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From: ben@moncol.UUCP (Bennett Broder)
Newsgroups: net.auto
Subject: Re: radar detectors and Newark Airport
Message-ID: <567@moncol.UUCP>
Date: Thu, 21-Nov-85 14:49:34 EST
Article-I.D.: moncol.567
Posted: Thu Nov 21 14:49:34 1985
Date-Received: Sat, 23-Nov-85 05:10:19 EST
References: <386@gcc-milo.ARPA> <558@moncol.UUCP> <466@mtunf.UUCP>, <631@mtung.UUCP>
Organization: Monmouth College, West Long Branch, NJ 07764
Lines: 24

>           Anyone out there know how police radar guns are
>built? It would be possible to modulate the 'send' signal
>and then compare this against the 'receive', hence doing
>away with this line of defence. Do any models actually do
>this?

Radar detectors work by emitting a microwave signal of a known
frequency, then comparing that frequency to the frequency of the
reflected signal.  Have you ever noticed that as a car approaches you,
it makes a high pitched sound which drops as it passes you, then makes
a low pitched sound as it heads off into the distance?  This is called
the doppler effect.  The faster the car approaches, the greater the
frequency will be shifted upwards.  Radar detectors measure the
frequency shift, then convert it into MPH.

If the police want to use a radar unit without worrying about door
openers and alarms causing interference, they can buy radar guns that
operate on the K band.  Since nearly all motion sensors are on the X
band, this is a much cheaper solution.

Ben Broder
..vax135!petsd!moncol!ben
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