Re: More laser stuff

[steveb@falcon.kla.com (Steven Buchholz)]

I (silly me for jumping into the middle of this thread) wrote:

> >It is my understanding that the "LASER" speed detectors do _not_ use the 
> >Doppler effect.  Doppler effect assumes that you can sense the frequency 
> >change that occurs due to the relative velocity difference between the 
> >source/receiver and the target.  The "LASER" speed detectors measure the 
> >amount of time it takes for light pulse to reach the target and return, 
> >and they use the change in this flight time to estimate speed.

To which Igor replied:
> 
> OFF COURSE it works on the Doppler Effect! The RED SHIFT occures on the 
> "relatively" slow pulse train the carrier frequency (IR light) is modulated 
> with, not on the frequency of light itself. 

Say what?  The red shift occurs in the relatively slow pulse train of the 
modulation frequency?  Any wavelength shift is going to occur in the carrier.
If I read your statement a certain way it almost sounds like you're saying 
the same thing I am.  You can sense the change in the return time to estimate 
speed.  
> 
> According to your postulate if a Smokey Bear is shooting at you @ 1km, the 
> detector should have the ability to MEASURE not even the time the beam goes 
> to the target and back, i.e. 1/300000 x 2 (sec), but the GRADIENT of this 
> time (the first order derivative). In other words, it should RESOLVE and 
> REPEAT at least one order of magnitude better (1/1500000sec).
> When was the last time you've seen a cop armed with the Michaelson 
> Interferometer!?
> 
1/1500000 of a second is 666.7 nanoseconds, which is ALL DAY where I work!
Using the changes in flight time rather than the doppler effect has been 
used for many years in standard RADAR systems at airports.  It is not rocket
science.  To use the Doppler effect you would *need* an interferometer to 
be able to get the beat frequency caused by the motion of the sampled object.
The other option would be to have a wavelength measuring device to sense the
difference in the returned beam.  My thinking is that simply having a timer
that started when you sent out a pulse and stopped when it got the return 
tells you distance, and collecting a couple of samples will allow you to 
compute velocity.  I cannot state for a fact what technology is used, clearly
either can be made to work, and I've read that the systems use the change in
light pulse flight time to measure velocity.  I could be wrong.  

... someone else made a comment about if you were moving tangent to the 
sensor that would be sufficient to get a zero speed with a single sample 
... since my theory was that it takes multiple samples to get the speed 
estimate you would need to be driving in a circle centered on the ossifer 
to actually get the zero speed reading.

Steve Buchholz
s_buchho@kla.com
San Jose, CA (USA)