1. ## Could a radar detector display speed?

We all know they can display the frequency, signal strength, and (for V1) direction. Could they also do what the cop's radar gun does, display speed? The cop's radar is reading a weak return signal, but our detectors would get a strong direct signal.

Displaying the relative speed between the cop and our cars could be useful. Is he getting closer or farther away? Is he speeding up behind me? Is he stationary or moving?

2. ## Re: Could a radar detector display speed?

Originally Posted by aspera
We all know they can display the frequency, signal strength, and (for V1) direction. Could they also do what the cop's radar gun does, display speed? The cop's radar is reading a weak return signal, but our detectors would get a strong direct signal.

Displaying the relative speed between the cop and our cars could be useful. Is he getting closer or farther away? Is he speeding up behind me? Is he stationary or moving?
Basically no, it couldn't. Without going into too many details: there isn't a doppler-shifted signal representing the target speed until the radar actually reflects off the moving target vehicle.

Jim

3. ## Re: Could a radar detector display speed?

Originally Posted by aspera
We all know they can display the frequency, signal strength, and (for V1) direction. Could they also do what the cop's radar gun does, display speed? The cop's radar is reading a weak return signal, but our detectors would get a strong direct signal.

Displaying the relative speed between the cop and our cars could be useful. Is he getting closer or farther away? Is he speeding up behind me? Is he stationary or moving?
Having the V1's arrows at least you can tell if the LEO is coming from behind and narrowing the gap with your car. That's about all you could hope to achieve. Radar guns compare the signal they transmit with the reflected one that comes back, so they have two signals to use to figure the speed. A V1 or any other detector has no way of knowing what the original signal was - though it would take a complete redesign in order to do something with the signal.

Would be cool, if it was do-able, but nope.

4. But, aren't the radar frequencies known values?

Laser is 904nm, right? Couldn't a laser detector compare the measured laser against 904nm?

5. ## Re: welcome

Welcome to the board aspera. Mk

6. So if I understand correctly you want a radar gun built in a V1?

7. Theoretically, for radar, yes. The doppler shift of the radar EM wave is still heard by the car. The detector (or whatever it might be called in this case) would have to measure the difference in the frequency heard and the reference frequency. However, it would have to know the reference frequency pretty accurately and I don't know if this is possible. I would assume that the cop's radar gun knows this -- it probably measures the reference (not just assumes a value) along with the return.

For laser, no. Laser measures the round trip time to determine distance. It does this on several occasions then computes speed by dividing change in this distance with time.

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