Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!vax5!mwh From: mwh@vax5.CIT.CORNELL.EDU Newsgroups: sci.electronics Subject: Police-radar countermeasures (and rockets) Summary: Radar-detector detectors, etc. Message-ID: <19212@vax5.CIT.CORNELL.EDU> Date: 1 Aug 89 14:10:54 GMT References: <414@ctycal.UUCP> Sender: news@vax5.CIT.CORNELL.EDU Reply-To: max@ee.cornell.edu (Max Hauser) Distribution: na Organization: Cornell University, Ithaca NY Lines: 62 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.)