Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!xanadu!michael
From: michael@xanadu.COM (Michael McClary)
Newsgroups: sci.electronics
Subject: Re: "Spybuster" AD COPY
Message-ID: <g651X#=michael@xanadu.COM>
Date: 17 Sep 89 17:09:47 GMT
References: <17100002@inmet>
Reply-To: michael@xanadu.UUCP (Michael McClary)
Organization: Xanadu Operating Company, Palo Alto, CA
Lines: 23

In article <17100002@inmet> newbie@inmet writes:
>     
>[] receives signals from the FAA's
>ground-based aircraft surveillance system.  As these signals
>reflect off  airplanes, Spybuster  locates and  tracks []

Ok, sounds like they're a radar receiver, taking advantage of
the illumination provided by the existing air traffic control
search radar, and requiring no active equipment in the plane.
Listen for the ping from the transmitter, then the echo from
the plane (which will always be later), and count the time
between them.  Doesn't give you true distance, but does
provide a lower bound, and something roughly proportional to
distance, for "is it closing" tests.

Such a system would become confused if the airport radar was
beside the road and the aircraft was between you and the
radar site.  (It would "sound" like it was right on top
of you.)  On the other hand, the reflection from the
plane still decays inverse square, so it wouldn't false
alarm very often.

Cute!