Okay, all you genii out there help me out on this. I’ve sent this question to The Master himself, but we all know how busy he is and I just can’t wait. I (think I) understand from college physics that radar guns gauge speed by measuring the Doppler blueshift of the returning wavefront - off of, say, the nose of your car - and calculating the closing velocity from that.
But if, as has happened to me on oh-so-many occasions, the porcine gentleman in question happens to be in a moving frame of reference (i.e., driving towards me in his cruiser), wouldn’t that skew the radar results? Or is the radar hooked into some sort of digital output from his own speedometer, subtracting and just reading the difference? Or, horror of horrors, is he supposed to subtract his own speed from mine in his head?
On a related note, let’s say I’m going past directly perpendicular to the beam of his gun. My approach vector, and hence the magnitude of the blueshift, would be very low. How’s this work?
Please help!
Oh, and please don’t try to assure me that I must have been mistaken, he was probably sitting still & I just missed it, etc., etc. It’s been too many times.
Let’s not call him a ‘porcine gentleman’ if you don’t mind. Some of us take offense to that word. “Gentleman” indeed!!
Besides, I used to be a police radar instructor so I’m not fond of ‘porcine’ either.
First of all, you may trust me that the officer does not have to do the math in his head. It would be difficult to get that through court if someone had even a cheap lawyer. So that’s not what happens. You remember pretty well from your college days about Doppler, right?. Radar does work on the Doppler effect, even the new laser radar. But imagine this incredible scenario: Suppose someone could figure out how to connect the radar unit to a computer which measured the speed of the police vehicle…sort of like a speedometer…well, exactly like a speedometer. In fact, since police speedometers are calibrated for precision, it IS the speedometer. Now further suppose that this computer could determine whether a target vehicle was approaching (and at what closing speed) or departing (at what departure speed)relative to the movement of the radar unit…Doppler will do that, eh? And then…here it comes…calculate the true speed if the target by adding or subtracting the speed of the police vehicle at the instant the radar calculates the relative speed. Science fiction? Nope.
Doppler radar. If you know what it is, there’s your answer. If you don’t, somebody else can explain it, 'cause I’m too sleepy and it’s hard to explain.
The OP makes it pretty clear that BigGiantHead does, in fact, understand Doppler radar. He then asks two questions: First, is the motion of the police car accounted for-- tcburnett covered that one pretty well. His second question remains unanswered, however: Since Doppler radar measures only radial velocity, how does the officer know the total velocity? I know plenty about the Doppler effect, but little about police methods, so I’m not sure of the answer here. I can hazard a guess, though, that since total velocity is always greater than or equal to radial velocity, that the cops just figure that if your radial velocity was over the limit, then you were speeding. Another possibility is that the guns also measure proper motion and range, and hence get both components of velocity.
Sorry, but the ‘porcine’ comment pissed me off. Police radar works at angles up to 45°. Ideally the target will be at 0° or 180°, but 22.5° from either of thise provides an acute angle which works out in favor of the target vehicle because angular velocity is incidental to the computed speed. To be perfectly accurate, if you read an approach higher than the safe speed at 45°, the target vehicle is going over twice the safe speed. We can do the math, but don’t because police radar does not measure offset angle, or calculate it into the result. But you can figure speed another way. Elapsed time between two points a known distance apart. Aircraft do it that way and officers on the ground can do it just as easily. But they usually have to be stopped, and have a wide field of view.
I can post the technical manuals which include theory, but the interested reader may research Doppler radar on his own.
Yes, but the functionality is degraded. Police don’t usually work speed enforcement in rain or snow because (1) they are busy handling traffic accidents and (2) they don’t want to stand in the rain or snow. But don’t be complacent, there is one in every crowd. You shouldn’t be speeding anyway, much less in the rain or snow. But if you must, get a top-of-the-line radar detector if they are legal in your state.
Here’s the latest from Kustom.
Sorry to nitpick, but at an approach angle of 45°, your target has an actual speed of only 1.4 x the indicated speed. Not double. But yes, any angle other than 0° or 180° works in the driver’s favor, because the indicated speed is equal to cos(Ø)*(total speed) where Ø is the angle you’re approching the radar (or laser) unit…
Joe, I love nitpickers as long as they are right. I hoped I had dodged the bullet by writing “we can do the math, but don’t” so nobody would, but you did. You are right as rain and you caught me. Thank you for the correction.
Yes, in exactly the same way as if the car is approaching on the level at an angle. It’s to the driver’s advantage, but as long as the angle is small, it’s reasonably close.
Having devoted a large part of my former driving career to sitting in traffic court, I always got a chuckle out of the ubiquitous chap who would ask, “when was the last time your radar gun was calibrated?” (The answer, at least here in VA, is that the radar guns are calibrated more-or-less daily.)
The question to ask, I think, would be “how far off was your radar gun when it was next calibrated?” Of course, if it tested 5mph slow at the end of the day, you could find yourself feeling a little embarassed.
Well, tc, how 'bout it? How far off does a radar gun drift in a given day? Are the calibration results recorded, and can they be asked for by the defendant in traffic court?
FWIW, photo radar (the kind that snaps a photo of your plate and results in a mailed ticket) sits at a uniform angle to the direction of travel (26 degrees, IIRC) and it does multiply the metered speed by the cosine of theta. Mr. Burnett can confirm or deny this, but I’m nearly certain.
Since the Doppler effect depends on the speed of the vehicle as compared to the speed of the wave , and the speed of the vehilce is much smaller than the speed of the wave, doesn’t a radar gun require an extremely high accuracy (somewhere around one in a million)?
Yup, it does require very high precision measurements, but the guns are quite capable of making those measurements. Using interferometric techniques, it’s possible to get down to one part in 10[sup]24[/sup] or so, which is by far more than you need to give out a speeding ticket.
Techically, the laser rader (lidar) does not use doppler to measure speed. Instead, it uses the speed of light and the time it takes for a pulse to fire from the gun to the target and back to measure the distance between the gun and target. It fires something like 50 shots per second and gets 50 different distances. Then it simply computes a distance over time formula to arrive at velocity.
This means that lidar guns can arrive at erroneous speeds via a few different ways:
Retargeting. If the gun is pointed at your windshield and then slips down to your license plate, your speed as going to look a lot higher because the distance between shots is decreasing in a hurry.
Movement of the gun. If the gun gets jarred a bit forward, this will also add to your speed.
#2 is what got the guns basically banned in New Jersey (according to a Car and Driver article - check their website) The defense attorney showed that the gun can’t be accurate because it measured the speed of the courtroom wall to be going at 4 MPH. This was, of course, after the expert witness for the lidar manufacturer insisted that they had software in the guns to handle such situations.
