my jail-guard (wannabe cop) friend and me are debating this right now, whenever a cop car is sitting on the side of the road facing perpendicular to the flow of traffic, both of us radar detector users notice that they never light up when passing in front of one, I say it's because the principles of a speed detecting radar itself that it doesn't work at a 90 degree angle like that and the cop has it turned off. He says because the horn-like antenna in a radar detector faces forward it only detects from the front, and that the cop is actively clocking people. I know he's at least partway right on that about detection mainly from the front I'd think even so bounced waves from my car and others around me would still make a low-level alert. only time I've ever alerted on a cop like that he was using a handheld radar-gun, pointing at cars down the road, not right in front of him. Tell me Please!

It depends on what type of radar he is using.  If he is using a doppler-type radar, he wont get any readings if you are moving perpendicular to the direction the radar gun is pointing, as the doppler affect requires movement toward or away from the detector to be applicable.

Kharn

Kharn is right.

Gonna stick my neck out here and say I don't think either doppler or plain radar alone could work right.

Plain old radar measures distance by the time it takes for the reflected wave to return.

Velocity can then be determined by measuring how much the distance changes in a given period of time.  The radar I learned about didn't do this calulation, it was done separately in a computer wired to it.  In older speed cop radar, I'd guess there's an processor in the housing.

For this to work most accurately, the target should be coming straight at the beam.  Otherwise, at an angle, the actual distance covered by the moving target will be a bit higher than what the radar measures.

You could figure the error using trigonometry.  If you did it for a few examples, you would find that the more broadside your target travels in relation to the beam, the greater the difference between the actual change in distance (traveled by the target) and the distance change as measured by the radar.

If the target was traveling at 90 degrees to the beam, the 'plain jane' radar would have trouble detecting/computing more than minor velocity, because there was little change in distance.

Don't plan on using this to get out of your next speeding ticket, because if you think about it, the error causes [u]underestimation[/u] of the target's actual speed.  And don't try making a sudden turn or change of lanes to get your measured speed to drop. Unsafe and unlikely to help you at any significant range.

In order to compensate for this error, you'd have to introduce technology that measured the sideways movement of the target as you panned the radar gun (kept it pointed at the vehicle).
Essentially this would mean measuring the angular change of the gun as you continued to paint with the beam.

There are ways this can be done in theory, but I don't know whether it's been implemented at the speed radar level, (I doubt it).  Certainly the military dealt with this in a variety of ways, beginning WWII or a bit before.

Doppler radar is different, and I believe the velocity calculation is actually intrinsic to the radar circuitry.  As the return time for the reflected signal changes (as the distance changes), the rate of that change correlates to a velocity.

However the angle problem you raised is no different for doppler than previous radars, AFAIK.

Even laser devices face the same issue.  They measure in to/from line of sight terms, or there have been advances I don't know about (entirely possible).

It's been a long time since I studied this stuff.  Maybe some P.O.'s, technicians, or engineers can add to the topic.

[red][size=4]P.R.K.

You exact question wasn't really met head on by the responses.

The radar gun is likely to use a tight beam, not a multi-directional or all-around pattern.

The operators are humans, and RF has its effects on the body, so it's to the officer's advantage to have the transmitted waves not coming out at all directions.

[red][size=4]P.R.K.

Maybe they are using LIDAR, which won't activate your radar detectors even if it's pointed right at them.  If they are using any sort of radar, it will activate your detector at some point if they're zapping cars going in your direction.

Don't forget it is possible for the police car to be pointing perpendicular but the antenna turned parallel.

Also, a good traffic cop won't activate the "instant on" feature unless he visually estimates a target vehicle to be speeding.  It's part of what's called "tracking history" and will be part of his testimony.

Police radar is directional.  Wherever the antenna is pointing is what he will be reading the speed off of.  Think about it, there's no other way to effectively work it.  If it just picked up speed from everything in all directions instead of a specific target, it would be pretty much useless as a device to detect the speed of a specific target.  

He's probably timing.  VASI is a speed detection device that's really just a sophisticated timer.  All he has to do is drive his car between two points (like the two white lines you see painted across the highway) and log each point.  The VASI now knows the distance between the two points.  He pulls off to the side and hits a button when you cross the first point, then hits it again when you cross the second.  The VASI does the speed calculation and shows the speed of the vehicle.  Simple, effective, reliable, and no signature (no radar/laser emissions).  

An even cheaper method is to just have a chart, and time the vehicle between two set points.  Then you just look at your watch, and look at the chart, and you have his speed.  That's how they do it with aircraft.

Ross

I've been qualified to perform the duties of the Radar Intercept Officer in the F-14 so I had to learn all about radar theory.

erick,

They don't have the radars turned on if your detector doesn't go off as you pass by them.  Radar is just microwaves and they bounce around a lot.  I used to be able to tell if an active police radar was pacing in front of or behind me by looking at when my detector indicated higher signal strength.  If he's behind me, then as I aproach fixed reflectors (bridge overpass, highway signs) the signal increases.  If he's in front, then the signal increases when HE approaches these reflectors.  

Radar is useless in heavy traffic as it's not directional enough.   A small car followed by a large truck will not be "seen" by the radar.  That's why they developed the laser systems.  

So the answer is: the cop's radar isn't transmitting.

he can turn his antenna or he may be running laser which will not set off any radar detector.

Question for you guys that have these toys....

How to you calibrate the gun?

The last time I saw one calibrated was with a tuning fork (during my last speeding ticket back in '86). I asked the officer to verify that the radar gun was working properly. He reached in and pulled out the tuning fork, stuck it and put it in front of the gun and it gave a reading.

The fun part about this story, was my physics teacher got a ticket the same day I did and we were swapping stories. When he hear of the tuning fork his eyes got REALLY BIG!!

About one week later we were given an assignment on radar waves, sound waves and light waves. Fun stuff [puke]. I did ask why we were doing this assignment early (wave functions were to start in 4 weeks), he explained to me that he had a court date to contest his speeding ticket.

In the end he took a white board with him and asked all the questions about calibration from the officer. He then went on to explain (in mathmatical functions) that the only thing that the radar guns could determine was that something was either comming towards or away the gun. It was not able to descern down to the mile per hour. The best it could do was maybe 20 mph increments.

He had the judge, the officer and all in the court room fully versed in physics and wave functions in about an hour and understanding everything! The judge agreed, and the officer agreed that the radar gun used worthless.

Unfortunately, my physics instructor was not very well versed in legal matters. This was not the court to be discussing this. The judge finally informed him that if he wanted to do this properly, he would have find him guilty and that he would have to appeal this case up several levels.

The end of the story is that the judge agreed that radar guns (at that time) were a joke. And that he would no longer use them as evidence. He dropped the fine, but took the points off his license. Even the officer agreed that the radar gun was worthless and that he would go out and get a good stop watch and a tape measure.

Just thought I would pass this story on.

Oh, btw, I paid my fine and took the points for my ticket.

Thank you for playing.

The tuning forks don't calibrate anything, they test the calibration. There is also a computer test button on the "counting unit".

As far as the radar, it sends out a "cone" of radio signal on a known frequency. When an object enters the cone the frequency is changed in direct proportion to the speed of the object. That part of the beam is reflectd back to the radar antenna. The reflected beam is recived at the radar antenna. The counting unit determines the change in frequency of the returned beam and calculates the speed of the object.

The principle that police radar works on is known as the doppler effect. A scientific principle that has been around for 300-400 years. The doppler effect origanally related to sound waves but also applies to radar waves. It's the priciple that's used by weather radar.

As others have stated the radar anrenna must be pointed at traffic. The closer to pointed directly at traffic (angle) the more accurate the reading. Traffic radar detects motion in relationship to the antenna. If the radar is at a  1 degree offset it is MORE accurate than if it is at a 20 degree offset. The closer to directly at the "target" the better. Oh and by the way the "error" would be the further offset, the LOWER the speed reading. So if a car is going 90 mph and ther is a 1 degree offset the speed may be read as 89 mph, a 20 degree offset may read the 90 mph speed as 76.

The person who said the radar times the reflection  from the object back to the radar was incorrect. That is the principle that military radar works on. It is also how LIDAR (laser) works.

Moving radar works by sending out one beam and detecting 2 returned signals. The beam goes out and is reflected off the ground, again change in freq proprtional to the speed. The ant. recieves this signal and calculates the "ground speed" of the patrol car. The second returned signal is the return from another moving object (approaching or going away from). The beam that is reflected again has changed frequency in proportion to speed. But the frequency has been changed so that it has the speed of the police car (moving) combined with the speed of the target (also moving). This is called closing speed. The counting unit subtracts the ground speed from the closing speed to determine the target speed.

Police radar, when activated, emits a continuous "cone" of radio waves at a set frequency. It will read a speed continuously if a moving object is within its beam, reflecting the beam, within range of the radar unit.

As to the origonal post that assumes the radar is mounted in the car and the antennas cant move. Often times they can be turned. There are also hand held radars and LIDARS that are pointed at "targets" like a gun by the operator. Also most radars have "hold" buttons so that the radar is not transmitting until the operator wants it to.

As to the physics teacher, wrong. radars are using 24.1 Ghz beam the shift for 1 mph is mathmetically signifigant and well within the radar antena's ability to detect and the counting unit to calculate. If neccesary radar could easily determine speed down to a thousandth of a mph. 20 mph......if that was true do you think radar would have ever been accepted as a valid speed measuring device by ANY court anywhere?

Learn about "Cosine Error".

No AR (yet) is correct.  The ultra high frequencies used makes very accurate readings [u]possible[/u].

Arock
Yeah, that's what I was trying to say...

[url]http://www.copradar.com/preview/chapt4/ch4d1.html[/url]

[red][size=4]P.R.K.

I think we both covered Cosine Error w/o saying it by name.

I should change my screen name to Got an M4gery [;D]

For the answer to your questions go to www.RadarTest.com

LIDAR will set off LIDAR detectors, but you've no time to react.

With the right colored vehicle (dark or black colors), no chrome up front and a laser absorbing license plate cover, you may have a few seconds of reaction time depending on what range he started lasing you.

For added protection, you need one of [url=http://www.lidatek.com/echo.html]these[/url]

The entire combination mentioned above works: I've had both my vehicles equipped like this for years, and used to get lased daily while living in Ohio.

Jay
Arizona

Aaaaaaa Thanks NO-AR:(

As for the tuning fork... With this new information the orginal premise is in error.

We were given the information that the tuning fork was used for calibration. Which then inturns fudges the caculations.

I stand corrected! But hey, what do you expect with 80's information! These things were still kinda new to us back then.

He can't closk you perpendicular because there is zero phase shift to calculate the speed by.  If your fuzzbuster didn't go off, then the officer had the radar off or it was operating on a set of frequencies that your fuzzbuster doesn't detect.  Either way he couldn't clock you.  Be aware that just because the car is perpendicular to the road doesn't necessarily mean that the radar antenna in the unit is also perpendicular to the road. Some of the guys have them cocked to one side or the other.  Some guys have hand helds that can be aimed to shoot in any direction.

Probably wasn't running radar at all.  I sit along the road and write reports where people can see my car, causing them to slow down.  No radar in the vehicle at all.  Not every squad car has a radar unit in it like it is foreskin or something.

Yep, officer presence goes a long way to slow people down.  I know some departments that put empty cruisers out just to trick people into slowing down.  Another department puts an empty squad car out with the radar left on to trip your "fuzzbuster".  You have to watch out, though because once they have you lulled into a false sense of security they put an officer in the "decoy" car and stick you for a speeding ticket.

Real basic here:
Keep the lock button on until you see a fast moving target then click it off to get a reading.
If the radar unit is handheld, you can aim it any direction you want regardless of vehicle direction.
On most current units like this that I have seen/used, the reading becomes questionable when outside of 30 degrees from the radar signal or over 300 or so yards from the gun.
You also cannot generally distinguish one vehicle out of a pack. Largest vehicle (surface) returns strongest signal. (Big semi-truck moving away from you and Mustang coming at you at excessive speed...you'll get the trucks speed)
Calibration is checked prior to each use with a tuning fork tuned to various specific speeds. (55MPH, 35 MPH, etc.) If it reads incorrectly, make sure you have the right fork for the right unit (they vary by type of radar, i.e. oncoming or stationary) or send it to the techs.
Did I miss anything?
[:)]
CS
Edit-Whoops...I now see NO-AR covered some of this...

I'm with Gun-Toter, He's not running radar he looking for other violations, writing reports, or drinking coffee, I do it all the time.  Makes you slow down though.