# Thread: How far off axis can a LEO be with laser?

1. ## How far off axis can a LEO be with laser?

I am wondering how off axis a LEO can be with a laser gun to legitimately give you a ticket? I know it has something to do with the cosine error. thanks!

2. ## Re: How far off axis can a LEO be with laser?

Originally Posted by HaZedXxX
I am wondering how off axis a LEO can be with a laser gun to legitimately give you a ticket? I know it has something to do with the cosine error. thanks!
Think of it this way for example: Your vehicle is taveling at 75 MPH. The officer shoots you at an angle of 45 degrees. Cos (45) = .707106781 .707106781 * 75m.p.h. = 53.03300859.

So the speed displayed would be 53 m.p.h.

Cosine error works in your favor (well, at least it does when the officer is NOT using moving radar). The speed displayed to the officer (radar or lidar) is lower than your actual speed. Hell, at 90 degrees, you may not even be moving at all ; )

3. ## Re: How far off axis can a LEO be with laser?

Does this imply the guns don't have some kind of compensating algorithm within them that compensates for off-axis? I'd have thought some mechanism is available to the LEO to adjust for this kind of error... either automatically or user-selectable?

4. ## Re: How far off axis can a LEO be with laser?

Isn't there also another source of inaccuracy with laser, which increases the reading? I recall reading that if the laser is swept horizontally across the vehicle (or also vertically, IIRC) the reading comes out higher. Could be accidental...or deliberate?

Does anybody have info on this?

MH

5. ## Re: How far off axis can a LEO be with laser?

Isn't there also another source of inaccuracy with laser, which increases the reading? I recall reading that if the laser is swept horizontally across the vehicle (or also vertically, IIRC) the reading comes out higher. Could be accidental...or deliberate?

Does anybody have info on this?

MH
if im not mistaken, i think that issue was specific to the PL3. not positive, or if it has been resolved.

^
video of one of our forum members using the LA, so i guess it applies to other guns.

6. ## Re: How far off axis can a LEO be with laser?

Sweeping the gun across the vehicle while trying to get a speed reading would most likely result in NO reading at all IMO. It also may depend on the speed of the sweep. A vveerryy slow sweep would probably get a reading.

The reading would always be lower than the actual speed for any situation other a a straight on position.

7. ## Re: How far off axis can a LEO be with laser?

Sweeping can, and does lead to different speeds (depending which way you sweep, it may actually be slower). There is software (I'd imagine one of the requirements to meet certification) in the gun that tries to minimize this from happening (that is usually what the error codes you see are for). The gun companies would like you to believe it's impossible to do, but it does happen.

There was a BBC show that dealt pretty much exclusively with this. It's been posted here a number of times.

As for the cosine thing, as has been mentioned before, it works in your favor. Technically, it would be compensated for, but the angle of the shot would have to be calculated, and you'd have to only aim at one spot in the road (or else the angle changes as you move with the car). It'd only be one more thing they would have to explain in court, since they'd have to show that they reset the angle when they shot again. Too much of a hassle considering the current system works pretty well.

The only case I've heard of compensation is for laser speed cameras, where the angle of attack is constant, and they always hit the same spot on the road. Wouldn't want people showing up as going slow than they really are to the camera, now would we?

I think one of the guns (UL or LA?) had an inclinometer(sp?) that compensated for when shooting up a hill, but I could be mistaken...

8. ## Re: How far off axis can a LEO be with laser?

"Cosine error" reduces speed, "pan and sweep error" can increase a speed reading from taking measurements off the hood then windshied. Its an effect that can be had with any lidar gun and why most come with the option for a tripod mount (required for IACP approval testing)

Do a search on the words in quotes, you'll find it discussed many times.

9. ## Re: How far off axis can a LEO be with laser?

Originally Posted by Jerry Lee
Sweeping the gun across the vehicle while trying to get a speed reading would most likely result in NO reading at all IMO. It also may depend on the speed of the sweep. A vveerryy slow sweep would probably get a reading.

The reading would always be lower than the actual speed for any situation other a a straight on position.
That would seem to contradict the videos above. Sweep the gun against a non-moving object, and you seem to get a reading.

(Lidar owners, does it happen that way?)

Also, I searched and found a page from the Stalker Lidar manual that says if the aim of the LIDAR device is permitted to wander up the back or front of the vehicle, the beam will "jump" some 3-4 feet from the plane of the license plate to the window area, which makes the gun think the beam has traveled farther than it actually did. The calculated vehicle speed could be to be as much as 8 miles higher than the true speed. Stalker reportedly tests for this condition and "suppresses" potentially inaccurate readings, although it doesn't say how it knows the operator has moved the gun.

MH

10. ## Re: How far off axis can a LEO be with laser?

By looking for unreasonable "jumps" in the distance readings.

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