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.

KR> police radar these days is usually in k or ku band, high frequency
KR> radar in these bands has a lot of problems with rain clutter,

Yeah, I said that thats the most likely reason, rather than
water on the car in drops. OTOH he appeared to be saying it
had always been a problem, which made it a bit questionable.

KR> this is because the rain drops are pretty close to dipole size at
KR> these frequencies. now spray coming off cars in the wet may present
KR> a bigger target than the vehicle because of this, and will probably
KR> be travelling at a much faster speed than the vehicle itself.

I think thats rather unlikely. Most of its actually coming off the
road, not the car, and so it doesnt have the speed the car has even.
Tho clearly the combination of the water coming off the road, the stuff
falling thru the air etc, could well be a significant problem overall.

KR> circular polarization helps reduce these effects, but may not
KR> be enough in these conditions, and complicate the antenna design,

Like Rot said, it should be possible to get around some of the effects,
particular the falling rain itself. Its not even clear if there really is
a problem that prevents the current ones being used in the wet anyway. It
wouldnt be surprising if it was a convenient excuse since its a pain in the
arse standing outside the victims car taking his details in the rain anyway.

RS> The detection of speed of the moving object has nothing at all to
RS> do with surface area with radar. And the surface area doesnt change
RS> at all anyway really, the bulk of the reflection is off the metal
RS> of the car, the water drops arent doing to change that much.

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

I doubt it, particular at the sorts of speeds that the radars are used.

KR> if they were vibrating in the slipstream, they may
KR> well put considerable jitter on the returned signal,

Yes, but I cant see that being a major problem.

KR> but what the overall effect would be is probably purely random,

Yes, so unlikely to be a major problem.

KR> and would depend on the particular signal
KR> processing in the model of radar used.

Still cant see its that much of a problem. I think its rather
unlikely it is actually a water droplet on the car problem myself.

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.

KR> in heavy rain, virtually all of the signal from
KR> the car would probably be blocked by clutter.

Sounds a tad implausible.

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

KR> weather radar is linearly polarized to maximise reflection
KR> from airbourne water, whereas interception radar is almost
KR> always circular polarization as the opposite effect is desired.

Yes, but that doesnt mean that the rain is irrelevant.

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,
RS> so they changed the law to shaft that approach.

KR> speed radar has been declared a precision instument in nsw.

Thats what I meant.

KR> that means that you cannot argue that an approved radar
KR> used properly by a licenced operator did not accurately
KR> measure your speed, but it is still possible to argue
KR> that the unit was not operated in the correct manner.

Yes, it was primarily intended to stop the smarty pants who
managed to dredge up an expert who proceeded to quibble on
the basics. Clearly it was never intended to stop you say
suggesting that they were pointing it the wrong way or something.

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

In fact bugger all.

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.

KR> hard to argue with the slant radar, as it is supposed to be able
KR> to detect which lane of a multilane road a speeding car is in.

Yes, but the same problem arises with normal radar too, unless
you have a fancy camera/radar combination, there is no reason
why they cant just get the speed of another car on purpose.

KR> in fact, some time ago i was driving down the right hand lane
KR> of the pacific highway, on the north shore, about 2 cars lengths
KR> behind another car, with a third car in the left hand lane just
KR> ahead of me. the cops pulled both the other cars over. presumably
KR> i was shielded by the car on my left, but they still got readings
KR> on the other two, almot side by side.

True.


(Continued to next message)

--- PQWK202