mwh@vax5.CIT.CORNELL.EDU (08/01/89)
In article <414@ctycal.UUCP>, ingoldsb@ctycal.COM (Terry Ingoldsby) writes: | While we're on the subject of automotive electronics, my curiosity | was whetted by an article in the paper the other night. There was | a picture of a device used by the Ontario Highway Police (Provincial | Police) of a gadget said to detect radar detectors in passing cars. | | How do these work? I presume that they attempt to detect the | oscillator in superhet receivers. ... Would not the metal cage | of the car shield out virtually all of the radiation emitted ... | the signal emitted from the detector must be quite small. When you have *any* kind of steady narrowband emitter, even a very small one like the local oscillator (LO) in a superhet receiver, a great deal of physics and mathematics aids the cause of anyone who wants to detect it. Approached from the proper signal-processing perspective, such emitters stick out in the electromagnetic environment like a magnesium flare in a darkened room, and can be detected at almost arbitrarily low amplitudes with processing-gain techniques. The problem of detecting continuous LOs is infinitely easier for example than the (still manageable) military problem of detecting frequency-agile sources or those that are inherently wideband or noiselike. (And less urgent besides, since a consumer radar detector is not normally a device trying its damnedest to kill you.) If you've been in metropolitan England you may well have noticed the receiver-detecting crews going around looking for unlicensed (and therefore untaxed) TV receivers (yes, receivers). They used the same principle, detecting the LO, in what is surely a much more cluttered electromagnetic environment than the highway police encounter. (I don't know if they still do this, but in 1972 the English TV-detector crews dressed in exotic uniforms and helmets like alien invaders from some low-budget Hammer-Studios film, or an episode of _The Avengers_). From past experience as an electronic-countermeasures engineer for the US government, I believe that detecting continuous narrowband emitters is not even considered an interesting, let alone a challenging, technical problem. If you wanted to build a car radar detector that was hard to find -- I stress that this is a hypothetical, purely technical discussion -- you'd make it completely passive, or at the very least, hop the IF frequency. Such measures are expensive (and imagine trying to sell such a product and describe it intelligently in a consumer advertisement). I have heard one account of custom microwave ICs actually designed to actively confuse police radar -- a more daring, not to mention sophisticated, approach -- by employees at a well-known US instruments firm, but I don't know about it firsthand, it may have been just a story. Copyright (c) 1989 by Max W. Hauser. All rights reserved. -- "As a ... dramatic illustration of the effect of larger specific-impulse variations, consider four identical, single-stage rockets each having a mass ratio of ten (initial to final weight) loaded with hypothetical propellants having specific impulses of 200, 300, 400 and 500 lb-sec/lb, respectively. The first can attain a maximum horizontal range of about 1300 nautical miles (theoretical, drag-free), the second about 4100 miles, the third can become an earth satellite, and the fourth can escape from the earth entirely." -- Holzmann, _Chemical Rockets and Flame and Explosives Technology_, Marcel Dekker 1969. (Quoted with respects to Norm Strong.)
dmt@PacBell.COM (Dave Turner) (08/03/89)
In article <19212@vax5.CIT.CORNELL.EDU> max@ee.cornell.edu (Max Hauser) writes:
.If you've been in metropolitan England you may well have noticed the
.receiver-detecting crews going around looking for unlicensed (and
.therefore untaxed) TV receivers (yes, receivers). They used the same
.principle, detecting the LO, in what is surely a much more cluttered
.electromagnetic environment than the highway police encounter. (I don't
.know if they still do this, but in 1972 the English TV-detector crews
.dressed in exotic uniforms and helmets like alien invaders from some
.low-budget Hammer-Studios film, or an episode of _The Avengers_).
.
There was a Monty Python skit to parody this.
The subject was pets (possibly about a license for a pet fish)
and the salesman (John Cleese) was talking about the "cat detector van"
which could detect a purr from quite a distance away.
--
Dave Turner 415/542-1299 {att,bellcore,sun,ames,decwrl}!pacbell!dmthugo@bigtime.fidonet.org (Hugo) (08/11/89)
Quite the interesting piece on ECM, and quite informative. Having
worked in ECM for a short while it is always interesting. I think,
however, that even if the passive systems or those with electronic
modifications are expensive, there sure seems to be a market for it.
Perhaps it is the modern-age answer to the 'rich ignoring the rules'.
Of such interest is the newest addition to the home ECM scene, the
aircraft detector. It is essentially the same as the radar detector in
that it is an alert device, but it operates by monitoring the police
aircraft communication freqs and with some minor discriminator
circuitry
is able to determine approximate distance and course. The latter must,
of
course, be relatively crude. It then 'assesses' whether the aircraft
poses a threat to your speed jaunt. It shouldn't take long before the
police alter the frequencies or go silent, but it is interesting the
amount of money people will pay to drive fast.
At the ultimate end of sophistication would be --hypothetically
speaking, naturally-- a CW doppler jammer computed to return the
correct
frequency to the radar to make it appear that you were really doing 55.
It would take a bit of integration with the car's electronics/speedo
and
some interesting programming, but although it seems feasible I don't
know
if it would really work as it should or just confuse the radar. Maybe
just a big chaff gun on the front of the car?
L.E.Hughes (BITNET - KQM@EPAVAX or ERP@ORSTATE)
--
Larry Hughes
Domain: hugo@bigtime.fidonet.org
UUCP: ...!{tektronix, hplabs!hp-pcd}!orstcs!bigtime!hugo
via Big Time Television (bigtime.fidonet.org, 1:152/201)larry@kitty.UUCP (Larry Lippman) (08/12/89)
In article <661.24E1D0E5@bigtime.fidonet.org>, hugo@bigtime.fidonet.org (Hugo) writes: > Of such interest is the newest addition to the home ECM scene, the > aircraft detector. It is essentially the same as the radar detector in > that it is an alert device, but it operates by monitoring the police > aircraft communication freqs and with some minor discriminator circuitry > is able to determine approximate distance and course. The latter must, of > course, be relatively crude. It then 'assesses' whether the aircraft > poses a threat to your speed jaunt. It shouldn't take long before the > police alter the frequencies or go silent, but it is interesting the > amount of money people will pay to drive fast. I trust that you intend the above to be a joke. For those readers to whom this is not obvious, virtually all local law enforcement agencies communicate on their regular assigned land mobile frequencies when engaged in aircraft operations. There are no assigned "police aircraft communication" frequencies in the VHF aircraft band. Wulfsberg Electronics and Motorola C&E, among other vendors, manufacture synthesized radios for VHF and UHF land mobile frequencies having ARINC mountings which are specifically intended for aircraft installation. Short of placing an Adcock antenna array or an equivalent device on the roof of a vehicle, there is no way to ascertain "course" of an aircraft based upon a radio emission. For the application at hand, there is no way whatsoever to ascertain "distance" of an aircraft. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"
georgep@vice.ICO.TEK.COM (George Pell) (08/15/89)
In article <3331@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: +In article <661.24E1D0E5@bigtime.fidonet.org>, hugo@bigtime.fidonet.org (Hugo) writes: +> Of such interest is the newest addition to the home ECM scene, the +> aircraft detector...... + + Short of placing an Adcock antenna array or an equivalent device on +the roof of a vehicle, there is no way to ascertain "course" of an aircraft +based upon a radio emission. For the application at hand, there is no +way whatsoever to ascertain "distance" of an aircraft. + Although I agree with you in the practical sense, there IS a way to ascertain distance from AIRCRAFT which are equiped with an ATC transponder, a device based upon a radio emmission. Through the process of pulsed interrogation and reply, distance, and course can be computed, and, with altitude encoding, altitude can also be determined. Now the problem becomes identifying which aircraft belongs to the law enforcement agency, and which one is a private aircraft sightseeing over the freeway. With a simpler method, you can determine that an aircraft is nearby (approx 1-2 miles) by monitoring the response transmission with a scanner such as the RS2004/5. You cannot tell if they are in front, beside, or behind you, and you can't tell if they are 500 or 5000 feet above you, and you can't tell if they are checking your speed, or just following the freeway. geo Cardinal 29531
hugo@bigtime.fidonet.org (Hugo) (08/15/89)
Actually, this is no joke and I am hoping that someone out there has
the catalog that I saw this in originally. It may have been the new
'Sharper Image' as they are often tuned (no pun intended) to this sort
of thing. I was more interested in the price - over $200, I believe
and the fact that anyone would go to this trouble. It was supposedly
designed by the makers of a popular radar detector.
It was rather vague on the circuitry, presumably from the point of
hiding the actual effectiveness, which without an Adcock array or
some other such directional ability as you said, would be impossible.
My assumption was that it probably used signal strength as one measure
and possibly the same type of antenna as the directional indoor FM
antennas (the BIC Beam Box, for instance), but then I am more familiar
with radar than radio.
As for the frequencies, I hope I can find that ad or someone out there
can, as I will put all the wonderful drivel about it up on the boards
and see what a fun discussion it will generate. Meanwhile a sun-roof
might actually be a cheaper option and it would also increase the value
of the car.
Hugo.
--
Larry Hughes
Domain: hugo@bigtime.fidonet.org
UUCP: ...!{tektronix, hplabs!hp-pcd}!orstcs!bigtime!hugo
via Big Time Television (bigtime.fidonet.org, 1:152/201)jeff@garth.UUCP (Jeff Kaskey) (08/18/89)
In article <19212@vax5.CIT.CORNELL.EDU> max@ee.cornell.edu (Max Hauser) writes: >When you have *any* kind of steady narrowband emitter, even a very small >one like the local oscillator (LO) in a superhet receiver, a great deal >of physics and mathematics aids the cause of anyone who wants to detect >it. Approached from the proper signal-processing perspective, such >emitters stick out in the electromagnetic environment like a magnesium >flare in a darkened room, and can be detected at almost arbitrarily low >amplitudes with processing-gain techniques. > >From past experience as an electronic-countermeasures engineer for the >US government, I believe that detecting continuous narrowband emitters >is not even considered an interesting, let alone a challenging, technical >problem. So then, following the thread, it should not be difficult to construct a circuit which detected the presence of a (just as an example :-)) nearby police radio, as is likely to be truned on in any cop car which might be pacing, clocking or radaring you. Interesting. Anyone got any idea of the LO used in your average (Motorola ?) police redio? Radio shack publishes a booklet detailing the transmitting frequencies of most major cop shops, but do they all use the same LO? How far is this from the LO in an FM radio? CB? celular phone? inquiring minds want to know.... -jeff
brian@ucsd.EDU (Brian Kantor) (08/18/89)
Receive-only anti-police ECM: The IF depends on the band and brand, but the majority of cop radios in SoCal are Motorola Micors and Syntors; they use an 11.7 MHz IF and you'll find the LO injecting into the mixer at +/- 11.7 from the receive frequency. Thus you can tune a sensitive receiver to the appropriate frequency and when the squelch starts to burble, there's a cop-car ["fuzz-mobile"] in the vicinity. The radio in the local police helicopter ["whirly-pig"] seems to radiate more, but they tend to hang out on the "tactical" or "air-support" channels so you might not be listening there. Another popular IF is 10.7 MHz; same technique applies. Older radios such as the Motrac used 12MHz. Ask a local technically-inclined ham; he'll probably know what's popular in your area. The CHP has a problem with walkies - they run low-band radios (around 41 MHz) to get really good coverage in mountain passes, valleys, and canyons. But nobody makes a really good low-band walkie, and the antenna is too big anyway - and they don't run enough power. So lots of CHP cars are equipped with cross-band .5w walkie repeaters that allow the officer to carry a high-band (150MHz) walkie with him and use the car's 100w+ low-band radio. So you just program the walkie talkback channel into your scanner and if you hear the dispatcher, you're probably within a 1/2 mile of a CHiPpie. Since they don't use radar, that's likely the only way you'll have some warning - those black mustangs with no external lights really aren't very obvious. And of course an X/K band ECM receiver (aka "radar detector") is essential in towns that enhance their revenues with speeding tickets. Snoop and listen - while you still can. "Those who forsake liberty for security deserve neither." -- Franklin