Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!jarthur!nntp-server.caltech.edu!fjs
From: fjs@nntp-server.caltech.edu (Fernando J. Selman)
Newsgroups: sci.electronics
Subject: Re: POLICE hand-held RADAR units
Message-ID: <1991Feb14.015812.14576@nntp-server.caltech.edu>
Date: 14 Feb 91 01:58:12 GMT
References: <1991Feb12.035201.16098@nntp-server.caltech.edu> <2470014@hp-vcd.HP.COM>
Organization: California Institute of Technology, Pasadena
Lines: 43

johne@hp-vcd.HP.COM (John Eaton) writes:

><<<
>< it was a K-band gunn. Because these devices actually compare the speed
>< of the car with the speed of light, it would imply an accuracy of
>----------
>Say What? 

>Where did you get this interesting little statement?

For a doppler radar, the formula to get the speed of the car is:

	Vcar = 2*(f_dopp/f_rad)*C    [that "2" might be 0.5, don't remember]

where f_dopp is the doppler shift in frequency, f_rad is the
radar frequency (10 GHz for an X band radar), Vcar is the car
speed, and C is the SPEED of LIGHT (all electromagnetic waves
do travel at this speed in vacuum). The doppler shift in frequency
is 29.8 Hz/mph at this frequency, thus, to claim a 1mph uncertainty
is equivalent to 29.8Hz/10GHz = 2.98 ppm! This is the power of the
doppler effect, that it allow you this type of accuracies, when
USED PROPERLY. But I doubt you can eliminate systematic errors to
this level using one of the toy guns produced by MPH Industries, Inc.
The cop in question had less than a second to make the measurement. I was
changing lanes from just passing another car. The uncertainty in the
cosine factor would be enough to reduce the speed measured from 71mph
to 65mph. The humidity of the day was 82%, even though the manufacturer of
the toy gun claim 90% for operational limit, I highly doubt this number
as I have never find electronics instruments rated for work
above 80% noncondensing.

	As for the main point in your posting, if the speed of light
were to be half that assumed by the logic of the instrument, no amount
of calibration would allow you to have a proper reading for all frequencies
not equal to the calibrating frequency. The fact that we measure a
frequency shift seem to confuse a lot of people into believing that
the speed of light is not the gauge in a dopple radar. Refresh your
physics! If I posted in this group was to get the knowledge that
people that have a lot of experience with chips, solder irons, and
bench experience on microwave measurements have about this topic, not
just to spread erroneous physics insights in a sarcastic way.

					- Fernando