Okay so I've read a lot here about side threats and...

[Jabomb9]

how they give a reading of zero MPH (or very low) but I have a question. I understand that it can quickly become a threat as soon as you pass the officer, but that's only if the radar gun is aimed at an angle, right? So if I drove by a LEO shooting radar perpendicular to me (at an intersection or something) and the LEO was stopped, could he get an accurate reading without moving the radar source? I know this probably makes no sense as I am having trouble wording it properly, so I drew a little picture (yes, you can thank me later ):



Basically, if I were to pass through his radar "view" would he get an accurate reading (assuming his radar was mounted parallel to his car)? Or would he have to physically move the radar to get an angle on me for it to produce an accurate speed?

Thanks and sorry for the crappy explanation/picture :roll:

[jdong]

In the arrangement you've drawn the officer would not be able to produce any reading at all. Not when approaching, not when going away. If he is perpendicular to your direction of motion, the speed reading is basically zero.

But be careful that an officer seemingly in that arrangement could POSSIBLY have a swivel-hinged radar head, or actually have radar pointed towards your direction of traffic, etc, which could quickly make him a threat.

[Jabomb9]

All right that's kinda what I figured.

Thanks! Always a lot of help.

[kuba]

What I have seen at intersections is that the LEO will take the radar head off the mount on his dash and just aim it with his hand. Seen this once, out of the 2 times I remember stopping next to a LEO.

[jdong]

I suspected that might happen and it surely does. Err on the side of caution, don't play with fire, and so on ;-)

[bart99gt]

I've witnessed an officer physically turning the antenna of a dash mounted radar a couple of times. Quite a while ago, I saw a radar on eBay where someone at one time had fashioned a handle that could be attached to an antenna to allow it to be used in a handheld manner. Truthfully, there's nothing that prohibits an officer from doing that and I wonder why its not done more often in cash strapped towns.

[jdong]

I guess laziness factor.... detector users are still a minority, and most people will even be blind to a simple constant-on encounter that even a cobra can protect against.

[Motor On]

Just echoing things here, mathematically if you travel perpendicular to him then he gets no doppler shift, but handheld or a rotated antenna and that changes in a hurry. If you're a little more interestedin the scence of things....

Cosine Effect and Radar Accuracy
If the target is in a direct line (collision course) with the police radar or sports radar gun the measured speed will be exact. As the angle of incidence increases, if you move either right or left of this direct line, the accuracy of radar guns will decrease. The measured speed will decrease as you move off this centerline. This phenomenon is called the Cosine Effect. It is called this because the measured speed is directly related to the cosine of the angle between the radar gun and the target’s direction of travel. As a quick reference to radar accuracy, remember to keep your targets direction of travel in a direct line with you, and not perpendicular.

The Cosine Effect refers to the angle of the target vehicle in relation to the patrol vehicle where the radar is mounted. The traffic radar should be operated as parallel as possible to the targets, although it is hardly possible to do perfectly. When the angle between the radar beam and target becomes too significant, the relative speed will be less than the true speed producing a lesser speed reading than what the vehicle is actually traveling. Thus, the cosine effect is always in the favor of the motorist. The greater the angle the lesser the speed will be recorded compared to the actual speed of the moving target.

From Here:http://www.radarguns.com/radar-and-cosine-effect.html

A Pro Laser III (LIDAR not RADAR) suggests keeping this to within 1-2% error, but the effect always reduces the speed reading of the vehicle being read.

[vliou2173]

And remember. Cosine of 90 = 0

So if you are directly perpendicular to him at that precise time (limit angle = 90), then your speed will approach infinitely close to 0.

Go calculus.

[jdong]

And remember. Cosine of 90 = 0

So if you are directly perpendicular to him at that precise time (limit angle = 90), then your speed will approach infinitely close to 0.

Go calculus.

In the OP's drawing, the antenna is perpendicular to his direction of motion, in which the speed is either zero or no-lock, never a non-zero number