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Thread: Radar Question

  1. #1 Radar Question 
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    I was walking home a few days ago and I saw one on those police radar carts that sit on the side of the road; the ones that list the speed limit and then tell you your speed. It was on my side of the road and there was a car coming towards me; as soon as it passed the cart and moved into the radar's field of view, it read the cars speed as 30mph, then as it moved further away from it and more into it's direct view the in a second it dropped from 30 to 24 (I would say 24mph was the actual speed the car was moving) without any decrease in the cars speed. I understand the basic concepts of how radar works, but I am unsure what would cause this; maybe the proximity of the car and it's angle caused more reflection back to the radar antenna? Any ideas?


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    Cooking Something Good MacGyver1968's Avatar
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    Probably the Doppler effect.
    Doppler effect - Wikipedia, the free encyclopedia


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    Quote Originally Posted by MacGyver1968 View Post
    I guess it would depend on if they were using RADAR or LIDAR.
    I don't think LIDAR 'cares' about Doppler effects.
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    It was def a radar antenna, not lidar. It was one of those unmanned carts that the police use; they tow them around and set them up and leave them.
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    Easy trig from angle transversed in a second,go figure
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    Quote Originally Posted by r_2016 View Post
    I was walking home a few days ago and I saw one on those police radar carts that sit on the side of the road; the ones that list the speed limit and then tell you your speed. It was on my side of the road and there was a car coming towards me; as soon as it passed the cart and moved into the radar's field of view, it read the cars speed as 30mph, then as it moved further away from it and more into it's direct view the in a second it dropped from 30 to 24 (I would say 24mph was the actual speed the car was moving) without any decrease in the cars speed. I understand the basic concepts of how radar works, but I am unsure what would cause this; maybe the proximity of the car and it's angle caused more reflection back to the radar antenna? Any ideas?
    Most, if not all, automotive radars depend on Doppler shift to infer speed. The greater the shift, the greater the speed. But what gets measured, of course, is the component of velocity in the direction of the beam. Head-on, you'll measure the full approach velocity. Side-on, you'll measure a much lower value (asymptotically zero). In between head-on and side-on, you'll measure a continuum of velocity values. Note that the geometry guarantees that they will never overestimate your speed, so you can't use a geometric dependence argument to talk your way out of a ticket. I'm sure that the police are trained to aim at vehicles far enough away that the needed geometric correction is negligible.
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  8. #7  
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    Quote Originally Posted by MacGyver1968 View Post
    Probably the Doppler effect.
    Doppler effect is how such radars work to begin with; it's not a source of error in the case he mentioned.

    However the following _can_ cause such errors:
    Trigonometric effects (radar only sees the velocity component away from the transmitter/receiver, not its actual speed)
    Different primary reflection (i.e. it "sees" the wrong car)
    Motion of the radar system itself
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    Radar I've heard of the stuff, it sends out a series of pulses and it recives a series of pulses back and measures the time it takes for the return pulse and it corralates to the distance of the object its bouncing back off of.It must use a computer to calculate the speed of a moving object. That I wouldn't know about, probably all hush hush only them and need to know don't ask them any questions and don't look at them offical goverment.The radar is rated at being harmless to people, but if you go throug five of them on your way to work, Idon't think you can sue them for frying your eyeballs out with it. Its somewhere in the microwave region.
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    Quote Originally Posted by sapien View Post
    Radar I've heard of the stuff, it sends out a series of pulses and it recives a series of pulses back and measures the time it takes for the return pulse and it corralates to the distance of the object its bouncing back off of.It must use a computer to calculate the speed of a moving object. That I wouldn't know about, probably all hush hush only them and need to know don't ask them any questions and don't look at them offical goverment.The radar is rated at being harmless to people, but if you go throug five of them on your way to work, Idon't think you can sue them for frying your eyeballs out with it. Its somewhere in the microwave region.
    Are you competing for an award for the least helpful post ever?
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    Why is that?
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    Brassica oleracea Strange's Avatar
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    Because your admitted lack of knowledge of how radar works does nothing to answer the question. And your fantasies about secrecy and fried eyes is just silly and irrelevant.
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    Anyone with the least amount of technical knowledge knows what radar is.
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    Quote Originally Posted by tk421 View Post
    But what gets measured, of course, is the component of velocity in the direction of the beam. Head-on, you'll measure the full approach velocity. Side-on, you'll measure a much lower value (asymptotically zero). In between head-on and side-on, you'll measure a continuum of velocity values.
    That was the first thing, I thought of. But then the speed would have appeared to increase not decrease.

    Quote Originally Posted by r_2016 View Post
    in a second it dropped from 30 to 24
    I wonder if the driver was braking, as they realised they were passing a radar?
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by sapien View Post
    Anyone with the least amount of technical knowledge knows what radar is.
    And how did your "I've heard of it" reply help answer the OP's question?
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    The discussion was about radar used to measure a cars speed.If you use the tan of 90' or1 and the 90' angle over a second theres a way to calculate mult cars speeds in one second without using arithmetic. Radar is easier to use. A true radar expert would be the guy who designed the latest one. Gibberish etc.......................!
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    Quote Originally Posted by r_2016 View Post
    I was walking home a few days ago and I saw one on those police radar carts that sit on the side of the road; the ones that list the speed limit and then tell you your speed. It was on my side of the road and there was a car coming towards me; as soon as it passed the cart and moved into the radar's field of view, it read the cars speed as 30mph, then as it moved further away from it and more into it's direct view the in a second it dropped from 30 to 24 (I would say 24mph was the actual speed the car was moving) without any decrease in the cars speed. I understand the basic concepts of how radar works, but I am unsure what would cause this; maybe the proximity of the car and it's angle caused more reflection back to the radar antenna? Any ideas?
    I don't understand this description. Wouldn't the car have been in the radar's field of view before it passed the cart, then gone out of the field of view after it passed?
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    Cooking Something Good MacGyver1968's Avatar
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by MacGyver1968 View Post
    Probably the Doppler effect.
    Doppler effect is how such radars work to begin with; it's not a source of error in the case he mentioned.

    However the following _can_ cause such errors:
    Trigonometric effects (radar only sees the velocity component away from the transmitter/receiver, not its actual speed)
    Different primary reflection (i.e. it "sees" the wrong car)
    Motion of the radar system itself
    Thanks for correcting me.
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    Quote Originally Posted by Harold14370 View Post
    I don't understand this description. Wouldn't the car have been in the radar's field of view before it passed the cart, then gone out of the field of view after it passed?
    I think the point is that car is not driving towards the radar but is coming the other way. So the radar is measuring its recessional velocity.

    If this is one of those things that just flashes up your speed as a warning, I'm surprised it didn't just ignore cars going away from it.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Harold14370 View Post
    I don't understand this description. Wouldn't the car have been in the radar's field of view before it passed the cart, then gone out of the field of view after it passed?
    I think the point is that car is not driving towards the radar but is coming the other way. So the radar is measuring its recessional velocity.

    If this is one of those things that just flashes up your speed as a warning, I'm surprised it didn't just ignore cars going away from it.
    That's what I thought too. You wouldn't have it tell the speed of the cars in the opposite lane going the other way.
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