RE: Do jammers work? NO Audi content

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> Principle is simple enough.  RADAR guns (at least the old 
> ones) calculate
> the Doppler shift between the signal they transmit and the one that is
> reflected back off the object they are illuminating.  The 
> frequency shift
> difference is converted to your mph.  All you have to do is inject the
> proper shift frequency onto the RF carrier at the RADAR 
> frequency you want.
> Using this method, you can make the RADAR read anything you 
> want.  I tested
> mine after I built it on an old X-band traffic unit I had 
> given to me by a
> friend who worked at the local PD.  It really did work.  The 
> read out on the
> RADAR unit was 53 mph no matter how fast, or slow I was going.

... given that you are moving at some velocity the Doppler effect will
continue to be acting.  If your signal generator is running at some fixed
offset, then the observer will see that frequecy further offset ... higher
if you're moving toward the sensor and lower if you're moving away ... good
ol' red and blue shift ...

The technique that I've heard is used to jam the receiver is to send a
constantly changing signal which is stronger than the returned signal from
your car.  If you vary the signal so fast that it is not likely to narurally
occur from a moving body then the radar gun cannot give a speed reading.  I
have heard that some of the newer guns to detect such occurrences and signal
a jamming attempt.  Here in California they did pass a law making it illegal
to jam radar signals.  I'm not sure, but I think that they are supposed to
be able to impound the jamming equipment and there's a fairly heavy fine.
That sort of law got passed because they argued that someone who had such a
device was intending to speed ...

Laser might actually be easier to jam, becuase rather than the Doppler
effect the gun simply takes several samples of distance over time and
computes speed.  If you can send a signal that is stronger than the return
from the gun, there's no way to compute speed ... or even detect that you're
being jammed.  Some time back there was some discussion about putting an IR
filter on a really bright incandescent light to jam laser ... I don't know
if anyone actually tried it out ...

Steve Buchholz
San Jose, CA (USA)