Radar Guns

[CycloWizard]

Being Labor Day weekend, there were lots of cops out and it got me thinking: how do they actually measure your 'speed'? More importantly, are they measuring speed or velocity?

1. I was thinking that they are probably measuring the component of velocity towards them.
2. I can also conceive of a way to measure the straight-ahead velocity (i.e. what I would call 'speed') by making multiple measurements, though this would make the 'gun' more tricky to design.

The important result of this I suppose is that you could influence the speed that they measure by changing lanes if they are measuring according to #1, whereas it doesn't matter if #2 is true. Anyone have any idea how these things actually work?

 

[TallBill]

Radar works by measureing a doppler shift. Yes, they measure velocity in regards to #1. Thats why cops park parallel to the road, anything greater then a 10 degree angle will drastically reduce the speed that shows up on the radar.

Extreme example. Cop is parked at an intersection with radar on. You are moving on cross traffic and fly through going 100 mph. His radar wouldn't even register a speed because the 90 degree angle. You would be moving 0 mph towards him.

<-- radar/lidar certified.

 

[Mr Fox]

Originally posted by: CycloWizard
Being Labor Day weekend, there were lots of cops out and it got me thinking: how do they actually measure your 'speed'? More importantly, are they measuring speed or velocity?

1. I was thinking that they are probably measuring the component of velocity towards them.
2. I can also conceive of a way to measure the straight-ahead velocity (i.e. what I would call 'speed') by making multiple measurements, though this would make the 'gun' more tricky to design.

The important result of this I suppose is that you could influence the speed that they measure by changing lanes if they are measuring according to #1, whereas it doesn't matter if #2 is true. Anyone have any idea how these things actually work?

Both Police Radar, and Laser both function using the Doppler effect.


Doppler Effect


Typically they require a minimum of Three separate locks, before you can be issued a Ticket. One of the issues that can come into play is confirmation of target especially with Radar, as it is designed to read the fastest moving object. Then it is up to Operator Discretion to visually establish the faster object.

Laser is more precise, as they see the target vehicle in the sights of the gun.

 

[TallBill]

I should say instructor certified. Its been a while since I've dealt with it. Anyways, changing lanes would make such a minor difference that it wouldn't even be noticeable. It would just change the angle of the reflecting surface to the radar unit. At a far distance, even moving over two lanes, or 20 feet, will be a fraction of a degree difference on the angle.

 

[CycloWizard]

Originally posted by: TallBill
I should say instructor certified. Its been a while since I've dealt with it. Anyways, changing lanes would make such a minor difference that it wouldn't even be noticeable. It would just change the angle of the reflecting surface to the radar unit. At a far distance, even moving over two lanes, or 20 feet, will be a fraction of a degree difference on the angle.

Well, that depends on my next question: what is the effective range? If I'm sufficiently close, then changing lanes has a dramatic effect on the angle. If, instead, the guy can clock me from a mile away, then changing lanes won't matter at all. Based on my empirical experience (i.e. slowing down when I see a cop - I'm pretty good at spotting them on the horizon ), I'm guessing that the range is relatively short - not more than a hundred meters or so.

Are radar or laser devices more common? If the cop is driving, how does the gun account for his velocity? Sorry for all the questions, just really curious about this all of a sudden and I've apparently found people who know what they're talking about.

 

[Gneisenau]

There is an angle range the car is supposed to be within I believe. I don't know what it is. The guns are calibrated to measure a car speed between say 10-20 deg. (I just WAGed those numbers BTW.) relative to the gun. If they shoot at an angle too shallow, their reading would be faster than your car is going, if the angle is too steep, their reading would be less than what you are really going. (like TallBill said.) Most cops I know, fudge the angle to read on the slow side. It's harder to argue that point in court.

The range from you to the gun becoming a problem shouldn't come into play because of the angle issue in general. A cop who is good at using the gun will trap you while you are moving straight and constant.

 

[Gneisenau]

Originally posted by: CycloWizard


Are radar or laser devices more common? If the cop is driving, how does the gun account for his velocity? Sorry for all the questions, just really curious about this all of a sudden and I've apparently found people who know what they're talking about.

Radar is more common I would think. If only because it's been in use so much longer.

As far as when the police are moving, that is a good questions. I don't know if the gun is tied into their speedometer or how it checks it's own speed.

 

[Mark R]

Originally posted by: Gneisenau

As far as when the police are moving, that is a good questions. I don't know if the gun is tied into their speedometer or how it checks it's own speed.

Radar is generally reserved for stationary applications.

For moving enforcement, speed measurement usually uses time-distance measurements. Two methods are used.

1) On roads where violations are common, the cops may paint special markings on the road at a calibrated 1/4 mile apart. Officers in a video pursuit vehicle record the chase, and a stopwatch device in the car, calculates the speed, my assuming 1/4 mile between clicks. The video together with calibrated markings provide irrefutable proof of speed.

2) In the absence of markings, a similar technique can be used. Except the stopwatch is used to record 4 events. Suspect passes landmark 1, cops pass landmark 1, supspect passes landmark 2, cops pass landmark 2. As the stopwatch gets distance data from the vehicle's calibrated speedometer, it can perform an accurate calculation. Again video evidence can be used to validate the measurement, but there is a slightly greater margin for error.

 

[spidey07]

Originally posted by: Mark R

Originally posted by: Gneisenau

As far as when the police are moving, that is a good questions. I don't know if the gun is tied into their speedometer or how it checks it's own speed.

Radar is generally reserved for stationary applications.

For moving enforcement, speed measurement usually uses time-distance measurements. Two methods are used.

1) On roads where violations are common, the cops may paint special markings on the road at a calibrated 1/4 mile apart. Officers in a video pursuit vehicle record the chase, and a stopwatch device in the car, calculates the speed, my assuming 1/4 mile between clicks. The video together with calibrated markings provide irrefutable proof of speed.

2) In the absence of markings, a similar technique can be used. Except the stopwatch is used to record 4 events. Suspect passes landmark 1, cops pass landmark 1, supspect passes landmark 2, cops pass landmark 2. As the stopwatch gets distance data from the vehicle's calibrated speedometer, it can perform an accurate calculation. Again video evidence can be used to validate the measurement, but there is a slightly greater margin for error.

meh, I've been in police cars where they have radar on the front, rear and side of the car and can measure the speed of any car, be it directly oncoming or not. Policeman even showed it off to me when he gave me a ride.

That's what I can't figure out, how does it work when the police car is moving? The radar must also measure the speed of the pavement and the car as well to come up with the speed of the car.

 

[silverpig]

Originally posted by: spidey07

Originally posted by: Mark R

Originally posted by: Gneisenau

As far as when the police are moving, that is a good questions. I don't know if the gun is tied into their speedometer or how it checks it's own speed.

Radar is generally reserved for stationary applications.

For moving enforcement, speed measurement usually uses time-distance measurements. Two methods are used.

1) On roads where violations are common, the cops may paint special markings on the road at a calibrated 1/4 mile apart. Officers in a video pursuit vehicle record the chase, and a stopwatch device in the car, calculates the speed, my assuming 1/4 mile between clicks. The video together with calibrated markings provide irrefutable proof of speed.

2) In the absence of markings, a similar technique can be used. Except the stopwatch is used to record 4 events. Suspect passes landmark 1, cops pass landmark 1, supspect passes landmark 2, cops pass landmark 2. As the stopwatch gets distance data from the vehicle's calibrated speedometer, it can perform an accurate calculation. Again video evidence can be used to validate the measurement, but there is a slightly greater margin for error.

meh, I've been in police cars where they have radar on the front, rear and side of the car and can measure the speed of any car, be it directly oncoming or not. Policeman even showed it off to me when he gave me a ride.

That's what I can't figure out, how does it work when the police car is moving? The radar must also measure the speed of the pavement and the car as well to come up with the speed of the car.

There's this handy little invention called a speedometer which I think might come in handy

 

[spidey07]

Originally posted by: silverpig
There's this handy little invention called a speedometer which I think might come in handy

But the angles don't work out right if it's purely linked to the speedo. Coming head on I can easily see that as trivial - subtract officer speed from offender speed (as measured coming toward officer)

 

[TallBill]

Ok, here goes.

I'm guessing that the range is relatively short - not more than a hundred meters or so.

Actually, stationary radar is good up to about 1/4 mile. The difficult part is spotting which car is moving the fastest and thus returning the highest number. However, proper tracking allows the officer to follow a car and match a speed to it. So if he gets you at 95 and then watches you decelerate to 75 then its an easy ticket at 95. The reason speeders some times get away is that the officer isn't 100% sure of which car is going the fastest, but to a trained eye its actually fairly easy to tell.

Are radar or laser devices more common?

There are probably more radar devices. Lidar is a handheld device, point and aim.

If the cop is driving, how does the gun account for his velocity?

New radar systems have both a high doppler and a low doppler. The high doppler goes out in a cone shape, reflects off a surface and returns to the unit. The radar does the math based on a change in wavelength. The low doppler is pointed at the ground and calculates the police cruiser's speed. Once again, the radar does all the math.

If they shoot at an angle too shallow, their reading would be faster than your car is going, if the angle is too steep, their reading would be less than what you are really going.

Actually, that is incorrect. It's impossible to have to shallow of an angle. Driving head on at the radar unit will give a true 100% speed. Results dimish as you move towards 90 degrees. I don't have an exact numbers, but at a 35 degree angle an 100 mph car would probably show up as like a 40-50 mph car.

For moving enforcement, speed measurement usually uses time-distance measurements. Two methods are used.

Not always, but there are several time-distance techniques that can be utilized and any inaccuracies favor the driver.

There's this handy little invention called a speedometer which I think might come in handy

Yes, the speedometer can be utilized in a time-distance technique. However, it must be calibrated and inspected just like the radar/lidar equipment if it is to be upheld in court.

 

[CycloWizard]

Originally posted by: TallBill
Actually, that is incorrect. It's impossible to have to shallow of an angle. Driving head on at the radar unit will give a true 100% speed. Results dimish as you move towards 90 degrees. I don't have an exact numbers, but at a 35 degree angle an 100 mph car would probably show up as like a 40-50 mph car.

This is fairly easy to calculate using simple geometry. That's why I was wondering whether this would actually work. The component of velocity that the cop will measure will be equal to your true speed multiplied by the cosine of the angle of measurement.

Thanks again for the info guys. It looks like the angle isn't terribly important, but a few degrees is a few percent, which can never hurt.

 

[TallBill]

lol, no problem Usually by the time you see the cop you're toast anyways

 

[silverpig]

Originally posted by: spidey07

Originally posted by: silverpig
There's this handy little invention called a speedometer which I think might come in handy

But the angles don't work out right if it's purely linked to the speedo. Coming head on I can easily see that as trivial - subtract officer speed from offender speed (as measured coming toward officer)

They work out just fine if it's coming head on or trailing from behind. How many roads do you know that are as wide as they are long? The only time two cars will come together at 90 degrees is at an intersection.

I'm not 100% sure but it seems to make sense. I'll ask my brother in law this weekend.