Re: [OT comment] Re: Doppler Ocsillation

Jerry Avins wrote:

Richard Owlett wrote:

Jerry Avins wrote:

[snip] In the early 50s, I made a "hood
ornament" that consisted of a pair of counter-rotating wind-driven pinwheels in front of a reflecting screen. The resulting Doppler indicated a speed of over 200 mph when I was going 60. That speed was so clearly ridiculous that I never got a ticket. Transmitters are illegal, but any group that wants to use radar can hardly object to a reflector.


Reminds me of a story I read when smaller police departments started using radar.

Speeder demanded trial and challenged accuracy of radar.

Prosecutor produced 'expert' witness who demonstrated calibration by reflection from a tuning fork.

Defense claimed prosecutor had proved radar was unreliable as it was evident that guy holding tuning for was NOT going 60 MPH.

Case dismissed.

I always thought the story apocryphal, but I do believe it represents some early reaction to radar.


But now you have me thinking. Could you have set it up such that Doppler said you were either standing still or perhaps going backwards?


The only way you can appear to be going backward is to radiate negative frequencies*. (That's not true, but it sounds good.) Most police radars, particularly the early ones, sense relative motion but not direction. What is actually detected is the beat between the CW oscillator (often a Gunn diode) and the return signal. Only a phase detector could distinguish direction, but a simple diode is used.


OK. Because of my background I tend to think in terms of Spectrum Analyzers. Thus 'positive offsets == approaching' and 'negative offsets == leaving' and appropriately adding/subtracting from patrol car velocity.

And as I wrote this I "realized error of my ways" ;}

Which leads me to "think" [perish the thought;]

Presume a moving patrol car with 'front' facing "radar device".

Using *only* Doppler and patrol cars 'speedometer' is it possible determine speed of both an approaching and receding target?

"gut" says yes. "thought" says no.

am i missing something?



.

Relevant Pages

  • Re: [OT comment] Re: Doppler Ocsillation
    ... ornament" that consisted of a pair of counter-rotating wind-driven pinwheels in front of a reflecting screen. ... but any group that wants to use radar can hardly object to a reflector. ... Could you have set it up such that Doppler said you were either standing still or perhaps going backwards? ... a sourpuss neighbor's service water pipe froze where it emerged from the ground in the garage. ...
    (comp.dsp)
  • [OT comment] Re: Doppler Ocsillation
    ... ornament" that consisted of a pair of counter-rotating wind-driven pinwheels in front of a reflecting screen. ... but any group that wants to use radar can hardly object to a reflector. ... Prosecutor produced 'expert' witness who demonstrated calibration by reflection from a tuning fork. ... Could you have set it up such that Doppler said you were either standing still or perhaps going backwards? ...
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