On (09 Jun 95) Rod Speed wrote to Bill Grimsley...



 BG> Even radar units are not unbeatable either.  Have you ever wondered

 BG> why they're not used in wet weather?  Coz the water droplets on the

 BG> surface of the vehicle being checked present a greater (varying)

 BG> surface area to the doppler beam, and cause it to read high.



 RS> That sounds pretty comprehensively garbled, makes no sense.



 BG> Well instead of suggesting that I've garbled the

 BG> explanation, why don't you attempt to explain it yourself?



 RS> Yeah, I usually would do that, but this time the original

 RS> makes no real sense and I cant actually see the real effect

 RS> it got garbled from. Sometimes its obvious which bit got

 RS> mangled, with this one I cant see it tho.



police radar these days is usually in k or ku band, high frequency radar

in these bands has a lot of problems with rain clutter, this is because

the rain drops are pretty close to dipole size at these frequencies. now

spray coming off cars in the wet may present a bigger target than the

vehicle because of this, and will probably be travelling at a much

faster speed than the vehicle itself. circular polarization helps reduce

these effects, but may not be enough in these conditions, and complicate

the antenna design,



 RS> The detection of speed of the moving object has nothing at all

 RS> to do with surface area with radar. And the surface area doesnt

 RS> change at all anyway really, the bulk of the reflection is off the

 RS> metal of the car, the water drops arent doing to change that much.



with a car covered in water drops, the bulk of the echo may well come

from the water, if they were vibrating in the slipstream, they may well

put considerable jitter on the returned signal, but what the overall

effect would be is probably purely random, and would depend on the

particular signal processing in the model of radar used.



 BG> I know next to fuck-all about radar, except that it costs me

 BG> money occasionally, but that's what I was told some time ago

 BG> by somebody who did know what he was talking about.  The precise

 BG> reason may not be accurately stated above, but the end result is.



 RS> Dunno, maybe he meant that the radar gets reflected off falling

 RS> water droplets in the AIR and you then get a returned signal which

 RS> has a mixture of signals which some have the effect of the car

 RS> speed, some have the effect of the rain drop speed, in the path

 RS> between the radar detector and the car. That makes some sense.



in heavy rain, virtually all of the signal from the car would probably

be blocked by clutter.



 RS> Aircraft airborn radar does use that effect for weather radar.



weather radar is linearly polarized to maximise reflection from

airbourne water, whereas interception radar is almost always circular

polarization as the opposite effect is desired.



 RS> Pity that the law just proclaims that they are accurate and fuck you.



 BG> Nope,



 RS> Yep, fraid so Bill. There was a big stink about it at the time

 RS> that law change was made, because it shafted those who wanted to use

 RS> that approach in the court. Thats why it was introduced like that.



 BG> not if you can scientifically prove that radar gives inaccurate

 BG> readings in the rain, or other conditions where the car may be wet.



 RS> Fraid so. Because some had tried stuff like that, so they changed the

 RS> law to shaft that approach.



speed radar has been declared a precision instument in nsw. that means

that you cannot argue that an approved radar used properly by a licenced

operator did not accurately measure your speed, but it is still possible

to argue that the unit was not operated in the correct manner. history

shows, though, that your chances of success aren't high.



 BG> Some people do actually challenge radar

 BG> tickets, and occasionally even get off.



 RS> Thats possible in a completely different situation where you

 RS> can show that the radar measured the speed of SOMETHING ELSE,

 RS> not you. Usually another car. Completely different problem.



hard to argue with the slant radar, as it is supposed to be able to

detect which lane of a multilane road a speeding car is in. in fact,

some time ago i was driving down the right hand lane of the pacific

highway, on the north shore, about 2 cars lengths behind another car,

with a third car in the left hand lane just ahead of me. the cops pulled

both the other cars over. presumably i was shielded by the car on my

left, but they still got readings on the other two, almot side by side.



in general, unless you can find a loophole, your only chance is to prove

that the cop is an incompetent dickhead who used the machine outside the

approved procedures, but then he is kindly mr plod who is only trying to

save you from killing yourself so you are probably going to end up

paying in the end.



                        Keith



ps. i think that the act cops are using laser speed traps, it looked

very much like one i saw there the other week.



... FASTER FASTER 'till the thrill of speed overcomes the fear of death.



--- PPoint 1.88