Rador while moving?

[Rob MacLennan]

I didnt write this although I did edit to make it clearer.

You are right cosine error between the 2 cars will also reduce the calculated speed so it all depends on the angles whether the net result is in favour of the driver or not. It's probably more of an issue with K band radar since the beam is larger than Ka and is more likely to reflect off other targets.

The scary thing about moving radar is there alot happening all at once and the curve in the road just increases the chances of error .... and the cop has to drive the car and verify the target which may be why the he didnt get a ticket in the incident.

What the angles are is immaterial. Cosine is a ratio that will ALWAYS result in a lower speed reading. It will always favour the target driver. The vehicle will always be measured as travelling at only one component (either X or Y component) of its total speed moment. The most that it could ever be RADARed at would be it's full speed, if travelling in the same plane as the police vehicle.

[spankayf]

What they are getting at is if the cosine error calculating the cruiser speed is larger than the cosine error calculating the closing speed then the net effect will be an inflated driver speed.

ie a reflecting billboard off axis to the road is used to calculate patrol speed (reducing it by 10) and then the closing speed is calculated (reducing it by 5) for a net of +5

[Team Lazy]

Fundamentally, you're right. But Rob is more right. Cosine error will cause the radar to err LOWER for the other vehicle as well. Whatever they see on the readout is usually lower, but never higher, than the actual vehicle target speed.

[spankayf]

What I dont seem to be able to get across is the radar unit calculates your speed by subtracting the patrol cars speed from the closing speed. The issue of the curve complicates it since there would be some cosine error on the closing speed as well.

So the cosine error on the patrol car ..... ahhh ferget about it

[rukus]

what if you are approaching an intersection and the cop is parandicular to the road, and you are radared at 90* to the vehicle

your car
V
| -- <-radar


is it accurate?

[Muchacho]

The question may be:

"Does moving radar work only when both vehicles are moving in parallel directions AND not work at any angle other than 0?"

[Team Lazy]

what if you are approaching an intersection and the cop is parandicular to the road, and you are radared at 90* to the vehicle

your car
V
| -- <-radar


is it accurate?

Nope. Look at one next time. They're bolted down inside, and face directly forward or back. Can't pivot. Unless it's a hand held jobby..

[Rob MacLennan]

what if you are approaching an intersection and the cop is parandicular to the road, and you are radared at 90* to the vehicle

your car
V
| -- <-radar


is it accurate?

It would read only a minute fraction of your actual speed as you hit the intersection.

[johnp]

In order for the police to win in court they would have to establish that there's a bias in favour of the driver i.e. if anything the driver was driving faster than recorded. So the cosine error definitely biases the closing speed error in favour of the driver. If the police want to win in court there better not be a cosine error like you guys are talking about when determining the police car's speed, since then the overall bias is not clear.

So either the police car gets it's own speed off the speedo (calibrated?) or something is pointing at the ground in front of the police car and no bias error there.

I should point out that I don't really know how the system works, but if they don't care of the bias in the police car speed you should be able to win in court.

[spankayf]

I would never utter the phrase "cosine error" in court since the JP would have heard people argue it and by now recognize that it reduces the calculated speed ... although it can inflate the caluclated speed as I have argued earlier.