Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!ll-xn!mit-eddie!bu-cs!madd
From: madd@bu-cs.BU.EDU (Jim Frost)
Newsgroups: sci.electronics
Subject: Re: radar countermeasures
Message-ID: <20271@bu-cs.BU.EDU>
Date: 29 Feb 88 17:48:48 GMT
References: <4596@pucc.Princeton.EDU>
Reply-To: madd@bu-it.bu.edu (Jim Frost)
Followup-To: sci.electronics
Organization: Boston University Distributed Systems Group
Lines: 39

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?
>(I hear your wheels turning already)
>[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.

Impractical:  Determining the proper time to send the
pseudo-reflection would be relatively difficult.

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!

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).  There are at
least two things that could be done about this kind of transponder.

The police could stop cars that appear to cause their radar guns to
screw up (I'd assume that it's not legal to transmit the signals in
the first place, and even if it is it's easy to pass a new law).

Second, a tagged radar signal would allow the gun to filter out
inappropriate signals, unless you make your transponder smart enough
to detect tagged signals and respond with the tag.  Unlikely at best,
and harder than that because it's real simple to make the gun transmit
tags that shift between transmissions (such as using a tag with a
clock value associated with it).  A transponder smart enough to handle
this would be slow in responding and cost more than the tickets would.

jim frost
madd@bu-it.bu.edu