New radar detection method?

**thestaton** · 04-07-2005, 08:19 PM

Ok, so I was eating lunch with my friend today who was telling me all about his friend who is a cop in Maine and has been apart of the testing method for a new type of speed detection.

According to him it can only be used at night time and it judges your speed by the officer aiming it at your headlight. Seriously I pulled out t the BS flag immediatly however if there was any place that would have the answer I knew it would be here.

Is he full of it or what?

Thanks

Staton

**SmaartAasSaabr** · 04-07-2005, 08:20 PM

LIDAR?

**thestaton** · 04-07-2005, 08:23 PM

well that is what I asked is it LIDAR and he said no this is new and totally undetectable. because it gets the reading from the acutal light not the reflection sorry I should of clarified.

thanks

staton

**SmaartAasSaabr** · 04-07-2005, 08:27 PM

?

How is it possible for it to do that, considering the light is a beam... ie it is travelling at let's say the speed of light ( or c)... so 299 792 458 m / s.

So this unit is precise enough to calculate a difference of a few km/h when the light moves at 1 079 252 849km/h??! :roll: (that's a billion 79 million, 252 thousand 849 kilometres per hour...)

**lordhamster** · 04-07-2005, 08:38 PM

Originally Posted by SmaartAasSaabr

?

How is it possible for it to do that, considering the light is a beam... ie it is travelling at let's say the speed of light ( or c)... so 299 792 458 m / s.

So this unit is precise enough to calculate a difference of a few km/h when the light moves at 1 079 252 849km/h??! :roll: (that's a billion 79 million, 252 thousand 849 kilometres per hour...)

Light always travels at the speed of light. If you measure the speed of light C. If you measure light coming from a car heading towards you, its speed will still be C not C + 55mph. HOWEVER.. the wavelength of the light changes depending if the light source is approaching you or going away from you and at what rate. This phenomenon is called the doppler effect, or redshift and blueshift. This is how scientists know the universe is expanding rather than contracting. Imagine staning on a street while a car is approaching you.. When he is approaching you the engine sound sounds higher pitched... as he passes and then goes away from you, the engine sounds changes and becomes lower pitch. Same thing happens with light. Measuring this type of doppler shift is how radar guns work. They shoot a radar signal at you, then wait for the return and measure it's doppler shift.

Theoretically if you have a good enough instrument, you might be able to use this "shift" method to passively (without sending out a signal) determine relative speed, but I don't think it would be accurate enough... since not all headlights will be exactly the same. Radar guns work since the exact frequency and wavelength of the outgoing signal are known and thus you can compare the return.

Current laser speed detection works by firing a laser beam (light) at you. Then measuring how long it takes to reflect back. Since light always travels at the speed of light C, they can calculate how far you are from the gun at the time they shot. They do this distance measurement a second time and they'll know how fast you covered a given distance.. your speed.

Other than the redshift method... the only other method I can think that may work would be to measure the apparant distance between headlights... as a car approaches, its headlights appear to be closer together... the closer it comes the further they get from one another... but again not all headlights are equidistant.

So I'm not sure what he is talking about, but the shifting measurement sounds the most promising... this is essentially how Radar works today, by measuring so called doppler shift.

**SmaartAasSaabr** · 04-07-2005, 08:45 PM

You're right. Nothing can exceed c. I must be drunk.

**Ghost** · 04-07-2005, 10:06 PM

I think your friend was just talking about LIDAR, and perhaps heard a few of the facts somewhat skewed.

Though the idea of clocking a car based on the blueshift is intriguing, it's also not really feasible. All headlights would have to be emitting the EXACT same frequency with extremely low variation, and all would have to have the exact same plastic/glass covering. And... come to think of it, the earth's rotation and orbit around the sun would affect it to, so it would have to base it off the exact date/time, and a 3-dimensional bearing of where you are in relation to the car. So yeah... only in sci-fi, for the foreseeable future.

Using distance between headlights would require that you know the distance of the car from you at one point, and then again at another. In which case you might as well use the variance in the two distances (adjusted a bit based on the angle you're taking the observation at) divided by the time between measurements instead of doing hoo-hah with the distance between the lights.

So, yeah... most likely he was thinking about LIDAR unless I'm just not thinking of something.

**crazyVOLVOrob** · 04-07-2005, 10:24 PM

thats what we call LIDAR!

**Veil Guy** · 04-08-2005, 08:50 AM

Could be VASCAR.

This is used a lot in PA. The trail of the headlight beam is measured between two arbitrary points, selected by the officer and compared to the officers travelling of the same distance.

Speed of Officer (Known) - Distance Traveled (Between Two Selected Points) :
Speed of Driver (Calculated) - Distance Traveled (Between Same Two Selected Points)

Its totally undetectable and totally lethal.

The Veil Guy 8)