Re: IR filters

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <m0zKHLl-000JpmC@post.cis.smu.edu> randrews@post.cis.smu.edu (Robert Paul Andrews) writes:

> That may be true, but what RADAR jammer do you know of that works.  I'd like
> if it does because it would be illegal even to operate anywhere as it would
> have to broadcast at such a high wattage, above for what the FCC allows
> citizens without a license for whatever they operate.

You're thinking of _swamping_ the return signal.  Not easy with RADAR,
because it's so easy to make highly selective filters.

What a jammer does is to emit signals of very much the same strength
as the return signal from the RADAR being jammed.  These signals are
modulated so as to defeat the measuring device attached to the
receivers output.  If it's a plan/position/ranging system using pulses,
the jammer returns pulses at erratic times making the RADAR 'believe'
it's seen a pulse from somewhere where there is, in fact, no target.
The trick is to do this on each pass so that the phantom target is
correctly simulated each time.  RADARs often incorporate an encrypted
key into each pulse, that also has to be simulated, and so it goes on.

The Doppler effect system that hand-helds use works by filtering out
the return signal and mixing it with a little bit of the outbound
signal.  The product of this mixing is a signal with a frequency
directly proportional to the subject's speed.  It's a neat trick,
because it doesn't require a calibrated microwave source - the
frequency errors cancel out.

To fool this device, you have to receive the signal, mix a couple of
spurious sine waves into it, and re-emit it.  The RADAR then sees
several speeds (a natural situation when there are multiple vehicles
about) and refuses to pick just one.  Of course, if you're all alone
on a desert road ...

Some police RADARs are also capable of filtering out _one_ spurious
speed signal, from the background if the RADAR is moving.  If the police
car is in motion, a signal is generated corresponding to the apparent
speed of the landscape and fed to the RADAR, effectively telling it that
it can ignore this particular component.  If only one component then
remains, that's the delta between the police vehicle's speed and the
target's speed.

Any physicists about?  I had the idea that you could mount something
like a cup-type anemometer head directly behind the radiator grill.  The
'cups' would be shaped like (and would be) miniature RADAR reflectors -
scaled down versions of the things you see on bouys and the masts of
yachts.  They would not have to be very big to generate a return as
large as the car's.  The device could be driven by the airstream, and
would have a Watt-style centripetal governor to generate a cup speed of
20mph relative to the car's motion.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
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