Why would a speed-measuring radar do this?

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Justin Kraus

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Why would a speed-measuring radar do this?
« on: 28/09/2009 08:30:03 »
Justin Kraus  asked the Naked Scientists:
   Dear Naked Scientists,

Hello from Lake Tahoe California USA. Awesome podcast; I'm a hopeless addict.  I was listening to an older podcast in which you made mention of the Doppler effect and how it's used in radar systems to detect a vehicle's speed.

At just the moment you mentioned a radar's capabilities I passed by a device which displays your vehicle's speed while driving through a construction zone and I was perplexed by what I saw.

I was travelling at 70mph and there was a lorry off to my right which I was about to overtake. I noticed the speed display on the radar system measured the lorry's speed first at 47mph. Then it measured my speed 70mph. Then, as I was just moving behind the lorry it appeared to measure the difference of our two speeds e.g. 23mph.

What's going on here?

Kind Regards,

Justin Kraus

What do you think?


Offline graham.d

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Why would a speed-measuring radar do this?
« Reply #1 on: 28/09/2009 09:35:52 »
This is possible because the radar signal can reflect from the front of the approaching lorry, on to the rear of your car, then back the lorry and finally return to the radar detector. The doppler shift will be from the difference between the two speeds.

This sort of multiple image reflection, if reflecting to a vehicle going in the opposite direction, could also result in a much increased reading comapred with the true speed of any of the vehicles. This has been used as a valid defence against speeding convictions and was successfully used by a friend of mine (a Canadian) in New York State. This was, however, back in the early days of the use of the technology and he happened to be an expert on radar and a very erudite and confident speaker.

As a cheap system just to warn drivers, I doubt that too much care is taken to avoid such false readings. Radar speed detection used to secure convictions have to be a lot more precise. I am unsure of the techniques used but I imagine that there are several ways to reject false readings - narrow beams and pulsed radar to only accept readings at a specific distance position from the detector etc. There are stories about accidentally detecting jet aircraft but these are probably apocryphal.


Offline techmind

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Why would a speed-measuring radar do this?
« Reply #2 on: 28/09/2009 22:46:43 »
While as graham.d says, it could be a multiple refection. Another possibility is overload and distortion in the radar receiver.

If the radar operated at 10.7GHz (they may operate at higher frequency these days) then the received Doppler frequencies are audio, e.g. 70mph = 2232Hz, 47mph = 1499Hz and 23mph = 733Hz.
I imagine they'd use a Fourier transform to analyse the jumble of signals being received, and then pick the strongest and/or most consistent peak to display as the "speed". Now under normal conditions, the combination of two tones do not produce a difference-tone - so you wouldn't expect to see 23mph flash up.
However, if the radar receiver amplifier overloaded and went into saturation (and a lorry at close range will give a BIG signal) then with the two input frequencies you do see distortion products at around 750Hz (23mph), among others.

With only a single significant target, the distortion frequencies are at three times the real speed, so can be safely ignored by the software.


Two signals from vehicles at different speeds should not normally interfere


...unless the receiver amplifier saturates!


Innards of a Doppler radar module which could be adapted for speed-measuring if wired to a laptop-computer "audio-in" connection!
« Last Edit: 28/09/2009 23:07:12 by techmind »
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