jhc@mtung.UUCP (Jonathan Clark) (11/21/85)
Actually I think that the reason that radar detectors (Escort and Passport, both of which feature the ST/O/P circuitry) go off at Newark Airport is either the large number of automatic door openers pointing towards the Pike, or just radar noise from the ground approach and other radar systems which are 'live'. Certainly there is no correlation between received signal strength and the way that that giant rotating antenna is pointing (I think that is the ground approach system). Driving along route 1/9 *behind* EWR results in no annoying beeps (except real ones!). I suspect that you would have a good chance of getting a radar ticket thrown out if you could prove that there was some other signal in the correct band in the area. In this case you might be able to prove 'reasonable doubt' that the radar gun was not reading back the wrong signal and getting confused. Anyone out there know how police radar guns are built? It would be possible to modulate the 'send' signal and then compare this against the 'receive', hence doing away with this line of defence. Do any models actually do this? -- Jonathan Clark [NAC]!mtung!jhc My walk has become rather more silly lately.
ben@moncol.UUCP (Bennett Broder) (11/21/85)
> Anyone out there know how police radar guns are >built? It would be possible to modulate the 'send' signal >and then compare this against the 'receive', hence doing >away with this line of defence. Do any models actually do >this? Radar detectors work by emitting a microwave signal of a known frequency, then comparing that frequency to the frequency of the reflected signal. Have you ever noticed that as a car approaches you, it makes a high pitched sound which drops as it passes you, then makes a low pitched sound as it heads off into the distance? This is called the doppler effect. The faster the car approaches, the greater the frequency will be shifted upwards. Radar detectors measure the frequency shift, then convert it into MPH. If the police want to use a radar unit without worrying about door openers and alarms causing interference, they can buy radar guns that operate on the K band. Since nearly all motion sensors are on the X band, this is a much cheaper solution. Ben Broder ..vax135!petsd!moncol!ben ..ihnp4!princeton!moncol!ben
jhc@mtung.UUCP (Jonathan Clark) (11/22/85)
In article <567@moncol.UUCP> ben@moncol.UUCP (Bennett
Broder), in response to a question of mine, treats us to a
small excerpt from a grade-school physics class, explaining
how radar guns use the Doppler effect to determine speed.
Unfortunately, that wasn't the question.
I'll ask it again: Is the transmission from a radar gun modulated
in any way so that the gun can distinguish the signal it
sent out from the signal coming back in IN THE SAME BAND?
Examples of extraneous noise are: door openers, other police
radar guns in the vicinity, radar jammers, and probably the
3K background.
--
Jonathan Clark
[NAC]!mtung!jhc
My walk has become rather more silly lately.fern@polyof.UUCP ( FTG ) (11/26/85)
> In article <567@moncol.UUCP> ben@moncol.UUCP (Bennett > Broder), in response to a question of mine, treats us to a > small excerpt from a grade-school physics class, explaining > how radar guns use the Doppler effect to determine speed. > Unfortunately, that wasn't the question. > > I'll ask it again: Is the transmission from a radar gun modulated > in any way so that the gun can distinguish the signal it > sent out from the signal coming back in IN THE SAME BAND? > Examples of extraneous noise are: door openers, other police > radar guns in the vicinity, radar jammers, and probably the > 3K background. > -- > Jonathan Clark > [NAC]!mtung!jhc While all of the above is true, the resolved signal is down in the FM range this means that any strong FM signal will couple beyond the detector stage of the radar gun. Since radar siganl stregth for these toys is in 100 milliwatt range, return signals are in the 10-1000 microwatt range. This implies that a strong signal would be less than a watt, if the source is very close. A handheld FM transmitter or the FM car radio (genrally fould in Police cars) would produce such a signal of the required stregnth. The toys which police currently use are too cheap to have the required sheilding and signal rejection to do away with this problem. Note that an escort or similar detector needs only to be 1% as effective, to have the same ablity to detect the signals presence, as the radar gun. Detection of a USEABLE signal requires more sofisticated circuitry, plus the transmitter stage plus a stage to resolve the speed. When one compares the price of a radar gun to an escort (or if one considers the cost of the components involved in making an ECM device) one is lead to the conclusion that these toys should be banned from use. enough of my ranting, FTG ps dietz etz
mikey@techsup (11/27/85)
Well, here it is again. NO flames or request for more info, if you want it, please do the research yourself! --->>> Hitchhikers guide to radar jamming <<<--- The following is a brief explanation of how police radar operates and how it can be used both against you and for you. It is NOT a complete guide and if you're really interested, get some technical information from the FCC radio telephone books ( there's one just for radar ). First off, police only use 2 bands for the radar signals, although at least one detector manufacturer tried to claim their detector covered the "THIRD" police band. Bullsh*t. The main band is called the X-Band. It is just above 10 GHz. It is the older and more common of the bands in current use and restriced to continuous modes only. Pulse is not allowed here. The other common band is the K-band. It is up around 24 GHz, I don't have the actual figure handy. Here the radar units can operate both continuous and pulse mode. Pulse mode is more difficult for detectors to pick up, therefore it is sometimes thought of as a new band. The new superhets have very little trouble detecting the pulse mode. How does the radar unit detect your speed? You should be aware of the doppler affect. To those who don't know, it's the same thing that makes a train, car, or bus change pitch as it passes you. What the radar units do is use a very small portion of the transmitted signal and they mix it with the signal reflected back from the object the radar unit is beamed at. If the time base is chosen correctly, the radar gan can count the beat frequency of the two signals and this is your speed. This has one very good side effect. Because the doppler effect is related to the relative velocity vector between the gun and the vehicle, it will read SLOW by the cosine of the angle of the direction your going to the direction toward the radar gun. For example, you're rounding a corner and the radar unit is off to the side of the road at about 45 degrees. You can be doing 77 mph and the gun will read 55 mph. Police are NOT allowed to compensate for the angle!!! Radar is not infalible, no matter what the police say. Aalmost any RF that gets into the gun will cause interference. There are stories of poeple putting a spark plug inside a cone aimed forward from the vehicle. The spark generates interference that the gun will wrongly use as the speed. This is the same effect that jammers use which I'll explain later. The radar guns only pick up the strongest signal, not the fastest, not the closest, but the strongest. This means that a Honda Ccivic at 400 feet is the same as a White Freightliner at 7000 feet, Yes 7000! BTW, the lowest profile car that I've heard of is the 78 Corvette. It has no flat metal parts and the radiator support bracket is tilted. Surprisingly, the fiberglass Lotus is not as good, probably because of some I beams across the roof. Car and Driver had some interesting articles on radar range in 1980 if your interested. How about jamming. OK, lets assume that you don't care about the legality and you just want to try to jam for funs sake. First thing, forget about trying to build a transponder that will calculate the required frequency shift and transmit a beam at the right frequency to read 55. You'd go broke. What if the cop is rolling radar, what if ...? There is an easy way. Lets assume that you have the capability to transmit. Figure out the doppler shift for 55 mph in the band that you want to jam. It will be different for each band. Now take the difference to the original frequency, this is your pulse rate. If you were to inject this into a stationary gun, you would read 55 on the display! Neat huh? How do you get this into the gun? That is also very easy. Sset up a 555 timer to the same pulse rate and use it to trigger a darlington to provide power to a gunn oscillator on the band that your jamming (Read the Febuary 1980 issue of Car and Driver). It turns out that by pulsing the front end of the radar gun you will overload the mixer and it will count your pulse rate instead of the difference frequency. If you put out a steady frequency, the radar unit will lock on it and display it. If you send out a constantly changing frequency, the radar unit should go display its last valid speed. This is because most of the guns have to get 3-5 readings within so much variance before it gives a display. Be forwarned that police are getting more adept at detecting jammers, and the penalty is not light. If you have a HAM ticket, you could lose it. There could even be criminal penalties in some states. I have even seen a KR-11 that had been modified with a switch to detect jammers. Even more popular are radar detectors in police cars. If you do jam, make sure that you trigger the device with a radar detector, that way the unit won't be transmitting all the time, and don't use the jammer to blow by a cop, just use it to give you that critical extra 3-4 seconds to get your speed down from warp 8 to 55. One final caution, don't forget that when the jammer is on, the radar will read that speed, even if you are going slower. I heard of a guy who got a ticket in NJ when he came into a small town and slowed down to 35, but the jammer told the cop 55. Try to talk your way out of that! mikey trsvax!techsup!bbimg!mikey