Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!trinity!max
From: max@trinity.uucp (Max Hauser)
Newsgroups: sci.electronics
Subject: Re: radar countermeasures
Message-ID: <1123@pasteur.Berkeley.Edu>
Date: 1 Mar 88 06:18:18 GMT
References: <4596@pucc.Princeton.EDU> <20271@bu-cs.BU.EDU> <912@spar.SPAR.SLB.COM>
Sender: news@pasteur.Berkeley.Edu
Reply-To: max@eros.UUCP (Max Hauser)
Organization: UC Berkeley
Lines: 72
Summary: Such a lot of nonsense.

In article <20271@bu-cs.BU.EDU>, madd@bu-cs.BU.EDU (Jim Frost) writes:

> In article <4596@pucc.Princeton.EDU> MJSCHMEL@pucc.Princeton.EDU writes:
> >Do you think it's feasible (forget about legal for the moment because I'm
> >speaking, of course, hypothetically) to build a radar TRANSPONDER? ...
> >[It] would send back a signal to your friendly Highway
> >Patrolman on his X or K or whatever that would register a perfectly
> >legal double nickels on the Law's satanic little radar gun's readout.
> >[...] LET ME KNOW WHY THIS COULD(N'T) WORK.
> 
> I can think of a couple of reasons why building it would be
> impractical and also why it probably wouldn't work.

I too would be happy to say why it "could" not work if I did not already
know that it's been done. With custom microwave chips, no less. Years ago. 

Or so I understand. I myself do not know the details. 

Of course, accomplishing such a task would demand that you both knew 
exactly what you were doing and also had an element of finesse, both of
which argue fundamentally against the possibility of finding any 
authoritative guidance on the Usenet.

> As for why it wouldn't work, your car will also send back an image.
> The radar gun would get back both your image and the pseudo image that
> your transponder is sending.  This would confuse the hell out of the
> radar gun!

Forgive me, but this sounds very much like a digital person's reaction
to a straightforward analog problem.

> The transponder idea is a pretty good one, actually, but the idea
> wouldn't be to send a "55" to the offending radar gun, but instead
> send a lot of extraneous signals to cause the radar gun to become
> confused ("hopelessly hosed" is probably a better term).  

No, the idea is to send 55. Or better still, 55.4 or 59 (let's be realistic).
4596@pucc was right on target. Indeed, the way this works so well is that 
local cops expect to see confused equipment, or bad readings, but they do
not expect to see loud and clear readings of fine legal speeds. The equipment
says 59, and the equipment cost $6700, so they must be going 59, Lieutenant.

At least, this is how I understand it works.


In article <912@spar.SPAR.SLB.COM>, malcolm@spar.SPAR.SLB.COM (Malcolm Slaney)
writes:

> In article <20271@bu-cs.BU.EDU> madd@bu-it.bu.edu (Jim Frost) writes:
> >Impractical:  Determining the proper time to send the
> >pseudo-reflection would be relatively difficult.
> 
> It is very unlikely that any radar gun could be built that times the
> arrival time of a reflected pulse.  Much more likely is a system based
> on doppler.  
> 
> Spoofing radar is very common in the defense community.  Contact your
> friends at TRW or any other of the military/electronice industry for
> more information.  (I can just conjecture what they do.)

I'm not obliged to conjecture, having done it. Arrival time vs. doppler
indeed. We are mixing apples and oranges (or, as it were, range and velocity).
(Do I need to point out that even pulsed radars examine doppler shift?
Evidently). Police radars, unlike "real" radars, are concerned with velocity,
not with range: a narrow slice of the ambiguity surface. It is sufficient --
in principle, mind -- to SSB-modulate an incoming carrier with a frequency 
corresponding to the doppler shift you want on the received band, correcting 
the modulation frequency as necessary from a speedometer or wheel-tachometer
link. This could, of course, be construed as obstructing justice.

Please drive safely.

Max Hauser / max@eros.berkeley.edu / ...{!decvax}!ucbvax!eros!max