Radar guns

[largejuan]

How can a police officer individually pick out a vehicle among a group driving appx. 65-70mph? I was behind a vehicle who was setting the speed in the lane at the time. What circumstances dictate my selection? I drive a nice truck, no record, etc. Also, if the officer uses a RADAR gun, are there state certifications that the officer has to have? Im sure it varies from state to state, but any information would be useful.

[Athas]

Ok....I'm not a cop and I've never used a radar gun in my life outside the lab....but I will try to answer your question until a real cop can.

How a radar gun works.... (the ones I've used)
A radar gun sends out a radar wave...which is part of the electromagnetic (EM) spectrum that corresponds to 10-100 m in wavelength. All EM wave, regardless of wavelength or freq travel in air at essentially C; the speed of light or 3.0x10^8 m/sec. (sorry I don't know the feet conversion off hand..only SI.) Anyway... most officer use a radar gun with traffic that is coming towards them. A properly trained officer simply has to point the radar at your car and do to the fast speed of the EM wave it reaches your vehicle very fast. Lets say he sees you a 50m it would take only 3.33x10^-7 seconds for the wave to make a round trip back to the officer....ignoring the fact you are moving. Providing he points it at the correct car, and at a high enough angle, their is really no way another vehicle can interfer unless they happen to cross the line of sight between your gun and the car...very unlikely given the speed of the wave. Now, how do they know your speed....this is doppler shift effect. Think of sound waves...as they move towards you ( an ambulance approaching) they get louder/ higher pitch. Why? because the wave front is being compressed, which means wavelength decreases and freq/pitch increases. As the ambulance passes you the wavefront gets stretched and the frequency/pitch drops. I think most have experienced this with trucks or something.
Samething happen with EM waves...as they hit your moving vehicle they their wavelengths are compressed, reflected, and returned to the gun.
Now your velocity is given by:
v = ((L-Lo)/Lo ) *c
where L= original wavelength sent out.
Lo= wavelength received back.
v= velocity of your vehicle
c= speed of light.
So Lo<L which implies the greater this is so the larger the numerator and the faster the speed you are going. So the faster you speed the greater L-Lo will be. The only hitch is that the officer has to aim for a very reflective portion of your vehicle...like the license plate or the bumper. He/she must also be careful of "back-scatter" from other vehicles...the radar has a certain spread and if other vehicles are caught in the spread this can cause a false reading. The training of the officer should prevent this.
Hope I answered what you were asking.

[abirtz]

Wow, real technical stuff there Athas. Most agencies (if not all) require their officers to be certified in order to run radar. But the fact is, even if they do not target your exact car, if your traveling in that pact they can only pull one person over. So if it is you, you can't really use that excuse that everyone was going the same so how can you tell. How does that saying go.

"Have you ever been fishing"

Just think about it....

[rare21]

what i say is if your'e hauling a next to a rig you're fine if you know radar ( as i am in the navy) you can know if you're hauling butt next to a large rig next to you then that cop car cant get nothing on you cuz its all messed up and your'e fine...if you're gonna haul *** next to a cop then haul next to a rig while your'e at it...lets see what they do to you..and if they do then you can fight it

[Stump]

The officers here have to take a class to be certified. They need to be able to judge the speed of a car by sight to verify what the gun is saying. Many of the officers here can judge the speed to within 1 mph. They also have to check the radar periodically to ensure it is working properly. I know we have little tuning forks that we use in some cars.
Because the officers are certified and can gauge by their eye the speed of a vehicle, rare's whole theory goes down the toilet...

*** Jenna - what happened to the profanity block? ***

[txinvestigator1]

Rare 21...WHAT????

original poster

The older radars only displayed one speed, the best target. The best target depended on several factors such as frontal area, reflective substance (metal v fiberglass) and paint.

An officer had to make a visual determination as to the faster vehicle.

The newer radars can be set to display the highest speed in a group.

Also radar does not have to be aimed directly at the vehicle. It has a very wide beam.

If you are driving along in a group of cars and an officer is running radar, he will either see the speed of the best target or the speed of the fastest vehicle, depending on the radar he is using. The officer must be able to determine that the vehicle he picks out matches what the radar indicates.

[Solego]

The answer is yes. Radars can determine the fastest or slowest of the group, while the operator is moving or stationry, and coming from behind or from the front. And yes, radar detectors work. They let you know that you've already been painted and clocked. I like writing people with detectors.

[motorpatrol110]

The department that I work for has supplied us with Stalker brand radars, with front and rear antennas in our cars. I am not only a certified operator of the Stalker, but a company certified instructor as well. Although I am not a radar technician (meaning I can't take the radar apart, repair it, or put it back together) I can testify to what it's functions are and how to properly operate it.

First of all, the model of radar that we have, has been equipped with a "fast" function, meaning that, say for instance, two cars approaching my location at the same time, with one travelling faster than the other, the radar will display both speeds. I can't count the number of times that a semi truck or other large target is coming towards me and some goofball on a motorcycle or other small target thinks that I can't determine his speed as he whizzes by the semi in either the same or opposite direction. WRONG! I just love writing those tickets! Anyway, just remember that if you are speeding, even behind someone who is "setting the pace", the speed of your vehicle CAN be determined. Isn't the real issue here the fact that if the "pace setter" is speeding, and you are following him at a comparable speed (that is probably over the limit), aren't you taking the risk of getting a ticket? Why whine about it. Just slow down and do the speed limit
- motorpatrol110

[TTS]

Tx

I have to agree with the first line of your response.

[abirtz]

Question:

You have a group of vehicle traveling 80 during rush hours downa major interstate. Lets say 15 cars...Which one are you going to stop, and why?

Also, lets say you have 2 cars, a Ferrari and a Ford Windstar doing 90 on the highway, which one then? Thanks.

[locokarma]

Just a quick question.... do you prefer laser/lidar over plain radar?

From what I've seen personally, it's more accurate. Just curious.

[txinvestigator1]

By accurate do you mean precise, as in able to be selective in the vehicle targeted?

[locokarma]

From my understanding... yes. It targets a specific vehicle with a laser beam.... Just like pointing a gun at a target... same concept.

[TTS]

Yes but the cost of lidar is way too high.