to apply a considerable degree of scepicism to anything on these forums.
 
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[ QUOTE ]
I would be interested to hear how you're friend fooled the gun in this way. As I understand it there is no information carried in the radar signal about a car's speed. Radar guns work, as I understand them, by measuring either the doppler effect of the returned signal or by carrying out a number of measurements of the round triptime of the signal. It seems to me that you would have to defeat some primary law of physics to mess with either method.


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The doppler manifests itself as a shift in frequency. Just have a system which receives the RADAR signal and re-transmits x Hz above.

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But then the radar would reject the altered freq. as it does a similar radar in vicinity.
The only radar rec. apparatus that I know that alters a return is a Racon - and as far as I know that does it by enhancing / increasing the strength of the return in a set pulse resembling morse dots - dashes, but freq. remains same.

It's one of those tales that does the rounds I think ... like why do people have CD's dangling from rear-view mirrors / stuck in truck windscreens ..... to deflect the laser speed gun !!

/forums/images/graemlins/laugh.gif
 
I suspect it would have to be rather more sophisticated than that.

To start with the frequency stability of these things is not that great so it would be essential to derive the signal you return from the incoming signal from the police.

Then you would have to calculate what shift would be applied by Doppler for the speed you actually are travelling.

Then you would have to add an additional shift in frequency to the calculated frequency to give the false reading.

Then you would have to actually switch on a trnsmitter to send out the signal.

All things being equal a wideband noise generator to simply swamp the receiver would be a much simpler proposition.
 
Actually, there's a much simpler option (although this is getting a bit away from the original question). In theory, if you can reduce the radar signature of the vehicle as a whole (not all that difficult with a motorcycle, at least), then add efficient reflectors in the wheels as close as practical to their rims, the reflected doppler reading would either approach twice the vehicle speed, or zero, depending on which way the reflectors were orientated.
 
Nigel

The Doppler effect is what makes the note of a train whistle change as it moves toward you or away from you - an apparent change of frequency of the sound as experienced by a stationary listener.

A more magnificent example is the magic deepening growl of a Merlin engine when a Spitfire passes overhead.

The same thing happens with radio waves that bounce off a moving object - there is an apparent shift in frequency. What a radar speed gun does is measure this change in frequency between its tranmitted and received signal and calculates (very accurately) the speed of the object.

Racons are a completely different thing they actually transmit a specific signal in response to being scanned by a radar signal.
 
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Nigel

The Doppler effect is what makes the note of a train whistle change as it moves toward you or away from you - an apparent change of frequency of the sound as experienced by a stationary listener.

A more magnificent example is the magic deepening growl of a Merlin engine when a Spitfire passes overhead.

The same thing happens with radio waves that bounce off a moving object - there is an apparent shift in frequency. What a radar speed gun does is measure this change in frequency between its tranmitted and received signal and calculates (very accurately) the speed of the object.

Racons are a completely different thing they actually transmit a specific signal in response to being scanned by a radar signal.

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I am aware of doppler effect ... having had to study it at college for tickets ... then onto Doppler Speed Logs, Doppler Docking Stations, Lightering ships close approach systems etc.

And it is an "apparent" shift.

Let's put it this way - a "modulator" as earlier attested to ... possible but quite an effort and I would imagine sophisticated piece of kit !!!

The racon has to enhance the return signal ... to produce the effect
 
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Sorry

Only trying to help

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No problem .... no offence ... honest !! Hope no offence to you either ... /forums/images/graemlins/cool.gif
 
As of 1 July 2002, ALL VESSELS regardless of size are required by Law (implementation of SOLAS V) to:

1. Voyage Plan
2. Fit RADAR reflector "if practicable". (IT ALWAYS IS PRACTICABLE what ever the vessel).
3.Carry on Board the vessel a copy of "LIFE SAVING SIGNALS" for communication with rescuer.
4.Provde assisstance to other craft.

amongst other thing.
 
[ QUOTE ]
As of 1 July 2002, ALL VESSELS regardless of size are required by Law (implementation of SOLAS V) to:

1. Voyage Plan
2. Fit RADAR reflector "if practicable". (IT ALWAYS IS PRACTICABLE what ever the vessel).
3.Carry on Board the vessel a copy of "LIFE SAVING SIGNALS" for communication with rescuer.
4.Provde assisstance to other craft.

amongst other thing.

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The "(IT ALWAYS IS PRACTICABLE what ever the vessel)." is your addition to the "rule" ... so in fact it is "if practicable" .... so NOT strictly legisltated or required.

I love it when text is taken and personal added / subtracted to make a point ... sort of destroys it !! /forums/images/graemlins/cool.gif
 
That's not that difficult - it is basically just a radar transponder (commonly available as SARs) with a small frequency shift - a fixed frequency shift will be fine to subtract a predictable amount from the speed.

I think there will be an analogue solution for that - so it is not much of an adaptation
 
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