john@stiatl.UUCP (John DeArmond) (02/23/89)
In article <8260@netnews.upenn.edu> depolo@eniac.seas.upenn.edu.UUCP (Jeff DePolo) writes: Maybe we ought to stop and apply some physics to this discussion. it's getting a bit awry. >In article <4057@alvin.mcnc.org> bishel@mcnc.org.UUCP (Geoffrey R. Bishel) writes: >>A laser will spread 3.5 feet in 1/5 mile? Spread is easily handled with simple optics. At any distance over perhaps 50 feet, the focal point will be infinity which means that the spot can be focused to an arbitrairly small dot limited by the optics and atmospherics. I'd expect to be able to hold under an inch at a half-mile. >Another thing to think about - a beam a few feet wide is going to reflect >a beam a bit bigger than that, but not much. If the copper is on the side >of the road, he's going to have to "wiggle" the laser around until it hits >a perpendicular plane on your car. If, for example, he hits the hood, most Not true. I have seen no specs on the new unit so this is speculation. I expect that they will be using interferometry instead of doppler. A recovered reflection is mixed with a sample of the incident beam and a detector is used to count interference edges. The countrate is convertable into speed. This is how such instruments designed for other applications work. Using interferometry, all the receiving optics need do is recover an image of the illumination spot on the target. I'd expect the operator to view the target thru the same scope, SLR-style. He would then only need to place the spot on a target to measure it. As I mentioned before, I've seen nothing on this instrument. However, for the prices quoted, I'd expect to see something of this sophistication. Such technology has been available to the metrologist and surveyer for years. I feel fairly safe in stating that this thing should be accurate and impossible to detect or defend against, at least passivly. I could imagine a powerful IR laser and suitable optics flooding the area in front of the car with enough diffuse light to blind the unit. But this would take serious money. I believe, gentlemen, that it is time to take the political remedy to this speed limit game once and for all. John -- John De Armond, WD4OQC | Manual? ... What manual ?!? Sales Technologies, Inc. Atlanta, GA | This is Unix, My son, You ...!gatech!stiatl!john | just GOTTA Know!!!
john@anasaz.UUCP (John Moore) (02/23/89)
In article <3349@stiatl.UUCP> john@stiatl.UUCP (John DeArmond) writes: >In article <8260@netnews.upenn.edu> depolo@eniac.seas.upenn.edu.UUCP (Jeff DePolo) writes: >Not true. I have seen no specs on the new unit so this is speculation. >I expect that they will be using interferometry instead of doppler. A ^^^^^^^^^^^^^^ == ^^^^^^^ >recovered reflection is mixed with a sample of the incident beam and a >detector is used to count interference edges. The countrate is convertable >into speed. This is how such instruments designed for other applications >work. Using interferometry, all the receiving optics need do is recover This is identical to doppler processing - you are just thinking of it in the phase domain rather than the frequency domain. To detect doppler on a radar signal, just mix the outgoing and incoming frequencies. The audio difference frequency is the doppler. Measuring the frequency of this difference frequency (=== counting the cycles == interference edges!) and doing simple math with the carrier frequency gives you the speed. This is how a police radar works. A simple radar uses a "gunnplexor" which has a circulator and a mixer diode. The circulator causes some of the transmitted energy to impinge on the mixer diode. The antenna causes the reflected signal to also hit the mixer diode. Audio comes off the diode and is processed for speed. -- John Moore (NJ7E) mcdphx!anasaz!john asuvax!anasaz!john (602) 861-7607 (day or eve) The opinions expressed here are obviously not mine, so they must be someone else's. :-)