Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!mcdphx!hrc!gtephx!pfluegerm
From: pfluegerm@gtephx.UUCP (Mike Pflueger)
Newsgroups: sci.electronics
Subject: Re: POLICE hand-held RADAR units
Message-ID: <231@alpine.gtephx.UUCP>
Date: 21 Feb 91 16:47:33 GMT
References: <2470014@hp-vcd.HP.COM> <37842@netnews.upenn.edu>
Organization: AG Communication Systems - Phoenix, AZ
Lines: 41

In article <37842@netnews.upenn.edu>, depolo@eniac.seas.upenn.edu (Jeff DePolo) writes:
> the speed of one of the vehicles behing clocked.  The problem is that
> you don't know which vehicle it is.  Hence, the accuracy is still within
> tolerance, only you don't know which vehicle is the target.  Consider
> one car passing another (55 MPH and 75 MPH).  While the passer is 
> behind the passee, chances are that the radar gun is going to return
> the lead car's speed -- 55 MPH.  When the passer passes the passee, 
> the radar will probably read 75 MPH.  Hence a 20 MPH difference.  This

I'd like to add a comment here - and one which CAN be used as a defense
for radar speeding tickets.  Possibly the best "technical" defense.

Due to the relative received signal strengths, the capture effect, and
radar guns' electronic bias, the stronger return will override the weaker
one.  Received signal strength being roughly equal, the FASTER speed will
be displayed.

So, if you're in front of (or behind) a semi rig in your compact car,
within say 150 feet, in a freeway situation, an oncoming cop may never
pick up your speed - the radar'll probably only see the truck.

I believe radar guns have about a 3 degree beamwidth.  At 1000'  (1/5
mile, roughly the detection range), the radar beamwidth is 105'. 
Consider the angles and the radar is illuminating a LONG stretch of
the oncoming lane.

Not only is the size of the vehicle a big factor in reflected signal,
but shape and even the construction play a part.  I can vouch for the
1977 corvette being noticably stealthy - it doesn't seem to return a
reliable reflection until about 500' for hand held K band units :-)

There are a lot of factors involved in reflected signal strength.  Suffice
it to say that if there are any cars near you and the cop is coming toward
you (or shooting your lane at a shallow angle), it can be very tough to
determine (for sure) which vehicle is being clocked.  Now, if he's shooting
more across the lane or no one else is around, that's a different story...
-- 
Mike Pflueger @ AG Communication Systems (formerly GTE Comm. Sys.), Phoenix, AZ
  UUCP: {...!ames!ncar!noao!asuvax | uunet!hrc | att}!gtephx!pfluegerm
  Work: 602-582-7049        FAX: 602-582-7624     Home: 602-439-1978
Packet: WD8KPZ @ W1FJI     Internet: PLEASE USE UUCP PATH (NOT INTERNET)!


Brought to you by Super Global Mega Corp .com